Terahertz broadband near-field imaging and spectroscopy device with tunable pump and its usage method

Abstract

The present invention discloses a tunable pump terahertz broadband near-field imaging and spectroscopy device and its usage method, which relates to the field of terahertz near-field microscopes. The device includes: a first fiber laser oscillator, a second fiber laser oscillator, an optical fiber beam splitter, a first delay line, a second delay line, a third delay line, a fourth delay line, a transmitting antenna, a first parabolic mirror, a second parabolic mirror, an atomic force microscope, a receiving antenna, a processor, a synchronous phase-locked loop, a first energy adjustment module, a second energy adjustment module, a first reflector and a second reflector; the present invention realizes the advantages of high pump light energy and tunable pump wavelength in a terahertz near-field microscope.

Description

Technical Field

[0001] The present invention relates to the field of terahertz near-field microscopes, and in particular to a terahertz broadband near-field imaging and spectroscopy device with tunable pumping and a method for using the same. Background Art

[0002] Terahertz near-field microscopy is one of the technologies that has emerged in recent years. Its outstanding advantage is that it combines terahertz time-domain spectroscopy with near-field microscopy, which can make use of the characteristics of terahertz waves with certain penetration and the super-resolution advantage of near-field microscopy, thus realizing super-resolution detection in the terahertz frequency band. However, there are still few studies on terahertz near-field microscopy based on laser pumping.

[0003] The applicant has found that the current optically pumped terahertz near-field microscope has the disadvantages of low pump light energy and non-adjustable pump wavelength. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a tunable pumped terahertz broadband near-field imaging spectroscopy device, the device comprising:

[0005] A first fiber laser oscillator, a second fiber laser oscillator, a fiber beam splitter, a first delay line, a second delay line, a third delay line, a fourth delay line, a transmitting antenna, a first parabolic mirror, a second parabolic mirror, an atomic force microscope, a receiving antenna, a processor, a synchronous phase-locked loop, a first energy regulating module, a second energy regulating module, a first reflector, and a second reflector;

[0006] The first fiber laser oscillator is used to output a laser with a wavelength of a first preset value. The first fiber laser oscillator has two laser outlets for outputting the first laser and the second laser respectively. The first laser is split into a transmitting laser and a receiving laser by a fiber beam splitter. The transmitting laser is processed by a first delay line and then input into a transmitting antenna to generate a terahertz wave. The terahertz wave is focused by a first parabolic mirror to the tip of an atomic force probe of an atomic force microscope to generate a scattered terahertz near-field signal. The terahertz near-field signal is focused by a second parabolic mirror and then collimated and emitted to a receiving antenna. The receiving laser is processed by a second delay line and then emitted into the receiving antenna. The terahertz near-field signal and the receiving laser act on the receiving antenna to generate an electrical signal. The electrical signal is transmitted to a processor for processing to obtain a terahertz near-field spectrum image of the sample.

[0007] The second laser is input into the first energy adjustment module after being processed by the third delay line, the third delay line is used to compensate for the optical path difference between the second laser and the terahertz optical path, the first energy adjustment module is used to adjust the pulse energy of the second laser, and the first reflector is used to reflect the energy-adjusted second laser toward the tip of the atomic force probe;

[0008] The second fiber laser oscillator is used to output a laser with an adjustable wavelength within a first preset range. The third laser output by the second fiber laser oscillator is input into the second energy adjustment module after being processed by a fourth delay line. The fourth delay line is used to compensate for the optical path difference between the third laser and the terahertz optical path. The second energy adjustment module is used to adjust the pulse energy of the third laser. The second mirror is used to reflect the third laser with adjusted energy towards the atomic force probe tip part;

[0009] The synchronous phase-locked loop is used to control the synchronous output of the first fiber laser oscillator and the second fiber laser oscillator.

[0010] The applicant's research found that in the prior art, the pump laser energy is relatively low. To implement terahertz near-field microscopy technology, the laser repetition frequency for generating terahertz must be high enough to meet the demodulation of the atomic force microscope tip jitter frequency. The jitter of the atomic force probe is generally in the range of dozens of kilohertz (kHz) to hundreds of kHz. Therefore, the laser repetition frequency must be close to the MHz level. However, for the terahertz time-domain spectroscopy generated by lasers with a repetition frequency close to MHz currently, its signal-to-noise ratio is much lower than that of the terahertz spectrum generated by the commonly used 100 MHz fiber laser. This severely restricts its application in near-field microscopes. To pursue a high signal-to-noise ratio, the currently commonly used terahertz spectrometer uses a 100 MHz fiber laser to generate laser light, with an effective spectral width of 0.1 - 5 THz and a multiple-averaged sampling signal-to-noise ratio of 80 dB. However, the energy of such fiber lasers is not large. Therefore, in addition to being used to generate terahertz spectra, the additional laser energy for pumping is not high, so the applicable scenario range will be much smaller, which also severely restricts the further development of the optical pumping - terahertz near-field detection technology.

