CN220730041U - A system for measuring terahertz pulse group velocity in a sample - Google Patents

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

A system for measuring terahertz pulse group velocity in a sample

Technical Field

The utility model relates to the technical field of terahertz band optical measurement, in particular to a measurement system for terahertz pulse group velocity in a sample.

Background technique

Dispersion is the property that the refractive index of a material changes with the frequency of the incident light. This dispersion property can cause pulse broadening and group phase mismatch, which restricts the development of free-space optical systems and fiber-optic communication systems towards high quality. The propagation speed of light pulses with rich frequency components in the sample is determined by both dispersion and refractive index. The essence of dispersion and refractive index measurement is to measure the time delay for light pulses of different frequencies to transmit the same distance in a dispersive medium, or to measure the group velocity of light pulses with rich frequency components in a dispersive medium.

At present, there are many methods for measuring the dispersion or group velocity of samples in the ultraviolet, visible light, infrared and other frequency bands, including the time delay method, phase shift method, interference method, etc. For example, the time delay method measures the time delay of light pulses of different frequencies passing through the sample, and then obtains the group velocity value through polynomial fitting; the phase shift method obtains the group velocity value by comparing the phase difference between the reference value and the measured value; the interference method uses a Michelson interferometer to adjust the relative distance between the reference arm and the measurement arm of the Michelson interferometer to obtain the phase difference between the reference value and the measured value. However, the above methods are difficult to use to measure the group velocity parameters of pulses in the terahertz frequency band. The factors that hinder the measurement of group velocity parameters in the terahertz frequency may be the following three points:

(1) Terahertz pulses usually contain a very rich frequency component, and their frequency coverage range usually exceeds 5 octaves. The method of measuring the time delay or phase of all single frequencies to ultimately obtain the terahertz pulse group velocity is extremely labor-intensive and places extremely high demands on the stability of the system.

(2) The spectral distribution of terahertz pulses is usually irregular and does not obey a strict Gaussian shape. The group velocity of the entire terahertz pulse can only be approximately calculated by the time delay or phase parameters of each frequency point.

(3) Due to the limitations of the technology level of terahertz sources and detectors, it is difficult to build a system that measures the velocity parameters of terahertz pulse groups by using the time delay method, phase shift method or interference method within the terahertz frequency band.

Therefore, a new measurement device is urgently needed to realize the direct measurement of the terahertz pulse group velocity parameters in the sample.

Utility Model Content

The utility model provides a system for measuring the group velocity of terahertz pulses in a sample, which uses the Kerr effect that is ubiquitous in materials to measure the group velocity parameters of terahertz pulses in the sample. The sensitivity of measuring the group velocity of terahertz pulses in the utility model can reach 0.001mm/ps, and can be applied to measuring the group velocity values of terahertz pulses with complex frequency components in samples.

To achieve the above-mentioned purpose, the utility model provides a system for measuring the group velocity of terahertz pulses in a sample, which comprises a laser, a spectroscope, an optical parametric amplifier, a chopper, a terahertz generating 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 sample to be measured, a second convex lens, a quarter-wave plate, a Wollaston prism, and a photoelectric balance detector arranged on an optical path; wherein:

The laser is preferably a titanium sapphire femtosecond laser amplifier, which is used to emit a laser with a wavelength of 800nm and a pulse width of 35fs. The 800nm femtosecond laser is separated into a beam as a pump light and a beam as a detection light through the beam splitter. The pump light is converted into a laser with a wavelength of any wavelength between 800 and 2600nm and a pulse width of 50fs through the optical parametric amplifier, and a terahertz pulse is generated through the chopper and the terahertz generating crystal; the terahertz pulse passes through the terahertz filter to filter out stray light, and then passes through the first off-axis parabolic reflector and the second off-axis parabolic reflector in turn for wavefront modulation, and then is reflected by the third off-axis perforated parabolic reflector and focused to the sample under test. The detection light passes through the optical delay device, the first convex lens, and the third off-axis perforated parabolic reflector in sequence, and is focused on the sample under test together with the terahertz pulse. The sample under test is excited by the third-order nonlinear Kerr effect of the strong terahertz pulse, causing the detection light focused on the sample under test to undergo polarization rotation; the detection light pulse modulated by the terahertz pulse is converged into parallel light by the second convex lens, and then passes through the quarter-wave plate and the Wollaston prism for polarization state splitting, and then the signal is collected by the photoelectric balanced detector; the optical delay device is adjusted to obtain time delay information for calculating the sample terahertz pulse group velocity.

