WO2010127386A1

WO2010127386A1 - Optical recording, storage and retrieval product, process, system and medium

Info

Publication number: WO2010127386A1
Application number: PCT/AU2010/000415
Authority: WO
Inventors: Min Gu; Peter Zijlstra; James Won Min Chon
Original assignee: Swinburne University Of Technology
Priority date: 2009-05-06
Filing date: 2010-04-15
Publication date: 2010-11-11

Abstract

An optical data storage product, including a data storage medium containing mutually spaced bodies selectively responsive to light of respective different wavelengths and respective different polarisations, said bodies being distributed in a plurality of selectably addressable regions of said data storage medium such that each region contains groups of said bodies selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each group of bodies in each region can be selectively modified to store digital data in said region using light of a corresponding one of a plurality of said wavelengths and a corresponding one of a plurality of said polarisations and having an intensity exceeding a threshold intensity.

Description

OPTICAL RECORDING, STORAGE AND RETRIEVAL PRODUCT, PROCESS,

SYSTEM AND MEDIUM

FIELD

The present invention relates to products, processes, systems and media for optical recording, storage and retrieval. The processes and systems may be for creating and using products and media, such as writable optical filters and/or optically writable and readable high density data storage. The products, processes, systems and media may use optically responsive bodies, e.g., nanoparticles.

BACKGROUND

The seemingly never ending desire to store ever increasing amounts of information, particularly in digital form, continues to push the limits of data storage technologies. There have been tremendous improvements in the capacities of removable and non-removable data storage media in recent years, and it has never before been possible to store so much information at such low cost. Terabyte capacity hard disk drives are now commodity items and are commonplace in household personal computers. Digital Versatile Disc (DVD) optical storage disc technology is currently being replaced by Sony's "Blu-Ray" optical storage disc technology, with commercially available Blu-Ray disks having a capacity of 50 GB, about six times greater than DVDs.

Yet even today's unprecedented data densities and the storage capacities of these technologies never seem to be enough to meet consumer demand, and thus there is a continuing need to develop new, higher capacity data storage technologies. As the rate at which information is generated, digitised, shared and accessed on a global scale continues to increase, the requirements for ever-higher density storage, e.g., in inexpensive, stable, easily written and easily readable recording media, will also continue to grow. Furthermore, information can be stored and applied in the form of optical components in optical systems, such as filters. Filters can be fabricated with high precision but there is a need for more easily and cheaply manufacturable optical filters with arbitrary, controllable and highly detailed patterns, e.g., for photonic and plasmonic circuits.

It is desired to address or ameliorate one or more disadvantages or limitations associated with the prior art, or to at least provide a useful alternative.

SUMMARY

In accordance with the present invention there is provided an optical data storage product, including a data storage medium containing mutually spaced bodies selectively responsive to light of respective different wavelengths and respective different polarisations, said bodies being distributed in a plurality of selectably addressable regions of said data storage medium such that each region contains groups of said bodies selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each group of bodies in each region can be selectively modified to store digital data in said region using light of a corresponding one of a plurality of said wavelengths and a corresponding one of a plurality of said polarisations and having an intensity exceeding a threshold intensity.

The different respective combinations of wavelength and polarisation can correspond to different respective combinations of orientation and dimensions of said bodies. The bodies can be distributed within a substance that is substantially unresponsive to said light. The data storage medium can be in the form of a circular disc. The modification can provide a persistent change due to photothermal reshaping of the bodies, wherein the reshaped bodies are no longer responsive to light of the corresponding wavelength and corresponding polarisation. The bodies can be held in a surrounding substance that resists mechanical forces from the bodies caused by the photothermal reshaping. The bodies can be initially elongate nanorods of an electrically conductive material, and the modification of the bodies can include changing their shape and thereby their optical responsiveness such that the modified bodies can be optically distinguished from the unmodified bodies to allow the stored data to be determined optically. Each body can be initially a gold nanorod with a responsive wavelength defined by a longitudinal surface plasmon resonance (SPR) of the gold nanorod. The bodies can be arranged in layers configured to allow separate optical addressing of different layers using the light. The groups of bodies can be substantially homogeneously distributed in each region.

The present invention also provides an information recording process including: receiving information to be stored; generating a plurality of input optical signals having respective different wavelengths and respective different polarisations representing the information to be stored; storing the information by selectively applying the input optical signals to a plurality of regions of an information storage medium having an initial configuration of mutually spaced bodies selectively responsive to each of said input optical signals, the intensity of the input optical signals exceeding a threshold intensity to modify said bodies.

The information recording process can include selecting the wavelengths and the polarisations of the input optical signals using one or more optical filters. Generating the input optical signals can include simultaneously generating a plurality of the wavelengths and/or the polarisations, and storing the information can include applying the generated plurality of wavelengths and/or polarisations simultaneously.

The present invention also provides an information retrieval process, including: generating a plurality of input optical signals having respective different wavelengths and respective different polarisations; directing the input optical signals to a plurality of regions of an information storage medium, each region having a plurality of mutually spaced bodies being selectively responsive to each of said input optical signals to represent stored information; and detecting output optical signals from an interaction of the input optical signals with the bodies in each region to determine the stored information. The intensity of the input optical signals can be selected to not exceed a threshold intensity to not modify said bodies. The information retrieval process can include selecting one or more wavelengths and one or more polarisations of light in the output optical signals using one or more optical filters. Generating the input optical signals can include simultaneously generating a plurality of the different wavelengths and/or the different polarisations, and detecting the output optical signals can include simultaneously detecting the output signals having a corresponding plurality of wavelengths and/or polarisations. Detecting the output optical signals can include simultaneously detecting the output optical signals in two spatial dimensions for each wavelength and each polarisation of the output optical signals. The bodies can respond to the input optical signals by emitting the output optical signals at an output wavelength different from the wavelength of the input optical signals, and detecting the output optical signals can include detecting the output optical signals at the output wavelength. The different output wavelength can be caused by two-photon luminescence (TPL) of the bodies, and the output wavelength can be shorter than the wavelength of the input optical signals.

The regions can be distributed in three spatial dimensions. Generating the input optical signals can include generating pulsed signals. Generating the input optical signals can include generating continuous-wave (CW) signals. The stored information can include any one of: a two-dimensional or three-dimensional filter pattern for a polarisation- and wavelength-selective optical filter; digital data for use in a digital processor and/or digital computing device; and a two-dimensional or three-dimensional image for display to a user. A system can be configured to perform the process, including an optical source configured to generate the input optical signals.

The present invention also provides an optical data storage product, including mutually spaced bodies selectively responsive to light of respective different wavelengths and respective different polarisations, wherein populations of said bodies are selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each population in a region of the data storage product can be selectively modified to store digital data in the region using light of a corresponding wavelength and polarisation having an intensity above a threshold writing intensity.

The present invention also provides an optical data storage product, including a plurality of mutually spaced bodies, including: first groups of said bodies selectively responsive to light of respective different wavelengths; and second groups of said bodies selectively responsive to light of respective different polarisations, said bodies in said groups being selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each of the bodies can be selectively modified in a selected region of the data storage product to store digital data in said region using light of a corresponding wavelength of the different wavelengths and a corresponding polarisation of the different polarisations and having an intensity exceeding a threshold intensity for writing data.

A process for forming the optical data storage product can include: forming the bodies; mixing the bodies in an unresponsive substance to form a mixture; forming an initial recording layer on a substrate using the mixture; and forming one or more combinations of a spacer layer adjacent a further recording layer, starting adjacent the initial recording layer, to provide three spatial dimensions for data storage. Forming of the initial recording layer can include spin-coating the initial recording layer on the substrate.

Also described herein is an information storage medium including a recording material having a plurality of bodies being selectively responsive to light of a plurality of respective frequencies and polarisations, including one or more first bodies selectively responsive to light of a first frequency and a first polarization, and one or more second bodies selectively responsive to light of a second frequency different to the first frequency and a second polarization different to the first polarization. The plurality of bodies can have a corresponding plurality of morphologies corresponding to respective frequencies, and a corresponding plurality of orientations corresponding to respective polarisations. Each body may be defined by one or more boundaries in the recording material between an unresponsive (substantially transmissive) substance and a responsive (substantially opaque) substance. The transmissive substance may be polyvinyl alcohol (PVA). The opaque substance may be a conductor at the plurality of frequencies, such as gold or silver, for example.

Also described herein is an optically writable medium including a recording material that responds to an incident beam of light having an intensity above a threshold intensity in a volume of the incident beam by persistently changing a first optical property of the recording material in the volume only if the incident beam has a first wavelength and a first polarization, and persistently changing a second optical property in the volume only if the incident beam has a second wavelength different to the first wavelength and a second polarization different to the first polarization. The volume may be a volume pixel (voxel) in a pattern. A "persistent" change is a change that endures after the incident beam of light is removed, preferably for a relatively long period of time, such as minutes, days or years. The first optical property can be the extinction of light with the first wavelength and first polarization. The second optical property can be the extinction of light with the first wavelength and second polarization. The information storage medium can include a plurality of mutually spaced elongate conductive bodies having different dimensions and orientations such that light of a selected wavelength and a selected polarisation selectively couples to one or more of said bodies having dimensions corresponding to the selected wavelength and an orientation corresponding to the selected polarisation. The configuration of the bodies represents stored information that can be determined by monitoring the interaction of light of selected wavelengths and selected polarisations with the information storage medium. The information can be stored by selectively coupling light of the selected wavelengths and selected polarisations to the corresponding mutually spaced elongate conductive bodies of the information storage medium, wherein the coupled light has a power sufficient to modify the morphology of those bodies and thereby modify the coupling of light thereto. The elongate bodies can be modified by melting to form substantially spherical bodies that do not substantially couple to the light of the selected wavelength and polarisation. The light can be directed to selected locations of the information storage medium, the locations being distributed three-dimensionally to provide the storage medium with five addressable dimensions.

