Systems and methods for improving sensitivity and efficiency of terahertz wave enhanced acoustic wave signals

Abstract

The application discloses a system and method for improving the sensitivity and efficiency of a terahertz wave enhanced sound wave signal, wherein the system comprises a laser, a light splitting prism, an optical parametric amplifier, a DAST organic crystal, a terahertz filter, a first terahertz polarizer, a second terahertz polarizer, a punched off-axis parabolic mirror, a first reflector, a light attenuation sheet, a second reflector, a quarter wave plate, a convex lens, a microphone and a lock-in amplifier arranged in sequence on an optical path; the quarter wave plate can change linear polarization of laser into circular polarization; sound wave signal intensities of plasmas formed by focusing laser with different polarization states through the punched off-axis parabolic mirror are different, and terahertz waves have different enhancements on different plasma sound waves. The application helps to understand the mechanism of air plasma sound emission and lays a good foundation for studying the interaction between dynamic plasma and electromagnetic radiation.

Description

Technical Field

[0001] This invention relates to the field of terahertz-induced acoustic enhancement technology, and more specifically, to a system and method for improving the sensitivity and efficiency of terahertz-enhanced acoustic signals. Background Technology

[0002] The applications of plasma in photoionization, laser-induced breakdown spectroscopy, spark-induced breakdown spectroscopy, and the generation and detection of broadband terahertz (THz) pulses have attracted much attention. During the process of laser breakdown of air to form plasma, the ionized electrons and ions recombine on a nanosecond timescale, releasing photons. The free electron temperature within the plasma can increase to 6 x 10⁻⁶. 4 At approximately Kelvin (K), free electrons transfer heat to surrounding air molecules through collisions. The air expands thermally, generating shock waves that attenuate into sound waves after traveling a short distance. By detecting these plasma sound waves, parameters such as the plasma's length and internal electron density can be obtained.

[0003] In recent years, with the rapid development of terahertz technology, the enhancement effect of terahertz waves on the acoustic and fluorescence emissions generated by plasma has been experimentally confirmed. Based on this, terahertz acoustic enhancement detection and terahertz fluorescence enhancement detection methods have been proposed. Both methods measure the acoustic or fluorescence emission intensity of air plasma and derive relevant information about the terahertz electric field by calculating the enhancement of the acoustic or fluorescence emission intensity by the terahertz electric field. Compared to fluorescence detection, acoustic detection has the advantage of using a lock-in amplifier to select a higher frequency acoustic signal for detection, reducing the influence of environmental noise on the acoustic signal and achieving higher response sensitivity and signal-to-noise ratio. Background light has a significant impact on fluorescence detection, which is determined by the sensitivity of the photomultiplier tube. Fluorescence detection requires a high-energy detection laser to reach the fluorescence excitation threshold of the plasma, which is related to the photon conversion efficiency. This makes acoustic detection, under the same experimental conditions, have higher response sensitivity and smaller experimental error than fluorescence detection.

[0004] Because water molecules in the air strongly absorb terahertz waves, traditional terahertz electro-optic sampling detection methods struggle to achieve long-distance detection. Acoustic detection methods, however, effectively overcome this difficulty. Furthermore, due to the long wavelength of sound waves, they can propagate around obstacles, enabling long-distance terahertz wave detection over a vast area. Since sound signals are correlated with the electron density of plasma, the principle of terahertz-enhanced sound waves allows for the characterization of parameters such as electron density and electron recombination rate without disrupting plasma integrity—a capability lacking in electro-optic sampling detection methods.

[0005] However, the efficiency of terahertz sound wave enhancement has been criticized. The enhancement efficiency observed in experiments is only about 10%, and the amplitude of terahertz sound pressure enhancement is small. In order to achieve higher sensitivity terahertz wave detection, how to improve the sensitivity and efficiency of terahertz sound wave enhancement under the same detection light conditions is a key problem to be solved. Summary of the Invention

[0006] In view of the above-mentioned prior art and the technical problems to be solved, the purpose of this invention is to provide a system and method for improving the sensitivity and efficiency of terahertz wave enhancement acoustic signal. By enhancing the electric field intensity of terahertz waves, the enhancement sensitivity of terahertz acoustic waves can be effectively improved. By changing the polarization state of the probe laser from linear polarization to circular polarization, the enhancement efficiency of terahertz acoustic waves can be improved, thereby achieving higher sensitivity terahertz wave detection.