[0011] Furthermore, in the prior art, the wavelength of the pump light is fixed. For the existing optical pumping terahertz near-field microscopes, the pump wavelength is often fixed, which greatly restricts the detection of different types of samples because different types of samples require different wavelengths of energy for pumping detection.

[0012] Aiming at the shortcomings of relatively low pump light energy and non-adjustable pump wavelength in the current optical pumping - terahertz near-field microscope, the present invention proposes a method of using a dual-oscillator design of a first fiber laser oscillator and a second fiber laser oscillator. The repetition frequencies of the first fiber laser oscillator and the second fiber laser oscillator are designed to be the same, and at the same time, a phase-locked loop is used to synchronize their clocks to ensure that the laser pulses emitted by the two are synchronous.

[0013] The synchronization of two laser oscillators is ensured based on a dual oscillator and a synchronous phase-locked loop, which is a prerequisite for conducting pump-probe experiments. At the same time, the second fiber laser oscillator used as the pump can ensure tunable laser wavelength because its output wavelength is adjustable. Moreover, the second fiber laser oscillator has undergone laser optimization energy processing at preset frequency points, enabling it to achieve high energy output.

[0014] Preferably, the maximum laser energy of the second fiber laser oscillator is 2.5 W (corresponding to a laser wavelength of 800 nm at this time). At some specially optimized frequency points, for a specific frequency point, a crystal with a specified thickness and crystal orientation is selected, which also has high-power pump energy. The output power at the 690 nm frequency point is 500 mW, the output power at the 710 nm frequency point is 1.35 W, the output power at the 920 nm frequency point is 1.35 W, and the output power at the 1040 nm frequency point is 300 mW. The above tuning range and the optimized energy at special frequency points can all achieve high-energy and wide-band pump, which is difficult to achieve in previous optical pump - terahertz near-field detection schemes.

[0015] Preferably, the first reflector is a foldable reflector. The purpose of designing it as a foldable reflector is to use the first reflector for reflection when pumping with the second laser, and to fold the first reflector to avoid blocking the optical path of the third laser when pumping with the third laser.

[0016] Preferably, the reflection optical path of the first reflector before folding coincides with the reflection optical path of the second reflector, and the first reflector is located outside the reflection optical path of the second reflector after the first reflector is folded. Among them, the purpose of designing the optical paths to coincide is that the area of the rear focusing lens is limited. For optical path adjustment, focusing on the tip requires relatively high optical path debugging requirements for the operator. If different optical paths are used, each optical path needs to be adjusted separately for the focusing lens, which is time-consuming and laborious. However, on the same optical path, after adjustment, different optical path pumping can be achieved by switching the folding mirror, and this kind of optical path is also more compact in structure and convenient for integration.

[0017] Preferably, the second delay line is a linear motor delay line, which is used to achieve optical path compensation with the emission optical path by adjusting the optical path of the receiving optical path, and is used for near-field demodulation imaging. The maximum movement speed of this delay line is 1 m / s, the accuracy of the built-in feedback grating scale is 100 nm, the motor stroke is 120 mm, corresponding to a round-trip optical path of 240 mm (converted to an optical path travel of 800 ps), the scanning speed is 10 waveforms / second, and the corresponding scanning range is 50 ps. This delay line is arranged on the receiving optical path, and the optical path compensation with the emission optical path is achieved by changing the optical path of the receiving optical path. At the same time, for use in a near-field environment, this motor can achieve a stepping movement with an accuracy of dozens of nanometers, so as to accurately find the maximum value of the near-field signal and achieve near-field demodulation imaging.

[0018] Preferably, the first delay line is a stepper motor compensation delay line for adjusting the distance between the transmitting antenna and the receiving antenna. The moving speed of the stepper motor delay line is 1 cm / s, the motor stroke is 30 cm, and it has a built-in 4-fold optical path, corresponding to a round-trip optical path of 120 cm (converted to an optical path travel of 4000 ps).

[0019] The applicant's research found that in traditional terahertz time-domain spectrometers, the delay line often has the ability of fast scanning and mostly uses a voice coil motor for fast scanning imaging. However, for use in combination with a near-field microscope, this type of terahertz spectrometer is not applicable because the high-speed moving voice coil motor causes the signal to be unable to be demodulated by the tip of the atomic force microscope. If it is a stepper motor, although it can achieve the demodulation of the tip frequency, it cannot perform fast scanning when performing near-field spectroscopy measurements. At the same time, for sample measurements in different situations, the required optical path length is also different, and traditional terahertz time-domain spectrometers cannot flexibly adjust the distance between the transmitting and receiving antennas.