Also described herein is a process for forming the information recording medium includes forming a plurality of layers of the material, wherein the layers are formed to be at least partially transparent to light of said frequencies and polarisations.

Also described herein is an information storage process including selecting a frequency and polarisation for an optical signal; generating an optical signal having the selected frequency and polarisation; and directing the optical signal to an information storage medium having a plurality of bodies selectively responsive to optical signals having respective frequencies and polarisations such that the optical signal selectively modifies the responsiveness of one or more of the plurality of bodies in the information storage medium to optical signals of the selected frequency and polarisation. The information storage process can include: receiving information to be stored; generating optical signals having respective frequencies and polarisations such that each of the optical signals has a unique combination of frequency and polarisation; and storing the information by applying the optical signals to an information storage medium having an initial configuration of mutually spaced elongate conductive bodies selectively responsive to each of said optical signals, the optical power of the optical signals being sufficient to modify the configuration of said bodies and being applied to the information storage medium such that the modified configuration represents the stored information.

Also described herein is an information retrieval process that includes interrogating the plurality of bodies with light of one or more of the selected wavelengths and polarisations. The information retrieval process can include: generating first optical signals having respective frequencies and polarisations such that each of the first optical signals has a unique combination of frequency and polarisation; directing the first optical signals to an information storage medium having a plurality of bodies selectively responsive to each of said optical signals, the arrangement of said bodies representing stored information; and detecting second optical signals from the information storage medium to determine the stored information, the second optical signals being generated by interaction of the first optical signals with the plurality of bodies in the information storage medium. The process can include directing the first optical signals to a plurality of selected locations of the information storage medium to selectively determine the respective information stored at said locations by monitoring the interaction of the optical signals with the plurality of bodies in the information storage medium. The locations may be distributed three dimensionally.

Also described herein is an information retrieval system including: an optical source to generate optical signals having respective frequencies and polarisations such that each of the optical signals has a unique combination of frequency and polarisation; an optical system to direct the optical signals to an information storage medium having a plurality of mutually spaced elongate conductive bodies selectively responsive to each of said optical signals, the configuration of said bodies representing stored information; and an detection system to monitor the interaction of the optical signals with the plurality of bodies in the information storage medium to determine the stored information.

Also described herein is an information storage system including: an optical source to generate optical signals having respective frequencies and polarisations such that each of the optical signals has a unique combination of frequency and polarisation; and an optical system to store the information by applying the optical signals to an information storage medium having an initial configuration of mutually spaced elongate conductive bodies selectively responsive to each of said optical signals, the optical power of the optical signals being sufficient to modify the configuration of said bodies and being applied to the information storage medium such that the modified configuration represents the stored information. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are hereinafter further described, by way of example only, with reference to the accompanying drawings, which are not to scale, in which: Figure 1 is a schematic diagram of a recording system in accordance with at least one embodiment of the present invention;

Figure 2 is a schematic diagram of a recording material of the recording system; Figure 3(a) is a scanning electron microscope (SEM) image of short bodies in an example recording material; Figure 3(b) is an SEM image of medium bodies in an example recording material;

Figure 3(c) is an SEM image of long bodies in an example recording material; Figure 3(d) is a plot of extinction spectra of the example recording materials including the short, medium and long bodies;

Figure 4 is an SEM image of an example recording material including a plurality of bodies with different morphologies and orientations;

Figure 5(a) is an SEM image of a plurality of bodies;

Figure 5(b) is a graph of a plurality of resonant linewidths of the bodies in Figure 5(a);

Figure 6 is a flow chart of a forming process of a recording medium used in the recording system;

Figure 7(a) is a schematic diagram of the recording material after writing with a first frequency and first polarisation;

Figure 7(b) is a schematic diagram of the recording material after writing with a second frequency and second polarisation; Figure 8 is a graph of a variation in extinction as a function of optical wavelength for a plurality of portions of an example recording material that has been written;

Figure 9(a) is an SEM image of the example recording material including a plurality of bodies for changing;

Figure 9(b) is an SEM image of the example recording material including a plurality of bodies having been changed by writing; Figure 10 is a flow chart of a writing process of the recording system;

Figure 1 1 is a normalised intensity spectrum of linear scattering and two-photon luminescence (TPL) of an example body;

Figure 12 is an angular scattering graph of linear scattering and TPL of the example body;

Figure 13(a) is a graph of normalised TPL excitation profiles for four example bodies with four different characteristic frequencies;

Figure 13(b) is a graph of four plots of TPL intensity versus polarization angle for the example bodies; Figure 14 is a plurality of images of an example pattern formed in an example recording medium;

Figure 15 is a flow chart of a reading process of the recording system;

Figure 16 is a pair of images written using continuous-wave light in an example recording medium; Figure 17 is a graph of the extinction spectra of three base materials with bodies having respective different response frequencies, and of a material in an example recording medium with a mix of the three base materials;

Figure 18 is a graph of detected TPL from an example recording medium having three layers and being scanned axially; Figure 19 is a plurality of images of nine example patterns formed in an example recording medium;

Figure 20 is a group of twenty images written in ten layers of an example recording medium;

Figure 21 is a group of three images written in three polarisations in the same layer of an example recording medium;

Figure 22 is an experimental example of a read-write system for white-light scattering spectroscopy and laser illumination of single gold nanoparticles;

Figure 23 is a graph of the scattering spectrum of four individual gold nanorods in unpolarised white light, and the inset is a histogram of statistics of the Lorentzian line width and the polarized scattering versus angle, with a dipolar cosine fit (solid line); Figure 24 is a graph of the correlation between an aspect ratio (measured from SEM images) of each rod and a corresponding longitudinal SPR energy (measured from the scattering spectrum) of each rod; the inserts are scanning electron microscope images of two individual nanorods with different aspect ratios, and the scale bars show 50 nm; Figure 25 (a) is a graph of scattering cross sections of a nanorod in unpolarised white light, measured before and after irradiation with a single laser pulse at 745 nm with an energy density of 1.93 mJ cm^'2; the inset is an SEM image of the nanorod after irradiation, and the scale bar shows 50 nm;

Figure 25(b) a graph of scattering cross sections of the nanorod in unpolarised white light, measured before and after irradiation with a single laser pulse at 745 nm with an energy density of 2.5 mJ cm^"2; the inset is an SEM image of the nanorod after irradiation, and the scale bar shows 50 nm;

Figures 25(c) and 25(d) are polar plots shows polarized scattering versus angle before (squares) and after (circles) irradiation, with dipolar cosine fits (solid lines), corresponding to the scattering cross section graphs in Figures 25(a) and 25(b);

Figure 26 includes sketches of the intermediate particle shapes observed at different pulse energy densities, and corresponding SEM images with dimensions of 200 x 150 nm; and

Figure 27 is a graph of the dependence of the longitudinal SPR energy after irradiation on the amount of absorbed laser energy, for two populations of responsive bodies (nanoparticles) with initial aspect ratios of 2.5 ± 0.2 (unfilled circles) and 3.0 ± 0.2 (filled circles); the errors indicate uncertainty (standard deviation) in the amount of absorbed energy; and the vertical dashed lines indicate the calculated energy range associated with no melting (T < 1330 K), partial melting (T = 1330 K), and full melting (T > 1330 K). DETAILED DESCRIPTION

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Recording and Retrieval System 100

As shown in Figure 1 , a recording and retrieval system 100 includes a recording medium 102 which is addressed (or written to) and interrogated (or read from) using a read-write system 104.

The recording medium 102 includes a plurality of recording layers 106 of a recording material 200.

The plurality of layers 106 includes a first layer 106A supported on a recording substrate 108, a second layer 106B separated from the first recording layer 106A by a first spacer layer 11 OA, and a third recording layer 106C separated from the second recording layer

106B by a second spacer layer HOB. The plurality of layers 106 may include further recording layers 106 alternating with spacer layers 110 following the pattern of the first and second recording and spacer layers. The maximum number of recording layers 106 is based on a desired storage volume and depth limitations of the read- write system 104, e.g., limitations on the depth into the recording medium 102 that the read- write system 104 can address.

The read-write system 104 (also referred to as a "patterning apparatus") includes a laser system 112 for generating an input optical signal in an input beam having a plurality of selectable (or controllable) laser frequencies (and thus wavelengths of light) of a plurality of selectable (or controllable) polarisations with a selectable (or controllable) beam power (or beam pulse energy for a pulsed beam).

The input beam 114 is directed by the read-write system 104 to an objective 116, including focussing optics, that focuses the input beam in a focal volume 118 in the recording medium. The role of the objective 116 is to direct the input beam 114 to at least one region (specifically a 2D area or a 3D volume) in the recording medium 102 where a sufficiently high optical intensity is to be provided for writing or for reading. The plurality of possible recording regions in each recording layer 106 are referred to as "volume pixels" or "voxels", i.e., spatially distinct regions of the recording medium 102 to be written to or read from. The voxels can be regarded as pixels when arranged in a two-dimensional array. The voxels can be arranged to form digital arrays (in one, two or three dimensions), or they can be arranged in an analogue form, i.e., without predefined boundaries between voxels. In examples, the digital arrangement can be used for storing digital data, whereas the analogue arrangement can be used for forming optical filtering components.

