CN220730041U - System for measuring terahertz pulse group velocity in sample - Google Patents
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Abstract
The utility model discloses a system for measuring terahertz pulse group velocity in a sample, and relates to the technical field of terahertz wave band optical measurement. The optical system comprises a laser, a spectroscope, an optical parametric amplifier, a chopper, a terahertz generation crystal, a terahertz filter, a first off-axis parabolic reflector, a second off-axis parabolic reflector, a third off-axis perforated parabolic reflector, an optical delay device, a first convex lens, a measured sample, a second convex lens, a quarter wave plate, a Wollaston prism and a photoelectric balance detector, wherein the laser, the spectroscope, the optical parametric amplifier, the chopper and the terahertz generation crystal are arranged on an optical path. The system has the advantages of simple structure, low construction cost, high stability, strong repairability and easy maintenance.
Description
Technical Field
The utility model relates to the technical field of terahertz wave band optical measurement, in particular to a terahertz pulse group velocity measurement system in a sample.
Background
Chromatic dispersion, i.e. the characteristic that the refractive index of a material changes with the change of the frequency of incident light, can cause the phenomena of pulse broadening, group phase mismatch and the like, and restricts the development of free space optical systems and optical fiber communication systems to high quality directions. The propagation speed of the optical pulse containing abundant frequency components in the sample is determined by the dispersion and the refractive index. Dispersion and refractive index measurements are essentially either measuring the time delay for optical pulses of different frequencies to travel the same distance in a dispersive medium, or measuring the group velocity of optical pulses with rich frequency components in a dispersive medium.
Currently, there are many methods for measuring dispersion or group velocity of samples in ultraviolet, visible, infrared, etc. frequency bands, including time delay method, phase shift method, interferometry, etc. For example, the time delay method is to measure the time delay of light pulses with different frequencies passing through a sample, and then obtain a group velocity value through polynomial fitting; the phase shift method obtains a group velocity value by comparing the phase difference between the reference value and the measured value; interferometry utilizes a Michelson interferometer to obtain a phase difference between a reference value and a measured value by adjusting the relative distance between a reference arm and a measurement arm of the Michelson interferometer. However, the above method is difficult to be used for measuring the group velocity parameter of the pulse in the terahertz frequency band, and the factors which obstruct the measurement of the group velocity parameter in the terahertz frequency band may have the following three points:
(1) Terahertz pulses typically contain very rich frequency components, with frequency coverage typically exceeding 5 octaves, with very large effort and extremely high stability requirements on the system by measuring the time delay or phase of all single frequencies to ultimately obtain the terahertz pulse group velocity.
(2) The spectral distribution of terahertz pulses is generally irregular, not subject to strict gaussian, and the group velocity of the entire terahertz pulse can only be approximately calculated by the delay or phase parameter of each frequency point.
(3) The system is limited by the technical level of the terahertz source and the detector, and the difficulty of setting up a system for measuring the terahertz pulse group velocity parameters by a time delay method, a phase shift method or an interferometry in the terahertz frequency band is high.
Therefore, a new measuring device is needed to realize the direct measurement of the terahertz pulse group velocity parameter in the sample.
Disclosure of Invention
The terahertz pulse group velocity measuring system in the sample provided by the utility model measures terahertz pulse group velocity parameters in the sample by utilizing the Kerr effect commonly existing in the substance, and the sensitivity for measuring the terahertz pulse group velocity in the utility model can reach 0.001mm/ps, and can be applied to measuring the group velocity value of terahertz pulses with complex frequency components in the sample.
