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51 pages, 13680 KiB  
Review
Roadmap of Terahertz Imaging 2021
by Gintaras Valušis, Alvydas Lisauskas, Hui Yuan, Wojciech Knap and Hartmut G. Roskos
Sensors 2021, 21(12), 4092; https://doi.org/10.3390/s21124092 - 14 Jun 2021
Cited by 202 | Viewed by 16785
Abstract
In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being [...] Read more.
In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging. Full article
(This article belongs to the Special Issue Terahertz Imaging and Sensors)
Show Figures

Figure 1

Figure 1
<p>Photo of fiber-coupled optoelectronic GaAsBi-based THz emitter mounted with a silicon lens. Courtesy of Laboratory of Ultrafast Optoelectronics Laboratory at Optoelectronics Department at FTMC and Teravil Ltd., Vilnius, Lithuania.</p>
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<p>Photo of silicon lens coupled FET-based THz detector equipped with low-noise amplifier. Courtesy of MB “Terahertz Technologies”, Vilnius, Lithuania.</p>
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<p>Photo of titanium-based microbolometer THz camera. It is composed of a linear array consisting of 2 lines with 16 pixels each. The left line is used for the detection of 700 GHz, and the right one for the detection of 300 GHz. It was realized via coupling of the sensors with antennas of relevant designs. The titanium bridge of 12 µm and 2 µm in length and width, respectively, was electrodeposited on the silicon nitride (SiN) membrane of 2-µm thick. The membrane was etched to reduce the heat capacitance and to increase the speed of operation. More details can be found in Refs. [<a href="#B188-sensors-21-04092" class="html-bibr">188</a>,<a href="#B190-sensors-21-04092" class="html-bibr">190</a>]. Courtesy of Laboratory for Microelectronics of Faculty of Electrical Engineering at University of Ljubljana and Luvitera Ltd., Vilnius, Lithuania. Photo: courtesy of Linas Minkevičius.</p>
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<p>Photo of high-resistivity silicon-based Bessel zone plate placed in the optical mount. The diffractive optical element is prepared by the laser ablation technology. More details on fabrication technology and the zone plate design can be found in Ref. [<a href="#B213-sensors-21-04092" class="html-bibr">213</a>]. Photo: courtesy of Domas Jokubauskis.</p>
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<p>Schematic of the Fourier imaging based on the heterodyne detection of nanometric FET detector at 300 GHz. Left inset: Sketch of the imaging setup. The scene is illuminated by radiation from source S1. The spatial Fourier spectrum of the scene is written in the focal plane of lens L2 (as indicated by the schematic on the lower right side of the figure) and recorded with the raster-scanned single-pixel detector. S2 works as local oscillator and is focused onto the front side of the detector by L3. L3 and S2 share the translation stage with the detector and are moved together with it (as symbolized by the dashed ellipse). Right inset: 3D imaging example (<b>a</b>) Left side: Washer-and-screw scene. The THz beam impinges from the left and hits first the screw and then the washer. Right side: The photograph of the washer and screw placed on a table with approximately correct projectional view. Middle panels: Intensity (<b>b</b>) and phase images (<b>c</b>) for a reconstruction distance equal to the position of the washer; lower two panels: Reconstructed intensity (<b>d</b>) and phase (<b>e</b>), but for the distance equal to that of the screw. Figure modified from Ref. [<a href="#B262-sensors-21-04092" class="html-bibr">262</a>].</p>
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<p>Schematic of an all-electronic homodyne s-SNOM measurement setup with a detector based on a Si CMOS field-effect transistor. Left inset: 3D layout of the detector with the monolithically integrated annular ring antenna and the Si substrate lens. Active devices were fabricated using either a 90 nm or a 180 nm technology process node. The high sensitivity enables demodulation of the s-SNOM signal at up to the 10th harmonic of the cantilever’s oscillation frequency. Right inset: (<b>a</b>) AFM topography of a Si surface (black) with dielectric islands (brown); (<b>b</b>) simultaneously measured s-SNOM amplitude images recorded at the 2nd harmonic of the cantilever’s oscillation frequency. Scale bars in (<b>a</b>,<b>b</b>): 1 µm. (<b>c</b>) Line scans along the red lines in (<b>b</b>) showing a spatial resolution down to &lt;50 nm. Figure modified from Ref. [<a href="#B318-sensors-21-04092" class="html-bibr">318</a>].</p>