In an embodiment of the present invention, the thickness of the sample to be measured is between 300 μm and 3 mm.

In one embodiment of the present invention, the laser is a titanium sapphire femtosecond laser amplifier, and the chopping frequency of the chopper is 30-400 Hz.

In one embodiment of the present invention, the output wavelength of the optical parametric amplifier is between 800 and 2600 nm.

In one embodiment of the present invention, the terahertz generating crystal is a DAST crystal, which can also be replaced by DSTMS, BNA, OH1 and other crystals.

In one embodiment of the present invention, the terahertz electric field strength generated by the terahertz generating crystal is 0.1-80 MV/cm.

In one embodiment of the present invention, the angle between the terahertz wave reflected by the third off-axis perforated parabolic reflector and the plane where the measured sample is located is between 80° and 100°.

In one embodiment of the present invention, the focal length ratio of the first off-axis parabolic reflector and the second off-axis parabolic reflector should be 1:4, and the preferred value should be less than 1:2.

In one embodiment of the utility model, the samples to be measured are quartz and sapphire. The samples that can be measured in the system include but are not limited to various window materials, substrate materials, biological samples, semiconductor materials, crystals, etc. for terahertz bands, such as quartz, diamond, silicon, gallium arsenide, silicon nitride, sapphire, polyethylene, organic solvents, protein solutions, etc.

In an embodiment of the present invention, the measured terahertz pulse frequency band covers 0.1~10THz.

The beneficial effects of the utility model are as follows: the utility model provides a system for measuring the group velocity of terahertz pulses in a sample, which innovatively applies the third-order nonlinear Kerr effect phenomenon that is commonly present in the sample, and realizes direct measurement of the group velocity of terahertz pulses in the sample. The terahertz pulse is focused on the sample to be measured and propagates in the sample to be measured, and the propagation process excites the anisotropy of the sample. The detection light is focused on the same position in the sample at the same time and propagates in the sample at the same time. When the terahertz pulse and the detection light pulse meet in the sample, the terahertz pulse will change 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 route, the detection light pulse and the terahertz pulse arrive at the sample successively at a certain time interval. Since the terahertz pulse and the detection light pulse have different group velocities in the sample, when the time interval is small, the terahertz pulse and the detection light pulse can still chase and meet in the sample and produce modulation of the polarization state of the detection light pulse. By adjusting the time delay device and measuring the time range in which the detection light pulse is modulated, the terahertz pulse group velocity parameters can be obtained based on the reference detection light pulse velocity, time range, sample thickness and other parameters. The sensitivity of measuring the terahertz pulse group velocity in the utility model can reach 0.001mm/ps, which is the first time to directly measure the group velocity of terahertz pulse propagation in a sample. The system structure of the utility model is simple, the construction cost is low, the stability is high, and it is easy to maintain. The utility model realizes the direct measurement of the terahertz pulse group velocity in the sample for the first time, and has a high sensitivity. It has great scientific research and application value in the field of terahertz components manufacturing, sample detection, substrate window material analysis, terahertz system construction in free space and instrument manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the utility model or the technical solutions in the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the utility model, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

FIG1 is a schematic diagram (top view) of a system for measuring the terahertz pulse group velocity in a sample provided by the present invention;

FIG2 is a spectrum of terahertz pulses used in the present invention;

FIG3 is a measurement result of a quartz sample by the system of the present invention;

FIG. 4 is a measurement result of a sapphire sample by the system of the present invention.

