Near-field terahertz nonlinear optical effect excitation detection method and system

Abstract

The invention discloses a near-field terahertz nonlinear optical effect excitation detection method and system. A femtosecond laser space-time shaping technology is combined, a strong terahertz pulse is generated based on a femtosecond laser filament, the strong terahertz pulse is coupled with a tapping mode atomic force microscope, and space-time matching of probe vibration and the terahertz pulse is accurately controlled by using a synchronous sampling method, so that the time-space matching of the probe vibration and the terahertz pulse is realized. Mismatch between the low-repetition-frequency strong-field terahertz pulse and the vibration frequency of the near-field microscope probe is overcome, and strong-field terahertz near-field coupling and detection are achieved. The local field enhancement effect of the tip of the nanoprobe can realize nanoscale focusing (peak field intensity reaches MV / cm magnitude) of a terahertz field at a probe-sample gap, the near-field nonlinear interaction is remarkably enhanced, the diffraction limit is broken through, and the nanoscale spatial resolution is realized. According to the method, the excitation detection efficiency and the spatial resolution of the terahertz nonlinear effect are remarkably improved, and accurate excitation, characterization and regulation of the nonlinear optical effect of the sample can be realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of instrument science, material science and nanotechnology, and relates to a method and system for near-field terahertz nonlinear optical effect excitation and detection, which is particularly suitable for the research of material terahertz nonlinear optical properties and the development of high-performance nonlinear photonic devices. BACKGROUND

[0002] In the field of material science and nanotechnology, terahertz technology provides a powerful tool for studying the electronic dynamics of materials as an emerging technology. With the development of modern ultra-high-speed electronics, there is an increasing demand for high-performance functional materials that support terahertz frequencies. Terahertz nonlinear optical technology not only provides a powerful detection means, but also can be used to control the electronic dynamics in crystalline solids, such as dynamic Bloch oscillation, quantum interference and intraband current. However, current terahertz nonlinear effects are usually excited on a macroscopic scale of about millimeter, making it difficult to capture terahertz nonlinear modes at the nanoscale, which severely limits the study of nanoscale quantum effects and the development of integrated high-performance nonlinear photonic devices.

[0003] Traditional terahertz scattering-type scanning near-field optical microscopy (THz s-SNOM) can detect carrier oscillation and lattice vibration at the nanoscale by coupling terahertz waves with a nanoprober. However, due to the weak field strength of commonly used continuous or high-repetition-frequency terahertz sources, it is difficult to excite nonlinear effects in materials at terahertz frequencies. In addition, the existing far-field excitation and detection technology for studying terahertz nonlinearities is limited by the diffraction limit to the excitation and detection precision, which is difficult to break through the micron level, further limiting the performance improvement and functional expansion of terahertz near-field imaging technology in high-end application fields.

[0004] In view of this, it is particularly urgent to develop a method that can accurately excite and detect terahertz nonlinear effects at the nanoscale. At present, although a variety of related technologies have been proposed, there is no mature solution to effectively solve the above problems in the field of nanoscale nonlinear optical detection based on strong terahertz fields, and related research and application are still in the exploratory stage. SUMMARY

[0005] The purpose of the present invention is to address the difficulty of achieving efficient excitation and detection at the nanoscale in existing terahertz nonlinear optical detection methods, and to provide an excitation and detection method and system based on the near-field terahertz nonlinear optical effect. This method generates femtosecond laser filaments and radiates strong terahertz pulses using femtosecond laser pulses. The strong terahertz pulses are coupled to a tapping-mode atomic force microscope (AFM), and the vibration of a nanoprobe is synchronized with the terahertz pulses. This allows the nanoprobe to approach the sample surface to generate a near-field scattering signal, which excites the terahertz nonlinear optical effect at the extremely small gap between the nanoprobe tip and the sample surface. The interaction between the terahertz field and the sample is enhanced by the local field enhancement effect, and the terahertz nonlinear optical signal is then analyzed using terahertz time-domain spectroscopy to determine the nonlinear optical response of the sample. The method of the present invention is applicable to various materials with nonlinear optical responses and can significantly improve the efficiency and spatial resolution of the terahertz nonlinear effect. It provides a new technical means for studying the nonlinear optical properties of materials in the terahertz band and developing high-performance terahertz nonlinear photonic devices.