[0020] To solve the above problems, in order to ensure both fast scanning measurement and the demodulation of the atomic force probe tip during near-field measurement, and at the same time be able to flexibly adjust the distance between the transmitting antenna and the receiving antenna to adapt to the needs of different types of samples, this device adopts a combination of a high-speed short-stroke linear motor and a long-stroke stepper motor. The short-stroke high-speed linear motor ensures fast scanning and can also achieve precise stepping functions to ensure the demodulation imaging of the tip frequency during near-field scanning; the long-stroke stepper motor is used for optical path compensation to adjust the distance between the terahertz transmitting and receiving antennas to adapt to the needs of different types of samples.

[0021] Preferably, the first energy adjustment module and the second energy adjustment module have the same structure and both include:

[0022] A half-wave plate, a first Brewster mirror, a second Brewster mirror, and a baffle

[0023] The rotation angle of the half-wave plate can be adjusted. The half-wave plate is used to adjust the component sizes between the S-polarized light and the P-polarized light in the incident laser. The laser adjusted by the electrically tunable half-wave plate is incident on the first Brewster mirror. Among them, the first Brewster mirror is used to reflect the S-polarized light in the incident laser to the second Brewster mirror, the first Brewster mirror is used to transmit the P-polarized light in the incident laser to the baffle, the second Brewster mirror is used to transmit the P-polarized light in the incident laser, and the second Brewster mirror is used to reflect the S-polarized light in the incident laser and then output it.

[0024] The applicant's research found that in pump-probe experiments, it is often necessary to perform high-precision continuous adjustment of the pump light energy. The traditional adjustment methods mainly include fixed attenuators, continuous attenuators, or beam splitting. For fixed attenuators, the attenuation multiple is fixed, making it difficult to achieve effective continuous adjustment. For continuous attenuators, continuous adjustment can be achieved, but like fixed attenuators, there is a situation where femtosecond lasers pass through the medium, which will broaden the pulse width of the laser and is not suitable for pump experiments with strict pulse width requirements. The beam splitting attenuation method also has a fixed attenuation multiple and is difficult to achieve continuous adjustment. Therefore, it is currently difficult to find a method in pump-probe that can continuously adjust the pump energy without affecting the laser pulse width.

[0025] To solve the above problems and address the issue of continuous and precise adjustment of pump energy in pump-probe experiments, a half-wave plate and two parallel first Brewster mirrors and second Brewster mirrors are used in this device to achieve continuous and precise adjustment of energy. Among them, the half-wave plate is adjusted using an electric rotary stage. The minimum rotation accuracy of the wave plate is 0.01 degrees. The role of the half-wave plate is to change the direction of the incident polarized light. The role of the Brewster mirror is to reflect the S-polarized light and transmit the P-polarized light. In this way, by electrically adjusting the half-wave plate, the components of the S-polarized light and P-polarized light are actually changed. Finally, after passing through the two Brewster mirrors, the remaining S-polarized light energy is used for pumping. The above adjustment method can ensure precise adjustment of the pump laser energy between 2% and 98%. At the same time, this method uses reflective beam splitting for energy adjustment, ensuring that the pulse width of the femtosecond laser is not affected.

[0026] Preferably, the half-wave plate is adjusted to change the rotation angle in an electrically adjustable manner.

[0027] Preferably, the transmitting antenna includes:

[0028] A focusing lens, a DAST organic crystal, and a hemispherical silicon focusing lens;

[0029] The focusing lens is used to focus the emitted laser onto the DAST organic crystal (4-methylpyridine p-toluenesulfonate crystal). The DAST organic crystal is used to generate a terahertz radiation source. The hemispherical silicon focusing lens is used to convert the divergent terahertz radiation source into parallel transmission.

[0030] The applicant's research found that traditional terahertz time-domain spectrometers, especially those applied to near-field optical microscopes, are almost all based on InGaAs photoconductive chips or Lt-GaAs photoconductive chips, with an effective spectral width generally ranging from 0.1 to 5 THz. The spectral range of traditional terahertz gaps generally refers to 0.1 to 10 THz, and it is difficult for such spectrometers to achieve effective spectral coverage. In the present invention, an organic crystal terahertz source is adopted. The crystal is in the form of optical rectification of DAST organic crystal. The laser excitation wavelength is 1560 nm, the power is 70 mW, and the pulse width is 100 fs. The detection crystal is also a DAST organic crystal. Utilizing the optical rectification effect of the DAST crystal, the generated effective spectral width is 0.1 to 18 THz, completely covering the current research frequency band of 0.1 to 10 THz, far higher than the current fiber-based terahertz time-domain spectroscopy system.