In the described embodiment, the objective 116 is a non-polarising broadband objective lens. In alternative embodiments, the objective 116 may include a plurality of optical components for directing the input beam 114 to at least one focal volume 118; for example, the objective 116 may include guiding optics to direct a plurality of portions of the input beam 114 from different directions to converge into the focal volume, such as from opposite directions or from transverse directions, such that the high optical intensity is achieved in the focal volume 118. In further alternative embodiments, the read- write system 104 focuses the input beam 114 into a plurality of focal volumes, e.g., using imaging principles to generate a two-dimensional image of varying optical intensity in the recording layer, such that sufficiently high optical intensity is achieved at certain pixels in the image but not at other pixels.

The focal volume 118 is the region in which the read- write system 104 addresses (i.e., reads from and writes to) the recording medium 102. The optical intensity for writing is higher than for reading. The position or location of the focal volume 1 18 in the recording medium 102 is determined by a spatial positioning system that controls and selects the position of the recording medium 102 relative to the read- write system 104 (and thus the focal volume 118); in Cartesian coordinates, this is referred to as the xyz position of the focal volume 118.

Emitted or scattered light from the focal volume 118 is collected by the read-write system using the objective 116 (or equivalent collection optics) to form an output beam 120, which is optically directed using a beam splitter 122 to a detection system 124, as shown in Figure 1. The detection system 124 includes a filter 126 for filtering the output beam 120 before it is converted to an electronic signal by a detector 128.

In the described embodiment the path of the output beam 120 from the focal volume and through the objective 116 is generally collinear with the path of the input beam 114 and the output beam 120 is reflected from the recording medium 102. In alternative embodiments, the output beam 120 is collected by a second objective, including collecting optics, and directed along an alternative beam path to the detection system 124, such as in a transmission geometry with a transmitted output beam transmitted through the recording medium 102.

The signals from the detection system 124 are collected and analysed by a control and analysis system (CAS) 130, which also controls the characteristics of the input beam 114 by controlling the laser system 112. The recording and retrieval system 100 is controlled using an interface 140, in communication with the CAS 130, provided by a personal computer. The interface 140 allows a user of the system 100 to enter information (data to) be written or stored in the recording medium 102, and to receive information read from the recording medium 102, via the read-write system 104.

The read- write system 104, together with the focal volume 118, operates using a plurality of light frequencies (corresponding to wavelengths in the optical beam) and polarisations, referred to as v and p respectively. In the writing process, described below, the light frequency v and polarisation p is selected by controlling the input beam 114: the laser system 112 generates the input signals with one or more selected single optical frequencies (or "colours") and one or more selected polarisations. T he input beam characteristics (colour, polarisation and power) are controlled directly by controlling the laser source, or by filtering the light from the laser. In the reading process, described below, the system power is still controlled by the laser system 112, but the polarisation and colour of the system can also be controlled by filtering the output beam 120, e.g., using the filter 126: for example, the input beam 114 may include a plurality of colours and polarisations that interact in the at least one focal volume 118, while the polarisation(s) and colour(s) being read by the read- write system 104 can be a subset of the input polarisations and colours. The subset is selected by the detection system 124 (e.g., by the filter 126).

In some embodiments, the read- write system 104 operates with a single colour and a single polarisation of the input beam 114 delivering input signals having a single colour and a single polarisation during writing and during reading. In multiplexed embodiments, the read-write system 104 operates during write with a plurality of independently controlled sub-signals in sub-beams of the input beam 114, each with controlled characteristics (colour, polarisation and power), and during read with a plurality of independent sub- signals in sub-beams of the output beam 120, each with characteristics that allow separate detection in the detection system 124 (i.e., separation by colour and polarisation to determine the power in each sub-beam). During read, the input beam 1 14 includes at least the same plurality of sub-beams as in the output beam 120, although the power need not be separately controlled for each beam for reading. The sub-beams in the input beam 114 are combined into one beam with optical multiplexing equipment, and the sub-beams in the output beam 120 are separated to separate detectors using optical de-multiplexing equipment (e.g., free space or fibre optic components) that can separate the output sub- beams.

The first recording layer 106A is formed by coating the recording material onto the substrate 108 (a glass plate) in a spin-coating process. The thickness of the first recording layer can be about 1 μm. Each spacer layer 110 can be formed using a transparent pressure-sensitive adhesive, with a thickness of about 10 μm. Recording Material 200

The recording material 200 responds to an incident beam of light with an intensity above a threshold intensity in a volume of the incident beam by persistently changing a first optical property of the recording material in the focal volume only if the incident beam has a first wavelength and a first polarization, and persistently changing a second optical property in the volume only if the incident beam has a second wavelength and polarization. The recording material 200 includes a plurality of bodies that are selectively responsive to light of a respective plurality of frequencies and polarisations. Each voxel includes at least two and preferably many (such as around 100, or more) bodies, each with a characteristic polarization and wavelength. The dimensions of each body define its morphology, and thus its aspect ratio.

The recording material 200 is stable in ambient conditions {e.g., at typical operating temperatures for electronics systems, or at room temperature, pressure and humidity), so that stored information — such as recorded patterns — is not lost or degraded.

Wavelength and Polarisation Selectivity

As shown in Figure 2, the recording material 200 is formed of a matrix of an optically unresponsive substance 202 {e.g., polyvinyl alcohol) doped with a plurality of optically responsive bodies 204 {e.g., gold nanorods) having a plurality of characteristic surface plasmon resonant frequencies (referred by using wavelength symbols λi, λ₂ and λ₃) and orientations (referred to by angular rotation symbols cpi and φ₂, representing angular position generally perpendicular to the input beam 114 and the output beam 120 when used in the system 100).

The recording material 200, as shown in Figure 2, may include six bodies 204 with six different characteristic responses, defined by the combinations of response frequency and polarisation: a first body 204A of the bodies 204 is responsive to light of a first optical frequency (corresponding to a wavelength λi) and a first polarisation (corresponding to an orientation angle φi); a second body 204B of the bodies 204 is responsive to light of a second optical frequency (corresponding to a wavelength λ₂) and the first polarisation (corresponding to the orientation angle φi); a third body 204C of the bodies 204 is responsive to light of a third optical frequency (corresponding to a wavelength λ₃) and the first polarisation (corresponding to the orientation angle φi); a fourth body 204D of the bodies 204 is responsive to light of the first optical frequency (corresponding to the wavelength λi) and a second polarisation (corresponding to an orientation angle φ₂); a fifth body 204E of the bodies 204 is responsive to light of the second optical frequency (corresponding to the wavelength λ₂) and the second polarisation (corresponding to the orientation angle φ₂); and a sixth body 204F of the bodies 204 is responsive to light of the third optical frequency (corresponding to the wavelength λ₃) and the second polarisation (corresponding to the orientation angle φ₂).

A body 204 is "responsive" to light if the body substantially absorbs the light (at least for higher optical intensities) and substantially scatters the light (at least for lower optical intensities).

As shown in Figure 3, populations or groups of bodies 204, such as gold nanorods, have characteristic surface plasmon resonance frequencies that are defined by their morphology, such as their aspect ratios: the extinction spectrum 302A of short rods 304A (aspect ratio 2 ± 1, e.g., 37 x 19 nm) is at a higher frequency (corresponding to a central optical wavelength of about 670 nm) than the extinction spectrum 302B of medium rods 304B (aspect ratio 4.2 ± 1, e.g., 50 x 12 nm), which has a central frequency corresponding to an optical wavelength of about 820 nm, which in turn is at a higher frequency than the extinction spectrum 302C of long rods 304C (aspect ratio 6 ± 2, e.g., 50 x 8 nm), which has a central frequency corresponding to an optical wavelength of about 960 nm.

Distribution of Bodies in the Recording Material 200

The bodies 204 may have generally unordered distributions of morphology, position and orientation through the recording material 200, as shown in the example SEM image in Figure 4, so that the recording material 200 has an extinction profile for each voxel that is substantially inhomogeneously broadened, i.e., all wavelengths and polarizations that will be used for writing and reading are approximately equally supported by each voxel. The bodies are distributed "isotropically", i.e., are generally randomly distributed in location, orientation and morphology.

Surface Plasmon Resonance (SPR)

In the described embodiment, the frequency and polarisation selectivity of the bodies 204 is provided by morphology and orientation-dependent charge oscillations supported by each body, such as longitudinal surface plasmon resonance (SPR) effects in gold nanorods. Longitudinal SPR exhibits high wavelength and polarization sensitivity and selectivity. Gold nanorods, for example, have narrow longitudinal SPR linewidths (about 100- 150 meV, or about 45-65 run in the near-infrared), combined with a polarization sensitive optical response, which allows selected nanorods, or small subpopulations of nanorods, to be addressed in each voxel 118, as described with reference to the reading and writing processes below.

In an example, a dilute solution of gold nanorods (average size 90x30 nm) in 1% polyvinyl alcohol (PVA) was spin coated onto a glass coverslip and characterised to directly determine the nanorods' plasmon line widths. To determine the locations of the nanorods, single-particle unpolarised white-light scattering spectroscopy was used to image the sample surface. Light from a spatially filtered quartz tungsten halogen lamp was reflected from the nanorods, collected by a focusing objective (1.4 numerical aperture NA, oil immersed), and directed to a photomultiplier tube (PMT, Oriel Instruments). The reflected light was bandpass-filtered in frequency before detection to improve sensitivity, thus the nanorods appeared dimmer when part of their scattering spectrum was outside the filter band. The focal spot was raster-scanned over the sample surface to generate an image. Diffraction-limited spots in the image indicated the location of isolated single gold nanorods, as shown in Figure 5(a). The unfiltered scattering spectra of five individual nanorods were dispersed by a spectrograph on a charge coupled device. The scattering spectra were background corrected and normalized to the spectral profile of the light source. As shown in Figure 5(b), the spectra exhibited a strong longitudinal plasmon resonance at optical frequencies (including the infrared) between 1.5 and 1.7 eV. The difference in scattered intensity may have been caused by differences in the volume of each body. Statistical Lorentzian fits, e.g., as shown in the insert of Figure 5(b), provided an estimate of an average linewidth of 107±15 meV. Gold nanorods that are similar in size — e.g., 37^χ 19 nm, 50^χ 12 nm or 50x8 nm — may have similar linewidths.