In order to achieve the above purpose, the utility model provides a system for measuring terahertz pulse group velocity in a sample, which comprises a laser, a spectroscope, an optical parametric amplifier, a chopper, a terahertz generation crystal, a terahertz filter, a first off-axis parabolic mirror, a second off-axis parabolic mirror, a third off-axis perforated parabolic mirror, an optical delay device, a first convex lens, a sample to be measured, a second convex lens, a quarter wave plate, a Wollaston prism and a photoelectric balance detector, wherein the laser, the spectroscope, the optical parametric amplifier, the chopper and the terahertz generation crystal are arranged on an optical path; wherein:
the laser is preferably a titanium sapphire femtosecond laser amplifier and is used for emitting laser with the wavelength of 800nm and the pulse width of 35fs, and the femtosecond laser with the wavelength of 800nm is separated into a beam serving as pump light and a beam serving as probe light through the spectroscope. The pump light is converted into laser with the pulse width of 50fs at any wavelength of 800-2600 nm through the optical parametric amplifier, and terahertz pulse is generated through the chopper and the terahertz generating crystal; and the terahertz pulse filters out stray light through the terahertz filter, sequentially passes through the first off-axis parabolic reflector and the second off-axis parabolic reflector to carry out wave front modulation, and then is focused to the tested sample after being reflected by the third off-axis perforated parabolic reflector. The detection light sequentially passes through the optical delay device, the first convex lens and the third off-axis perforated parabolic reflector and then is focused on the sample to be detected together with terahertz pulses, and the sample to be detected is excited by the strong terahertz pulses to generate third-order nonlinear kerr effect, so that the detection light focused on the sample to be detected simultaneously generates polarization rotation; the detection light pulse modulated by the terahertz pulse is converged into parallel light through the second convex lens, polarized beam splitting is carried out through the quarter wave plate and the Wollaston prism, and then signal acquisition is carried out through the photoelectric balance detector; and adjusting the optical delay device to obtain time delay information for calculating the terahertz pulse group velocity of the sample.
In one embodiment of the utility model, the thickness of the sample to be measured is 300 μm-3 mm.
In an embodiment of the utility model, the laser is a titanium sapphire femtosecond laser amplifier, and the chopping frequency of the chopper is 30-400 Hz.
In an embodiment of the present utility model, the output wavelength of the optical parametric amplifier is 800-2600 nm.
In an embodiment of the utility model, the terahertz-generating crystal is a DAST crystal. It may also be replaced by a crystal of DSTMS, BNA, OH1 or the like.
In an embodiment of the utility model, the terahertz electric field strength generated by the terahertz generating crystal is 0.1-80 mv/cm.
In an embodiment of the present utility model, an included angle between the terahertz wave reflected by the third off-axis perforated parabolic mirror and a plane of the sample to be measured is between 80 ° and 100 °.
In one embodiment of the present utility model, the first off-axis parabolic mirror and the second off-axis parabolic mirror should have a focal length ratio of 1:4, its preferred value should be less than 1:2.
in one embodiment of the utility model, the sample to be measured is exemplified by quartz and sapphire. Examples of the sample that can be measured in the present system include, but are not limited to, various window materials for terahertz wave bands, substrate materials, biological samples, semiconductor materials, crystals, etc., such as quartz, diamond, silicon, gallium arsenide, silicon nitride, sapphire, polyethylene, organic solvents, protein solutions, etc.
In an embodiment of the present utility model, the terahertz pulse frequency band to be measured covers 0.1 to 10thz.
The beneficial effects of the utility model are as follows: the terahertz pulse group velocity measuring system provided by the utility model innovatively applies the phenomenon of the three-order nonlinear kerr effect commonly existing in the sample, and realizes the direct measurement of the terahertz pulse group velocity in the sample. The terahertz pulse is focused on the sample to be tested and propagates in the sample to excite the anisotropy of the sample. The probe light is focused at the same time at the same location in the sample and propagates within the sample at the same time. When the terahertz pulse and the detection light pulse meet in the sample, the terahertz pulse changes the polarization state of the detection light pulse due to the third-order nonlinear kerr effect of the sample. By adjusting the time delay device on the detection light path, the detection light pulse and the terahertz pulse reach the sample successively at a certain time interval. Because the terahertz pulses and the detection light pulses have different group velocities within the sample, the terahertz pulses and the detection light pulses can still track and meet within the sample and produce modulation of the polarization state of the detection light pulses when the time interval is small. And adjusting the time delay device to measure the modulated time range of the detection light pulse, namely acquiring terahertz pulse group velocity parameters based on parameters such as the reference detection light pulse velocity, the time range, the sample thickness and the like. The sensitivity of the terahertz pulse group velocity measurement method can reach 0.001mm/ps, and the method is used for directly measuring the terahertz pulse propagation group velocity in a sample for the first time. The system of the utility model has simple structure, low construction cost, high stability and easy maintenance. The utility model realizes the direct measurement of the terahertz pulse group velocity in the sample for the first time, has higher sensitivity, and has great scientific research and application value in the aspects of manufacturing components in the terahertz field, detecting the sample, analyzing the substrate window materials, constructing a terahertz system in a free space, manufacturing instruments and the like.