Explanation of the accompanying drawings: 1-laser; 2-beamsplitter; 3-optical parametric amplifier; 4-chopper; 5-terahertz generating crystal; 6-terahertz filter; 7-first off-axis parabolic reflector; 8-second off-axis parabolic reflector; 9-third off-axis perforated parabolic reflector; 10-optical delay device; 11-first convex lens; 12-sample to be measured; 13-second convex lens; 14-quarter wave plate; 15-Wollaston prism; 16-photoelectric balance detector.

Detailed ways

The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

FIG1 is a schematic diagram (top view) of a system for measuring the group velocity of terahertz pulses in a sample provided by the present invention, which includes a laser 1, a beam splitter 2, an optical parametric amplifier 3, a chopper 4, a terahertz generating crystal 5, a terahertz filter 6, a first off-axis parabolic reflector 7, a second off-axis parabolic reflector 8, a third off-axis perforated parabolic reflector 9, an optical delay device 10, a first convex lens 11, a sample to be measured 12, 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 to emit a laser with a wavelength of 800nm and a pulse width of 35fs. The 800nm femtosecond laser is separated into a beam as a pump light and a beam as a detection light through a beam splitter 2. The pump light is converted into a laser with a wavelength of any wavelength between 800 and 2600nm and a pulse width of 50fs through an optical parametric amplifier 3, and a terahertz pulse is generated through a chopper 4 and a terahertz generating crystal 5. The terahertz pulse passes through a terahertz filter 6 to filter out stray light, and then passes through a first off-axis parabolic reflector 7 and a second off-axis parabolic reflector 8 in sequence for wavefront modulation, and then is reflected by a third off-axis perforated parabolic reflector 9 and focused on a sample 12 to be measured. The detection light passes through the optical delay device 10, the first convex lens 11, and the third off-axis perforated parabolic reflector 9 in sequence, and is focused on the sample 12 under test together with the terahertz pulse. The sample 12 under test is excited by the third-order nonlinear Kerr effect by the strong terahertz pulse, causing the detection light focused on the sample 12 under test to rotate in polarization. The detection light pulse modulated by the terahertz pulse is converged into parallel light by the second convex lens 13, and then polarization-state beam splitting is performed by the quarter-wave plate 14 and the Wollaston prism 15, and then the signal is collected by the photoelectric balanced detector 16. By adjusting the optical delay device, the time delay information used to calculate the sample terahertz pulse group velocity can be obtained.

In FIG. 1 , the equivalent focal length of the first off-axis parabolic reflector 7 may be 1 inch, and the equivalent focal length of the second off-axis perforated parabolic reflector 8 may be 4 inches.

In this embodiment, the thickness of the sample 12 to be tested is between 300 μm and 3 mm.

In this embodiment, the laser 1 is a titanium sapphire femtosecond laser amplifier, and the chopper frequency of the chopper 4 is 30-400 Hz.

In this embodiment, the output wavelength of the optical parametric amplifier 3 is between 800 and 2600 nm.

In this embodiment, the terahertz generating crystal 5 is a DAST crystal, which can also be replaced by DSTMS, BNA, OH1 and other crystals.

In this embodiment, the terahertz electric field strength generated by the terahertz generating crystal 5 is 0.1~80MV/cm.

In this embodiment, the angle between the terahertz wave reflected by the third off-axis perforated parabolic reflector 9 and the plane where the sample 12 to be measured is located is between 80° and 100°.

In this embodiment, the focal length ratio of the first off-axis parabolic reflector 7 and the second off-axis parabolic reflector 8 should be 1:4, and the preferred value should be less than 1:2.

In this embodiment, quartz and sapphire are used as examples of the sample 12 to be measured. The samples that can be measured by this system include, but are not limited to, various window materials, substrate materials, biological samples, semiconductor materials, crystals, etc. for terahertz bands, 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 to 10 THz.