Forming Process 500

As shown in Figure 6, a forming process 500 for making the recording medium 102 commences with forming the responsive bodies 204 of the recording material 200 (step 502). The bodies 204 with the desired responsivity characteristics may be formed separately, and subsequently mixed with the non-responsive substance (step 504), as in the described embodiment. The recording material 200 forms the recording layer 106 on the substrate 108, or on some supporting body, or has some supporting material incorporated into the recording material as a form of integrated substrate 108 (step 506). If it is determined that an additional recording layer 106 is desired (step 508), a spacer layer 110 is formed adjacent the recording layer 106, e.g., on top of, or underneath or next to the recording layer 106 depending on the process used (step 510). Subsequently, the additional recording layer 106 is formed adjacent the spacer layer 1 10, with the spacer layer 1 10 separating the two recording layers 106 (step 512). The forming process 500 ceases when no further additional recording layers 106 are desired to be formed.

"Five-Dimensional" Recording

The recording medium 200 provides substantial selectivity to wavelength and polarisation. Furthermore, as the bodies 204 are distributed in three dimensions in the recording material 200, at least when in a plurality of layers 108, and the bodies 204 are responsive to light that can be focussed to at least one confined focal volume 118 or voxel, the recording medium 200 provides selectivity (or "orthogonality") in five "domains" or "dimensions": frequency (and thus optical wavelength), polarisation, and the three spatial dimensions. Wavelength, polarization and spatial dimensions are thus integrated into single techniques for writing and reading. Multiplexed optical recording using the recording material 200 and the system 100 may increase a storable information density of the recording medium 102 beyond 10¹² bits per cm³ (1 Tbit cm^"3) by storing multiple, individually addressable patterns or digital bits or data within each voxel.

Writing (also referred to as "recording" and "patterning")

Each body 204 responds to incident light in the input beam 1 14 corresponding to its characteristic frequency and polarisation by absorbing a portion of the incident light. The absorbed light is converted in part to heat in the body 204. Each type of body 204 has a distinct energy threshold required for photothermal recording. For sufficient absorbed light (above the threshold intensity), the generated heat is sufficient to at least partially melt the body 204, which undergoes photothermal reshaping. For example bodies 204 formed of gold, the temperature is at least above the melting point of gold, which is T_men =1337 K. The temperature rise is determined by: the mass and thermal properties of the body 204 and the surrounding "unresponsive" substance 202; and the amount of energy absorbed by the body 204, which is based on the absorption cross section of the body 204 at the laser wavelength and the light energy density at the body 204 (related to the intensity in the focal volume 118). The light energy may be laser pulse energy for a pulsed input beam, or laser power for a continuous-wave (CW) input beam.

The reshaping of the bodies 204 occurs substantially in the at least one focal volume 118 defined by the focusing objective 1 16. The reshaping is selective in terms of morphology and orientation (defined by the wavelength and polarisation of the input beam 114), and selective in terms of spatial position (defined by the position of the focal volume 118 in the recording medium 102). For example, a linear polarized laser pulse will only be absorbed substantially by gold nanorods that are aligned to the laser light polarization and which have an absorption cross-section that matches the laser wavelength. The selective reshaping results in a depleted population of nanorods with a certain aspect ratio and orientation, and hence a polarization and wavelength dependent bleaching occurs in the extinction profile.

The reshaping causes a change in the responsivity of the body 204. For example, the reshaping may cause a rod-shaped body 204 to be reshaped as a shorter rod or a ball- shaped body 204, similar to a sphere. For example, as shown schematically in Figures 2 and 7(a), the rod-shaped body 204 A, when in the focal volume 118 of the input beam 114 including the wavelength λ_\ at the polarisation corresponding to the orientation φi and power/energy above the thermal threshold for writing to the body 204A, is re-shaped into a ball-shaped body 604A, which is no longer responsive to its characteristic light (at the wavelength λi and the polarisation corresponding to the orientation φi) due to its changed morphology. As another example, as shown schematically in Figures 2 and 7(b), the rod- shaped body 204E, when in the focal volume 1 18 of the input beam 114 including the wavelength λ₂ at the polarisation corresponding to the orientation φ₂ and sufficient power/energy for writing to the body 204E, is re-shaped into a ball-shaped body 604E, which is no longer responsive to its characteristic light (at the wavelength λ₂ and the polarisation corresponding to the orientation φ₂) due to its changed morphology. As shown in Figures 2, 7(a) and 7(b), the wavelengths and polarisations in the input beam 114 may be selected to affect one type of body 204 (e.g., the bodies 204A and 204E respectively), defined by its optical characteristics, while not affecting (i.e., not writing to) the other types of bodies 204 in the same focal volume 118 with substantially different optical characteristics (e.g., the bodies 204B, C, D, E and F in Figure 7(a) and the bodies 204A, B, C, D and F in Figure 7(b)).

Photothermal melting of the selected bodies occurs only above the bodies' characteristic incident power or energy threshold, thus the writing process is spatially confined to within the at least one focal volume 118 when the input beam 114 is tightly focussed in the recording material 200. This allows recording in three spatial dimensions. In photothermal melting, unlike other methods used for photon recording, such as photobleaching or photo- isomerisation, out-of-focus laser light, i.e., input light outside the focal volume, has a generally negligible effect on the surrounding recording material 200 and thus generally does not induce unwanted recording.

In an example, gold nanorods in a spin-coated PVA film experienced a change in their extinction spectrum ("Δext"), as shown in Figure 8, caused by irradiation with light of a selected wavelength and polarisation: irradiation with a single polarised femtosecond pulse of 710-nm light caused a change (a bleaching) in the extinction spectrum 1502 primarily around 710 nm (marked with a black arrow in Figure 8), whereas irradiation with a single polarised femtosecond pulse of 840-nm light caused a change (a bleaching) in the extinction spectrum 1504 primarily around 840 nm (marked with a grey arrow in Figure 8).

As shown in the insert to Figure 8, the measured extinction modulation (circles) was also polarization dependent, with a dipolar cosine fit (solid line).

Following photothermal reshaping, a reshaped body 204 has a new characteristic optical responsivity, i.e., the reshaped body 204 has a new extinction spectrum and thus a new characteristic optical frequency and polarisation to which it responds, and thus no longer reflects or scatters a signal into output beam 120 at the original frequency and polarisation. In the described embodiment, the extent of reshaping is selected to be sufficient such that the new responsivity of a reshaped body 204 lies outside the detection range (in wavelength and/or polarisation) of the detection system 124, and thus the reshaped bodies 204 "disappear" from the recording medium from the perspective of the detection system 124. In alternative embodiments, the input beam 114 and detection system 124 are configured for reading such that the reshaped bodies 204 "appear", e.g., the reshaped bodies 204 form shorter rods which are detected after re-shaping.

For example, for bodies 204 in the form of gold nanorods with a range of morphologies, as shown in Figures 3 (a) to (c), which are spin-coated on an indium tin oxide (ITO) coated glass coverslip, the recording material 200 is formed with the rods in a range of orientations, as shown in Figure 4. As shown in Figures 9(a) and 9(b), irradiation with a single, linearly horizontally polarized femtosecond laser pulse (λ = 840 nm and pulse energy 0.28 nJ in the focal plane of the objective), only affects nanorods 702 with an aspect ratio of 3.4 ± 0.9 within an angular range of 25° with respect to the horizontal laser light polarization (averaged over 20 reshaped particles). As shown in Figure 9(b), the reshaped nanorods 702 are no longer responsive to the detector system 124, and are thus "invisible".

For recording, using a few, or a single laser pulse per voxel (or pixel), rather than a high repetition rate pulse train, prevents (or at least reduces) adverse accumulative thermal effects in the recording medium 200.

Mechanical Stability During Reshaping

During writing, the bodies 204 can experience a rapid change in their centre of mass during the reshaping process, which may exert forces that tend to move the reshaped bodies 204 to new locations, e.g., causing lift-off from the substrate 108. These reshaping forces are substantially resisted by resistive mechanical properties of the unresponsive substance 202, such as a sufficiently thick surrounding layer of substance, e.g., at least thicker than about 20 nm, or at least about 200 run thick to resist any substantial movement of the bodies 204 during re-shaping.

Writing Process

As shown in Figure 10, a writing process 800 for writing information into the medium 102 commences with the CAS 130 receiving the information to be written, e.g., in the form of bits and bytes, or an image, or a file etc. (step 802). The CAS 130 then determines a location to write and the optical characteristics of the input beam 114 to use for each bit (step 804). Light energy is transmitted to the recording medium 102 to write each bit, as required, e.g., only "0" bits need to be written while "1" bits are not written, or vice versa (step 806). If the CAS 130 determines that another bit needs to be written (step 808), the process of writing a bit is repeated (steps 804 and 806). The writing process 800 may be performed using multiplexed techniques, as discussed below, to write a plurality of bits in parallel: in some cases, a many bits may be written generally simultaneously using a plurality of wavelengths, polarisations and XYZ locations. Multiplexed Writing

A supercontinuum light source may be used for simultaneous recording using a plurality of wavelengths (referred to as "wavelength channels") of the input beam 114 simultaneously. Simultaneous use of multiple channels for writing may be allowed as only a low pulse energy {e.g. , <0.5 nJ per pulse) is required for writing.

Alternatively, a plurality of input beams 114 and objectives 116, or an input beam 114 with an intensity modulated pattern in the focal plane, may operate simultaneously to write a plurality of bits in parallel.

