Microscopic imaging system for measuring second harmonic phase by using polarization interference

Abstract

The invention discloses a microscopic imaging system for measuring a second harmonic phase based on polarization interference. In order to overcome the problems of insufficient precision and limited application scene of the traditional interference type second harmonic technology, a material (such as a two-dimensional material single crystal meeting D3h symmetry) with a specific second harmonic polarization response characteristic is creatively adopted as a reference sample, and the reference sample and a sample to be tested construct a coaxial interference light path. The system changes the polarization direction of the reference second harmonic by regulating and controlling the orientation angle of the reference sample, so that a controllable polarization angle difference is formed between the reference second harmonic and a signal to be detected, and implicit phase information is converted into an imaging contrast ratio capable of being quantitatively analyzed. According to the system, in-situ dynamic measurement of the phase can be realized with submicron resolution and high imaging contrast. The method has important application value in the field of characterization of novel semiconductor photoelectric materials and quantum devices, and a standardized technical scheme is provided for defect detection, intrinsic symmetry analysis and domain structure dynamics research of crystals.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical microscopic imaging technology, and particularly relates to a microscopic imaging system for measuring the second harmonic phase by polarization interference. BACKGROUND

[0002] In recent years, the second harmonic microscopic imaging technology has become an important nonlinear optical microscopic characterization method due to its high sensitivity detection capability for material symmetry and non-destructive detection characteristics, and has shown great application value in the fields of micro-nano structure accurate characterization, interface science and biomedical high-quality imaging. The technology is based on nonlinear optical principle, utilizes the frequency-doubled signal intensity generated by femtosecond laser excitation and detection to realize the visualization and non-destructive detection of the microstructure of the material. However, the traditional intensity-type second harmonic microscopic imaging technology has obvious limitations: only the intensity information of the nonlinear optical response can be obtained, and the phase information containing the key material property parameters cannot be obtained. These missing phase information is crucial in many frontier research fields. For example, in three-dimensional holographic reconstruction, it is the core basis for accurately reconstructing the spatial morphology of the object; in spintronics research, it is the key basis for determining the polarity reversal of logic devices, characterizing the magnetic domain structure and spin transport characteristics; in the characterization of new materials, it is the core criterion for identifying the breaking of inversion symmetry and revealing the intrinsic physical properties of the material. Therefore, breaking through the limitations of traditional intensity detection, developing a second harmonic microscopic imaging technology capable of accurately analyzing the optical phase information is not only crucial for deepening the scientific understanding of the above fields, but also provides a core driving force for the application innovation, and has great scientific significance and application value.

[0003] In order to obtain phase information, an interferometric second harmonic microscopy technology emerges as the times require. The technology interferes the second harmonic signal generated by the sample with the reference light, analyzes the interference fringe intensity distribution to extract the phase information, so as to realize the comprehensive characterization of the nonlinear optical response of the material. However, the existing interferometric second harmonic technology still faces many technical challenges in practical application. Although the mainstream frequency domain interference method can obtain the phase, it needs a broadband light source. The inherent dispersion effect of the broadband light source can significantly reduce the spatial resolution and phase accuracy of the system, which is difficult to meet the high resolution characterization demand. The phase adjustment method adopts a monochromatic laser, and the phase accuracy is higher, but the mechanical instability of the double light path design of the method is easily affected by environmental vibration and temperature fluctuation, which restricts the application of the method in precise imaging. The existing improvement scheme generally adopts a collinear geometric configuration, and uses a delay crystal or an electro-optic modulator to control the phase. However, due to the group velocity mismatch between the fundamental light and the second harmonic light, time walk-off is generated. The order of the time walk-off is often more than the laser pulse width, which significantly destroys the time domain coherence. Although it can be partially corrected by a dispersion compensation device, under the condition of ultrashort pulse, the high-order dispersion compensation error will further amplify the jitter of the carrier envelope phase, so that the phase stability is difficult to maintain. In addition, the system is limited by strict polarization constraints, and needs to be frequently adjusted and calibrated when facing samples with different polarization characteristics, so the applicability is poor.