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<p>Block diagram of the THz dual-comb imaging system. The first optical frequency comb is generated from the output of the master laser by two phase modulators, and then two teeth (with a frequency spacing equal to the central THz signal to be generated) are filtered by optical injection locking and a dual-comb signal is created from one of them; both signals are recombined on an uni-traveling-carrier (UTC) photodiode. The emitted THz signal is focused on the sample plane and detected later by a FET detector. The insets show (from left to right) the spectrum of the THz dual-comb, the responsivity of the FET detector and the spectrum of the multi-heterodyne signal after detection. The imaging example presents the transmittance spectra through the complex plastic sample at several locations, enabling a straightforward separation between plastic layers (differences in the overall absorbance and the frequency dependent slope) and the edge diffraction. Reproduced from Ref. [<a href="#B361-sensors-21-04092" class="html-bibr">361</a>].</p>
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<p>Detection of thermal radiation with a broadband nanometric FET [<a href="#B419-sensors-21-04092" class="html-bibr">419</a>]. The main panel shows the setup for the detection of radiation emitted from (<b>Case 1</b>) a ceramic heat source (embedded in a reflector) or (<b>Case2</b>) the human body, here, the palm of a hand. The heater, respectively, the hand, was placed on a <math display="inline"><semantics> <mrow> <mi>x</mi> <mi>y</mi> </mrow> </semantics></math>-translation stage for raster-scan imaging. The radiation was guided by two parabolic mirrors and through a mechanical chopper to the detector, where it impinged via a Si substrate lens, attached to the backside of the nanometric FET chip, onto the antenna-embedded transistor. The angles shown are various acceptance angles used for the calculation of the expected noise-equivalent temperature difference (NETD). Inset: Raster-scan image of three fingers of a hand. The measurement was performed at ambient temperature. The black-and-white plot of the 25 × 75-pixel image (white indicating an elevated temperature; black representing room temperature) is superimposed on a photograph of the hand. Figure modified from Ref. [<a href="#B419-sensors-21-04092" class="html-bibr">419</a>].</p>
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<p>Map of room temperature emitting and sensing devices in a logarithmic THz frequency scale. The considered devices are plotted with respect to the emitting power in sources section (left) and the noise equivalent power <math display="inline"><semantics> <mrow> <mo>(</mo> <mi>N</mi> <mi>E</mi> <mi>P</mi> <mo>)</mo> </mrow> </semantics></math> in sensors one (right). Taken data references: Schottky diodes multipliers [<a href="#B87-sensors-21-04092" class="html-bibr">87</a>,<a href="#B439-sensors-21-04092" class="html-bibr">439</a>] CMOS-based and SiGe-based electronic emitters [<a href="#B62-sensors-21-04092" class="html-bibr">62</a>]; CMOS and SiGe detectors parameters [<a href="#B159-sensors-21-04092" class="html-bibr">159</a>], parameters of Schottky detectors [<a href="#B440-sensors-21-04092" class="html-bibr">440</a>], microbolometers values [<a href="#B167-sensors-21-04092" class="html-bibr">167</a>], conventional THz QCL [<a href="#B41-sensors-21-04092" class="html-bibr">41</a>]; frequency-difference THz QCLs (FD THz QCLs) parameters—from publications by M. Razeghi [<a href="#B51-sensors-21-04092" class="html-bibr">51</a>,<a href="#B52-sensors-21-04092" class="html-bibr">52</a>] and M. Belkin’s [<a href="#B53-sensors-21-04092" class="html-bibr">53</a>] groups. Optoelectronic THz systems (denoted as red solid lines) are attributed for the emitters section only. The left red solid line depicts facilities of optoelectronic InGaAsBi-based systems [<a href="#B36-sensors-21-04092" class="html-bibr">36</a>], the right one—the system relying on InGaAs:Rh compound [<a href="#B37-sensors-21-04092" class="html-bibr">37</a>]. Resonant tunneling diodes data are taken from publications by M. Asada [<a href="#B119-sensors-21-04092" class="html-bibr">119</a>,<a href="#B121-sensors-21-04092" class="html-bibr">121</a>], M. Feiginov [<a href="#B117-sensors-21-04092" class="html-bibr">117</a>] and H. Yokoyama [<a href="#B441-sensors-21-04092" class="html-bibr">441</a>] groups. Shaded ares denote schematically possible complementary components in a design of compact THz imaging systems in respect to the THz frequency scale. One can note that compact room temperature THz imaging systems can be constructed, for instance, using FD THz QCLs and silicon nanometric transistors or microbolometers. It is seen that RTD devices can be used together with Schottky diodes, silicon nanoFETs, microbolometers and bow-tie diodes [<a href="#B181-sensors-21-04092" class="html-bibr">181</a>]. One can mention that CMOS technology-based mixers and oscillators in imaging can be nicely fitted together with nanometric FETs and microbolometers, as well as Schottky or bow-tie diodes. Graphene-based room-temperature THz detector’s parameters are taken from publications of the H. G. Roskos [<a href="#B163-sensors-21-04092" class="html-bibr">163</a>], J. Stake [<a href="#B442-sensors-21-04092" class="html-bibr">442</a>], and M. S. Vitiello groups [<a href="#B164-sensors-21-04092" class="html-bibr">164</a>].</p>