Reading

The information written into the recording material 200 is read out non-destructively using the input beam 114 with less power at the characteristic location (voxel or pixel), wavelength and polarisation than that required to write to the recording material 200.

In a reading process 1300, recorded information in the recording medium 102 is accessed, and corresponding information is generated by the read- write system 104. In accessing the recorded information, the read- write system 104 deconvolves the measured optical signal in the output beam 120 with a response of the objective 116, and any other optical components that filter the scattered light from the bodies 204.

Two-Photon Luminescence (TPL)

In a reading process 1300, information in the recording medium 102 is read (accessed or read) using two-photon luminescence (TPL) of the bodies 204 in the recording medium. The TPL is excited at the characteristic frequency of the bodies 204 {e.g., the longitudinal SPR wavelength of the nanorods), and has an enhanced wavelength and angular selectivity compared to linear emission / scattering from the bodies 204 due to its nonlinearity. For example, as shown in Figure 11 , the TPL excitation linewidth 902 of a single gold nanorod (average aspect ratio 3, average size 90 x 30 nm), is almost 60% narrower than the linewidth of the linear scattering spectrum 904 for normalized white light, determined as a function of photon energy hω, and centred around the longitudinal SPR energy HΩ_LSP- Furthermore, as shown in Figure 12, the TPL angular excitation profile 1002 for the single gold nanorods is almost 50% narrower than the angular width of the scattering profile 1004, normalised to the polarization of the excitation light. Further exemplary normalised TPL excitation profiles, for four more individual gold nanorods, were measured as having linewidths of about 70±10 meV (averaged over 10 particles), and similar TPL intensity versus polarization angle widths, as shown in Figure 13(a) and 13(b).

Having narrower spectral and angular excitation profiles significantly reduces interference in the readout between neighbouring recording channels, which are associated with bodies 204 having adjacent responses within the frequency domain or within the polarisation domain. Furthermore, axial sectioning due to the spatial selectivity of two-photon excitation allows for crosstalk-free readout of closely spaced layers, i.e., reduced interference between spatially adjacent bodies 204 with different responsivities.

TPL is most efficiently excited on resonance with the linear plasmon absorption band, enabling single photon recording and multi-photon readout using the same wavelength of input light.

The TPL brightness of gold nanorods may be determined by calculating the TPL action cross-section (ησi, where η is the luminescence quantum yield and σ₂ is the two-photon absorption cross-section) of a single gold nanorod. From a TPL raster scan of isolated gold nanorods (average aspect ratio 4, average size 44 x 12 nm), it was determined that the TPL action cross-section was about 3 * 10⁴ GM (Gόppert-Mayer) for excitation on resonance with the longitudinal SPR. From the TPL, the two-photon absorption cross-section, σ₂, can be determined using η, based on the following relationship, which expresses the number F of fluorescence photons detected for a single body:

F = gφησj² (1) where g is the degree of second-order temporal coherence of the excitation source, φ is the collection efficiency of the optical setup, / is the incident photon flux (in photons/s/cm²), and ησ₂ is the two-photon action cross section (in cm⁴ s/photon), with η the fluorescence quantum yield and σ₂ the two-photon absorption cross section. Typical values are g = 1.9 x 10⁵, φ = 0.17, and / = 1 x 10²³ photons/s/cm². The number of detected fluorescence photons F may be extracted from the peak value in the TPL excitation profile, taking into consideration the sensitivity of the photomultiplier tube, and the TPL action cross section may then be determined based on the relationship in Equation (1).

The fluorescence quantum yield η of a rod-shaped body 204 is substantially higher than the quantum yield of spherical particles or films, e.g., an example rod-like nanoparticle (or nanorod) may have a quantum yield of about 10^~4, while a film of similar dimensions may have a quantum yield of about 10^~10. A so-called "lightning-rod effect" around a nanorod enhances the local field strength and a radiative decay rate via coupling to the SPR, which may cause the increase in quantum yield in the rods. For a quantum yield of about 10 , the two-photon absorption cross-section σ₂ is about 3 x 10⁸ GM. The quantum yield, and thus the two-photon absorption cross-section, is related to the volume of the bodies. Having a large TPL cross-section for each body 204 allows non-destructive reading (or imaging) of the recorded information using very low excitation power in the input beam 114.

Using the read- write system 104, a recorded pattern can be retrieved by raster scanning the sample and detecting a TPL signal from the bodies 204, where the TPL is excited using the same input wavelength and polarization that was used for writing the pattern.

An example pattern was written into an example recording medium 1 106 using a single laser pulse per pixel with a wavelength of 840 nm and vertical polarisation, and then imaged / read using vertically polarised laser light (indicated by the vertical double-headed arrow in Figure 14(a)) with a wavelength of 840 nm (i.e., the same wavelength and polarisation), resulting in an image, as shown in Figure 14(a), with lower TPL signal (shown as a darker colour) per pixel where the responsive bodies 204 have been reshaped, and a higher TPL signal (shown as a lighter colour) per pixel where the bodies 204 remain responsive (i.e., those still having a longitudinal SPR on resonance with the readout laser light). The pattern included 75 x 75 pixels, with a pixel spacing of about 1.33 μm. An average pixel size was about 500 ± 100 nm, which was in good agreement with an expected diffraction limit of about 470 nm for light of this wavelength in the recording medium 102. The contrast of the pixels is defined as |/_Pj_xei - /_bg|/(/_p,_xei + /_bg), with I_pιxeι and h_g representing the intensities of the pixel and background TPL signals, respectively. As the concentration of bodies 204 in the example recording medium 1106 was about 400 nM, the contrast in the resulting image of Figure 14(a) arises from reshaping of about 30 bodies 204 in the focal volume 118 of each pixel.

For a pattern or image written with input light having a first wavelength and a first polarisation, the pattern or image is not read when interrogating with input light having a different wavelength or polarisation (for sufficient separation between the characteristics writing and reading lights). For example, as shown in Figure 14(b), no image was read from the example recorded medium 1106 when using horizontally polarized light (indicated by the horizontal double-headed arrow in Figure 14(b)). Furthermore, as shown in Figures 14(c) and 14(d), no image was read when using light of the same polarisation as the image (vertical polarisation, indicated by the vertical double-headed arrow in Figures 14(c) and 14(d)), but sufficiently different wavelengths, namely 710 nm and 980 nm, respectively. In the example medium 1106, only the population of bodies 204 with a longitudinal SPR on resonance with the recording laser light, /. e. , matching frequency and polarisation, was reshaped in each pixel region to represent the recorded image.

Multiplexed Reading

For a recording medium 102 with data recorded in multiple layers 106, the data can be read out using a two-dimensional detector, e.g., a charge coupled device (CCD), and a white light source (i.e., a multi-wavelength source) to provide "one-shot" (i.e., two-dimensional multiplexed) readout of data such as patterns or images. When multiple CCDs are used, multiplexed reading can provide simultaneous readout of all recorded patterns (e.g., separated by wavelength, polarisation and/or focal depth (z) to provide three-, four- or five- dimensional multiplexed readout).

Reading Process 1300

The information written into the recording material 200 of the recording medium 102 is read or accessed in the reading process 1300 that commences, as shown in Figure 15, with the medium 102 being positioned relative to the read- write system 104 so the input beam 114 is incident on the recorded parts of the recording material (step 1302). The input beam 114 then illuminates the at least one pixel or voxel with optical power, below the threshold required for writing, including the one or more wavelength-polarisation combinations used to write the information (step 1304). Light scattered or transmitted by the one or more voxels, of at least one selected wavelength and polarisation, is detected by the detection system 124 (step 1306). After deconvolving the response of the optical components in the output beam path, the read- write system 104 determines the optical output signal(s) from the voxel(s) being read with each wavelength-polarisation combination representing the extinction of the voxel at that wavelength-polarisation combination (step 1308). The information in the recording medium, which may be an optical filter, an image and/or a digital data stream, is assembled and stored or transmitted or displayed by the CAS 130.

Pulsed or Continuous Input Light

In the described embodiment, the input beam 114 includes pulses of light, e.g., from a femtosecond pulsed laser. In alternative embodiments, the recording process may be performed with a continuous-wave (CW) laser or laser diode, which may allow for a lower-cost laser system 112.

In an example, multiplexed recording of patterns was performed using a CW titanium sapphire laser source, operating at 840 nm with a controllable or selectable linear polarization. Two patterns were written: the first using vertical polarisation and the second using horizontal polarisation. The CW laser power was about 80 mW at the back aperture of the 0.95-NA objective and 60 mW in the focal plane. The exposure time on the sample was controlled by an electro-optic modulator, and was about 10 ns per pixel.

Using a CW laser source to write data to and from the recording medium 102 may significantly reduce the cost of embodiments of the read-write system 104 through the use of low-cost, readily available laser diodes. Combined with transmission-based readout, the read- write system 104 may be integrated with current optical disc drive technology, such as CD or DVD disc drives.

In reading, the TPL effects of the bodies 204 were excited using an input beam from a pulsed titanium sapphire laser and with the same wavelength and polarization as used for recording (840 nm with a linear polarization), and detected at the two-photon wavelength

(in the wavelength range 400-600 nm). The recording medium 102 was raster scanned, and the detection system 124 generated two two-dimensional images (75 x 75 pixels and

100 x 100 μm each) multiplexed in polarization: the first used vertical polarisation and the second used horizontal polarisation, as indicated by the arrows in Figure 16.