[0007] To achieve the above objectives, the present invention provides a system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals, comprising, in sequence, a laser, a beam splitter, an optical parametric amplifier, a DAST organic crystal, a terahertz filter, a first terahertz polarizer, a second terahertz polarizer, a perforated off-axis parabolic mirror, a first reflector, an optical attenuator, a second reflector, a quarter-wave plate, a convex lens, a microphone, and a lock-in amplifier, wherein:

[0008] The laser emits a laser beam with a wavelength of 800nm, which is split into a pump beam and a probe beam by the beam splitter.

[0009] The pump light is incident on the optical parametric amplifier and outputs a laser with a wavelength of 1550nm. The laser with a wavelength of 1550nm is incident on the DAST organic crystal to generate a terahertz wave. The terahertz wave passes through the terahertz filter to remove stray light and then passes through the first terahertz polarizer and the second terahertz polarizer in sequence. After being reflected by the perforated off-axis parabolic mirror, it is focused.

[0010] The probe light passes sequentially through the first reflector, the light attenuator, the second reflector, the quarter-wave plate, and the convex lens, and is then focused into the air through a small hole on the back of the perforated off-axis parabolic mirror, exciting the air to form plasma. The terahertz wave is focused onto the plasma. The quarter-wave plate is used to change the laser polarization state from linear polarization to circular polarization.

[0011] The sound signal generated from the plasma is detected and collected by the microphone, and then amplified by the lock-in amplifier.

[0012] In one embodiment of the present invention, the laser is a femtosecond laser amplifier.

[0013] In one embodiment of the present invention, the first terahertz polarizer is a rotatable polarizer used to change the polarization state of the terahertz wave, and the second terahertz polarizer is a non-rotatable polarizer used only to allow the vertical component of the terahertz wave to pass through. By rotating the first terahertz polarizer, the angle between the terahertz wave and the vertical direction is changed, thereby changing the electric field strength of the terahertz wave.

[0014] In one embodiment of the present invention, the electric field strength of the terahertz wave generated by the DAST organic crystal can be controlled by the first terahertz polarizer and the second terahertz polarizer, and the electric field strength can be adjusted in the range of 2.75 to 11 MV / cm.

[0015] In one embodiment of the present invention, the optical attenuator is capable of regulating the energy of the probe light, wherein the energy intensity of the probe light can be adjusted within the range of 100 to 500 μJ.

[0016] In one embodiment of the present invention, the frequency response range of the microphone is 20Hz to 20kHz.

[0017] In one embodiment of the present invention, the distance between the microphone and the plasma is 3 mm.

[0018] In one embodiment of the present invention, the lock-in amplifier uses the 20th harmonic of the laser repetition frequency as a reference signal to measure a 20kHz acoustic signal.

[0019] This invention also provides a method for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals, which is implemented through the above-mentioned system and specifically includes the following steps:

[0020] Step S1: Set the laser to emit a laser with a wavelength of 800nm;

[0021] Step S2: Adjust the beam splitter so that the 800nm ​​wavelength laser is split into a pump beam and a probe beam.

[0022] Step S3: The pump light is incident on the optical parametric amplifier and outputs a laser with a wavelength of 1550nm. The laser with a wavelength of 1550nm is incident on the DAST organic crystal to generate a terahertz wave. After the terahertz wave passes through the terahertz filter to remove stray light, it passes through the first terahertz polarizer and the second terahertz polarizer in sequence, and is then focused after being reflected by the perforated off-axis parabolic mirror.

[0023] The probe light passes sequentially through the first reflecting mirror, the light attenuator, the second reflecting mirror, the quarter-wave plate, and the convex lens, and then enters through a small hole on the back of the perforated off-axis parabolic mirror.

[0024] Step S4: When the probe light is focused on the air, the air is excited by the probe photoionization in a very short time to form plasma. The plasma radiates shock waves outward and decays rapidly into plasma sound waves. When the terahertz wave is focused on the plasma, it can enhance the intensity of the plasma sound waves.

