Device and method for measuring solid higher harmonic phase

Abstract

The invention discloses a device and a method for measuring a solid higher harmonic phase, and belongs to the technical field of ultrafast laser. According to the invention, a single beam of driving laser is adopted, two spatially separated harmonic light sources are formed through higher harmonic response of two different harmonic targets, and a complex beam splitting light path design is not needed; by changing the laser wavelength, the influence of the single variable of the laser wavelength on the interference fringes under the condition of fixed interference spacing can be observed; single-order higher harmonics are screened out through a band-pass filter, and far-field interference of the single-order higher harmonics is directly measured on a camera; the distance, the material type and the micro-nano structure of the harmonic target material can be customized through a modern mature technology, and the customizable parameters are closely related to the period, the resolution ratio, the light intensity response and other properties of interference fringe measurement; according to the invention, the resolution of stripe movement measurement is obviously improved by reducing the harmonic target distance; the method is particularly suitable for measuring the two-dimensional layered material which is difficult to prepare the bulk single crystal.

Description

Technical Field

[0001] This invention relates to ultrafast laser technology, and more specifically to an apparatus and method for measuring the phase of higher harmonics in solids. Background Technology

[0002] Higher harmonic generation is the physical process by which high-frequency coherent radiation is produced when ultrafast intense lasers interact with matter. It is the main approach to generating extreme ultraviolet attosecond pulses in the laboratory and is at the forefront of research in strong-field physics and attosecond physics. Accurate measurement of the phase of higher harmonics is a core requirement for attosecond pulse characterization and also provides a platform for studying the interaction between ultrafast intense lasers and matter and exploring the band structure and other physical properties of harmonic targets.

[0003] Currently, the measurement of high-harmonic phase in solids mainly references the measurement schemes for high-harmonic gases, which can be divided into two categories. One category is based on photoelectronic measurement, and the other is based on far-field interferometry all-optical schemes for high-harmonics. Photoelectronic measurement schemes include attosecond beat frequency reconstruction using two-photon interferometry and attosecond fringe camera technology. These methods measure the dependence of the photoelectron energy spectrum generated by the interaction of high-harmonics and the fundamental frequency light field on the delay of the two beams, thereby measuring the harmonic phase and attosecond pulse width. Photoelectronic measurement schemes require precise attosecond timing control of the delay between the high-harmonics and the fundamental frequency light, placing extremely high demands on the stability of the excitation optical path, the luminous flux of the high-harmonic to be measured, and the detection device. The far-field interferometry all-optical scheme for high-harmonics splits the driving laser beam, forming two spatially separated focal points on the sample surface. These two focal points generate two spatially separated high-harmonic sources that coherently superimpose in the far field to form interference fringes. By measuring the movement of the interference fringes, the phase difference information of these two harmonic sources can be obtained. However, the material dispersion of the beam splitter that achieves dual focal points limits wavelength-tuned interferometric measurements with a fixed focal spacing; beam pointing jitter and the jitter of the optical path difference between the two beams reduce fringe stability and signal-to-noise ratio; the diffraction limit of the selected focusing lens limits the lower limit of the focal spacing (about 100 μm), resulting in a small angular period of the interference fringes, making experimental measurements more difficult.

[0004] Therefore, there is an urgent need to develop a new method and device for measuring the phase of higher harmonics in solids, so as to advance research on the measurement of phase of higher harmonics in solids. Summary of the Invention

[0005] To overcome the shortcomings of existing solid-state high-harmonic far-field interferometric all-optical schemes in terms of system complexity, stability, and resolution, this invention proposes a device and method for measuring the phase of solid-state high-harmonics, providing an easy-to-build device for high-precision and high-stability measurement of the phase of solid-state high-harmonics.

[0006] One object of the present invention is to provide an apparatus for measuring the phase of higher harmonics in a solid.

[0007] The device for measuring the phase of higher harmonics in solids according to the present invention includes: a driving laser module, a power control module, a focusing lens, a sample holder, a filtering device, an illumination source, an imaging lens, a camera, and a computer; wherein, the sample includes two different harmonic targets, which are set on the sample holder, and the two harmonic targets have different micro-nano structures or are thin films of different materials, and the surface of the sample is perpendicular to the optical axis.

[0008] The illumination source emits illumination light, which is focused by the focusing lens to illuminate the sample. The surface morphology of the sample is focused onto the camera by the imaging lens. The position of the sample holder is adjusted so that the sample is located at the center of the camera's field of view, and the center of the sample is located on the optical axis.

[0009] The driving laser module emits a femtosecond laser as the driving laser; the power control module adjusts the intensity of the driving laser; the light is incident on the sample through a focusing lens, which adjusts the spot radius of the incident light so that the spot covers the sample; the incident light excites two harmonic targets to generate multiple higher harmonics; a filtering device filters out the higher harmonics of the order to be measured; the phase lock of the same order of higher harmonics generated by the two harmonic targets results in interference in the far field; the spatial difference in the phase response of the solid higher harmonics is converted into changes in the higher harmonic interference fringes; a camera collects the interference fringes and transmits them to a computer; the computer obtains the phase difference of the higher harmonics generated by the two harmonic targets based on the interference fringes.

