Terahertz imaging method and imaging system based on gas plasma dynamic aperture

Abstract

The invention discloses a terahertz imaging method and an imaging system based on a gas plasma dynamic aperture. Two femtosecond lasers with mutually orthogonal propagation directions are focused to form two crossed gas plasmas near the focus to form a cross filament, which propagates through a sample in a grazing incidence manner. The terahertz beam is vertically incident on the sample surface. Due to the shielding effect of the plasma, the terahertz intensity is modulated by the cross filament. By using a double modulation technology, only the terahertz signal modulated by the middle part of the cross filament is measured. In the entire near-field detection process, the method of the invention does not require an actual terahertz detector or a terahertz source to be close to the sample, and terahertz imaging with sub-wavelength resolution can be achieved. The sample is not directly irradiated, and the damage of the gas plasma to the sample is greatly reduced. The method of the invention can also support transmission and reflection measurements at the same time, and greatly improves the ability to obtain terahertz near-field information.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz near-field imaging, and in particular relates to a terahertz imaging method and an imaging system based on a gas plasma dynamic aperture. Background Art

[0002] Due to the advantages of terahertz (THz) radiation, such as non-ionization, high transmittance to non-polar substances, and wide spectrum characteristics, THz imaging technology has irreplaceable application potential in many basic research and industrial fields. However, since THz radiation belongs to far-infrared radiation (the wavelength of 1THz radiation is 300μm), its imaging resolution has been limited to the millimeter level, which seriously limits the application of THz imaging. To solve this problem, in 1998, Hunsche et al. first introduced a metal aperture (that is, placing a metal aperture at the THz focus to allow the THz beam to pass through the aperture, so that the THz focal spot size is only related to the size of the metal aperture, and the imaging resolution is improved by reducing the metal aperture) into the THz imaging system (as shown in Figure 1(a)), achieving an imaging resolution of 1 / 4 wavelength and successfully measuring the THz image of the resolution test card, as shown in Figure 1(b). Since then, terahertz near-field detection technology has gradually attracted people's attention, and various improvement schemes have been introduced into terahertz near-field technology. For example, Kersting, Cocker, and Yoshioka's respective research teams used nanoscale probes in atomic force microscopes or tunnel scanning microscopes as scatterers or detection points in terahertz near-field detection, making the resolution of terahertz near-field imaging reach the nanometer level, and can successfully detect the morphology of metal or semiconductor nanostructures and the transport process of carriers; Bhattacharya et al. used microstructured photoconductive antennas to realize terahertz near-field detection, and successfully measured the electromagnetic resonance process caused by the interaction between metal rods and terahertz waves; Zhang Weili and Minin's respective research teams introduced high numerical aperture dielectric spheres and cubes to realize terahertz near-field detection, and successfully measured the fine structures inside gratings and magnetic cards. Now, the types of terahertz near-field technology are very rich, and the performance of the system is still improving. Its application areas are mainly concentrated on the performance characterization of semiconductor devices.

[0003] In 2000, Zhang Xicheng et al. first proposed the concept of dynamic aperture and applied it to terahertz near-field technology. As shown in Figure 2(a), a control light is introduced into the ordinary terahertz time-domain spectroscopy system, and the control light is focused on a semiconductor substrate, which is placed close to the sample. Photoinduced carriers are generated on the exposure area of ​​the semiconductor substrate, which increases the conductivity of the exposure area and modulates the transmitted terahertz intensity. A mechanical chopper is inserted into the control light path, and the terahertz signal of the modulated area can be independently extracted using the phase-locked technology. Since the wavelength of the control light is 800nm, it can be focused to the micron level, so this exposure area is equivalent to opening a micron-scale aperture for imaging measurement, which is a dynamic aperture. Using this technology, a metal structure on a semiconductor chip is measured, as shown in Figure 2(b). It can be seen that the morphology of the metal structure is accurately characterized. The introduction of this technology provides a new idea for the development of terahertz near-field imaging, and many technologies have been introduced to improve its performance. In this technology, a semiconductor substrate needs to be placed close to the sample, which limits the choice of samples. Furthermore, since the laser needs to be focused onto the surface of the semiconductor substrate, damage to the substrate is inevitable.