[0025] Step S5: After detecting and collecting the sound signal of the plasma using a microphone, the sound signal of the plasma is amplified by a lock-in amplifier, wherein:

[0026] When the terahertz wave is not focused on the plasma, the microphone detects and collects the sound signal of the plasma, and then the sound signal of the plasma is amplified by the lock-in amplifier to obtain the local sound pressure intensity.

[0027] Step S6: After the terahertz wave is focused onto the plasma, the first terahertz polarizer is rotated to change the electric field strength of the terahertz wave. At the same time, the terahertz enhanced sound pressure intensity amplified by the lock-in amplifier under different electric field strengths of the terahertz wave is measured, and the terahertz enhanced sound pressure difference is obtained by calculating the enhanced sound pressure intensity minus the local sound pressure intensity.

[0028] Step S7: Change the polarization state of the probe laser by adjusting the quarter-wave plate, regulate the energy of the probe laser by adjusting the optical attenuator, and measure the efficiency of terahertz acoustic wave enhancement under different probe laser energies in both linear and circular polarization states. The efficiency is the ratio of the terahertz enhanced sound pressure difference to the local sound pressure intensity.

[0029] The system and method for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals provided by this invention, compared with existing technologies, innovatively utilizes the principle of terahertz wave enhancement. By enhancing the electric field strength of the terahertz wave, the sensitivity of terahertz wave enhancement can be effectively improved, and the terahertz enhancement sound pressure difference is proportional to the square of the electric field strength of the terahertz wave. This invention also innovatively changes the polarization state of the probe laser using a quarter-wave plate, changing the laser from linear polarization to circular polarization. The terahertz wave enhancement efficiency of the plasma formed by the circularly polarized probe laser is higher than that of the linearly polarized state. Compared to the 10% terahertz wave enhancement efficiency provided by previous experiments, the efficiency provided by this invention can reach up to 160%. The higher terahertz wave enhancement sensitivity and efficiency result in higher detection sensitivity when using plasma acoustic waves to detect terahertz waves. Furthermore, the system of this invention has a simple structure, low cost of sound signal detection device, high system stability, long service life, and is easy to maintain. It helps to understand the mechanism of air plasma acoustic emission and lays a good foundation for studying the interaction between dynamic plasma and electromagnetic radiation, possessing significant scientific research and application value. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram (top view) of the system composition according to an embodiment of the present invention.

[0032] Figure 2 This is a schematic diagram showing the relationship between the square of the terahertz wave electric field intensity and the enhanced sound pressure difference in one embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram illustrating the relationship between the terahertz acoustic enhancement efficiency and the probe laser energy under both linear and circular polarization states of the probe laser in one embodiment of the present invention.

[0034] Figure labeling: 1-Laser; 2-Beam splitter prism; 3-Optical parametric amplifier; 4-DAST organic crystal; 5-Terahertz filter; 6-First terahertz polarizer; 7-Second terahertz polarizer; 8-Drilled off-axis parabolic mirror; 9-First reflecting mirror; 10-Optical attenuator; 11-Second reflecting mirror; 12-Quarter-wave plate; 13-Convex lens; 14-Plasma; 15-Microphone; 16-Lock-in amplifier. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Figure 1 This is a schematic diagram (top view) of the system composition according to an embodiment of the present invention, as shown below. Figure 1 As shown, this embodiment provides a system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals. It includes a laser 1, a beam splitter prism 2, an optical parametric amplifier 3, a DAST organic crystal 4, a terahertz filter 5, a first terahertz polarizer 6, a second terahertz polarizer 7, a perforated off-axis parabolic mirror 8, a first reflector 9, an optical attenuator 10, a second reflector 11, a quarter-wave plate 12, a convex lens 13, a microphone 15, and a lock-in amplifier 16, arranged sequentially in the optical path.

[0037] Laser 1 emits a laser beam with a wavelength of 800nm, which is split into a pump beam and a probe beam by a beam splitter prism 2.

[0038] In this embodiment, the laser 1 can be a femtosecond laser amplifier.