[0006] The technical solutions of the present invention are as follows:

[0007] A method for exciting and detecting near-field terahertz nonlinear optical effects, characterized by comprising the following steps:

[0008] Step 1: Combine femtosecond laser spatiotemporal shaping technology to generate strong terahertz pulses based on femtosecond laser filaments generated by femtosecond laser pulses;

[0009] Step 2: coupling the strong terahertz pulse with a tapping mode atomic force microscope (AFM);

[0010] Step 3: Using the synchronous sampling method (SSD), the vibration of the atomic force microscope (AFM) probe is synchronized with the terahertz pulse, so that the AFM probe approaches the sample surface and generates a near-field scattering signal.

[0011] Step 4: Stimulate the terahertz nonlinear optical effect at the extremely small gap between the AFM probe tip and the sample surface;

[0012] Step 5: Utilize the local field enhancement effect at the gap to enhance the interaction between the terahertz field and the sample;

[0013] Step 6: Analyze the terahertz nonlinear optical signal through a time-domain spectroscopy system based on a terahertz antenna to determine the nonlinear optical response of the sample.

[0014] The femtosecond laser spatiotemporal shaping technology is implemented through specific optical crystals or components to control the spatiotemporal and polarization characteristics of the femtosecond light field, compensate for spatiotemporal walk-off, optimize filament formation, and radiate strong terahertz pulses.

[0015] The femtosecond laser pulses for generating the femtosecond laser filaments have tunable parameters, including but not limited to pulse width, pulse energy, repetition frequency and central wavelength, to adapt to the needs of different samples and experimental conditions.

[0016] The synchronization of the vibration of the AFM probe and the terahertz pulse is achieved by using a synchronization signal generator to generate a synchronization signal and a digital delay signal generator to control the timing of the signal, so as to ensure that the vibration frequency of the AFM probe is synchronized with the repetition frequency of the terahertz pulse; the piezoelectric ceramic is controlled to drive the AFM probe to vibrate in the tapping mode, so that the probe approaches the surface of the sample and generates a near-field scattering signal.

[0017] The material and size of the nanoprobes can be selected according to specific application requirements to optimize the terahertz pulse coupling efficiency, local field enhancement effect and signal acquisition efficiency.

[0018] The sample is a material with nonlinear optical response, including but not limited to three-dimensional Dirac semimetals, two-dimensional materials and any nanostructured materials.

[0019] The frequency of the terahertz nonlinear optical signal can be selected and detected according to specific experimental requirements.

[0020] A system for realizing the near-field terahertz nonlinear optical effect excitation and detection method of the application, characterized by comprising:

[0021] (1) a femtosecond laser filament generation and pulse space shaping and control device for generating strong terahertz pulses;

[0022] (2) a synchronization sampling module for realizing the synchronization of the vibration of the AFM probe and the terahertz pulse, facilitating the extraction of near-field terahertz signals;

[0023] (3) a tapping mode atomic force microscope (AFM) for controlling the vibration and position of the AFM probe;

[0024] (4) a local field enhancement module for enhancing the interaction between the terahertz field and the sample;

[0025] (5) a terahertz time-domain spectroscopy analysis module for analyzing the terahertz nonlinear optical signal.

[0026] The dual-color femtosecond laser filament generation device comprises:

[0027] (1) a laser for generating femtosecond laser pulses;

[0028] (2) a specific optical crystal or element for controlling the space-time and polarization characteristics of the femtosecond light field and compensating for space-time walk-off.