Experimental Example 1

An example recording medium 200 was prepared using gold nanorods with average aspect ratios of 2.3 ± 1, 4.3 ± 1 and 6 ± 2 using a wet chemical synthesis process, as described in the publication by Nikoobakht, B. and El-Sayed, M. A. entitled "Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method" (in Chem. Mater. 15, pp. 1957-1962, in 2003), and the publication by Zijlstra, P., Bullen, C, Chon, J.W.M. and Gu, M. entitled "High-temperature seedless synthesis of gold nanorods" (in J. Phys. Chem. B I lO, pp. 19315-19318, in 2006). After preparation, the solutes were diluted by three orders of magnitude through centrifugation. The nanorod solutions, with different individual extinction profiles 2002A, 2002B, 2002C, were combined {i.e., different types were mixed pseudo randomly) to obtain a "flat" extinction profile 2004A {i.e., with a substantially constant magnitude over the range of desired frequencies) in the 700- 1,000 nm wavelength range, as shown in Figure 17. The nanorods were mixed with a 15 wt% polyvinyl alcohol (PVA) solution, and spin-coated on a glass coverslip. The thickness of this layer was 1 ± 0.2 μm, as measured using an atomic force microscope. The nanorod concentration in the film was about 400 ± 50 nM, i.e., about 200 nanorods in a focal volume defined by a 0.95 NA objective lens in the objective 116. A transparent pressure sensitive adhesive (from LINTEC Co.) with a thickness of about 10 ± 1 μm and a refractive index of about 1.506 was laminated onto the spin-coated layer to form a spacer layer 110. This process was repeated until the desired number of layers was reached. As the number of layers was increased, the extinction profile increased generally linearly, from 2004A to 2004B to 2004C, as shown in Figure 17, indicating a consistent layer thickness and nanoparticle concentration for each recording layer 106.

The layer spacing was measured by exciting the TPL of the gold nanorods at 760 nm and scanning the sample in the z-direction. The highest peak 2102 in the z-profile corresponded to TPL from the first recording layer, as shown in Figure 18. Measured z-scans on three different positions in the sample spaced by about 5 mm (shown as the three curves in Figure 18) indicated that the layer spacing did not vary by more than 0.5 μm over this distance. The peak TPL intensity reduced by about 50% between the first 2102 and the third 2104 recording layers, as shown in Figure 18. This reduction in TPL intensity was caused by a combination of spherical aberration and extinction by the shallower recording layers. The extinction of a recording layer was about 0.07 at the excitation wavelength of 760 nm, and thus each recording layer reduced the light transmission by about 15%. Due to the two-photon nature of the excitation, extinction by the shallower recording layers reduced the TPL intensity by about 30% for one layer, and by about 50% for two layers, thus the reduction in detected TPL intensity may be mainly due to the extinction by the shallower recording layers (rather than spherical aberration).

An example recording process included an electro-optic modulator (ConOptics Inc., 350- 160) selecting single pulses from the pulse train of a broadband tunable femtosecond pulse laser (Spectra Physics "Tsunami", with a 100-fs pulse duration, a 82-MHz repetition rate, and wavelength tunable between 690 and 1,010 nm). The laser pulses were focused onto the sample recording medium 102 through a high NA objective lens in the form of an "Olympus" 0.95 NA 4Ox, coverslip-corrected lens. Writing or patterning was conducted using a single femtosecond laser pulse per pixel at wavelengths of 700 nm, 840 nm and 980 nm, and both horizontal and vertical polarizations, using pulse energies (determined in the focal plane) of 0.21 nJ at 700 nm, 0.22 nJ at 840 nm, and 0.32 nJ at 980 nm. The images were patterned in three layers, with a layer spacing of 10 μm and a bit spacing of 1.33 μm. For patterning in the deeper layers, the pulse energy was increased by 20% per layer to compensate for extinction by the shallower layers. The laser pulse energy and wavelength used for patterning were selected to minimize crosstalk between the different recording channels. The size of all images was 100 x 100 μm, and the patterns were 75 x 75 pixels.

An example reading process included exciting the TPL of the bodies 204 using the 82- MHz output pulse train from the femtosecond laser. The pulse train was focused on the sample through the same objective used for writing. The recordings were retrieved by detecting the TPL excited with the same wavelength and polarization as employed for the recording. The TPL signal was directed to a photomultiplier tube (Hamamatsu, H7422P40), which detected the signals in the 400-600 nm wavelength range. To prevent erasure of the patterns, the pulse energy of the readout laser was almost three orders of magnitude lower than the patterning pulse energy, i.e., about 1 pJ/pulse in the focal plane, and 85-100 μW average power.

Information was recorded and imaged in "five dimensions" (5D): as shown in Figure 19, where TPL raster scans of 18 images, all patterned in the same area using three layers, two laser light polarizations and three different laser wavelengths, are reproduced as images. The wavelengths were 700 nm, 840 nm and 980 nm; the layers were "layer 1", "layer 2" and "layer 3"; and the polarisations were horizontal and vertical as marked by orthogonal double-headed arrows in Figure 19. Experimental Example 2

In an example of high-density (5D) recording, 20 patterns (sized 100 x 100 μm, and 75 x 75 pixels) were recorded using single femtosecond laser pulses at 840 nm in ten layers (spaced by about 10 μm) and at two orthogonal polarisations, as shown in Figure 20. The recording pulse energy for each layer was selected to yield sufficient contrast without noticeable cross talk. The recorded patterns were detected by exciting the TPL at 840 nm, and detecting in the 400—600 nm window. As shown by the inset in Figure 20, which is a 6 x 6 μm high-magnification image of the area indicated by the square, the tenth layer (the deepest) was recorded and read without noticeable loss of spatial resolution and sensitivity.

Experimental Example 3

In an example of three-state polarisation multiplexing, three patterns were recorded in each pixel (or voxel) using three different recording light polarizations. The recording used femtosecond laser pulses at 840 nm with pulse energies of 160 pj in the focal plane of the objective 116. Patterns were recorded using linear polarizations of 0, 60, and 120 degrees relative to the laboratory frame. The recorded patterns (sized 100 x 100 μm, and 75 x 75 pixels), as shown in Figure 21, were imaged by detecting the TPL excited with the same wavelength and polarization used for the recording, and detected at 400-600 nm.

Experimental Example 4

In an example of writing information to a region in the recording material 200, examples of bodies 204 for the recording material 200 in form of the gold nanorods were prepared using a wet-chemical synthesis method (as described in B. Nikoobakht and M. A. El- Sayed, Chem. Mater., 2003, 15, 1957-1962). The rods had an ensemble average size of 92 x 30 nm, and an ensemble average volume of 5.8 x 10⁴ ran³. To ensure that the nanorods were in a homogeneous environment, an initial thin layer of polyvinyl alcohol (PVA) was spin coated onto a substrate in the form of an indium tin oxide (ITO)-coated coverslip. A dilute solution of the nanorods was mixed with a 3% PVA solution, and spin coated onto the initial PVA layer. The sample was then sealed with another pure PVA layer identical to the first layer. The thickness of the individual layers were determined to be about 80 nm, 30 nm, and 80 nm, respectively, using an atomic force microscope. Grids were fabricated in the PVA layer by femtosecond laser writing for use in locating the same nanorods for spectroscopy and electron microscopy.

An optical microscope and the example recording medium were arranged to form single- particle white-light scattering microscopy system 2200 (described in part in K. Lindfors, T.

Kalkbrenner, P. Stoller and V. Sandoghdar, Phys. Rev. Lett., 2004, 93, 037401), as shown in Figure 22. The output from a high power quartz tungsten halogen light source was spatially filtered using a 30 mm pinhole, and was focused onto the sample through a 1.4-

NA oil-immersed objective 2202. The reflected light was collected by the same objective 2202 and directed to a photomultiplier tube (PMT) 2204 from Oriel Instruments (no.

77348). To increase the visibility of the rods, the reflected light was filtered using a bandpass filter 2206, with a pass band of 760 ± 60 nm, before detection. The unfiltered scattering spectrum of each individual nanorod was dispersed by a spectrograph 2208

(Acton Instruments, SpectraPro 300i) onto a charge coupled device 2210 (Princeton Instruments, PIXIS 100). Except for when making polarization measurements, the white light was selected to be randomly polarized.

To induce melting and reshaping, the nanorods were illuminated with an input beam from a femtosecond pulsed laser source (a SpectraPhysics Tsunami, pulse width 100 fs, repetition rate 82 MHz, tunable from 700 nm to 1000 nm). An electro-optic modulator 2212 (ConOptics Inc., 350-160) selected single pulses on demand from the 82 MHz pulse train. A half wave plate was used to rotate the polarization of the linearly polarised input beam to match the orientation of a selected population of nanorods on the sample surface.

In general, the nanorods were irradiated with a laser wavelength corresponding to the peak of their longitudinal SPR, and with a polarization parallel to their long axis. In white-light scattering measurements in the experimental example, the measured intensity Im at the detector 2208, 2210 was a superposition of the reflected field off the interface Er and the light field Es scattered by the particle (as described in K. Lindfors, T. Kalkbrenner, P. Stoller and V. Sandoghdar, Phys. Rev. Lett., 2004, 93, 037401), according to the relationships of Equation ( 1 ):

The field at the detector was thus composed of a pure scattering signal, a signal due to the reflection of the interface, and component due to the interference between the reflected and scattered fields. The pure scattering signal scales as R⁶ and is the dominant term for our large nanorods (r_e/f - 25 nm). The detected intensity can then be expressed using the relationships of Equation (2):

where the reflection term was included to correct for the weak reflection of the oil- polymer-glass interface. In the experimental example, a correction was made for the reflection signal by subtracting a background spectrum h_g, which was recorded several microns away from the particle. The spectral characteristics of the white light profile were taken into account by normalizing each spectrum to I_WL, which was recorded at the glass- air interface at the back of the coverslip. The absolute scattering cross section σ_sca was extracted by taking into account the focal spot size A/_oc and the collection efficiency φ of the setup using the relationships of Equation (3):

This approach is valid in an index matched geometry where h_g(λ) is much smaller than the scattered intensity: otherwise, the interference between the scattered and reflected waves should be included in Equation (2). To enable comparison between the individual nanorods, several selection criteria were applied to the nanorods to improve the output information. Only nanorods with comparable areas under the longitudinal SPR in the scattering spectrum (standard deviation 10%) were included: because the total scattered field scales as V², this selects a subpopulation with a standard deviation of 5% in volume. Only nanorods with a peak value of σ_sca = 1.55 ± 0.2 x 10^"15 m² in randomly polarized light (standard deviation 12%) were included: because σ_sca scales as V² and σabs scales as V, this criterion isolates nanorods with a standard deviation of 5.8% in σabs. The irradiated nanorods were divided into two separate groups according to their longitudinal SPR energy, which were selected corresponding to average initial aspect ratios of 2.5 ± 0.2 (longitudinal SPR 750 ± 20 nm) and 3 ± 0.2 (longitudinal SPR 805 ± 15 nm).