[0004] Therefore, in view of the above technical bottlenecks, a new phase-sensitive microscopic imaging system with high sensitivity, high phase accuracy, fast measurement capability, and non-contact and non-destructive detection advantages has become a key scientific problem and technical challenge to be broken through in the current nonlinear optical microscopic technology field. SUMMARY

[0005] In view of the problems of insufficient accuracy and limited application scenarios in the existing technology in the second harmonic phase detection, the present application innovatively proposes a microscopic imaging system based on polarization interferometric measurement of second harmonic phase. The system successfully realizes non-contact and non-destructive phase detection by introducing polarization interferometric measurement technology, significantly improves the test efficiency while maintaining the integrity of the sample.

[0006] The phase recognition principle of the system is based on the polarization interference effect of the second harmonic signal, and high-precision characterization of the microstructure is realized through a phase-intensity information conversion mechanism. Specifically, the system first excites the second harmonic signals generated by the sample to be measured and the reference sample. When the two harmonic waves with a fixed phase relationship interfere, the phase difference of each region inside the sample to be measured will directly modulate the intensity distribution of the interference field, thereby converting the implicit phase information into a bright-dark contrast image that can be directly observed. Further, the system precisely controls the polarization direction of the reference light by rotating the reference sample, and uses the sample orientation angle sensitivity of the second harmonic polarization state to actively build a continuously adjustable polarization angle difference between the second harmonic signals of the reference sample and the sample to be measured. Changing the polarization angle difference can enhance the visibility of the interference signal, thereby improving the imaging contrast of different phase regions and achieving more accurate phase recognition. This dynamic polarization modulation strategy breaks through the sensitivity limit of traditional static interference and provides a new way for the visualization detection of sub-micron phase characteristics.

[0007] The micro-imaging system device for measuring the second harmonic phase by polarization interference provided by the application comprises two modules: a second harmonic imaging module and a white light imaging module.

[0008] The laser emission module comprises a femtosecond laser, a first lens, a second lens and a neutral density attenuation sheet arranged in sequence. The femtosecond laser is used to emit femtosecond pulse laser, serving as the base frequency excitation light of the system; the first lens and the second lens are used to expand the laser beam, and the distance between the two lenses is the sum of their focal lengths; the neutral density attenuation sheet is used to adjust the laser power.

[0009] Optionally, the neutral density attenuation sheet is a manually variable neutral density attenuation sheet.

[0010] Preferably, the neutral density attenuation sheet is an electrically variable neutral density attenuation sheet.

[0011] The scanning focusing module is located behind the laser emitting module, and comprises an XY scanning galvanometer, a scanning lens, a sleeve lens, a dichroic mirror and an objective lens arranged in sequence. The XY scanning galvanometer changes the propagation direction of the laser beam by means of a fast rotating mirror to realize two-dimensional scanning of the laser on the target plane. The scanning lens is a telecentric scanning lens, which is located behind the XY scanning galvanometer and has a distance from the XY scanning galvanometer equal to the focal length of the scanning lens. The main function of the scanning lens is to correct aberration and realize flat-field focusing, so as to ensure that the laser beam forms a uniform focused spot on the scanning plane. The sleeve lens is a telecentric sleeve lens, which is located behind the scanning lens and has a distance from the scanning lens equal to the sum of the focal lengths of the scanning lens and the sleeve lens. The sleeve lens is used to convert the angular displacement of the XY scanning galvanometer into the translation of the light beam, and ensure that the laser beam is collimated to the target plane. The dichroic mirror is located behind the sleeve lens, and is used to transmit the excitation light into the objective lens and reflect the signal light into the signal receiving module. The objective lens is located behind the dichroic mirror and has a distance from the scanning lens equal to the focal length of the scanning lens. The objective lens is used to focus the excitation light to reach the diffraction-limited size of the spot, and collect the signal light. The XY scanning galvanometer, the scanning lens, the sleeve lens and the objective lens jointly constitute a 4f imaging system. The system forms a beam fixed point at the entrance pupil of the objective lens, ensures that the laser focal spot forms a flat focal field on the image plane when scanning the entire field plane, and the spot size distortion is negligible.