[0029] The synchronization sampling module comprises:

[0030] (1) a synchronization signal generator for generating a synchronization signal;

[0031] (2) a digital delay signal generator for controlling the timing of the signal;

[0032] (3) a phase-locked amplifier for improving the signal-to-noise ratio and extracting the near-field scattering signal.

[0033] The advantages and beneficial effects of the present application are as follows:

[0034] The method and system for near-field terahertz nonlinear optical effect excitation and detection provided by the present application, by coupling the strong terahertz pulse (peak field strength up to MV / cm order) generated by the femtosecond laser filament with the tapping mode atomic force microscope, using the local field enhancement effect of the nanoprober tip to realize the nanoscale focusing of the terahertz field at the probe-sample gap, and through the synchronous sampling method, the spatial and temporal matching of the probe vibration and the terahertz pulse is precisely controlled, and the near-field nonlinear interaction is significantly enhanced; at the same time, the near-field scattering microscopy system breaks through the diffraction limit and realizes a spatial resolution of up to nanometer level. The method significantly improves the efficiency and spatial resolution of the terahertz nonlinear effect, and realizes the precise regulation of the nonlinear optical response of the sample. Compared with the traditional method, the present application overcomes the problem that the low repetition frequency strong terahertz pulse is incompatible with the near-field detection technology, realizes efficient energy transfer and signal acquisition through the synchronous sampling technology, greatly improves the sensitivity and reliability of the detection, and provides a new technical approach for the research on the nonlinear optical properties of nanomaterials in the terahertz wave band and the development of high-performance terahertz nonlinear photonic devices. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 A strong terahertz field nanoscale nonlinear optical detection system in the specific embodiment of the present application is shown, wherein (a) is a strong terahertz scattering scanning near-field optical microscope (THz s-SNOM) system schematic diagram, and (b) is the coupling of the terahertz pulse and the near-field system, wherein: 1, 800 nm and 400 nm dual-color pulse laser, 2, femtosecond laser filament, 3, off-axis parabolic mirror, 4, Teflon baffle, 5, strong terahertz pulse, 6, probe of atomic force microscope (AFM), 7, near-field scattering signal, 8, Cd3As2 film and CdTe substrate, 9, 635 nm continuous laser, 10, Z-axis position feedback, 11, piezoelectric ceramic, and 12, terahertz antenna.

[0036] Figure 2 A piezoelectric ceramic driven atomic force microscope AFM probe vibration and terahertz pulse signal synchronization schematic diagram in the specific embodiment of the present application is shown.

[0037] Figure 3The local electric field intensity distribution of the nanoprobe tip-Cd3As2 system in two different positions (P1 and P2) in Example 1 of the present application is shown, wherein (a) is a schematic diagram of the local electric field distribution between the nanoprobe tip and the Cd3As2 film, (b) is a schematic diagram of the local electric field distribution at the P1 position and the P2 position. Figure 3 (a) The local electric field intensity at the P1 position and the P2 position varies with the horizontal distance.

[0038] Figure 4 The near-field terahertz scattering spectra with and without a 0.5 THz filter in Example 1 of the present application are shown, wherein (a) is the near-field THz scattering spectrum of the Cd3As2 film and the CdTe substrate without a filter, and the inset is the intensity ratio of the two within the dashed line frame, (b) is the near-field THz scattering spectrum of the Cd3As2 film and the CdTe substrate with a 0.5 THz filter.

[0039] Figure 5 The time-domain curve of the THz near-field scattering signal obtained from the interaction of the nanoprobe tip and Cd3As2 in Example 1 of the present application is shown, wherein (a) is the time-domain curve of the third harmonic, (b) is the time-domain curve of the fundamental wave.