[0004] In 2020, Zhu Liguo and others developed dynamic aperture technology. They used two digital micromirrors (DMDs) to form a spatially coded laser and irradiated it onto an electron spin terahertz emitter, as shown in Figure 3(a). This emitter is composed of a ferromagnetic metal film and a non-ferromagnetic metal film. Under the action of laser pumping and an external magnetic field, the spin current can be converted into a charge current, thereby generating terahertz radiation. Since the laser irradiated on the emitter is spatially encoded, the generated terahertz radiation is also spatially encoded. The sample is placed close to the emitter, and the spatially encoded pattern is continuously changed by two DMDs to complete the ghost imaging measurement of the sample. Finally, the compressed sensing algorithm is used to accurately restore the terahertz image of the sample. In this technology, since the pixel size of the laser can be controlled to be very small and the emitter is very close to the sample, high-precision sampling of terahertz near-field signals can be achieved. Using this technology, they measured a series of gold structures on a silicon oxide substrate. The measurement results are shown in Figure 3(b), and the sample morphology is accurately characterized. In this technology, the sample still needs to be placed close to the emitter, which also limits the choice of samples. In addition, this technology is only suitable for transmission measurement and difficult to perform reflection measurement. It is not applicable to samples with low transmittance in the terahertz band.

[0005] In 2014, Liu Weiwei and others used a gas plasma terahertz radiation source to achieve terahertz near-field imaging, as shown in Figure 4(a). The femtosecond laser is focused to generate gas plasma near the focus. Due to the nonlinear effect inside the plasma, terahertz radiation is generated. This is a popular technology for generating strong terahertz pulses in the laboratory. Since the diameter of the plasma is at the micron level, the emission aperture of the terahertz radiation source is also at the micron level. They used this feature to achieve terahertz near-field imaging. In their technology, the femtosecond laser is first focused, and the second harmonic is generated by using a barium metaborate crystal. The interaction between the fundamental frequency light and the doubled frequency light in the plasma generates terahertz radiation. Subsequently, they placed the sample at the plasma position for scanning imaging. To avoid damage to the sample by the plasma, they placed a ceramic plate in front of the sample. The selected sample is a series of small holes processed on a printed circuit board, forming the "NK" letter pattern. Figure 4(b) shows the measurement results, and it can be seen that this technology can accurately characterize the morphology of the small holes. In this technology, since plasma is used as a terahertz radiation source, it needs to be directly irradiated onto the sample surface, which inevitably causes damage to the sample. Therefore, they introduced a ceramic plate to shield the plasma, which requires the ceramic plate to be placed close to the sample, which also limits the choice of samples. In addition, the ceramic plate is also very easily damaged by plasma, so the repeatability of this technology is not very high. Summary of the invention

[0006] In view of this, the purpose of the present invention is to provide a terahertz imaging method and imaging system based on gas plasma dynamic aperture, which can complete subwavelength imaging without any actual terahertz detector or terahertz source approaching the sample while ensuring that the gas plasma does not damage the sample surface, and can also realize terahertz subwavelength imaging of non-solid samples.

[0007] Terahertz imaging method based on gas plasma dynamic aperture, including:

[0008] Focus two femtosecond laser beams with mutually orthogonal propagation directions to form two crossed gas plasmas at the focus on the sample surface to form a cross filament; use a terahertz beam to pass through the cross filament and vertically incident on the sample surface, and use a double modulation method so that only the terahertz signal modulated by the middle part of the cross filament is measured, thereby realizing terahertz imaging;

[0009] The specific method of the dual modulation method is:

[0010] The two femtosecond laser beams are modulated by a first chopper (23) and a second chopper (27), respectively; the output frequency of the first chopper (23) is input to a first lock-in amplifier (32) as a reference frequency; the terahertz signal reflected from the surface of the sample (9) is input to the first lock-in amplifier (32) as an input signal for the first step; the output signal of the first lock-in amplifier (32) is input to a second lock-in amplifier (33) as an input signal for the second step, and the output frequency of the second chopper (27) is input to the second lock-in amplifier (33) as a reference frequency; and finally, the terahertz near-field information of the sample is obtained according to the signal output by the second lock-in amplifier (33).

[0011] Preferably, the terahertz modulation intensity of the cross filaments is adjusted by adjusting the power and / or beam diameter of the two femtosecond laser beams.

[0012] Preferably, the terahertz modulation intensity of the cross filament is adjusted by adjusting the focal length of the two femtosecond laser beams.