[0012] Optionally, the scanning lens and the sleeve lens have the characteristics of flat field, achromatism and distortion elimination.

[0013] Optionally, the objective lens has good transmittance in the excitation light waveband and the signal light waveband.

[0014] The reference and sample to be tested module comprises a reference sample plate, a right-angle type XYZR four-axis displacement table, a sample to be tested plate and an XYZ three-axis sample table, and is used for realizing interference and adjustment of the second harmonic signal of the reference sample and the sample to be tested. The reference sample plate is a thin layer material with a center inversion symmetry break. The reference sample plate is a D 3hSymmetrical single-layer two-dimensional material film. The right-angle XYZR four-axis displacement table is assembled and fixed by a triangular plate with an XYZ three-axis displacement table, a horizontal cantilever and a hollow rotary displacement table. The horizontal cantilever is fixed on the XYZ three-axis displacement table, the rotary displacement table is inverted below the horizontal cantilever, and the reference sample plate is fixed downward below the rotary displacement table. The reference sample is located in the center of the field of view by adjusting the XY direction of the right-angle XYZR four-axis displacement table, and the Z direction is adjusted to keep the reference sample and the sample to be measured at a proper distance to ensure that the second harmonic signal strengths of the two are equivalent. The direction of the reference sample plate is rotated by adjusting the R direction to change the polarization direction of the second harmonic of the reference sample, and then the intensity of the interference signal is adjusted. The sample to be measured is located on the XYZ three-axis sample table. The XYZ three-axis sample table is used to fix the sample to be measured and adjust the position of the sample to be measured, so that the sample to be measured is located in the center of the field of view in the XY direction and at the position of the focusing point of the objective lens in the Z direction.

[0015] Optionally, the reference sample plate is a D 3h Symmetrical single-layer two-dimensional material film.

[0016] Optionally, the sample to be measured is a sample deposited on a light-transmitting substrate.

[0017] Optionally, when the sample to be measured is a sample deposited on a light-transmitting substrate, in an alternative configuration, the sample to be measured can be placed in the position of the reference sample table, and the reference sample can be placed in the position of the sample to be measured table.

[0018] Optionally, the XYZ three-axis displacement table in the right-angle XYZR four-axis displacement table is a manually adjusted XYZ three-axis displacement table.

[0019] Preferably, the XYZ three-axis displacement table in the right-angle XYZR four-axis displacement table is a piezoelectric precision-adjusted XYZ three-axis displacement table.

[0020] Optionally, the XYZ three-axis sample table is a manually adjusted displacement table.

[0021] Preferably, the Z-axis of the XYZ three-axis sample table is a piezoelectric precision-adjusted Z-axis displacement table.

[0022] The signal receiving module comprises the objective lens and the dichroic mirror in the scanning focusing module, and a third lens, a fourth lens, an optical filter, a collection lens and a photomultiplier tube (PMT) arranged in sequence. The objective lens is used to collect the second harmonic interference signal generated by the sample and the reference sample in the module. The dichroic mirror is used to reflect the signal light collected by the objective lens into the subsequent detection light path. The distance between the third lens and the entrance pupil of the objective lens is the focal length of the third lens. The third lens and the fourth lens are used to realize beam shrinking of the second harmonic interference signal, and form a light beam scanning fixed point at the collection lens to ensure that the signal collection efficiency remains constant during the light beam scanning. The distance between the third lens and the fourth lens is the sum of the focal lengths of the two lenses. The optical filter is used to eliminate the fundamental frequency signal and linear and nonlinear noise signals irrelevant to the measurement. The collection lens is used to focus the shrunk second harmonic interference signal onto the PMT. The PMT is used to collect the pure second harmonic interference signal and measure its intensity. The distance between the collection lens and the PMT is the focal length of the collection lens.