[0031] Preferably, the first fiber laser oscillator is used to output a laser with a wavelength of 1560 nm, and the second fiber laser oscillator is used to output a laser with a wavelength ranging from 690 nm to 1040 nm.

[0032] The present invention also provides a usage method of the tunable pump terahertz broadband near-field imaging and spectroscopy device, and the method includes:

[0033] Step 1: Turn on the second fiber laser oscillator, then turn on the first fiber laser oscillator, and then use the synchronous phase-locked loop to synchronize the first fiber laser oscillator and the second fiber laser oscillator;

[0034] Step 2: Turn on the first delay line, the second delay line, the transmitting antenna, the atomic force microscope, and the receiving antenna;

[0035] Step 3: Adjust the atomic force probe of the atomic force microscope to perform the needle-down operation until the atomic force probe finishes the needle-down;

[0036] Step 4: Adjust the first delay line and the second delay line so that the distance between the transmitting antenna and the receiving antenna matches the atomic force microscope, and debug the terahertz time-domain pulse signal by adjusting the first parabolic mirror and the second parabolic mirror;

[0037] Step 5: Select the required laser pump according to the type of the sample:

[0038] If the second laser with the wavelength of the first preset value is required, turn on the second laser optical path, adjust the optical path difference through the third delay line, adjust to the required pulse energy through the first energy adjustment module, and reflect the second laser to between the atomic force probe tip and the sample through the first mirror;

[0039] If a third laser with a wavelength in the first preset range is required, turn on the third laser optical path, turn off the second laser optical path, and flip the first mirror. Adjust the optical path difference through the fourth delay line, adjust the pulse energy through the second energy adjustment module, and reflect the third laser to between the atomic force probe tip and the sample through the second mirror.

[0040] One or more technical solutions provided by the present invention have at least the following technical effects or advantages:

[0041] The present invention realizes the advantages of high pump light energy and adjustable pump wavelength in a terahertz near-field microscope.

[0042] The present invention realizes that the pump light energy in a terahertz near-field microscope can be adjusted precisely.

[0043] The present invention can not only perform rapid scanning measurement, but also realize the demodulation of the atomic force probe tip during near-field measurement. At the same time, it should also be able to flexibly adjust the distance between the transmitting antenna and the receiving antenna to adapt to the needs of different types of samples.

[0044] The effective spectral width generated by the present invention is 0.1 - 18 THz, which completely covers the current research frequency band of 0.1 - 10 THz, far higher than the current fiber-based terahertz time-domain spectroscopy system. Description of the Drawings

[0045] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, form a part of the present invention, and do not constitute a limitation on the embodiments of the present invention;

[0046] Figure 1 It is a schematic structural diagram of a terahertz broadband near-field imaging and spectroscopy device for tunable pumping;

[0047] Figure 2 It is a schematic structural diagram of the first energy adjustment module;

[0048] Figure 3 It is a schematic structural diagram of the transmitting antenna;

[0049] Among them, 1 - the first fiber laser oscillator, 2 - the fiber beam splitter, 3 - the second delay line, 4 - the first delay line, 5 - the transmitting antenna, 6a - the first parabolic mirror, 6b - the second parabolic mirror, 7 - the atomic force probe, 8 - the receiving antenna, 9 - the second fiber laser oscillator, 10 - the synchronous phase-locked loop, 11 - the third delay line, 12 - the first energy adjustment module, 13 - the first mirror, 14 - the fourth delay line, 15 - the second energy adjustment module, 16 - the second mirror, 5a - the emitted laser, 5b - the focusing lens, 5c - the DAST organic crystal, 5d - the hemispherical silicon focusing lens, 12a - the incident pump laser, 12b - the electrically tunable half-wave plate, 12c - the first Brewster mirror, 12d - the baffle, 12e - the second Brewster mirror. Specific embodiments

[0050] In order to more clearly understand the above-mentioned objects, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0051] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0052] Embodiment 1

[0053] Please refer to Figure 1 , Figure 1 which is a schematic structural diagram of a terahertz broadband near-field imaging and spectroscopy device with tunable pumping. Figure 1 In , pump light is the pump laser. Embodiment 1 of the present invention provides a terahertz broadband near-field imaging and spectroscopy device with tunable pumping, and the device includes:

[0054] The first fiber laser oscillator 1, the second fiber laser oscillator 9, the fiber beam splitter 2, the first delay line 4, the second delay line 3, the third delay line 11, the fourth delay line 14, the transmitting antenna 5, the first parabolic mirror 6a, the second parabolic mirror 6b, the atomic force microscope, the receiving antenna 8, the processor, the synchronous phase-locked loop 10, the first energy adjustment module 12, the second energy adjustment module 15, the first mirror 13, and the second mirror 16;