[0040] Figure 6 The schematic diagram of the non-equilibrium band dynamics process driven by a terahertz pulse in Example 1 of the present application is shown. The electron distribution of the Dirac cone conduction band at different time delays is depicted, as well as the schematic diagram of the near-field coupling of the nanoprobe tip and the Cd3As2 system. Wherein (a) is the electron distribution of the Dirac cone conduction band at a delay time of 0 ps, (b) is the electron distribution of the Dirac cone conduction band at a delay time of 13 ps, (c) is the electron distribution of the Dirac cone conduction band at a delay time of 16 ps, (d) is the electron distribution of the Dirac cone conduction band at a delay time of 21 ps. Wherein: 13, Dirac cone conduction band, 14, point with zero momentum in the z direction.

[0041] Figure 7 The change of the near-field coupling dipole moment intensity with the probe-sample distance in Example 1 of the present application is shown. Wherein (a) is that the probe tip approaches the sample surface, resulting in significant local field enhancement, (b) is that the probe slightly moves away from the sample surface, and the local field enhancement is weakened, (c) is that the probe further moves away from the sample surface, and the local field enhancement continues to weaken.

[0042] Figure 8 The dependence of the near-field induced dipole moment and the local field enhancement on the distance between the probe and the sample in Example 1 of the present application is shown.

[0043] Figure 9 The relationship between the normalized third harmonic electric field intensity and the pump field intensity in Example 1 of the present application is shown.

[0044] Figure 10 Atomic force microscope (AFM) imaging results and THz s-SNOM images measured by Example 1 of the present application are shown, wherein (a) is the AFM imaging result, (b) is the 0.5 THz near-field scattering image, and (c) is the 1.5 THz near-field scattering image.

[0045] Figure 11 Atomic force microscope (AFM) imaging results and THz s-SNOM images measured by Example 1 of the present application are shown, wherein (a) is the AFM imaging result, (b) is the 0.5 THz near-field scattering image, and (c) is the 1.5 THz near-field scattering image. Figure 10 THz near-field scattering signal distribution and first derivative curve of the black line in a.

[0046] Figure 12 Atomic force microscope (AFM) imaging results and THz s-SNOM images measured by Example 1 of the present application are shown, wherein (a) is the AFM imaging result, (b) is the 0.5 THz near-field scattering image, and (c) is the 1.5 THz near-field scattering image. Figure 10 THz near-field scattering signal distribution and first derivative curve of the black line in a. DETAILED DESCRIPTION

[0047] The present application will be further described in detail below with reference to the accompanying drawings and examples, so that those skilled in the art can more clearly understand the present application.

[0048] The present application proposes a near-field terahertz nonlinear optical effect excitation and detection system, a device schematic diagram of which is shown in Figure 1 The system includes 800 nm and 400 nm dual-color pulse laser 1, femtosecond laser filament 2, off-axis parabolic mirror 3, Teflon baffle 4, terahertz pulse 5, atomic force microscope (AFM) probe 6, near-field scattering signal (THz signal scattered from the probe) 7, Cd3As2 film and CdTe substrate 8, 635 nm continuous laser 9, Z-axis position feedback 10, piezoelectric ceramic 11, and terahertz antenna 12.

[0049] By combining the strong terahertz pulse 5 generated by the dual-color femtosecond laser filament 2 and the tapping mode atomic force microscope (AFM) probe 6, the excitation and detection of the nonlinear optical effect of the sample on the nanoscale are realized. The system utilizes the extremely small gap between the atomic force microscope (AFM) probe 6 and the sample surface of the Cd3As2 film and CdTe substrate 8, enhances the near-field scattering signal by a synchronous sampling method (SSD), and significantly improves the efficiency of the nonlinear optical response by using the local field enhancement effect.

[0050] The following gives an example of the system. Figure 1 The system realizes a near-field terahertz nonlinear optical effect excitation and detection method, which includes the following steps:

[0051] Step one, generating femtosecond laser filaments 2 by 800 nm and 400 nm dual-color pulsed laser 1, and then generating strong terahertz pulses 5: femtosecond laser pulses are generated by a laser, with a pulse width of 45 fs, a pulse energy of 4.4 mJ, and a wavelength of 800 nm. The laser pulses pass through a β-BBO crystal to generate second harmonic waves with a wavelength of 400 nm. A dual-wavelength wave plate (DWP) and an α-BBO crystal are used to control the polarization and compensate for the time delay, respectively, and the generated strong terahertz pulses 5 are focused onto the surface of the probe 6 and the Cd3As2 film and CdTe substrate 8 sample.