[0023] The white light imaging module comprises an LED white light point source, a collimating lens, a half-transmissive half-reflective mirror, a white light imaging sleeve lens, a plane mirror, the objective lens in the scanning focusing module and a CCD camera arranged in sequence, realizes real-time white light imaging positioning of the sample, and is used for in-situ second harmonic measurement. The LED white light point source is used as the light source for white light imaging. The collimating lens is used to collimate the white light. The half-transmissive half-reflective mirror is used to reflect the white light onto the sample and transmit the reflected light beam of the sample to the CCD camera for imaging. The white light imaging sleeve lens is used to ensure uniform illumination of the white light in the field of view and correct aberration.

[0024] The second harmonic imaging and white light imaging module are integrated on a set of microscopic imaging systems, are used for observing and positioning the sample under white light, and are used for phase-sensitive second harmonic imaging in the same detection region (in-situ). The second harmonic imaging and white light imaging are switched by switching the dichroic mirror and the plane mirror placed above the objective lens. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of the provided drawings.

[0026] Figure 1 The optical path schematic diagram of the microscopic imaging system for measuring the second harmonic phase by using polarization interference according to the present application.

[0027] Figure 2 Structure diagram of reference and sample module in the polarized interferometric second harmonic phase microscopy system of the present application.

[0028] Figure 3 Structure diagram of reference and sample module in the polarized interferometric second harmonic phase microscopy system of the present application.

[0029] Figure 4 Operation flow diagram of the polarized interferometric second harmonic phase microscopy system of the present application.

[0030] Figure 5 Second harmonic imaging interface diagram of the polarized interferometric second harmonic phase microscopy system of the present application.

[0031] Figure 6 Optical microscopic imaging diagram of the present application in which the reference sample is covered on the sample to be measured.

[0032] Figure 7 Optical microscopic imaging diagram of the present application in which the reference sample is covered on the sample to be measured.

[0033] In the figure, the reference signs correspond to:

[0034] Solid line part: second harmonic imaging module

[0035] 1-laser emission module: 11-femtosecond laser; 12-first lens; 13-second lens; 14-neutral density attenuation sheet.

[0036] 2-scan focusing module: 21-XY scan galvanometer; 22-scan lens; 23-sleeve lens; 24-dichroic mirror; 25-objective lens.

[0037] 3-reference and sample module: 31-XYZR four-axis displacement table; 32-reference sample plate; 33-sample plate to be measured; 34-XYZ three-axis sample table.

[0038] 4-signal receiving module: 41-third lens; 42-fourth lens; 43-optical filter; 44-collecting lens; 45-photomultiplier tube (PMT).

[0039] Dotted line part: white light imaging module

[0040] 5-white light module: 51-LED white light source; 52-collimating lens; 53-semi-transparent mirror; 54-sleeve lens; 55-flat mirror; 25-objective lens; 56-CCD camera. Specific implementation method:

[0041] The present invention is described in one of its embodiments below in conjunction with specific implementation methods. The accompanying drawings are only for illustrative purposes and are schematic diagrams rather than actual drawings, and should not be understood as limiting this patent. In order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the size of the actual product. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Example 1

[0043] like Figure 1 As shown, the present invention discloses a microscopic imaging system for measuring the second harmonic phase using polarization interferometry, including a second harmonic imaging module and a white light imaging module. The second harmonic imaging module is used to extract the phase information of the sample, such as Figure 1 The solid line part shows the white light imaging module. The white light imaging module is used to align and position the sample and the reference sample, as shown in FIG. Figure 1 The dotted line portion is shown;

[0044] The second harmonic imaging module includes a laser emission module 1 , a scanning and focusing module 2 , a reference and sample module 3 , and a signal receiving module 4 .

[0045] The laser emission module 1 includes a femtosecond laser 11, a first lens 12, a second lens 13, and a neutral density attenuation plate 14, which are arranged in sequence. The femtosecond laser 11 is used to emit femtosecond pulsed laser light, the first lens 12 and the second lens 13 are used to expand the laser beam, and the neutral density attenuation plate 14 is used to adjust the laser power.