[0013] Preferably, the frequency of the first chopper (23) is set to 200 Hz, and the frequency of the second chopper (27) is set to (18) Hz.

[0014] A terahertz imaging system based on gas plasma dynamic aperture comprises a laser light source, a concave lens (4), a generating crystal (5), a first parabolic mirror (6), a first high-resistance silicon beam splitter (7), a second parabolic mirror (8), a third parabolic mirror (10), a first electric translation stage (12), an adjustable neutral filter (13), a first lens (14), a detection crystal (15), a second lens (16), a quarter wave plate (17), a Wollaston prism (18), a differential detector (19), a sixth electric translation stage (31), a first chopper (23), a second chopper (27), a first lock-in amplifier (32), a second lock-in amplifier (33) and a computer (34);

[0015] The laser light source divides a beam of 800nm ​​laser into four beams, namely, generation light ②, detection light ③, first control light ④ and second control light ⑤;

[0016] The generated light ② is expanded by a concave lens (4) and then irradiated onto a generating crystal (5) to generate a terahertz wave ⑥. After being collimated by a first parabolic mirror (6), the terahertz wave ⑥ is focused onto a sample (9) by a second parabolic mirror (8); the terahertz wave ⑥ reflected by the sample (9) is guided by the second parabolic mirror (8) and a first high-resistance silicon beam splitter (7), and is focused onto a detection crystal (15) by a third parabolic mirror (10);

[0017] The detection light ③ first passes through the first electric translation stage (12) to adjust the time delay between the detection light ③ and the generated light ②, and then passes through the first adjustable neutral filter (13) and the first lens (14) in sequence to be focused onto the detection crystal (15);

[0018] In the detection crystal (15), the information of the terahertz wave ⑥ is coherently loaded onto the polarization state of the detection light ③; the modulated detection light ③ continuously passes through the second lens (16), the quarter wave plate (17) and the Wollaston prism (18), and is finally received by the differential detector (19); the differential detector (19) outputs a light intensity difference signal between the detection light ③ and the terahertz wave ⑥;

[0019] The sample (9) is mounted on a sixth electric translation stage (31); the first chopper (23) and the second chopper (27) modulate the first control light ④ and the second control light ⑤ respectively; the output frequency of the first chopper (23) is input to the first phase-locked amplifier (32) as a reference frequency, and the signal output by the differential detector (19) is input to the first phase-locked amplifier (32) as an input signal for the first step; the output signal of the first phase-locked amplifier (32) is input to the second phase-locked amplifier (33) as an input signal for the second step, and the output frequency of the second chopper (27) is input to the second phase-locked amplifier (33) as a reference frequency; finally, the computer (34) obtains the terahertz near-field information of the sample according to the signal output by the second phase-locked amplifier (33).

[0020] Furthermore, it also includes a third lens (24), a fourth lens (28), a fourth electric translation stage (26) and a fifth electric translation stage (29); the first control light ④ and the second control light ⑤ are focused near the surface of the sample (9) through the third lens (24) and the fourth lens (28) respectively; wherein the third lens (24) and the fourth lens (28) are respectively mounted on the electric translation stage (25) and the fifth electric translation stage (29) to adjust the front and rear positions of the third lens (24) and the fourth lens (28).

[0021] Furthermore, it also includes an adjustable neutral filter (20) and an adjustable diaphragm (21) for simultaneously adjusting the intensity and diameter of the first control light ④ and the second control light ⑤.

[0022] Furthermore, it also includes a first high-resistance silicon beam splitter (7) placed in front of the detection crystal (15) to respectively receive the light beam reflected by the third parabolic mirror (10) and the light beam emitted by the first lens (14), so that the two light beams are collinear.

[0023] Furthermore, the laser light source comprises a first beam splitter (1), a second beam splitter (2) and a third beam splitter (3); the incident 800 nm laser is split into two beams by the first beam splitter (1), one of which is further split into a first control light ④ and a second control light ⑤ by the third beam splitter (3); and the other beam is split into a generated light ② and a detection light ③ by the second beam splitter (2).