[0052] Step two, coupling the strong terahertz pulses 5 with the tapping mode atomic force microscope (AFM) probe 6: the generated strong terahertz pulses 5 are focused near the AFM probe 6 by an off-axis parabolic mirror 3, so that the strong terahertz pulses 5 are coupled with the tip of the AFM probe 6 to form a near-field interaction region.

[0053] Step three, controlling the synchronization of the AFM probe 6 and the strong terahertz pulses 5 to make the nano-probe approach the sample surface to generate near-field scattering signals 7: a synchronization signal generator is used to generate a synchronization signal, and a digital delay signal generator controls the timing of the signal to ensure that the vibration frequency of the AFM probe 6 is synchronized with the repetition frequency of the strong terahertz pulses 5 (as shown). Figure 2 The piezoelectric ceramic 11 is controlled to drive the AFM probe 6 to vibrate in tapping mode, so that the probe approaches the sample surface to generate near-field scattering signals 7.

[0054] Step four, exciting terahertz nonlinear optical effects at the extremely small gap between the tip of the AFM probe 6 and the sample surface of the Cd3As2 film and CdTe substrate 8: at the extremely small gap between the tip of the AFM probe 6 and the sample surface, the local field enhancement effect and the strong terahertz pulses 5 are used to excite terahertz nonlinear optical effects in the sample.

[0055] Step five, using the local field enhancement effect to enhance the interaction of the terahertz field with the sample: by adjusting the distance between the AFM probe 6 and the sample surface, the local field enhancement effect is used to significantly enhance the interaction of the terahertz pulses 5 with the sample, and the efficiency of the nonlinear optical response is improved.

[0056] Step six, analyzing the terahertz nonlinear optical signals based on the terahertz antenna 12-based terahertz time-domain spectroscopy system to determine the nonlinear optical response of the sample: the time-domain curve data are processed by Fourier transform to extract the frequency and intensity information of the signals, thereby determining the nonlinear optical response characteristics of the sample.

[0057] Example 1

[0058] According to the above method, a system is built as shown in Figure 1The system for near-field terahertz nonlinear optical effect excitation and detection shown in the figure comprises a dual-color femtosecond laser filament generation device, a synchronous sampling module, a tapping mode atomic force microscope (AFM), a local field enhancement module and a terahertz time-domain spectrum analysis module.

[0059] First, a femtosecond laser filament 2 is generated by a dual-color pulse laser 1 with a wavelength of 800 nm and 400 nm, and then an intense terahertz pulse 5 is radiated. A femtosecond laser with a wavelength of 800 nm is used to generate a laser pulse with a pulse width of 45 fs and a pulse energy of 4.4 mJ. The laser pulse is passed through a β-BBO crystal with a thickness of 107 μm to generate a second harmonic pulse with a wavelength of 400 nm. The dual-wavelength wave plate (DWP) and α-BBO crystal are used to control the polarization and time delay compensation of the 800 nm fundamental wave and 400 nm second harmonic wave respectively, so as to accurately adjust the intensity and phase of the terahertz pulse. The generated intense terahertz pulse 5 is focused on the surface of the sample of the needle tip and Cd3As2 film and CdTe substrate 8 by an off-axis parabolic mirror 3, so as to ensure the stability of the pulse energy and phase. The process is as shown in Figure 1 (a).

[0060] Then, the generated intense terahertz pulse 5 is coupled with the tapping mode atomic force microscope (AFM) probe 6. The AFM probe selects an aluminum nano probe with a tip radius of 10 nm, and the distance between the probe and the sample surface is 50 nm. The nano probe is installed on the AFM. The process is as shown in Figure 1 (b).