[0046] The scanning focusing module 2 comprises an XY scanning galvanometer 21, a scanning lens 22, a sleeve lens 23, a dichroic mirror 24 and an objective lens 25 arranged in sequence. The XY scanning galvanometer 21 changes the propagation direction of the laser beam by rapidly rotating the mirror to realize two-dimensional scanning of the laser on the target plane. The scanning lens 22 is a telecentric scanning lens, which is located behind the XY scanning galvanometer 21 and has a distance equal to the focal length of the scanning lens 22, mainly functions to correct aberration and realize flat-field focusing, and ensures that the laser beam forms a uniform focused spot on the scanning plane. The sleeve lens 23 is a telecentric sleeve lens, which is located behind the scanning lens 22 and has a distance equal to the sum of the focal lengths of the scanning lens 22 and the sleeve lens 23, which is used to convert the angular displacement of the XY scanning galvanometer 21 into the translation of the light beam, and ensures that the laser beam is collimated to the target plane. The femtosecond pulsed laser scanning beam is transmitted through the dichroic mirror 24 and then focused onto the surfaces of the reference sample plate 32 and the sample plate 33 through the objective lens 25, so that both surfaces generate a second harmonic signal.

[0047] The structure of the reference and sample module 3 comprises a reference sample and sample stage and a sample and sample stage arranged in sequence, but the functions of the devices are different. The reference sample and sample stage is composed of a right-angle type XYZR four-axis displacement stage 31 and a reference sample plate 32, which aims to make the second harmonic generated between the reference sample and the sample have good interference and introduce an arbitrary size of polarization angle difference.

[0048] Specifically, the reference sample plate is a single crystal with a D 3h point group symmetry atomic thin layer, and the second harmonic polarization direction has a sample orientation angle dependence, which is used to modulate the size and contrast of the interference signal. Please refer to Figure 3 , the right-angle type XYZR four-axis displacement stage 31 is assembled by a triangular plate, an XYZ three-axis displacement stage 311, a triangular plate 312, a horizontal cantilever 313 and a rotary displacement stage 314. The horizontal cantilever 313 is fixed on the XYZ three-axis displacement stage 311, and the rotary displacement stage 314 is inverted below the horizontal cantilever 313. The reference sample plate 32 is fixed downward below the rotary displacement stage 314.

[0049] The sample and sample stage is another XYZ three-axis displacement stage 34, and the sample plate 33 is placed horizontally on the upper surface of the XYZ three-axis displacement stage 34, which is placed horizontally below the reference sample plate 32.

[0050] The XYZ three-axis displacement table 311 and the XYZ three-axis displacement table 34 are adjusted in one aspect to enable the laser pulse to be focused on the sample surface and the reference sample surface at the same time, and in another aspect to adjust the distance between the sample and the reference sample to generate a good interference phenomenon. By rotating the rotary displacement table 314, the orientation angle of the reference sample is adjusted, the polarization angle difference between the reference signal and the sample signal is changed, and the size of the interference signal is adjusted, so that the second harmonic imaging of different orientation regions has an arbitrary and adjustable contrast.

[0051] The signal receiving module 4 includes an objective lens 25, a dichroic mirror 24, a third lens 41, a fourth lens 42, an optical filter 43, a collection lens 44, and a photomultiplier tube (PMT) 45 arranged in sequence. The objective lens 25 is used in this module to collect the second harmonic interference signal generated by the sample and the reference sample, the dichroic mirror 24 is used in this module to reflect the second harmonic interference signal into the subsequent detection light path, the third lens 41 and the fourth lens 42 are used to realize beam shrinking of the second harmonic interference signal, the optical filter 43 is used to eliminate the fundamental frequency signal and linear and nonlinear signal noise irrelevant to measurement, the collection lens 44 is used to focus the second harmonic interference signal, and the PMT 45 is used to collect pure second harmonic interference signal and measure its intensity.