Drawings
In order to more clearly illustrate the embodiments of the present utility model or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It is evident that the drawings in the following description are only some embodiments of the present utility model and that other drawings may be obtained from these drawings without inventive effort for a person of ordinary skill in the art.
Fig. 1 is a schematic diagram (top view) of a measurement system of terahertz pulse group velocity in a sample provided by the present utility model;
FIG. 2 is a terahertz pulse spectrum used in the present utility model;
FIG. 3 is a graph showing the results of a system of the present utility model for measuring a quartz sample;
fig. 4 is a graph showing the measurement results of the sapphire sample by the system of the present utility model.
Reference numerals illustrate: 1-a laser; 2-spectroscope; 3-optical parametric amplifier; 4-chopper; 5-terahertz generation crystals; 6-terahertz filter plate; 7-a first off-axis parabolic mirror; 8-a second off-axis parabolic mirror; 9-a third off-axis perforated parabolic mirror; 10-an optical delay device; 11-a first convex lens; 12-the sample to be tested; 13-a second convex lens; 14-quarter wave plate; 15-Wollaston prism; 16-photo balance detector.
Detailed Description
The technical solutions in the embodiments of the present utility model will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present utility model. It will be apparent that the described embodiments are only some, but not all, embodiments of the utility model. All other embodiments, which can be made by those skilled in the art based on the embodiments of the utility model without any inventive effort, are intended to be within the scope of the utility model.
Fig. 1 is a schematic diagram (top view) of a measurement system of terahertz pulse group velocity in a sample provided by the utility model, which includes a laser 1, a spectroscope 2, an optical parametric amplifier 3, a chopper 4, a terahertz generating crystal 5, a terahertz filter 6, a first off-axis parabolic mirror 7, a second off-axis parabolic mirror 8, a third off-axis perforated parabolic mirror 9, an optical delay device 10, a first convex lens 11, a sample 12 to be measured, a second convex lens 13, a quarter-wave plate 14, a wollaston prism 15, and a photoelectric balance detector 16, wherein:
the laser 1 is used for emitting laser light with a wavelength of 800nm and a pulse width of 35fs, and the femtosecond laser light with the wavelength of 800nm is separated into a beam serving as pump light and a beam serving as detection light through the spectroscope 2. The pump light is converted into laser with the pulse width of 50fs at any wavelength of 800-2600 nm through the optical parametric amplifier 3, and terahertz pulse is generated through the chopper 4 and the terahertz generating crystal 5. The terahertz pulse filters stray light through the terahertz filter 6, carries out wave front modulation through the first off-axis parabolic reflector 7 and the second off-axis parabolic reflector 8 in sequence, and focuses on a tested sample 12 after being reflected by the third off-axis perforated parabolic reflector 9. The detection light sequentially passes through the optical delay device 10, the first convex lens 11 and the third off-axis perforated parabolic reflector 9 and is focused on the sample 12 to be detected together with the terahertz pulse, and the sample 12 to be detected is excited by the strong terahertz pulse to generate a third-order nonlinear kerr effect, so that the detection light focused on the sample 12 to be detected simultaneously generates polarization rotation. The terahertz pulse modulated detection light pulse is converged into parallel light through the second convex lens 13, polarized beam splitting is performed through the quarter wave plate 14 and the Wollaston prism 15, and then signal acquisition is performed through the photoelectric balance detector 16. And adjusting the optical delay device to obtain time delay information for calculating the terahertz pulse group velocity of the sample.
In fig. 1, the equivalent focal length of the first off-axis parabolic mirror 7 may be 1 inch and the equivalent focal length of the second off-axis perforated parabolic mirror 8 may be 4 inches.
In this embodiment, the thickness of the sample 12 is 300 μm to 3mm.
In the embodiment, the laser 1 is a titanium sapphire femtosecond laser amplifier, and the chopping frequency of the chopper 4 is 30-400 Hz.
In this embodiment, the output wavelength of the optical parametric amplifier 3 is 800-2600 nm.