[0061] In the tapping mode, the vibration frequency of the AFM probe 6 is synchronized with the repetition frequency of the terahertz pulse 5. A synchronization signal generator is used to generate a synchronization signal to ensure that the vibration frequency of the nano probe is synchronized with the repetition frequency of the terahertz pulse Figure 2 The time sequence of the signal is controlled by a digital delay signal generator to ensure that the distance between the AFM probe 6 and the sample surface of the Cd3As2 film and CdTe substrate 8 is in the optimal position when the terahertz pulse arrives. The AFM probe 6 is driven to vibrate in the Z-axis direction by a piezoelectric ceramic 11. When the AFM probe 6 approaches the sample surface, the intense terahertz pulse 5 interacts with the sample surface to generate near-field scattering signals 7. A lock-in amplifier is used to extract these signals to ensure the stability and reliability of the signals. In the small gap between the tip of the AFM probe 6 and the sample surface, the local field enhancement effect is used to significantly enhance the interaction between the terahertz field 5 and the sample. By adjusting the distance between the AFM probe 6 and the sample surface, the local field intensity is accurately controlled, so as to optimize the nonlinear optical response. The process is as shown in Figure 2As shown, the synchronization mechanism of the strong terahertz pulse 5 and the AFM probe 6 vibration controlled by the piezoelectric ceramic 11 is demonstrated, which ensures the precise spatiotemporal matching of the AFM probe 6 vibration and the arrival of the terahertz pulse 5 on the sample surface, thereby realizing the effective collection of the near-field scattering signal 7.

[0062] Figure 3 The local electric field intensity distribution of the nanoprobe tip-Cd3As2 system in two different positions (P1 and P2) in Example 1 of the present application is shown, wherein (a) is a schematic diagram of the local electric field distribution between the nanoprobe tip and the Cd3As2 film, (b) is the local electric field intensity distribution at the P1 position and the P2 position, and (c) is the local electric field intensity distribution at the P1 position and the P2 position. Figure 3 (a) The local electric field intensity at the P1 position and the P2 position varies with the horizontal distance. From the above results, it can be seen that the local field enhancement effect is significantly enhanced when the AFM probe 6 tip approaches the sample surface; while the local field enhancement effect is weakened when the AFM probe 6 is slightly away from the sample surface. This phenomenon indicates that by precisely controlling the distance between the nanoprobe and the sample surface, the local field intensity can be effectively adjusted, thereby optimizing the nonlinear optical response. Figure 3

[0063] By adjusting the distance between the AFM probe 6 and the sample surface, the terahertz third harmonic (THG) is successfully excited at the extremely small gap between the AFM probe 6 tip and the sample surface. In the experiment, by optimizing the excitation conditions through a 0.5 THz narrowband filter, the THz third harmonic with a center frequency of 1.5 THz is successfully excited in the Cd3As2 film, while no third harmonic generation is observed in the CdTe substrate. The experimental results show that the scattering spectrum of the Cd3As2 film in the range of 1-2 THz is significantly enhanced (as shown in the inserted diagram), indicating that the local field enhancement effect significantly improves the THz nonlinear response efficiency of the 3D Dirac semimetal. This result is as shown in Figure 4 Figure 4 (a) is the near-field THz scattering spectrum of the Cd3As2 film and the CdTe substrate without filter, and the intensity ratio of the two in the dashed box range is shown in the inserted diagram, (b) is the near-field THz scattering spectrum of the Cd3As2 film and the CdTe substrate with a 0.5 THz filter.

[0064] The terahertz third harmonic signal scattered from the sample surface is analyzed by the terahertz antenna-based time-domain spectroscopy system. By Fourier transform processing of the time-domain curve data, the frequency and intensity information of the terahertz third harmonic is extracted, and the nonlinear optical response characteristics of the sample are determined. This result is as shown in Figure 5 (a) is the third harmonic time-domain curve, and (b) is the fundamental time-domain curve. After Fourier transform processing of the time-domain curve data, the THz third harmonic signal can be more clearly observed.