To locate the nanorods, the white light spot was raster scanned over the sample surface. White light scattering spectra 2302 of the individual gold nanorods were acquired before irradiation, e.g., as shown in Figure 23. The vertical axis in Figure 23 shows the absolute scattering cross section of the nanorods, σ_sca, determined use Equation (3). The scattering spectra 2302 showed a longitudinal SPR ranging from 1.4-1.7 eV (equivalent to 840-740 nm). The Lorentzian linewidth was about T = 105 ± 20 meV. The polarized scattered intensity followed the expected dipolar angle dependence, as shown in the inset in Figure 23. These scattering spectra 2302 allow location of isolated gold nanorods, for determination of their orientation on the sample surface, and selection of an appropriate irradiation wavelength for each individual nanorod.

To establish a correlation between the physical and optical properties, scattering spectra and scanning electron microscopy (SEM) images were acquired of about 20 nanorods. The correlation between aspect ratio and longitudinal SPR energy for all the particles was strong, as shown in Figure 24. The small spread of the data points, shown by error bars in Figure 24, was caused by a different (effective) refractive index of the local environment around each nanorod, possibly caused by impurities or local air/water content in the polymer matrix. The error bars indicate the uncertainty in the dimensions obtained from the SEM images. There was good agreement between the measured SPR energy and modelled longitudinal SPR energy (shown as a dotted line in Figure 24) for a spherically capped cylinder with a width of 30 run energy embedded in a medium with ε_m = 2.25 (as described in S. W. Prescott and P. Mulvaney, J. Appl. Phys., 2006, 99, 123504), indicating that the rods can be accurately modelled as spherically capped cylinders.

Individual gold nanorods were irradiated with single femtosecond laser pulses. An example nanorod was selected with an initial longitudinal SPR at about 1.68 eV (745 nm), as shown in the "before" plot of Figure 25(a). The estimated initial aspect ratio of the example nanorod was 2.5 ± 0.2. This nanorod was irradiated with a single laser pulse with a pulse energy density of 1.93 mJ cm^"2 in the focal plane (Λ,=745 nm). After irradiation, the longitudinal SPR energy of the example nanorod was blue-shifted to about 1.96 eV (640 nm), as shown in the "after" plot of Figure 25(a). A significant broadening of the longitudinal SPR occurred, caused by the interband transitions in gold which have an onset energy of about 2 eV. The polarized scattered intensity did not change significantly due to the remaining elongated particle shape. The aspect ratio of the particle was reduced from about 2.5 to about 1.6, as shown in the SEM image in the inset of Figure 25(a).

Another example nanorod was selected with an initial longitudinal SPR at 1.67 eV or 750 nm, as shown in the "before" plot of Figure 25(b), and an aspect ratio 2.5 ± 0.2. This example nanorod was illuminated with a higher pulse energy density of 2.5 mJ cm^'2 (λ = 745 nm), upon which the nanorod completely relaxed / reshaped to a spherical geometry, forming a spherical particle with a radius of about 24 nm, as shown in the SEM image in the inset of Figure 25(b). The scattering spectrum after irradiation displayed a peak at about 2.3 eV (540 nm), corresponding to the sphere SPR, as shown in the "after" plot of Figure 25(b). The initial dipole character of the polarized scattered intensity was replaced with a monopole character due to the loss of anisotropy in the particle shape.

Upon melting, the nanorods (partly) reshape to the energetically favoured spherical geometry. Particle shapes were observed for three different particles after irradiation with increasing laser pulse energy: the particles had an initial aspect ratio of 2.5 ± 0.2, and were illuminated with a laser wavelength on resonance with their longitudinal SPR peak. Increasing pulse energy from 1.6 mJ/cm² to 1.9 mJ/cm² to 2.1 mJ/cm² caused shape changes from a long nanorod to a shorter and wider nanorod, to the spherical shape, as shown in the diagrams and SEM images in Figure 26.

Longitudinal SPR after irradiation gradually decreases as the amount of absorbed energy increases, as shown in Figure 27. The amount of energy absorbed by a nanorod can be estimated or determined by considering the pulse energy density, /, in the focal plane, and the absorption cross section σ^f , where the superscript indicates that the value is averaged over the narrow sub-population we considered. The amount of energy absorbed by the rod can be expressed as Q_abs = I x σ^f . An estimate for σ^f can be obtained from a theoretical ratio between the scattering and absorption cross section, based on an electrostatic approach and Gans' theory (as described in C. F. Bohren and D. R. Huffman,

Absorption and Scattering of Light by Small Particles, John Wiley & Sons. Inc., New

York, 1998) with the estimated shape factors L described in S. W. Prescott and P.

Mulvaney (J. Appl. Phys., 2006, 99, 123504) for a spherically capped cylinder with a width of 30 nm. Using typical parameters for the nanorods embedded in PVA with an aspect ratio of 2.7, σj^a _bs ^g I σζξ is determined to be about 4.3. The expression relating Q_abs and / is then:

β_βΛl = 4.3 x /x O . (4)

Using the relationship shown in Figure 27, it can be determined that both aspect ratios require about 260 fJ to completely relax to the spherical geometry. From thermodynamic considerations the amount of energy required to just melt a nanorod can be determined (as described in A. Bejan, Heat Transfer, John Wiley & Sons, Inc., New York, 1993):

PV[c_p{T_meirT₀)+

(5)

where p is the density of bulk gold, V is the volume of the nanorod, c_p is the specific heat capacity of bulk gold (129 J kg^"1 K^"1), Tmelt is the melting temperature of bulk gold (1330 K), T₀ is the ambient temperature, and ΔHfus is the heat of fusion for bulk gold (6.5 x 10⁴ J kg^"1), as described at least in part in: R. C. Weast and D. R. Lide, CRC Handbook of Chemistry and Physics, CRC press, Boca Raton, Florida, 85th edn, 2004. 38 S. Williamson, G. Mourou and J. C. M. Li, Phys. Rev. Lett., 1984, 52, 2364-2367. Employing the ensemble average volume of 5.8 x 10⁴ nm³, it can be determined that Q_met, = 225 ± 15 fj for a typical nanorod in the experimental example. The error originates from the uncertainty in the typical volume of the rods of 4.8%.

Inconsistencies in the theoretical and measured melting energies can be due to the viscoelasticity of the polymer matrix, which counters the surface energy driven migration of the molten gold atoms to form a spherical particle, and can increase the amount of energy required for structural changes to occur. Also cooling of the particle during reshaping could contribute to the observed discrepancy: whereas C. Y. Ruan, Y. Murooka, R. K. Raman and R. A. Murdick (Nano Lett., 2007, 7, 1290-1296) reported that a gold sphere undergoes full melting within 100 ps, an estimated 1 ns cooling time could implies that approximately 10% of the heat has dissipated before melting, which may explain the difference between the measured and calculated melting energies (as described in F. Cooper, Int. J. Heat Mass Transfer, 1977, 991-993).

The initial stages of the reshaping at energies below about 200 fj exhibit a gradual structural change into shorter aspect ratio particles. In this pulse energy regime the nanorod undergoes partial melting, and only a thin shell of surface atoms migrates to form a lower energy geometry. Intermediate, stable particles can be formed with lower aspect ratios. This intermediate shape can be the result of a surface reorganization from { 110} and { 100} facets to the energetically more stable { 111 } facets (e.g., as described in Y. T. Wang and C. Dellago, J. Phys. Chem. B, 2003, 107, 9214-9219; J. K. Diao, K. Gall and M. L. Dunn, Phys. Rev. B, 2004, 70, 075413; and Y. T. Wang, S. Teitel and C. Dellago, Nano Lett., 2005, 5, 2174-2178). The intermediate product covered with { 111 } facets is stable up to the bulk melting point of gold. The temperature that caused formation of the intermediate geometry increases with increasing particle size, and approached the homogeneous melting temperature for large particles. The intermediate stable particle shape should be precluded in large nanorods. When the nanorod absorbed more than about 220 fJ (the "total reshaping threshold"), it collapsed to a spherical geometry. The total reshaping threshold could be defined by the bulk melting point, at which the internal atomic structure of the nanorod suddenly becomes disordered and the nanorod collapses to the spherical geometry.