[0024] The present invention has the following beneficial effects:

[0025] Compared with the previous technology, the method of the present invention has the advantage that, in the entire near-field detection process, there is no need for an actual terahertz detector or terahertz source to be close to the sample, and terahertz imaging with sub-wavelength resolution can be achieved. This feature greatly broadens the selection of samples. In traditional terahertz near-field technology, since it is always necessary to use a certain device to be close to the sample, such as an aperture, a probe, a phase change film, a micro-antenna, etc., it is very easy to change the sample morphology when detecting non-solid samples (such as grease, liquid), and at the same time cause equipment pollution. In this technology, it is only necessary to properly control the intensity and height of the gas plasma to easily complete the measurement. Even if the sample is accidentally touched, that is, the sample is irradiated by the laser, it will not cause changes in the sample morphology and equipment pollution. At the same time, due to the measurement method adopted by this technology, the gas plasma propagates through the sample in a grazing incidence manner, and the sample is not directly irradiated, which greatly reduces the damage of the gas plasma to the sample, making this technology have a wide range of applications. In addition, this technology can also support transmission and reflection measurements at the same time, which greatly improves the ability to obtain terahertz near-field information. Therefore, this technology greatly expands the application field of terahertz microscopy, especially in biosensing and chemical detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1(a) is a terahertz imaging system of Hunsche et al. that uses a metal aperture to adjust the size of the terahertz focal spot; FIG1(b) is a terahertz image using the system shown in FIG1(a);

[0027] FIG2(a) is a terahertz near-field technology of dynamic aperture proposed by Zhang Xicheng et al., and FIG2(b) is an imaging effect diagram using the technology of FIG2(a);

[0028] Figure 3(a) shows the dynamic aperture technology proposed by Zhu Liguo et al., and Figure 3(b) shows the measurement result using Figure 3(a);

[0029] FIG4(a) shows the terahertz near-field imaging achieved by Liu Weiwei et al. using a gas plasma terahertz radiation source; FIG4(b) shows the measurement result obtained using the technique of FIG4(a);

[0030] Figure 5 The schematic diagram of the terahertz imaging method of the gas plasma dynamic aperture of the present invention;

[0031] Figure 6 A diagram of a terahertz imaging system of a gas plasma dynamic aperture according to the present invention;

[0032] Figure 7 The imaging result of the metal resolution test card using the terahertz near-field microscopy system of the present invention;

[0033] Figure 8 The imaging result of semiconductor chip using the terahertz near-field microscopy system of the present invention;

[0034] Fig. 9 The imaging result of plastic patterns using the terahertz near-field microscopy system of the present invention;

[0035] Fig.10 This is the imaging result of oil droplets using the terahertz near-field microscopy system of the present invention.

[0036] Among them, 1-first beam splitter, 2-second beam splitter, 3-third beam splitter, 4-concave lens, 5-generation crystal, 6-first parabolic mirror, 7-first high-resistance silicon beam splitter, 8-second parabolic mirror, 9-sample, 10-third parabolic mirror, 11-second high-resistance silicon beam splitter, 12-first electric translation stage, 13-first adjustable neutral filter, 14-first lens, 15-detection crystal, 16-second lens, 17-1 / 4 wave plate, 18-Wollaston prism Mirror, 19-differential detector, 20-second adjustable neutral filter, 21-adjustable aperture, 22-second electric translation stage, 23-first mechanical chopper, 24-third lens, 25-third electric translation stage, 26-fourth electric translation stage, 27-second mechanical chopper, 28-fourth lens, 29-fifth electric translation stage, 30-cross filament, 31-sixth electric translation stage, 32-first phase-locked amplifier, 33-second phase-locked amplifier, 34-computer. DETAILED DESCRIPTION

[0037] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0038] The principle of the terahertz near-field technology of the present invention is as follows Figure 5As shown, two femtosecond lasers with orthogonal propagation directions are focused by lenses (L1, L2), and two crossed gas plasmas are formed near the focus to form a cross filament. The cross filament propagates through the sample in a grazing incidence manner. While ensuring that the sample is not damaged, the distance between the sample and the cross filament is as small as possible. At this time, the terahertz beam is vertically incident on the sample surface, and the terahertz intensity is modulated by the cross filament due to the shielding effect of the plasma. Two mechanical choppers (Chopper1, Chopper2) are placed on the two laser light paths, and the dual modulation technology is used to measure only the terahertz signal modulated by the middle part of the cross filament. Since the gas plasma has a length of millimeters and a diameter of micrometers, it is also called a filament morphology. Therefore, the middle part of the cross filament opens a dynamic aperture of only tens of square microns for the incident terahertz beam. Since the cross filament is very close to the sample and the dynamic aperture size is very small, the near-field terahertz signal can be detected. This technology has two characteristics: 1) Under the premise of ensuring that terahertz subwavelength imaging can be achieved, no real device touches or approaches the sample during the entire process, which provides great freedom in the selection of samples. The sample can not only be solid, but also colloidal or liquid, which is difficult to achieve with traditional terahertz near-field technology; 2) Since the gas plasma does not directly irradiate the sample, but only grazingly passes over the sample, the sample is not damaged by the plasma, and this measurement technology ensures sufficient safety.