[0065] ​​The electron distribution in the Dirac cone conduction band changes significantly under the action of a strong THz field. The electrons are accelerated and oscillated in momentum space, resulting in a non-equilibrium electron distribution, which in turn produces a highly efficient THz nonlinear effect. Figure 6 As shown, (a) is the electron distribution of the Dirac cone conduction band when the delay time is 0ps, (b) is the electron distribution of the Dirac cone conduction band when the delay time is 13ps, (c) is the electron distribution of the Dirac cone conduction band when the delay time is 16ps, and (d) is the electron distribution of the Dirac cone conduction band when the delay time is 21ps.

[0066] By adjusting the distance between the AFM probe 6 and the sample surface, the local field intensity can be precisely controlled, thereby optimizing the nonlinear optical response. At the optimal distance, the local field intensity is significantly enhanced, thereby improving the nonlinear optical response efficiency. This result is shown in Figure 2. Figure 7 As shown, (a) is when the tip of the AFM probe 6 approaches the sample surface of the Cd3As2 film and the CdTe substrate 8, resulting in a significant local field enhancement; (b) is when the AFM probe 6 is slightly away from the sample surface of the Cd3As2 film and the CdTe substrate 8, and the local field enhancement is weakened; (c) is when the AFM probe 6 is further away from the sample surface, and the local field enhancement continues to weaken. Figure 8 We further demonstrate the dependence of the near-field induced dipole moment and local field enhancement on the distance between the probe and the sample. Figure 9 The relationship between the normalized third harmonic electric field intensity and the pump field intensity is demonstrated, showing an obvious cubic dependence, verifying the nonlinear characteristics of THz third harmonic generation.

[0067] Finally, the terahertz third harmonic signal 7 scattered from the sample surface 8 is analyzed by a time-domain spectroscopy system based on a terahertz antenna to determine the nonlinear optical response characteristics of the sample. The measured atomic force microscope (AFM) imaging results and THzs-SNOM images are shown in Figure 2. Figure 10 As shown, (a) is the AFM imaging result, (b) is the 0.5THz near-field scattering image, and (c) is the 1.5THz near-field scattering image. As can be seen from the figure, the contrast between the THz s-SNOM image and the AFM image is opposite, which is due to the different responses of the two technologies to the sample surface morphology and near-field coupling. In addition, nonlinear harmonic imaging ( Figure 10 c) Compared with linear imaging ( Figure 10 b) With higher spatial resolution, nanoscale near-field THz THG imaging of 200nm (λ / 3000) was successfully achieved. Figure 11 and Figure 12 The 0.5THz and 1.5THz near-field scattering signals are shown. Figure 10(a) The distribution of the black line in the middle and the distribution of the first derivative curve. The spatial resolution of the nonlinear harmonic imaging Figure 10 c) Significantly higher than linear imaging Figure 10 b), due to the stronger dependence of the nonlinear process on the local field.

[0068] By the above steps, the application overcomes the problem that low repetition rate THz pulse and near-field detection technology are incompatible by combining strong THz pulse generated by dual-color femtosecond laser filaments and light tapping mode AFM, and realizes efficient excitation and detection of THz third harmonic in three-dimensional Dirac semimetal Cd3As2 film on a nanometer scale. By precisely controlling the local field enhancement effect, the efficiency and spatial resolution of the THz nonlinear effect are significantly improved, which provides a new technical means for studying the nonlinear optical properties of nanomaterials and developing high-performance nonlinear photonic devices.

[0069] Finally, it should be noted that the purpose of publishing the embodiments is to help further understand the application, and those skilled in the art should understand that various replacements and modifications are possible without departing from the spirit and scope of the application and the appended claims. Therefore, the application should not be limited to the disclosed content, and the scope of protection claimed by the application is defined by the scope defined by the claims.