The utility model adopts the following method for measurement, which can be applied to the system shown in Figure 1, for example, and includes the following steps:

S1: The laser emits laser light with a wavelength of 800 nm.

S2: The 800nm laser is separated into a pump light and a detection light through the beam splitter.

S3: The pump light is converted into a laser with a wavelength of any wavelength between 800 and 2600 nm and a pulse width of 50 fs through the optical parametric amplifier, and a terahertz pulse is generated through the chopper and the terahertz generating crystal. The terahertz pulse passes through the terahertz filter to filter out stray light, and then passes through the first off-axis parabolic reflector and the second off-axis parabolic reflector in turn for wavefront modulation, and then is reflected by the third off-axis perforated parabolic reflector and focused to the sample under test.

S4: The detection light passes through the optical delay device, the first convex lens, and the third off-axis perforated parabolic reflector in sequence and is focused on the sample under test together with the terahertz pulse. The sample under test is excited by the third-order nonlinear Kerr effect by the strong terahertz pulse, causing the detection light focused on the sample under test to rotate in polarization. The detection light pulse modulated by the terahertz pulse is converged into parallel light by the second convex lens, and then passes through the quarter-wave plate and the Wollaston prism to split according to the polarization state, and then the signal is collected by the photoelectric balanced detector. By adjusting the optical delay device, the time delay information used to calculate the sample terahertz pulse group velocity can be obtained.

S5: Based on the measured time delay information, the time range t is obtained, and the reference velocity of the detection light pulse in the sample is obtained from the refractive index table. Then, for the sample with a thickness of l , the group velocity parameter of the terahertz pulse in the sample can be obtained by the formula.

FIG3 shows the measurement result of a quartz sample with a thickness of 0.6 mm using the system described in FIG1 . It can be observed that the time range t is approximately 1 ps. The refractive index n of the detection light pulse in the quartz sample obtained from the refractive index table is 1.4533, and the corresponding reference velocity is 0.2064 mm/ps. According to the formula calculation, the propagation group velocity parameter of the terahertz pulse in the quartz sample in this embodiment is 0.154 mm/ps.

FIG4 shows the measurement results of a sapphire sample with a thickness of 0.6 mm using the system described in FIG1 . It can be observed that the time range t is approximately 2.6 ps. The refractive index n of the detection light pulse in the sapphire sample obtained from the refractive index table is 1.7601, and the corresponding reference velocity is 0.1704 mm/ps. According to the formula calculation, the propagation group velocity parameter of the terahertz pulse in the sapphire sample in this embodiment is 0.098 mm/ps.

The utility model innovatively applies the third-order nonlinear Kerr effect phenomenon that is commonly present in samples, and for the first time realizes the high-sensitivity direct measurement of the terahertz pulse group velocity in the sample, with a measurement sensitivity of up to 0.001mm/ps. Compared with the traditional time delay method and phase method that measure the refractive index of each frequency point one by one and then indirectly approximate the calculation of dispersion and group velocity, the utility model can directly obtain the group velocity of terahertz pulses composed of any frequency components in the sample. The system structure of the utility model is simple, the construction cost is low, the stability is high, and it is easy to maintain. The utility model realizes the direct measurement of the terahertz pulse group velocity in the sample for the first time, and has a high sensitivity. It has great scientific research and application value in the field of terahertz components manufacturing, sample detection, substrate window material analysis, free space terahertz system construction and instrument manufacturing.

Those skilled in the art can understand that the accompanying drawings are only schematic diagrams of an embodiment, and the modules or processes in the accompanying drawings are not necessarily required for implementing the present utility model.

Those skilled in the art can understand that the modules in the embodiments can be distributed in the devices of the embodiments according to the description of the embodiments, or can be located in one or more devices different from the embodiments with corresponding changes. The modules in the above embodiments can be combined into one module, or can be further divided into multiple sub-modules.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the utility model, rather than to limit it. Although the utility model has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the 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.