[0052] The white light module includes an LED white light point source 51, a collimating lens 52, a half-transmission half-reflection mirror 53, a sleeve lens 54, a plane mirror 55, the objective lens 25 in the scanning focusing module, and a CCD camera 56 arranged in sequence. The LED white light point source 51 is used as a light source for white light imaging. After collimation by the collimating lens 52, the white light passes through the half-transmission half-reflection mirror 53. The reflected part of the white light passes through the sleeve lens 54 and the plane mirror 55, and then passes through the objective lens 25 to reach the sample surface, thereby achieving illumination of the sample. The white light reflected from the sample surface passes through the objective lens 25 and the plane mirror 55, and then passes through the sleeve lens 54, and then transmits through the half-transmission half-reflection mirror 53 to the CCD camera 56. It should be noted that the transmitted part of the white light should pass through the focal plane of the sleeve lens 54, which should coincide with the entrance pupil of the objective lens 25. In this way, uniform illumination in the entire field of view is achieved while ensuring the correct magnification, and aberration is reduced.

[0053] The reference and sample module 3 comprises a reference sample and a reference sample table, a sample and a sample table arranged in sequence.

[0054] Embodiment 2

[0055] The application also provides a reverse structure phase identification method of embodiment 1, comprising the following steps:

[0056] S1, positioning the reference sample and the sample by using the white light imaging module:

[0057] The specific process of step S1 is as follows:

[0058] S11, switching to the white light imaging mode: placing a light barrier behind the sleeve lens 23 to block the laser from entering the optical path system, switching the dichroic mirror 24 above the objective lens 25 to a plane mirror 55, and turning on the LED white light source 51;

[0059] S12, installing and positioning the reference sample: fixing the reference sample plate 32 horizontally under the rotary displacement table 314 of the right-angle XYZR four-axis displacement table 31 with the face downward, adjusting the Z axis of the right-angle XYZR four-axis displacement table 31 to preliminarily locate the reference sample 32 near the working distance of the objective lens 25, and adjusting the XY direction of the right-angle XYZR four-axis displacement table to accurately locate the sample 33 in the center of the field of view of the CCD camera 56;

[0060] S13, installing and positioning the sample: placing the reference sample plate 32 horizontally on the XYZ three-axis displacement table 34, adjusting the XYZ three-axis displacement table 34 to move the sample to the top of the reference sample, finely adjusting the Z axis of the XYZ sample table 34, and slightly adjusting the Z axis of the right-angle XYZR four-axis displacement table 31 to ensure that the reference sample 33 is completely covered by the reference sample 32 in the field of view of the CCD camera 56, and both are in a clear imaging state. This covering state is a necessary condition for subsequent effective interference;

[0061] S2, switching the optical path to the second harmonic imaging module and performing interference calibration:

[0062] The specific process of step S2 is as follows:

[0063] S21, switch to second harmonic imaging mode: turn off LED white light source 51, switch the plane mirror 55 above the objective lens 25 back to the dichroic mirror 24, and remove the light shield plate;

[0064] S22, adjust excitation light power: use neutral density attenuation sheet 14 to adjust the laser power output by femtosecond laser 11 to an appropriate level;

[0065] S23, focus excitation and signal generation: adjust the Z axis of XYZR four-axis displacement stage 31 and the Z axis of XYZ three-axis displacement stage 34 to ensure that the objective lens 25 accurately focuses the femtosecond pulse laser on the surface of the reference sample plate 32 and the sample to be tested 33, and confirms that both generate second harmonic signals;

[0066] S24, optimize distance to generate interference: fine-tune the Z axis of XYZR four-axis displacement stage 31 and the Z axis of XYZ three-axis displacement stage 34 while maintaining the laser focus state, change the distance between the reference sample plate 32 and the sample to be tested 33, the purpose of this adjustment is to optimize the optical path matching and spatial overlap of the two second harmonic signals, so that stable and high visibility interference occurs in the signal receiving module. The best state can be determined by monitoring the interference signal intensity or contrast output by PMT 45;

[0067] S3, identify the inversion structure phase by polarization modulation;

[0068] S31, rotate the reference sample to introduce polarization angle difference: rotate the reference sample plate 32 by controlling the R axis of the right-angle XYZR four-axis displacement stage 31 to change its orientation angle. Since the polarization state of the second harmonic signal of the sample to be tested 33 has angle dependence, and the polarization state of the second harmonic signal of the sample to be tested 33 is determined by its inherent structure, the rotation operation actively introduces the polarization angle difference between the reference signal and the test signal;