Claims

1. A method for exciting and detecting near-field terahertz nonlinear optical effects, characterized in that: The following steps are involved: Step 1: Combine femtosecond laser spatiotemporal shaping technology to generate strong terahertz pulses based on femtosecond laser filaments generated by femtosecond laser pulses; Step 2: coupling the strong terahertz pulse with a tapping mode atomic force microscope (AFM); Step 3: Using the synchronous sampling method (SSD), the atomic force microscope (AFM) probe vibration is synchronized with the terahertz pulse, and the AFM probe is brought close to the sample surface to generate a near-field scattering signal. Step 4: Stimulate the terahertz nonlinear optical effect at the extremely small gap between the AFM probe tip and the sample surface; Step 5: Utilize the local field enhancement effect at the gap to enhance the interaction between the terahertz field and the sample; Step 6: Analyze the terahertz nonlinear optical signal through a time-domain spectroscopy system based on a terahertz antenna to determine the nonlinear optical response of the sample.

2. The method according to claim 1, characterized in that The femtosecond laser spatiotemporal shaping technology is implemented through specific optical crystals or components to control the spatiotemporal and polarization characteristics of the femtosecond light field, compensate for spatiotemporal walk-off, optimize filament formation, and radiate strong terahertz pulses.

3. The method according to claim 1, characterized in that The femtosecond laser pulses used to generate femtosecond laser filaments have tunable parameters, including but not limited to pulse width, pulse energy, repetition frequency and central wavelength, to meet the needs of different samples and experimental conditions.

4. The method according to claim 1, wherein To control the synchronization of the vibration of the atomic force microscope (AFM) probe and the terahertz pulse, a synchronization signal generator is used to generate a synchronization signal, and a digital delay signal generator controls the timing of the signal to ensure that the vibration frequency of the AFM probe is synchronized with the repetition frequency of the terahertz pulse; by controlling the piezoelectric ceramic to drive the AFM probe to vibrate in tapping mode and bringing the probe close to the sample surface, a near-field scattering signal is generated.

5. The method according to claim 1, wherein The material and size of the nanoprobe are selected according to specific application requirements to optimize terahertz pulse coupling efficiency, local field enhancement effect and signal acquisition efficiency. 6 . The method according to claim 1 , wherein the sample is a material having nonlinear optical response, including but not limited to three-dimensional Dirac semimetals, two-dimensional materials, and any nanostructured materials.

7. The method according to claim 1, wherein the frequency of the terahertz nonlinear optical signal is selected and detected according to specific experimental requirements.

8. A system for realizing the near-field terahertz nonlinear optical effect excitation and detection of the method according to any one of claims 1 to 7, characterized in that: include: (1) Femtosecond laser filament generation and pulse spatiotemporal shaping control device for generating strong terahertz pulses; (2) Synchronous sampling module, used to synchronize the AFM probe vibration with the terahertz pulse, facilitating near-field terahertz signal extraction; (3) Tapping mode atomic force microscopy (AFM), used to control the vibration and position of the AFM probe; (4) Local field enhancement module, used to enhance the interaction between the terahertz field and the sample; (5) A terahertz time-domain spectroscopy analysis module, used to analyze the terahertz nonlinear optical signal of the near-field scattering.

9. The system according to claim 8, characterized in that The dual-color femtosecond laser filament generating device comprises: (1) a laser for generating femtosecond laser pulses; (2) Specific optical crystals or components used to control the spatiotemporal and polarization characteristics of the femtosecond light field and compensate for spatiotemporal walk-off.

10. The system according to claim 8, wherein: The synchronous sampling module includes: (1) a synchronization signal generator, used to generate a synchronization signal; (2) Digital delay signal generator, used to control the timing of the signal; (3) Phase-locked amplifier, used to improve the signal-to-noise ratio and extract near-field scattered signals.

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