[0055] The first fiber laser oscillator is used to output laser with a first preset wavelength. The first fiber laser oscillator has two laser outlets respectively used to output a first laser and a second laser. The first laser is split by an optical fiber beam splitter into a transmitted laser and a received laser. The transmitted laser is processed by a first delay line and then input into a transmitting antenna to generate a terahertz wave. The terahertz wave is focused by a first parabolic mirror onto the tip of the atomic force probe 7 of the atomic force microscope to generate a scattered terahertz near-field signal. The terahertz near-field signal is focused by a second parabolic mirror and then directly emitted to a receiving antenna. The received laser is processed by a second delay line and then injected into the receiving antenna. The terahertz near-field signal and the received laser act on the receiving antenna to generate an electrical signal. The electrical signal is transmitted to a processor for processing to obtain a terahertz near-field spectral image of the sample;

[0056] The second laser is processed by a third delay line and then input into a first energy adjustment module. The third delay line is used to compensate for the optical path difference between the second laser and the terahertz optical path. The first energy adjustment module is used to adjust the pulse energy of the second laser. The first reflector is used to reflect the second laser with adjusted energy towards the tip of the atomic force probe;

[0057] The second fiber laser oscillator is used to output laser with an adjustable wavelength within a first preset range. The third laser output by the second fiber laser oscillator is processed by a fourth delay line and then input into a second energy adjustment module. The fourth delay line is used to compensate for the optical path difference between the third laser and the terahertz optical path. The second energy adjustment module is used to adjust the pulse energy of the third laser. The second reflector is used to reflect the third laser with adjusted energy towards the tip of the atomic force probe;

[0058] The synchronous phase-locked loop is used to control the synchronous output of the first fiber laser oscillator and the second fiber laser oscillator.

[0059] Such as Figure 1As shown in the figure, 1 is the first fiber laser oscillator with a repetition frequency of 80 MHz, an output wavelength of 1560 nm, and a pulse width of 65 fs at the laser outlet. After passing through 5 m of optical fiber, the pulse width becomes 100 fs. The first fiber laser oscillator has a total of two output ports. One is used to generate terahertz time-domain spectroscopy, and the other is used as the pump light at 1560 nm. 2 is an optical fiber beam splitter, which is divided into transmitted laser and received laser, and the splitting ratio is 1:1. 3 is the second delay line, namely a fast linear motor delay line, which is used for fast spectral scanning and imaging functions in a near-field detection environment. 4 is the first delay line, namely a long-stroke stepping motor compensation delay line, which is used to adjust the distance between the transmitting and receiving antennas to meet the requirements of different types of samples. 5 is the transmitting antenna, which is a combination of DAST organic crystals and is used to transmit broadband terahertz waves. 6a is the first parabolic mirror, which is used to focus the collimated terahertz waves onto the atomic force tip. 7 is the atomic force probe, which is used to scan and image the sample. 6b is the second parabolic mirror, which is used to collect and collimate the scattered near-field signals and reflect them to the receiving part. 8 is the receiving antenna, which is used to receive the scattered near-field signals and send them to the signal processing part. 9 is the second fiber laser oscillator, namely a high-energy pump laser oscillator, which is used to provide pump laser with wavelengths tunable from 690 to 1040 nm. 10 is a synchronous phase-locked loop, which is used to synchronously output the pulses of the two lasers. 11 and 14 are the third delay line and the fourth delay line, which are used to compensate for the optical path differences of the two pump lasers and the terahertz optical path. 12 and 15 are the first energy adjustment module and the second energy adjustment module, namely the pump laser energy adjustment modules, which are used to finely adjust the pulse energy of the pump laser. 13 is the first mirror, namely a foldable mirror, which is used to reflect the pump laser at 1560 nm and can be folded to allow the pump light at 690 - 1040 nm to pass through. 16 is the second mirror, namely a broadband dielectric film mirror, which reflects the pump laser at 690 - 1040 nm. The broadband dielectric film is a commonly used film system for laser mirrors. Its advantage lies in that the film is deposited on a quartz glass substrate, and the film layer is an alternating multi-layer film of oxide rings with different refractive indices. It is deposited on a flat substrate by ion-assisted film formation technology. Compared with metal mirrors, the reflectivity of the broadband multi-layer dielectric film mirror is over 99% in the wavelength band of 600 - 1700 nm.