[0069] S32, scanning imaging and contrast analysis: under different reference sample orientation angles, start the XY scanning galvanometer 21 for two-dimensional scanning, use PMT 45 to collect the second harmonic interference signal intensity at the corresponding position, and generate a second harmonic imaging map;

[0070] S33, identifying the phase of the inversion structure: if the sample 33 to be measured is an inversion structure (such as ferroelectric domain, magnetic domain, etc.) with 0-π phase distribution, the intrinsic second harmonic phase difference of different regions thereof is π. In this case, when the orientation angle of the reference sample is changed (rotated by a certain angle such as 60°), the amplitude of the light-dark contrast of the second harmonic imaging of the region with an original phase difference of π in the sample to be measured remains approximately unchanged, but the relative light-dark relationship of the adjacent region is reversed (i.e. the contrast sign is changed). The observation of this phenomenon that the contrast amplitude is constant and the sign is reversed with the change of the orientation angle of the reference sample can be used as the core experimental basis for determining whether the sample to be measured has an inversion structure with 0-π phase distribution.

[0071] The above is only the preferred embodiment of the present application, and the protection scope of the present application is not limited to the above-mentioned embodiment. Any technical scheme falling within the concept of the present application shall fall within the protection scope of the present application. It should be noted that, for ordinary skilled in the art, some improvements and refinements without departing from the principles of the present application shall be considered as falling within the protection scope of the present application.

Claims

1. A microscopic imaging system for measuring second harmonic phase using polarization interferometry, characterized in that: include: Second harmonic imaging module and white light imaging module; wherein: The second harmonic imaging module is used for exciting and collecting phase-sensitive second harmonic interference signals, and comprises: a laser emission module (1), a scanning and focusing module (2), a reference and sample module (3) and a signal receiving module (4).

2. The microscopic imaging system for measuring the second harmonic phase using polarization interferometry according to claim 1, characterized in that: The laser emission module (1) further comprises: A femtosecond laser (11), a first lens (12), a second lens (13), and a neutral density attenuation plate (14); the distance between the first lens (12) and the second lens (13) is the sum of the focal lengths of the two lenses, and the neutral density attenuation plate (14) is used for laser beam expansion; the neutral density attenuation plate (14) is a manually or electrically variable neutral density attenuation plate, and is used for adjusting laser power.

3. The microscopic imaging system for measuring the second harmonic phase using polarization interferometry according to claim 1, characterized in that: The scanning and focusing module (2) further comprises: An XY scanning galvanometer (21), a scanning lens (22), a sleeve lens (23), a dichroic mirror (24) and an objective lens (25); the XY scanning galvanometer (21), the scanning lens (22), the sleeve lens (23) and the objective lens (25) form a 4f optical system for correcting aberrations and realizing laser surface scanning; the dichroic mirror (24) is used to transmit femtosecond laser light to the objective lens (25) and reflect a second harmonic interference signal from a sample to a signal receiving module (4).

4. The scanning focusing module (2) according to claim 3, characterized in that: The scanning lens (22) is a telecentric scanning lens. When the incident light angle changes relative to the lens optical axis, the telecentric scanning lens can generate a flat image plane, which is used to eliminate the spot size distortion. The sleeve lens (23) is a telecentric sleeve lens, which is used for laser scanning and wide-field imaging. The scanning lens (22) and the sleeve lens (23) form a telecentric system, which scans the object plane point by point through the XY scanning galvanometer (21).

5. The microscopic imaging system for measuring the second harmonic phase using polarization interferometry according to claim 1, characterized in that: The reference and test sample module (3) further comprises: A right-angle XYZR four-axis translation stage (31), a sample plate to be tested (32), a reference sample plate (33) and a sample stage (34); the right-angle XYZR four-axis translation stage (31) is composed of an XYZ three-axis translation stage (311), a horizontal cantilever (313) and a rotational translation stage (314) fixedly assembled by a triangle plate (312), and is used to adjust the spatial position and orientation angle of the reference sample; the rotational translation stage (314) is installed below the horizontal cantilever; the sample plate to be tested (32) is fixed below the rotational translation stage (314) and is made of a material with second harmonic polarization angle dependence, wherein the polarization direction of the second harmonic changes with the orientation angle of the reference sample and the intensity remains constant; the sample stage (34) is an XYZ three-axis translation stage, and is used to fix the reference sample plate (33) and adjust its position so that the second harmonic signals of the sample to be tested and the reference sample generate interference.