[0039] The pump light is incident on the optical parametric amplifier 3 and outputs a laser with a wavelength of 1550nm. The laser with a wavelength of 1550nm is incident on the DAST organic crystal 4 to generate a terahertz wave. The terahertz wave passes through the terahertz filter 5 to filter out stray light, and then passes through the first terahertz polarizer 6 and the second terahertz polarizer 7 in sequence. After being reflected by the perforated off-axis parabolic mirror 8, it is focused.

[0040] In this embodiment, the first terahertz polarizer 6 is a rotatable polarizer that can change the polarization state of the terahertz wave, while the second terahertz polarizer 7 is a non-rotatable polarizer that only allows the vertical component of the terahertz wave to pass through. Therefore, the angle between the terahertz wave and the vertical direction can be changed by rotating the first terahertz polarizer 6, thereby changing the electric field strength of the terahertz wave.

[0041] In this embodiment, the electric field strength of the terahertz wave generated by the DAST organic crystal 4 can be controlled by the first terahertz polarizer 6 and the second terahertz polarizer 7, and the range of its electric field strength can be adjusted is 2.75 to 11 MV / cm.

[0042] The probe light passes sequentially through the first reflector 9, the light attenuator 10, the second reflector 11, the quarter-wave plate 12 and the convex lens 13, and then enters and focuses into the air through a small hole on the back of the perforated off-axis parabolic mirror 8, exciting the air to form plasma 14. The terahertz wave is focused onto the plasma 14. The quarter-wave plate 12 is used to change the laser polarization state from linear polarization to circular polarization.

[0043] In this embodiment, the light attenuator 10 can regulate the energy of the probe light, and the energy intensity of the probe light can be adjusted within the range of 100 to 500 μJ during the experiment.

[0044] The sound signal generated from plasma 14 is detected and collected by microphone 15, and then amplified by lock-in amplifier 16.

[0045] In this embodiment, the frequency response range of the microphone 15 is 20Hz to 20kHz.

[0046] In this embodiment, the distance between the microphone 15 and the plasma 14 is 3mm.

[0047] In this embodiment, the lock-in amplifier 16 uses the 20th harmonic of the laser repetition frequency (1 kHz) as a reference signal to measure the 20 kHz acoustic signal.

[0048] Another embodiment of the present invention also provides a method for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals, which is achieved by means of... Figure 1 The system implementation specifically includes the following steps:

[0049] Step S1: Set laser 1 to emit a laser with a wavelength of 800nm;

[0050] Step S2: Adjust the beam splitter prism 2 so that the 800nm ​​wavelength laser is split into a pump beam and a probe beam by the beam splitter prism 2.

[0051] Step S3: The pump light is incident on the optical parametric amplifier 3 and outputs a laser with a wavelength of 1550nm. The laser with a wavelength of 1550nm is incident on the DAST organic crystal 4 to generate a terahertz wave. The terahertz wave passes through the terahertz filter 5 to filter out stray light, and then passes through the first terahertz polarizer 6 and the second terahertz polarizer 7 in sequence. After being reflected by the perforated off-axis parabolic mirror 8, it is focused.

[0052] The probe light passes sequentially through the first reflector 9, the light attenuator 10, the second reflector 11, the quarter-wave plate 12 and the convex lens 13, and then enters through the small hole on the back of the perforated off-axis parabolic mirror 8.

[0053] Step S4: When the probe light is focused on the air, the air is excited by the probe photoionization in a very short time to form plasma 14. Plasma 14 radiates shock waves outward and decays rapidly into plasma sound waves; after the terahertz wave is focused on plasma 14, it can enhance the intensity of plasma sound waves.

[0054] Step S5: After detecting and collecting the sound signal of the plasma 14 using microphone 15, the sound signal of the plasma 14 is amplified by lock-in amplifier 16, wherein:

[0055] When the terahertz wave is not focused onto the plasma 14, the microphone 15 detects and collects the sound signal of the plasma 14, and then the lock-in amplifier 16 amplifies the sound signal of the plasma 14 to obtain the local sound pressure intensity.

[0056] Step S6: After the terahertz wave is focused onto the plasma 14, the first terahertz polarizer 6 is rotated to change the electric field strength of the terahertz wave. At the same time, the terahertz enhanced sound pressure intensity amplified by the lock-in amplifier 16 under different electric field strengths of the terahertz wave is measured, and the terahertz enhanced sound pressure difference is obtained by calculating the enhanced sound pressure intensity minus the local sound pressure intensity.