[0060] Among them, the 1560-nm pump laser and the 690 - 1040-nm pump laser enter the optical path corresponding to the atomic force probe through two separate optical paths. In a specific experiment, which optical path to choose is determined according to the sample. Only one of the two can be selected during the experiment. Therefore, when the 690 - 1040-nm laser is needed, a folding mirror is required to transmit this part of the laser because they share the same optical path later. The purpose of designing the optical paths to coincide is that the area of the rear focusing lens is limited. For optical path adjustment, focusing on the tip requires relatively high optical path debugging skills for the operator. If different optical paths are used, the focusing lens needs to be adjusted separately for each optical path, which is time-consuming and laborious. However, on the same optical path, after adjustment, different pump lasers can be achieved by switching the folding mirror, and this optical path is also more compact in structure and convenient for integration.

[0061] The repetition frequencies of the first fiber laser oscillator and the second fiber laser oscillator are both 80 MHz. The two are synchronized through the phase-locked loop technology. The principle of the phase-locked loop is that the frequency of the output signal is equal to the frequency of the input signal, and the phase of the output signal follows the change of the input signal. Once the phase-locked loop enters the locked state, if the phase of the input signal changes, the loop adjusts the control voltage of the voltage-controlled oscillator so that the phase of its output signal follows the phase change of the input signal, that is, maintains a constant stable phase difference. This ensures the synchronization of the two lasers and meets the requirements of the pump-probe experiment.

[0062] Figure 2 It is a schematic structural diagram of the first energy adjustment module. The structures of the first energy adjustment module and the second energy adjustment module are the same. Here, the first energy adjustment module 12 is taken as an example for introduction:

[0063] As Figure 2 shown, 12a is the incident pump laser, 12 and 15 respectively correspond to a pump laser, which are the 1560-nm single-wavelength pump laser and the 690 - 1040-nm tunable pump laser respectively. 12b is an electrically tunable half-wave plate, which is used to adjust the rotation angle of the half-wave plate and adjust the component sizes between the S-polarized light and the P-polarized light. 12c is the first Brewster mirror, which reflects the S-polarized light and transmits the P-polarized light. 12d is a baffle, which blocks the excess P-polarized light. 12e is the second Brewster mirror, which has the same effect as 12C. After passing through two such mirrors, the finally output S-polarized light will be purer. Through the action of the electrically tunable half-wave plate and two parallel Brewster mirrors, the continuous and precise adjustment of the pump laser energy can be achieved.

[0064] This part is the precise energy adjustment part, and its principle is as follows: The polarization plane of the incident linearly polarized laser changes after passing through an electrically adjustable half-wave plate. It then passes through two parallel Brewster mirrors. The function of these mirrors is to reflect the s-polarized light and transmit the p-polarized light. The transmitted p-polarized light is blocked by a baffle. The reflected s-polarized light reaches the system for use after passing through another Brewster mirror. Here, the half-wave plate is adjusted by an electric rotary stage with an adjustment accuracy of 0.01°, which is much higher than the manual adjustment accuracy. In fact, after the incident linearly polarized laser passes through the electrically adjustable half-wave plate, the ratio of s-light and p-light is adjusted. After passing through the parallel Brewster mirrors, the energy is precisely controlled, and the controllable range is 2% - 98%. The control accuracy is as high as one-thousandth, which is much higher than the manual adjustment accuracy.

[0065] Adjusting the rotation angle of the half-wave plate can adjust the component sizes of the s-polarized light and the p-polarized light. Then, the rotation accuracy of the half-wave plate can determine the energy distribution accuracy. Assuming rotation within a 90-degree range, the s-polarized light can be adjusted from 0 to the maximum (correspondingly, the p-polarized light changes from the maximum to 0). Assuming the maximum amplitude is A, then rotating 1° corresponds to A / 90. Generally, the manual adjustment accuracy is only 1°, and the electrically adjustable half-wave plate can move 0.01° at a time. Then, the corresponding minimum change amount each time is A / 9000. This energy adjustment accuracy cannot be achieved manually at all. At the same time, the efficiency of the Brewster mirror reflecting the s-polarized light and transmitting the p-polarized light is constant. Therefore, by superimposing in the form of two parallels, the finally obtained s-polarized light is purer and closer to the limit value A / 9000.

[0066] This adjustment method adopts a reflective adjustment. Compared with the previous transmissive adjustment method, not only is the adjustment accuracy higher, but continuous energy adjustment is achieved. Moreover, the pulse width of the laser does not change, which is very beneficial for pump-probe experiments sensitive to the pulse width.

[0067] Figure 3 It is a schematic structural diagram of the transmitting antenna, as Figure 3 shown. 5a is the transmitting laser, 5b is a 1560nm focusing lens for focusing the transmitting laser into the crystal. 5c is a DAST organic crystal for generating a broadband terahertz radiation source. 5d is a super-hemispherical silicon focusing lens for converting the generated divergent terahertz source into parallel transmission, which is convenient for subsequent focusing on the atomic force tip. The principle of the receiving part is similar to that of the transmitting part and will not be elaborated here.