In the present embodiment, the terahertz-generating crystal 5 is a DAST crystal, which can also be replaced by a crystal of DSTMS, BNA, OH or the like.
In the embodiment, the intensity of the terahertz electric field generated by the terahertz generating crystal 5 is 0.1-80 MV/cm.
In this embodiment, the included angle between the terahertz wave reflected by the third off-axis perforated parabolic mirror 9 and the plane of the sample 12 to be measured is between 80 ° and 100 °.
In this embodiment, the ratio of focal lengths of the first off-axis parabolic mirror 7 and the second off-axis parabolic mirror 8 should be 1:4, its preferred value should be less than 1:2.
in this embodiment, the sample 12 to be measured is exemplified by quartz and sapphire. Samples that may be measured by the present system include, but are not limited to, various window materials for terahertz wave bands, substrate materials, biological samples, semiconductor materials, crystals, etc., such as quartz, diamond, silicon, gallium arsenide, silicon nitride, sapphire, polyethylene, organic solvents, protein solutions, etc.
As shown in fig. 2, the terahertz pulse frequency band measured in this embodiment covers 0.1-10 thz.
The utility model is measured using a method that can be applied in a system such as that shown in fig. 1, comprising the steps of:
s1: the laser emits laser light having a wavelength of 800 nm.
S2: the 800nm laser beam is separated into a pump light beam and a detection light beam by the spectroscope.
S3: the pump light is converted into laser with the pulse width of 50fs at any wavelength of 800-2600 nm through the optical parametric amplifier, and terahertz pulse is generated through the chopper and the terahertz generating crystal. And the terahertz pulse filters out stray light through the terahertz filter, sequentially passes through the first off-axis parabolic reflector and the second off-axis parabolic reflector to carry out wave front modulation, and then is focused to the tested sample after being reflected by the third off-axis perforated parabolic reflector.
S4: the detection light sequentially passes through the optical delay device, the first convex lens and the third off-axis perforated parabolic reflector and then is focused on the sample to be detected together with terahertz pulses, and the sample to be detected is excited by the strong terahertz pulses to generate third-order nonlinear kerr effect, so that the detection light focused on the sample to be detected simultaneously generates polarization rotation. And after the detection light pulse modulated by the terahertz pulse is converged into parallel light through the second convex lens, the parallel light is split into beams according to polarization states through the quarter wave plate and the Wollaston prism, and then the signal acquisition is performed through the photoelectric balance detector. And adjusting the optical delay device to obtain time delay information for calculating the terahertz pulse group velocity of the sample.
S5: acquiring a time range based on measured time delay informationtAnd obtaining the reference speed of the detection light pulse in the sample from the refractive index table, the reference speed is equal to the thicknesslThe group velocity parameter of terahertz pulse in the sample can be obtained by a formula.
FIG. 3 is a graph of thickness using the system of FIG. 1lMeasurement results for a 0.6mm quartz sample, a time frame can be observedtAbout 1ps; obtaining refractive index of probe light pulse in quartz sample from refractive index metern1.4533 corresponding to the ginsengThe test speed is 0.2064mm/ps; the propagation group velocity parameter of the terahertz pulse in the quartz sample in this embodiment is 0.154mm/ps through formula calculation.
FIG. 4 is a graph of thickness using the system of FIG. 1lMeasurement results for a 0.6mm sapphire sample, a time frame can be observedtAbout 2.6ps; obtaining refractive index of probe light pulse in sapphire sample from refractive index metern1.7601, corresponding reference speed is 0.1704mm/ps; the propagation group velocity parameter of the terahertz pulse in the sapphire sample in the embodiment is 0.098mm/ps through calculation of a formula.
The utility model innovatively applies the three-order nonlinear Kerr effect phenomenon commonly existing in the sample, and directly measures the terahertz pulse group velocity high sensitivity in the sample for the first time, wherein the measuring sensitivity can reach 0.001mm/ps. Compared with the traditional time delay method and the phase method which measure the refractive index of each frequency point one by one so as to indirectly approximate the dispersion and the group velocity, the utility model can directly obtain the group velocity of terahertz pulses formed by any frequency component in a sample. The system of the utility model has simple structure, low construction cost, high stability and easy maintenance. The utility model realizes the direct measurement of the terahertz pulse group velocity in the sample for the first time, has higher sensitivity, and has great scientific research and application value in the aspects of manufacturing components in the terahertz field, detecting the sample, analyzing the substrate window materials, constructing a terahertz system in a free space, manufacturing instruments and the like.