[0039] The terahertz near-field microscopy system based on gas plasma dynamic aperture and the working process of the system are as follows:

[0040] Figure 6 The specific optical path schematic diagram used in the present invention. As shown in the figure, the system includes beam splitters 1, 2, 3, a concave lens 4, a generating crystal 5, a parabolic mirror 6, a high-resistance silicon beam splitter 7, a parabolic mirror 8, a sample 9, a parabolic mirror 10, a high-resistance silicon beam splitter 11, an electric translation stage 12, an adjustable neutral filter 13, a lens 14, a detection crystal 15, a lens 16, a 1 / 4 wave plate 17, a Wollaston prism 18, a differential detector 19, an adjustable neutral filter 20, an adjustable aperture 21, an electric translation stage 22, a mechanical chopper 23, a lens 24, an electric translation stage 25, 26, a mechanical chopper 27, a lens 28, an electric translation stage 29, a cross filament 30, a two-dimensional electric translation stage 31, a lock-in amplifier 32, 33, a computer 34, and incident light ①, generated light ②, detection light ③, a first control light ④, a second control light ⑤, a terahertz wave ⑥, and the rest are reverse reflectors.

[0041] The incident 800nm ​​laser ① is divided into four beams by three beam splitters 1, 2, and 3, including the generated light ②, the detection light ③, the first control light ④, and the second control light ⑤. After the generated light ② is expanded by the concave lens 4, it is irradiated on the generated crystal 5, and the terahertz wave ⑥ is generated through the nonlinear optical rectification process. The terahertz wave ⑥ is collimated by the parabolic mirror 6, and then the terahertz wave ⑥ is focused on the sample 9 by the parabolic mirror 8. The terahertz wave ⑥ reflected by the sample 9 is guided by the parabolic mirror 8 and the high-resistance silicon beam splitter 7, and is focused on the detection crystal 15 by the parabolic mirror 10. The detection light ③ is first adjusted to the time delay between the generated light ② by the electric translation stage 12, and then focused on the detection crystal 15 by the adjustable neutral filter 13 and the lens 14, wherein the high-resistance silicon beam splitter 11 is used to make the detection light ③ and the terahertz wave ⑥ collinear. In the detection crystal 15, due to the first-order electro-optical effect, the information of the terahertz wave ⑥ is coherently loaded onto the polarization state of the detection light ③. The modulated detection light ③ passes through the lens 16, the 1 / 4 wave plate 17, the Wollaston prism 18 and the differential detector 19 successively. The function of the Wollaston prism 18 is to separate the detection light ③ into two linearly polarized light beams with mutually perpendicular polarization directions; the 1 / 4 wave plate 17 is used to adjust the light intensity of the two linearly polarized light beams after separation to make the light intensity of the two light beams equal; the differential detector 19 outputs the difference between the two light intensities, which is proportional to the electric field of the terahertz wave ⑥, and then extracts the terahertz time domain signal.

[0042] By using the adjustable neutral filter 20 and the adjustable aperture 21 to simultaneously adjust the intensity and diameter of the first control light ④ and the second control light ⑤, the terahertz modulation intensity of the cross filament can be adjusted, or the dynamic aperture can be achieved by adjusting the focal length of the two femtosecond laser beams, that is, replacing the lenses 24 and 28.

[0043] The first control light ④ and the second control light ⑤ are respectively adjusted to have a time delay with the terahertz wave ⑥ through the electric translation stages 22 and 26, and the first control light ④ and the second control light ⑤ are respectively focused by the lenses 24 and 28 to generate two gas plasmas with orthogonal propagation directions, forming a cross filament 30. The time delay of the first control light ④, the second control light ⑤ and the terahertz wave ⑥ is controlled to ensure that the two plasmas reach the sample before the terahertz beam, so that the terahertz beam is modulated by the plasma.