[0068] This part is the generation part of broadband terahertz time-domain spectroscopy. The DAST organic crystal is used in the generation part and is excited by a 1560 nm femtosecond laser with a laser repetition frequency of 80 MHz. Using the optical rectification effect of the DAST crystal, the effective spectral width generated is 0.1 - 18 THz, which completely covers the current research frequency band of 0.1 - 10 THz and is much higher than the current fiber-based terahertz time-domain spectroscopy system. In the figure, the incident laser is focused on the DAST crystal after passing through the focusing lens. Due to the optical rectification effect of the crystal, broadband terahertz is generated. There is an integrated hemispherical silicon lens after the crystal. The purpose is to convert the divergent terahertz radiation into a parallel beam for convenient subsequent collection and testing. Its detection principle is similar to that of emission and is the reverse process of the above process.

[0069] Embodiment 2

[0070] Based on Embodiment 1, Embodiment 2 of the present invention provides a usage method of the tunable pump terahertz broadband near-field imaging and spectroscopy device in Embodiment 1, specifically as follows:

[0071] First, turn on the second fiber laser oscillator 9, and then turn on the first fiber laser oscillator 1. The order of the two cannot be changed. The purpose is to use the pump laser as the excitation light source first. At this time, the sample will show some laser-induced property changes, such as changes in carrier concentration, such as changes in conductivity, or some ultrafast kinetic characteristics. Then, use the terahertz near-field atomic force to detect, and these microscopic changes in properties can be captured. The synchronization of the lasers after startup is ensured by the phase-locked loop 10. At this time, for the phase-locked loop 10, the repetition frequency of the second fiber laser oscillator 9 is the input information, and finally, the output pulse of the first fiber laser oscillator 1 is synchronized with the second fiber laser oscillator 9.

[0072] Turn on the terahertz near-field microscope part, which includes the delay line 3 control part, the compensation delay line 4 control part, the transmitting and receiving antenna part, and the atomic force microscope part. There is no specific order for this part.

[0073] Adjust the atomic force microscope probe and perform the needle-down operation until the atomic force probe finishes entering the sample, that is, enter the atomic force measurement stage.

[0074] Adjust the second delay line 3 and the first delay line 4 so that the distance between the transmitting antenna 5 and the receiving antenna 8 matches the atomic force main unit. Debug the terahertz time-domain pulse signal, specifically by adjusting the first parabolic mirror 6a and the second parabolic mirror 6b to make the focusing of the terahertz wave reach the best state and the signal reach the maximum value.

[0075] For different samples, if 1560 nm laser pumping is required, turn on the 1560 nm pumping optical path, adjust the optical path difference through the third delay line 11 so that it is equal to the optical path of the terahertz optical path, adjust the first energy adjustment module 12 to achieve the required pulse energy, and reach between the atomic force tip and the sample by flipping the first mirror 13. Thus, the effect of 1560 nm pumping is achieved.

[0076] If pump laser in the 690 - 1040 nm frequency band is required, turn on this part of the optical path, adjust the optical path difference through the fourth delay line 14 so that it is equal to the optical path of the terahertz optical path, adjust the second energy adjustment module 15 to achieve the required pulse energy, and transmit through the second mirror 16. At this time, flip the first mirror 13 to ensure that the reflected light passing through the second mirror 16 reaches between the atomic force tip and the sample, thus achieving the pumping effect of 690 - 1040 nm.

[0077] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0078] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these changes and modifications.