[0057] Step S7: The polarization state of the probe laser is changed by adjusting the quarter-wave plate 12, and the energy of the probe laser is controlled by the optical attenuator 10. The efficiency of terahertz acoustic wave enhancement under different probe laser energies is measured under both linear and circular polarization states. The efficiency is the ratio of the terahertz enhanced sound pressure difference to the local sound pressure intensity.

[0058] Figure 2 This is a schematic diagram illustrating the relationship between the square of the terahertz wave electric field intensity and the enhanced sound pressure difference in one embodiment of the present invention, as shown below. Figure 2 As shown, the small squares represent the enhanced sound pressure difference values ​​under different electric field intensities of terahertz waves measured in the experiment. The vertical axis represents the enhanced sound pressure difference value, the horizontal axis represents the electric field intensity of the terahertz wave, and the curve is a quadratic fitting curve of the enhanced sound pressure difference value and the electric field intensity of the terahertz wave. Figure 2 The results show that the terahertz-enhanced sound pressure difference is proportional to the square of the electric field strength of the terahertz wave.

[0059] Figure 3 This is a schematic diagram illustrating the relationship between terahertz acoustic enhancement efficiency and probe laser energy under both linear and circular polarization states of the probe laser in one embodiment of the present invention. Figure 3 As shown, the solid and dashed lines represent the results when the probe laser is in a linearly polarized or circularly polarized state, respectively. The vertical axis represents the terahertz acoustic enhancement efficiency, and the horizontal axis represents the energy front of the probe light. As the energy of the probe laser decreases, the efficiency of the terahertz acoustic enhancement gradually increases, and this efficiency is even greater when the probe laser is in a circularly polarized state.

[0060] In summary, the system and method for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals provided by this invention innovatively utilize the principle of terahertz wave enhancement. By enhancing the electric field strength of the terahertz wave, the sensitivity of terahertz wave enhancement can be effectively improved, and the terahertz enhancement sound pressure difference is proportional to the square of the electric field strength of the terahertz wave. This invention also innovatively changes the polarization state of the probe laser using a quarter-wave plate, changing the laser from linearly polarized to circularly polarized. The terahertz wave enhancement efficiency of the plasma formed by the circularly polarized probe laser is higher than that of the linearly polarized state. Compared to the 10% terahertz wave enhancement efficiency provided by previous experiments, this invention can provide an efficiency of up to 160%. The higher terahertz wave enhancement sensitivity and efficiency result in higher detection sensitivity when using plasma acoustic waves to detect terahertz waves. Furthermore, the system of this invention has a simple structure, low cost of sound signal detection device, high system stability, long service life, and is easy to maintain. It contributes to understanding the mechanism of acoustic emission from air plasma and lays a good foundation for studying the interaction between dynamic plasma and electromagnetic radiation, possessing significant scientific research and application value.

[0061] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of one embodiment, and the modules or processes shown in the drawings are not necessarily essential for implementing the present invention.

[0062] Those skilled in the art will understand that the modules in the apparatus of the embodiments can be distributed in the apparatus of the embodiments as described in the embodiments, or they can be located in one or more devices different from this embodiment with corresponding changes. The modules of the above embodiments can be combined into one module, or they can be further divided into multiple sub-modules.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals, characterized in that, The optical path includes, in sequence, a laser, a beam splitter, an optical parametric amplifier, a DAST organic crystal, a terahertz filter, a first terahertz polarizer, a second terahertz polarizer, a perforated off-axis parabolic mirror, a first reflecting mirror, an optical attenuator, a second reflecting mirror, a quarter-wave plate, a convex lens, a microphone, and a lock-in amplifier, wherein: The laser emits a laser beam with a wavelength of 800nm, which is split into a pump beam and a probe beam by the beam splitter. The pump light is incident on the optical parametric amplifier and outputs a laser with a wavelength of 1550 nm. The 1550 nm laser is incident on the DAST organic crystal to generate a terahertz wave. After the terahertz wave passes through the terahertz filter to remove stray light, it passes through the first terahertz polarizer and the second terahertz polarizer in sequence, and is focused after being reflected by the perforated off-axis parabolic mirror. The first terahertz polarizer is a rotatable polarizer used to change the polarization state of the terahertz wave, and the second terahertz polarizer is a non-rotatable polarizer used only to allow the vertical component of the terahertz wave to pass through. By rotating the first terahertz polarizer, the angle between the terahertz wave and the vertical direction is changed, thereby changing the electric field strength of the terahertz wave. The probe light passes sequentially through the first reflector, the light attenuator, the second reflector, the quarter-wave plate, and the convex lens, and is then focused into the air through a small hole on the back of the perforated off-axis parabolic mirror, exciting the air to form plasma. The terahertz wave is focused onto the plasma. The quarter-wave plate is used to change the laser polarization state from linear polarization to circular polarization. The sound signal generated from the plasma is detected and collected by the microphone, and then amplified by the lock-in amplifier.