[0044] Among them, lenses 24 and 28 are respectively mounted on electric translation stages 25 and 29 to adjust the front and rear positions of lenses 24 and 28 to ensure that the central parts of the two gas plasmas overlap with each other. The cross filament 30 is placed close to the upper surface of the sample 9 to form a dynamic aperture. The sample 9 is mounted on a two-dimensional electric translation stage 31 for two-dimensional point-by-point scanning imaging. In order to measure only the terahertz signal modulated by the middle part of the cross filament 30, a dual modulation technology is used for signal detection. The mechanical choppers 23 and 27 are respectively placed in the optical paths of the first control light ④ and the second control light ⑤. The frequency of the mechanical chopper 23 is set to 200Hz, and its output frequency is input to the phase-locked amplifier 32 as the reference frequency. The output signal of the differential detector 19 is input to the phase-locked amplifier 32 as the input signal of the first step. The output signal of the phase-locked amplifier 32 is input to the phase-locked amplifier 33 as the input signal of the second step, and the frequency of the mechanical chopper 27 is set to 18Hz, and its output frequency is input to the phase-locked amplifier 33 as the reference frequency. Finally, the signal output by the lock-in amplifier 33 is read by the computer. With this modulation technology, only the terahertz signal modulated by the middle area of ​​the cross filament 30 is measured, while the background signal modulated by other areas of the cross filament 30 is suppressed, thereby obtaining terahertz near-field information with high precision.

[0045] Among them, the difference in the numerical values ​​of the above-mentioned two choppers should be as large as possible. Since the output of the phase-locked amplifier 32 is used as the input of the phase-locked amplifier 33, the response of the output signal of the phase-locked amplifier 32 should be as fast as possible, and its reference signal is required to be as large as possible. The phase-locked amplifier 33 receives the signal of the phase-locked amplifier 32, and its response should be as slow as possible. Therefore, its reference signal should be as small as possible. However, if it is too small, the phase cannot be locked. Therefore, the frequencies of the two phase-locked amplifiers are set according to actual needs. In the present invention, since the repetition frequency of the laser is 1kHz, it is more appropriate to set the modulation frequencies of the two phase-locked amplifiers to 200Hz and 18Hz.

[0046] Embodiment 2:

[0047] The above-mentioned terahertz near-field microscopy system is used to image the metal resolution test card, and the experimental data are recorded and processed.

[0048] The details are as follows:

[0049] The sample is a metal resolution test card, such as Figure 7 (a) and Figure 7 As shown in (c), it consists of three chrome strips with a spacing of 80 μm. The sample is processed by standard photolithography technology. Figure 7As shown in (c), the incident terahertz beam has a polarization direction perpendicular to the chrome strip. Using the imaging system, the sample is scanned point by point with a step size of 10 μm, and the imaging area is 56×32 pixels. During the imaging process, the distance between the cross filament and the sample is 50 μm, and the energy of the two control beam pulses is 0.3 mJ. At each scanning point, the integral value of the terahertz spectrum intensity ∫|E THz (v)| 2 dv is calculated as imaging data, where E THz (v) is the terahertz spectrum amplitude, and v is the corresponding frequency. The terahertz near-field image of this sample is shown in Figure (b), which can clearly show the morphological characteristics of the sample. Figure 7 (b) and Figure 7 (c) The terahertz and optical intensity envelopes of the samples were extracted for comparison, e.g. Figure 7 As shown in (d), it can be seen that the two are perfectly matched. Since the terahertz center wavelength in this system is 222μm, the fine structure of 80μm scale on the sample can be clearly resolved, thus proving the sub-wavelength imaging capability of this system.

[0050] Embodiment three:

[0051] The above-mentioned terahertz near-field microscopy system is used to image the semiconductor chip and record and process the experimental data. The details are as follows:

[0052] The sample is a semiconductor chip, which is actually a terahertz photoconductive antenna, such as Figure 8 (a) and Figure 8 (c) The electrodes of the photoconductive antenna are made of AuGeNi alloy and are processed by standard photolithography technology. The electrode spacing is 150μm, and the substrate material of the photoconductive antenna is a semi-insulating GaAs substrate. The sample is scanned point by point with a step length of 20μm. The range of the two imaging areas A and B is 40×40 pixels. Figure 8 (c) is marked by purple and light blue dashed boxes. During the imaging process, the distance between the cross filament and the sample is 100μm, and the energy of the two control beam pulses is 0.3mJ. The terahertz near-field imaging of this sample is shown in Figure 2. Figure 8 (b) and Figure 8 As shown in (d), it can be seen that the structure of the antenna and the letter "Z" are clearly characterized. This experiment proves that the system can perform high-precision measurement of the fine structure of similar samples.