Partial structural changes occurred at lower pulse energy densities for longer aspect ratio particles. For example, for a Q_abs of about 220 fJ, the particles with an initial aspect ratio of about 3 underwent significant structural changes to an aspect ratio of about 1.8, but the particles with an initial aspect ratio of about 2.5 were hardly affected, exhibiting an aspect ratio of about 2.2 after reshaping. Due of the selection criteria outlined above, this effect is probably not caused by differences in particle volume or absorption cross section. The difference in energy required for reshaping depends on the difference in surface energy between the two aspect ratios. The shape of each particle after laser-induced heating is driven by the tendency of surfaces to reduce their surface energy. As surface energy is proportional to surface area, the total surface energy difference, relative to the lowest energy geometry of a sphere, is almost 20% larger for an aspect ratio of 3 compared to an aspect ratio of 2.5 (assuming a constant volume). Due to this difference in surface energy, an aspect ratio 3 particle may exhibit a higher rate of reshaping in order to minimize its surface energy. The homogeneous melting temperature still occurs at a similar temperature for both aspect ratios because this is determined by the bulk melting temperature of gold.

Applications

In optical filtering applications, an image in the recording medium 102 may be used as a polarization- and wavelength-dependent signal modulator for optical devices. For example, the large extinction of certain reactive bodies, such as gold nanorods, allows for customized modulation in multiple wavelength bands and polarizations of a light source. This can be accomplished in a single filter, which does not suffer from bleaching and exhibits a high damage threshold of >10 mJ cm^"2 (based on the threshold energy required for recording a pixel). The recorded medium may be part of a device integrated in a chip, such as an optical integrated circuit, or a plasmonic integrated chip. The wavelength polarization and spatial selectivity of the recorded filter can be written into the chip in situ using the non-contact optical writing process 800.

In security imprinting and encryption applications, the multiple wavelengths, polarization and spatial dimensions can be used for extended and counterfeit-resistant encryption keys, in particularly using multiplexed writing and reading methods.

In high-density optical data storage applications, the incorporation of two polarization and three wavelength channels, a 10-μm spacer layer, and a bit spacing equal to the bit diameter of about 0.75 μm could provide a bit density of about 1.1 Tbit cm^"3, and thus a disk capacity of about 1.6 Tbyte for the recording medium 102 in a disc form similar to a CD or DVD. For a recording medium with 280 layers, three-state polarization encoding, and a 3.3-μm thick spacer, a DVD-sized disk could have a capacity of about 7.2 Tbyte. Bit-by-bit recording may be compatible with existing disk drive technology (e.g., mechanical mounting and actuation, optical paths, electronic drives and electronic/digital signal processing), and may provide recording speeds up to 1 Gbit s^"1 with a high repetition rate laser source, or even higher with a supercontinuum light source for simultaneous multiplexed recording and reading in all channels.

An example personal computer (PC), with one or more standard commercial off-the-shelf processor(s) and user interface (UI) connections, can include the CAS 130 as a hardware card (e.g., with application-specific integrated circuits and/or dedicated digital signal processors and digital memory), or as program modules executed by the processors. The PC's UI can include the interface 140. The PC can include an optical disc drive with other components of the read- write system 104. The optical disc drive can include a drive for receiving the recording medium 102 and moving it in three spatial dimensions relative to the focal volume 118 (e.g., by moving the focal length or position of the objective 1 16 and/or the medium 102). For example, a disc-shaped product including the recording medium 102 can be rotated by the drive, while the objective 116 is moved radially across the disc, and the depth of the focal volume 1 18 is moved perpendicular to the plane of the disc, all under control of the CAS 130, to address different regions in the recording medium. The stored information on the recording medium 102 can include any one of: a two-dimensional or three-dimensional filter pattern for a polarisation- and wavelength- selective optical filter; digital data for use in a digital processor and/or digital computing device; and a two-dimensional or three-dimensional image for display to a user of the PC.

Interpretation

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.

Related Applications

The described is related to Australian Provisional Application No. 2009901994, filed on 6 May 2009, in the name of Swinburne University of Technology, the disclosure of which is hereby incorporated herein by reference.

Claims

CLAIMS:

1. An optical data storage product, including a data storage medium containing mutually spaced bodies selectively responsive to light of respective different wavelengths and respective different polarisations, said bodies being distributed in a plurality of selectably addressable regions of said data storage medium such that each region contains groups of said bodies selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each group of bodies in each region can be selectively modified to store digital data in said region using light of a corresponding one of a plurality of said wavelengths and a corresponding one of a plurality of said polarisations and having an intensity exceeding a threshold intensity.

2. The optical data storage product of claim 1 , wherein the different respective combinations of wavelength and polarisation correspond to different respective combinations of orientation and dimensions of said bodies.

3. The optical data storage product of any one of claims 1 and 2, wherein the bodies are distributed within a substance that is substantially unresponsive to said light.

4. The optical data storage product of any one of claims 1 to 3, wherein the data storage medium is in the form of a circular disc.

5. The optical data storage product of any one of claims 1 to 4, wherein the modification provides a persistent change due to photothermal reshaping of the bodies, and wherein the reshaped bodies are no longer responsive to light of the corresponding wavelength and corresponding polarisation.

6. The optical data storage product of claim 5, wherein the bodies are held in a surrounding substance that resists mechanical forces from the bodies caused by the photothermal reshaping.

7. The optical data storage product of any one of claims 1 to 4, wherein the bodies are initially elongate nanorods of an electrically conductive material, and the modification of the bodies includes changing their shape and thereby their optical responsiveness such that the modified bodies can be optically distinguished from the unmodified bodies to allow the stored data to be determined optically.

8. The optical data storage product of claim 7, wherein each body is initially a gold nanorod with a responsive wavelength defined by a longitudinal surface plasmon resonance (SPR) of the gold nanorod.

9. The optical data storage product of any one of claims 1 to 8, wherein the bodies are arranged in layers configured to allow separate optical addressing of different layers using the light.

10. The optical data storage product of any one of claims 1 to 9, wherein the groups of bodies are substantially homogeneously distributed in each region.

11. An information recording process including: receiving information to be stored; generating a plurality of input optical signals having respective different wavelengths and respective different polarisations representing the information to be stored; storing the information by selectively applying the input optical signals to a plurality of regions of an information storage medium having an initial configuration of mutually spaced bodies selectively responsive to each of said input optical signals, the intensity of the input optical signals exceeding a threshold intensity to modify said bodies.

12. The information recording process of claim 11, including selecting the wavelengths and the polarisations of the input optical signals using one or more optical filters.

13. The information recording process of any one of claims 11 and 12, wherein generating the input optical signals includes simultaneously generating a plurality of the wavelengths and/or the polarisations, and storing the information includes applying the generated plurality of wavelengths and/or polarisations simultaneously.

14. An information retrieval process, including: generating a plurality of input optical signals having respective different wavelengths and respective different polarisations; directing the input optical signals to a plurality of regions of an information storage medium, each region having a plurality of mutually spaced bodies being selectively responsive to each of said input optical signals to represent stored information; and detecting output optical signals from an interaction of the input optical signals with the bodies in each region to determine the stored information.

15. The information retrieval process of claim 14, wherein the intensity of the input optical signals is selected to not exceed a threshold intensity to not modify said bodies.

16. The information retrieval process of any one of claims 14 and 15, including selecting one or more wavelengths and one or more polarisations of light in the output optical signals using one or more optical filters.

17. The information retrieval process of any one of claims 14 to 16, wherein generating the input optical signals includes simultaneously generating a plurality of the different wavelengths and/or the different polarisations, and detecting the output optical signals includes simultaneously detecting the output signals having a corresponding plurality of wavelengths and/or polarisations.

18. The information retrieval process of any one of claims 14 to 17, wherein detecting the output optical signals includes simultaneously detecting the output optical signals in two spatial dimensions for each wavelength and each polarisation of the output optical signals.

19. The information retrieval process of any one of claims 14 to 18, wherein the bodies respond to the input optical signals by emitting the output optical signals at an output wavelength different from the wavelength of the input optical signals, and detecting the output optical signals includes detecting the output optical signals at the output wavelength.

20. The information retrieval process of claim 19, wherein the different output wavelength is caused by two-photon luminescence (TPL) of the bodies, and the output wavelength is shorter than the wavelength of the input optical signals.

21. The process of any one of claims 11 to 20, wherein the regions are distributed in three spatial dimensions.

22. The process of any one of claims 11 to 21, wherein generating the input optical signals includes generating pulsed signals.

23. The process of any one of claims 11 to 21, wherein generating the input optical signals includes generating continuous-wave (CW) signals.

24. The process of any one of claims 11 to 23, wherein the stored information includes any one of: a two-dimensional or three-dimensional filter pattern for a polarisation- and wavelength-selective optical filter; digital data for use in a digital processor and/or digital computing device; and a two-dimensional or three-dimensional image for display to a user.

25. A system configured to perform the process of any one of claims 11 to 24, including an optical source configured to generate the input optical signals.

26. An optical data storage product, including mutually spaced bodies selectively responsive to light of respective different wavelengths and respective different polarisations, wherein populations of said bodies are selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each population in a region of the data storage product can be selectively modified to store digital data in the region using light of a corresponding wavelength and polarisation having an intensity above a threshold writing intensity.

27. An optical data storage product, including a plurality of mutually spaced bodies, including: first groups of said bodies selectively responsive to light of respective different wavelengths; and second groups of said bodies selectively responsive to light of respective different polarisations, said bodies in said groups being selectively responsive to light of respective different combinations of wavelength and polarisation, wherein each of the bodies can be selectively modified in a selected region of the data storage product to store digital data in said region using light of a corresponding wavelength of the different wavelengths and a corresponding polarisation of the different polarisations and having an intensity exceeding a threshold intensity for writing data.

28. A process for forming the optical data storage product of any one of claims 1 to 10, 26 and 27 including: forming the bodies; mixing the bodies in an unresponsive substance to form a mixture; forming an initial recording layer on a substrate using the mixture; and forming one or more combinations of a spacer layer adjacent a further recording layer, starting adjacent the initial recording layer, to provide three spatial dimensions for data storage.

29. The process of claim 28, wherein forming of the initial recording layer includes spin-coating the initial recording layer on the substrate.