2. The system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals according to claim 1, characterized in that, The laser is a femtosecond laser amplifier.

3. The system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals according to claim 1, characterized in that, The electric field strength of the terahertz wave generated by the DAST organic crystal can be controlled by the first terahertz polarizer and the second terahertz polarizer, and the range of the electric field strength can be adjusted is 2.75 to 11 MV / cm.

4. The system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals according to claim 1, characterized in that, The optical attenuator can regulate the energy of the probe light, wherein the energy intensity of the probe light can be adjusted within the range of 100–500 μJ.

5. The system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals according to claim 1, characterized in that, The microphone has a frequency response range of 20Hz to 20kHz.

6. The system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals according to claim 1, characterized in that, The distance between the microphone and the plasma is 3 mm.

7. The system for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals according to claim 1, characterized in that, The lock-in amplifier uses the 20th harmonic of the laser repetition frequency as a reference signal to measure a 20kHz acoustic signal.

8. A method for improving the sensitivity and efficiency of terahertz wave-enhanced acoustic signals, implemented by the system described in any one of claims 1 to 7, characterized in that, Specifically, the following steps are included: Step S1: Set the laser to emit a laser with a wavelength of 800nm; Step S2: Adjust the beam splitter so that the 800nm ​​wavelength laser is split into a pump beam and a probe beam. Step S3: The pump light is incident on the optical parametric amplifier and outputs a laser with a wavelength of 1550nm. The laser with a wavelength of 1550nm is incident on the DAST organic crystal to generate a terahertz wave. After the terahertz wave passes through the terahertz filter to remove stray light, it passes through the first terahertz polarizer and the second terahertz polarizer in sequence, and is then focused after being reflected by the perforated off-axis parabolic mirror. The probe light passes sequentially through the first reflecting mirror, the light attenuator, the second reflecting mirror, the quarter-wave plate, and the convex lens, and then enters through a small hole on the back of the perforated off-axis parabolic mirror. Step S4: When the probe light is focused on the air, the air is excited by the probe photoionization in a very short time to form plasma. The plasma radiates shock waves outward and decays rapidly into plasma sound waves. When the terahertz wave is focused on the plasma, it can enhance the intensity of the plasma sound waves. Step S5: After detecting and collecting the sound signal of the plasma using a microphone, the sound signal of the plasma is amplified by a lock-in amplifier, wherein: When the terahertz wave is not focused on the plasma, the microphone detects and collects the sound signal of the plasma, and then the sound signal of the plasma is amplified by the lock-in amplifier to obtain the local sound pressure intensity. Step S6: After the terahertz wave is focused onto the plasma, the first terahertz polarizer is rotated to change the electric field strength of the terahertz wave. At the same time, the terahertz enhanced sound pressure intensity amplified by the lock-in amplifier under different electric field strengths of the terahertz wave is measured, and the terahertz enhanced sound pressure difference is obtained by calculating the enhanced sound pressure intensity minus the local sound pressure intensity. Step S7: Change the polarization state of the probe laser by adjusting the quarter-wave plate, regulate the energy of the probe laser by adjusting the optical attenuator, and measure the efficiency of terahertz acoustic wave enhancement under different probe laser energies in both linear and circular polarization states. The efficiency is the ratio of the terahertz enhanced sound pressure difference to the local sound pressure intensity.

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