[0053] Embodiment 4:

[0054] The above-mentioned terahertz near-field microscopy system is used to image the plastic pattern and record and process the experimental data. The details are as follows:

[0055] The sample is a plastic pattern, such as Fig. 9As shown in (a), a portion of a petal pattern with a size of 1.1 mm × 2.7 mm was cut and pasted onto a metal mirror for testing. First, the flat and raised portions of the petal pattern were selected for terahertz signal measurement. The measurement positions are marked by blue and green dots. flat and I convex is the measured terahertz time domain signal, such as Fig. 9 (d) As shown. It can be seen that flat In comparison, I convex There is an obvious time delay and waveform distortion, which is because the convex part of the petals introduces a longer optical path and greater dispersion to the terahertz pulse. flat and I convex The sample was scanned with a step size of 50 μm, the imaging area was 40 × 80 pixels, the distance between the cross filament and the sample was 200 μm, and the energy of the two control beam pulses was 0.3 mJ. Fig. 9 (b) and Fig. 9 (c) shows the intensity and phase images of the petal pattern at 0.7 THz. It can be seen that both clearly characterize the fine structure of the petals, and the edges of the petals show greater intensity and higher phase. The completion of this experiment has two meanings. First, it proves that the terahertz intensity and phase signals obtained by this system can be used for sample characterization. Second, it shows that this technology has good safety. Even plastic samples are not damaged by gas plasma.

[0056] Embodiment five:

[0057] The above-mentioned terahertz near-field microscopy system is used to image the oil droplets and record and process the experimental data. The details are as follows:

[0058] The sample is a drop of oil, which is actually a small drop of Vaseline cream, such as Fig.10 As shown in (a), it is placed on a metal mirror for testing, and its diameter is about 300μm. Using this system, the terahertz signal I in the metal mirror and oil droplet area is first measured. metal and I greasy ,like Fig.10 (c) As shown. It can be seen that relative to I metal , I greasy It shows a significant delay, which is caused by the refractive index of vaseline cream being greater than that of air. greasy It also shows a higher amplitude, which may be due to the quasi-hemispherical shape of the oil droplet causing a kind of lens focusing effect on the terahertz wave, thereby increasing the reflectivity. metal and I greasyThe sample was imaged with a scanning step of 30 μm, an imaging area of ​​30 × 30 pixels, a distance of 200 μm between the cross filament and the sample, and two control beam pulse energies of 0.3 mJ. Fig.10 (b) and Fig.10 (d) shows the intensity and phase images of the oil droplet at 1.0 THz. It can be seen that both accurately characterize the morphology of the sample. In addition, in the phase image, the bottom of the oil droplet shows a higher phase value, which may be caused by the inhomogeneity of the sample. The completion of this experiment proves that this technology can be used to perform terahertz near-field detection on non-solid samples, such as colloidal samples or liquid samples.