Claims

1. A terahertz broadband near-field imaging and spectroscopy device with a tunable pump, characterized in that The device includes: a first fiber laser oscillator, a second fiber laser oscillator, an optical fiber beam splitter, a first delay line, a second delay line, a third delay line, a fourth delay line, a transmitting antenna, a first parabolic mirror, a second parabolic mirror, an atomic force microscope, a receiving antenna, a processor, a synchronous phase-locked loop, a first energy adjustment module, a second energy adjustment module, a first reflector and a second reflector; The first fiber laser oscillator is used to output a laser with a wavelength of a first preset value. The first fiber laser oscillator has two laser outlets respectively for outputting a first laser and a second laser. The first laser is split by the optical fiber beam splitter into a transmitting laser and a receiving laser. The transmitting laser is processed by the first delay line and then input into the transmitting antenna to generate a terahertz wave. The terahertz wave is focused by the first parabolic mirror onto the tip of the atomic force probe of the atomic force microscope to generate a scattered terahertz near-field signal. The terahertz near-field signal is focused by the second parabolic mirror and then directly emitted to the receiving antenna. The receiving laser is processed by the second delay line and then injected into the receiving antenna. The terahertz near-field signal and the receiving laser act on the receiving antenna to generate an electrical signal, and the electrical signal is transmitted to the processor for processing to obtain a terahertz near-field spectral image of the sample; The second laser is processed by the third delay line and then input into the first energy adjustment module. The third delay line is used to compensate for the optical path difference between the second laser and the terahertz optical path. The first energy adjustment module is used to adjust the pulse energy of the second laser. The first reflector is used to reflect the second laser with adjusted energy towards the tip of the atomic force probe; The second fiber laser oscillator is used to output a laser with an adjustable wavelength within a first preset range. The third laser output by the second fiber laser oscillator is processed by the fourth delay line and then input into the second energy adjustment module. The fourth delay line is used to compensate for the optical path difference between the third laser and the terahertz optical path. The second energy adjustment module is used to adjust the pulse energy of the third laser. The second reflector is used to reflect the third laser with adjusted energy towards the tip of the atomic force probe; The synchronous phase-locked loop is used to control the synchronous output of the first fiber laser oscillator and the second fiber laser oscillator.

2. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 1, wherein The first reflector is a foldable reflector.

3. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 2, characterized in that, Before the first reflector is folded, its reflection optical path coincides with that of the second reflector. After the first reflector is folded, the first reflector is located outside the reflection optical path of the second reflector.

4. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 1, wherein, The second delay line is a linear motor delay line, which is used to achieve optical path compensation with the transmitting optical path by adjusting the optical path of the receiving optical path, and is used for near-field demodulation imaging.

5. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 1, characterized in that, The first delay line is a stepping motor compensation delay line, which is used to adjust the distance between the transmitting antenna and the receiving antenna.

6. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 1, characterized in that, The first energy adjustment module and the second energy adjustment module have the same structure and both include: a half-wave plate, a first Brewster reflector, a second Brewster reflector and a baffle; The rotation angle of the half-wave plate can be adjusted. The half-wave plate is used to adjust the component sizes between the S-polarized light and the P-polarized light in the incident laser. The laser adjusted by the electrically tunable half-wave plate is incident on the first Brewster mirror. Among them, the first Brewster mirror is used to reflect the S-polarized light in the incident laser to the second Brewster mirror, and the first Brewster mirror is used to transmit the P-polarized light in the incident laser to the baffle. The second Brewster mirror is used to transmit the P-polarized light in the incident laser, and the second Brewster mirror is used to reflect the S-polarized light in the incident laser and then output it.

7. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 6, wherein The rotation angle of the half-wave plate is adjusted by an electric adjustment method.

8. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 1, characterized in that The transmitting antenna includes: a focusing lens, a DAST organic crystal, and a hemispherical silicon focusing lens; The focusing lens is used to focus the transmitted laser on the DAST organic crystal. The DAST organic crystal is used to generate a terahertz radiation source. The hemispherical silicon focusing lens is used to convert the divergent terahertz radiation source into parallel transmission.

9. The tunable pump terahertz broadband near-field imaging and spectroscopy device according to claim 1, wherein The first fiber laser oscillator is used to output a laser with a wavelength of 1560 nm, and the second fiber laser oscillator is used to output a laser with a wavelength ranging from 690 nm to 1040 nm.

10. A method for using a tunable pump terahertz broadband near-field imaging and spectroscopy device according to any one of claims 1-9, characterized in that, The method includes: Step 1: Turn on the second fiber laser oscillator, then turn on the first fiber laser oscillator, and then use a synchronous phase-locked loop to synchronize the first fiber laser oscillator and the second fiber laser oscillator; Step 2: Turn on the first delay line, the second delay line, the transmitting antenna, the atomic force microscope, and the receiving antenna; Step 3: Adjust the atomic force probe of the atomic force microscope to perform the needle-down operation until the atomic force probe finishes needle-down; Step 4: Adjust the first delay line and the second delay line so that the distance between the transmitting antenna and the receiving antenna matches the atomic force microscope, and debug the terahertz time-domain pulse signal by adjusting the first parabolic mirror and the second parabolic mirror; Step 5: Select the required laser pump according to the type of sample: If a second laser with a wavelength of the first preset value is required, turn on the second laser optical path, adjust the optical path difference through the third delay line, adjust the pulse energy through the first energy adjustment module, and reflect the second laser to between the atomic force probe tip and the sample through the first mirror; If a third laser with a wavelength within the first preset range is required, turn on the third laser optical path, turn off the second laser optical path, and fold the first mirror. Adjust the optical path difference through the fourth delay line, adjust the pulse energy through the second energy adjustment module, and reflect the third laser to between the atomic force probe tip and the sample through the second mirror.

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