6. The reference and test sample module (3) according to claim 5, characterized in that: The XYZ three-axis displacement stage in the right-angle XYZR four-axis displacement stage (31) is a piezoelectric precision adjustment displacement stage; and the Z axis of the sample stage (34) is a piezoelectric precision adjustment displacement stage.

7. The reference and test sample module (3) according to claim 5, characterized in that: The sample plate (32) to be tested comprises a material having second harmonic polarization angle dependence, including but not limited to D 3h Symmetric single crystal films of two-dimensional materials.

8. The reference and test sample module (3) according to claim 5, characterized in that: The rotational translation stage (314) changes the polarization direction of the second harmonic of the reference sample by adjusting the reference sample orientation angle, thereby forming a controllable polarization angle difference between the second harmonic signals of the reference sample and the sample to be measured, thereby modulating the imaging contrast of different areas.

9. The reference and test sample module (3) according to claim 5, characterized in that: When the sample to be tested (33) is a sample deposited on a light-transmitting substrate, the sample to be tested (33) can be placed at the position of the reference sample stage (31), and the reference sample plate (32) can be placed at the position of the sample to be tested (34).

10. The microscopic imaging system for measuring the second harmonic phase using polarization interferometry according to claim 1, characterized in that: The signal receiving module (4) further includes: An objective lens (25), a dichroic mirror (24), a third lens (41), a fourth lens (42), an optical filter (43), a collecting lens (44) and a photomultiplier tube (45); the objective lens (25) is used to collect a second harmonic interference signal; the dichroic mirror (24) is used to transmit a fundamental frequency laser and reflect a second harmonic signal; the third lens (41) and the fourth lens (42) constitute a beam reduction system, which is used to form a beam scanning fixed point at the collecting lens (44) so ​​that the signal collection efficiency remains consistent during the beam scanning process; the optical filter (43) is used to filter out non-second harmonic band noise; the collecting lens (44) focuses the second harmonic interference signal after beam reduction onto the photosensitive surface of the photomultiplier tube (45); and the photomultiplier tube (45) is used to detect the intensity of the second harmonic signal.

11. The microscopic imaging system for measuring the second harmonic phase using polarization interferometry according to claim 1, characterized in that: The white light imaging module (5) is used for positioning and optical imaging of a sample, and comprises: an LED point light source (51), a collimating lens (52), a semi-transparent and semi-reflective mirror (53), a sleeve lens (54), a plane reflector (55), an objective lens (25) and a CCD camera (56); the second harmonic imaging module and the white light imaging module share the objective lens (25), and the conversion between the second harmonic imaging mode and the white light imaging mode is achieved by switching the dichroic mirror (24) or the plane reflector (55) located above the objective lens (25).

12. A 0-π phase inversion structure measurement method based on the system according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1. Use the white light imaging module to position the reference sample and the sample to be tested at the center of the field of view; S2. Switch to the second harmonic imaging module and adjust the laser power and focus position so that the reference sample and the surface of the sample to be measured generate second harmonic signals; S3. The orientation angle of the reference sample is changed by rotating the translation stage (314) to obtain second harmonic interference images at different orientation angles; when the sample to be tested has a 0-π phase inversion structure, after rotating a specific angle, the imaging contrast amplitude of the same area remains stable while the light and dark polarities are reversed; if this phenomenon is detected, it is determined that the sample has a 0-π phase inversion structure.

13. The method according to claim 10, wherein: The specific angle includes 60° and can be extended to other angles, and the light and dark polarity reversal is reflected as a contrast sign change, wherein the contrast sign change indicates a reversal of the brightness relationship between the light and dark areas.

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