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<p>Schematic presentation of important milestones in the evolution of room temperature THz imaging systems within the last two decades. The systems are plotted with respect to the systems size reduction, power consumption, and enhanced functionality. Pioneering works in continuous wave [<a href="#B1-sensors-21-04092" class="html-bibr">1</a>] and optoelectronic THz imaging [<a href="#B2-sensors-21-04092" class="html-bibr">2</a>], as well as THz microscopy [<a href="#B273-sensors-21-04092" class="html-bibr">273</a>], and THz metamaterials [<a href="#B199-sensors-21-04092" class="html-bibr">199</a>] are denoted via blue circles and light-green labels. Meanings of other colors are depicted in the top-left corner of the plot. The first THz image recorded using optically-pumped molecular THz laser which is more than 2 meters long and uses of about 7 kW of electrical power, while state-of-art modern electronic sources, e.g., CMOS emitters are compact, in cm scale, and require only up to 1 W of power. No cryogenic cooling is needed for their operation. Invention of THz QCLs (still cooled cryogenically, below 50 K) unveiled an elegant solid-state-based compact solution for THz emitters—a route to reduce dimensions and power consumption in THz imaging systems [<a href="#B40-sensors-21-04092" class="html-bibr">40</a>]. First experiments on non-resonant THz detection using nanometric field-effect transistors open a new trend in development of sensitive detectors [<a href="#B139-sensors-21-04092" class="html-bibr">139</a>]. THz real-time imaging system using uncooled microbolometers array [<a href="#B184-sensors-21-04092" class="html-bibr">184</a>] demonstrated their potential for real time THz image recording. Room-temperature THz detection using silicon nanoFETs [<a href="#B141-sensors-21-04092" class="html-bibr">141</a>] was a breakthrough paper in the development of silicon-based THz and their sensors. Room-temperature generation of THz radiation in nanometric InAlAs/InGaAs and AlGaN/GaN HEMTs stimulated further research on nanotransistors. Self-resistive mixing mechanism in THz detection and development of CMOS technology-based THz focal-plane arrays provided a deeper understanding in physics behind the detection and opened a route for future cost-effective THz imaging solutions [<a href="#B144-sensors-21-04092" class="html-bibr">144</a>]. Femtosecond fiber lasers-based optoelectronics THz systems [<a href="#B35-sensors-21-04092" class="html-bibr">35</a>] demonstrated compact realization for optoelectronic THz imaging and increased convenience in their use. Room-temperature CMOS- [<a href="#B58-sensors-21-04092" class="html-bibr">58</a>,<a href="#B59-sensors-21-04092" class="html-bibr">59</a>,<a href="#B60-sensors-21-04092" class="html-bibr">60</a>], SiGe HBT-based [<a href="#B63-sensors-21-04092" class="html-bibr">63</a>] THz sources, and InP heterojunction bipolar transistors [<a href="#B64-sensors-21-04092" class="html-bibr">64</a>] revealed an interesting purely electronic approach to develop imaging systems. Resistive-distributed-plasmonic mixing models allowed gaining of wider insight into THz detection mechanism in nanometric FETs and extend their detection range up to 9 THz [<a href="#B148-sensors-21-04092" class="html-bibr">148</a>]. CMOS-based sensors and metamaterials absorbers can successfully be monolithically integrated [<a href="#B238-sensors-21-04092" class="html-bibr">238</a>]. Silicon-based diffractive optics can be a rational way for the integration of passive components [<a href="#B211-sensors-21-04092" class="html-bibr">211</a>,<a href="#B212-sensors-21-04092" class="html-bibr">212</a>] with active devices. Room temperature intracavity frequency difference tunable THz QCLs [<a href="#B51-sensors-21-04092" class="html-bibr">51</a>] exhibited a monolithic solution for THz spectroscopy, sensing and imaging systems. All electronic realization of THz nanoscopy allowed for creation of laser- and cryogenic cooling-free electronics-based near-field optical microscope [<a href="#B315-sensors-21-04092" class="html-bibr">315</a>]. Conventional THz QCLs has reached an operating temperature of 250 K, with the size of just a few millimeters [<a href="#B49-sensors-21-04092" class="html-bibr">49</a>].</p>
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