Those of ordinary skill in the art will appreciate that: the drawing is a schematic diagram of one embodiment and the modules or flows in the drawing are not necessarily required to practice the utility model.
Those of ordinary skill in the art will appreciate that: the modules in the embodiments may be distributed in the apparatus of the embodiments according to the embodiment description, or may be located in one or more apparatuses different from the present embodiment with corresponding changes. The modules of the above embodiments may be combined into one module, or may be further split into a plurality of sub-modules.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present utility model, and are not limiting; although the utility model has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present utility model.
Claims (8)
1. The system for measuring the terahertz pulse group velocity in the sample is characterized by comprising a laser, a spectroscope, an optical parametric amplifier, a chopper and a terahertz generation crystal which are arranged on an optical path, a terahertz filter, a first off-axis parabolic reflector, a second off-axis parabolic reflector, a third off-axis perforated parabolic reflector, an optical delay device, a first convex lens, a measured sample, a second convex lens, a quarter-wave plate, a Wollaston prism and a photoelectric balance detector, wherein:
the laser is a titanium sapphire femtosecond laser amplifier and is used for emitting laser with the wavelength of 800nm and the pulse width of 35fs, and the femtosecond laser with the wavelength of 800nm is split into a beam serving as pump light and a beam serving as probe light through the spectroscope; the pump light is converted into laser with the pulse width of 50fs at any wavelength of 800-2600 nm through the optical parametric amplifier, and terahertz pulse is generated through the chopper and the terahertz generating crystal; the terahertz pulse filters stray light through the terahertz filter, sequentially passes through the first off-axis parabolic reflector and the second off-axis parabolic reflector to carry out wave front modulation, and then is focused to the sample to be measured after being reflected by the third off-axis perforated parabolic reflector; the detection light sequentially passes through the optical delay device, the first convex lens and the third off-axis perforated parabolic reflector and then is focused on the sample to be detected together with terahertz pulses; the three-order nonlinear kerr effect is excited by the strong terahertz pulse of the tested sample, so that the detection light focused on the tested sample simultaneously rotates in polarization; the detection light pulse modulated by the terahertz pulse is converged into parallel light through the second convex lens, polarized beam splitting is carried out through the quarter wave plate and the Wollaston prism, and then signal acquisition is carried out through the photoelectric balance detector; and adjusting the optical delay device to obtain time delay information for calculating the terahertz pulse group velocity of the sample.
2. The system for measuring terahertz pulse group velocity in a sample according to claim 1, wherein the thickness of the measured sample is 300 μm to 3mm.
3. The system for measuring the terahertz pulse group velocity in a sample according to claim 1, wherein the laser is a titanium sapphire femtosecond laser amplifier, and the chopper has a chopping frequency of 30-400 hz.
4. The system for measuring the terahertz pulse group velocity in a sample according to claim 1, wherein the terahertz electric field strength generated by the terahertz generating crystal is 0.1-80 mv/cm.
5. The system for measuring the group velocity of terahertz pulses in a sample according to claim 1, wherein an included angle between the terahertz wave reflected by the third off-axis perforated parabolic mirror and a plane in which the sample to be measured is located is between 80 ° and 100 °.
6. The system of claim 1, wherein the first off-axis parabolic mirror and the second off-axis parabolic mirror have a focal length ratio of less than 1:2.
7. the system of claim 1, wherein the sample comprises a window material for terahertz wave band, a base material, a biological sample, a semiconductor material, a crystal.
8. The system for measuring the terahertz pulse group velocity in a sample according to claim 1, wherein the terahertz pulse frequency band can cover 0.1-10 thz.
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| CN202222840249.6U CN220730041U (en) | 2022-10-27 | 2022-10-27 | System for measuring terahertz pulse group velocity in sample |
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| CN202222840249.6U CN220730041U (en) | 2022-10-27 | 2022-10-27 | System for measuring terahertz pulse group velocity in sample |
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