[0059] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A terahertz imaging method based on gas plasma dynamic aperture, characterized in that: Based on a terahertz imaging system, the terahertz imaging system comprises a laser light source, a concave lens (4), a generating crystal (5), a first parabolic mirror (6), a first high-resistance silicon beam splitter (7), a second parabolic mirror (8), a third parabolic mirror (10), a first electric translation stage (12), an adjustable neutral filter (13), a first lens (14), a detection crystal (15), a second lens (16), a 1 / 4 wave plate (17), a Wollaston prism (18), a differential detector (19), a sixth electric translation stage (31), a first chopper (23), a second chopper (27), a first lock-in amplifier (32), a second lock-in amplifier (33) and a computer (34); The laser light source divides a beam of 800nm ​​laser into four beams, namely, generation light ②, detection light ③, first control light ④ and second control light ⑤; The generated light ② is expanded by a concave lens (4) and then irradiated onto a generating crystal (5) to generate a terahertz wave ⑥. After being collimated by a first parabolic mirror (6), the terahertz wave ⑥ is focused onto a sample (9) by a second parabolic mirror (8); the terahertz wave ⑥ reflected by the sample (9) is guided by the second parabolic mirror (8) and a first high-resistance silicon beam splitter (7), and is focused onto a detection crystal (15) by a third parabolic mirror (10); The detection light ③ first passes through the first electric translation stage (12) to adjust the time delay between the detection light ③ and the generated light ②, and then passes through the first adjustable neutral filter (13) and the first lens (14) in sequence to be focused onto the detection crystal (15); In the detection crystal (15), the information of the terahertz wave ⑥ is coherently loaded onto the polarization state of the detection light ③; the modulated detection light ③ continuously passes through the second lens (16), the quarter wave plate (17) and the Wollaston prism (18), and is finally received by the differential detector (19); the differential detector (19) outputs a light intensity difference signal between the detection light ③ and the terahertz wave ⑥; The sample (9) is mounted on a sixth electric translation stage (31); the first chopper (23) and the second chopper (27) modulate the first control light ④ and the second control light ⑤ respectively; the output frequency of the first chopper (23) is input to the first lock-in amplifier (32) as a reference frequency, and the signal output by the differential detector (19) is input to the first lock-in amplifier (32) as an input signal for the first step; the output signal of the first lock-in amplifier (32) is input to the second lock-in amplifier (33) as an input signal for the second step, and the output frequency of the second chopper (27) is input to the second lock-in amplifier (33) as a reference frequency; finally, the computer (34) obtains the terahertz near-field information of the sample according to the signal output by the second lock-in amplifier (33); The terahertz imaging method comprises: Focus two femtosecond laser beams with mutually orthogonal propagation directions to form two crossed gas plasmas at the focus on the sample surface to form a cross filament; use a terahertz beam to pass through the cross filament and vertically incident on the sample surface, and use a double modulation method so that only the terahertz signal modulated by the middle part of the cross filament is measured, thereby realizing terahertz imaging; The specific method of the dual modulation method is: The two femtosecond laser beams are modulated by a first chopper (23) and a second chopper (27), respectively; the output frequency of the first chopper (23) is input to a first lock-in amplifier (32) as a reference frequency; the terahertz signal reflected from the surface of the sample (9) is input to the first lock-in amplifier (32) as an input signal for the first step; the output signal of the first lock-in amplifier (32) is input to a second lock-in amplifier (33) as an input signal for the second step, and the output frequency of the second chopper (27) is input to the second lock-in amplifier (33) as a reference frequency; and finally, the terahertz near-field information of the sample is obtained according to the signal output by the second lock-in amplifier (33).

2. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The terahertz modulation intensity of the cross filaments is adjusted by adjusting the power and / or beam diameter of the two femtosecond lasers.

3. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The terahertz modulation intensity of the cross filament is adjusted by adjusting the focusing distance of the two femtosecond laser beams.

4. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The frequency of the first chopper (23) is set to 200 Hz, and the frequency of the second chopper (27) is set to (18) Hz.

5. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The terahertz imaging system further comprises a third lens (24), a fourth lens (28), a fourth electric translation stage (26) and a fifth electric translation stage (29); the first control light ④ and the second control light ⑤ are focused near the surface of the sample (9) via the third lens (24) and the fourth lens (28) respectively; wherein the third lens (24) and the fourth lens (28) are respectively mounted on the electric translation stage (25) and the fifth electric translation stage (29) to adjust the front and rear positions of the third lens (24) and the fourth lens (28).

6. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The terahertz imaging system further comprises an adjustable neutral filter (20) and an adjustable diaphragm (21), which are used to simultaneously adjust the intensity and diameter of the first control light ④ and the second control light ⑤.

7. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The terahertz imaging system also includes a first high-resistance silicon beam splitter (7), which is placed in front of the detection crystal (15) and receives the light beam reflected by the third parabolic mirror (10) and the light beam emitted by the first lens (14) respectively, so that the two light beams are collinear.

8. The terahertz imaging method based on gas plasma dynamic aperture according to claim 1, characterized in that: The laser light source comprises a first beam splitter (1), a second beam splitter (2) and a third beam splitter (3); the incident 800nm ​​laser is split into two beams by the first beam splitter (1), one of which is further split into a first control light ④ and a second control light ⑤ by the third beam splitter (3); and the other beam is split into a generated light ② and a detection light ③ by the second beam splitter (2).

Images