[0010] Micro- and nanostructures are fabricated on harmonic targets using micro- and nanofabrication techniques. These structures are on the order of micrometers and take shapes such as hexahedrons, disks, or microcavities. Different harmonic targets with different micro- and nanostructures exhibit different field enhancement factors (FEMs). The distance between two harmonic targets is on the order of micrometers. The two harmonic targets can be either thin films of two different materials or micro- and nanostructures of the same material with different FEMs. The horizontal dimension of each harmonic target ranges from several micrometers to tens of micrometers, and the thickness ranges from nanometers (microscopic single-atom layers) to tens of micrometers (mesoscopic scale). For two harmonic targets of the same material with different FEMs, the excitation intensity is calculated as incident intensity × the square of the FEM. Because the excitation intensities for higher harmonics differ between the two targets, interference of these higher harmonics generated by different excitation intensities in the far field is obtained. By changing the intensity of the driving laser using a power control module, the interference at different intensities is measured. The movement of the interference fringes reflects the change in the phase difference between the higher harmonics generated by the two harmonic targets.

[0011] Within the laser beam, since the focal point of the Gaussian beam is an equiphase surface, the electric field phases of the driving lasers on the two harmonic targets are identical. The definite relationship between the solid-state higher harmonic phase and the driving laser phase allows the two harmonic targets on the sample to form a mutually phase-locked harmonic source. This causes the generated harmonics to coherently superimpose in the far field and interfere, forming interference fringes. This device can use a harmonic target with a known solid-state higher harmonic phase or a target whose solid-state higher harmonic phase is independent of the excitation light intensity (such as silicon dioxide). The phase difference obtained by measuring the interference fringes directly reflects the solid-state higher harmonic phase generated by the other harmonic target material, thus achieving the measurement of the solid-state higher harmonic phase.

[0012] It also includes a folding mirror, positioned before the focusing lens, to image and position the sample before solid-state high-harmonic phase measurement. The folding mirror guides the illumination light onto the sample for bright-field illumination. During solid-state high-harmonic phase measurement, the folding mirror is moved out of the optical path. The illumination source is a white light source, emitting white light.

[0013] The driving laser module generates a wavelength-tunable mid-infrared femtosecond laser; it includes a femtosecond laser and an optical parametric amplification device with a tunable laser wavelength in the mid-infrared band. The femtosecond laser emits a low-photon-energy mid-infrared femtosecond laser as the driving laser to ensure that the sample is not damaged when high-intensity laser irradiation generates high-order harmonics.

[0014] The power control module includes two rotatable broadband metal wire grid polarizers to generate mid-infrared polarized femtosecond lasers with variable power.

[0015] The focusing lens has high transmittance in the mid-infrared band, focusing mid-infrared laser or illumination light onto the harmonic target. The focusing lens adjusts the spot radius of the incident light to tens of micrometers, covering two harmonic targets.

[0016] It also includes an adjustment platform, on which the sample holder is mounted for translational adjustment in three dimensions. The position of the sample is adjusted by means of the adjustment platform and the sample holder; the sample holder can rotate around the optical axis. The sample holder uses a transparent substrate, such as fused silica or sapphire.

[0017] The filtering device includes a mechanical rotating wheel and multiple filters; each filter corresponds to a different order of higher harmonics; multiple filters and an imaging lens are mounted on the mechanical rotating wheel, the axis of rotation of the mechanical rotating wheel is parallel to the optical axis; when imaging and positioning the sample, the mechanical rotating wheel inserts the imaging lens into the optical path; when measuring the phase of solid-state higher harmonics, the mechanical rotating wheel inserts the filter corresponding to the higher harmonic of the order to be measured into the optical path.

[0018] The camera uses an enhanced charge-coupled device (ICCD) area array camera to record interference fringes of higher harmonics.

[0019] By changing the laser wavelength, interference fringes at different laser wavelengths were obtained. The influence of the laser wavelength on the interference under a fixed target spacing was observed.

[0020] Another object of the present invention is to provide a method for measuring the phase of higher harmonics in a solid.

[0021] The method for measuring the phase of higher harmonics in a solid according to the present invention includes the following steps:

[0022] 1) Two different harmonic targets are set as samples on the sample holder. The two harmonic targets have different micro-nano structures or are thin films of different materials. The surface of the sample is perpendicular to the optical axis.

[0023] 2) The illumination source emits illumination light, which passes through the focusing lens to illuminate the sample. The surface morphology of the sample is imaged onto the camera by the imaging lens. The position of the sample holder is adjusted so that the sample is located in the center of the camera's field of view, and the center of the sample is located on the optical axis.

[0024] 3) The driving laser module emits a femtosecond laser as the driving laser, and the intensity of the driving laser is adjusted by the power control module;

[0025] 4) The driving laser is incident on the sample through the focusing lens. The focusing lens adjusts the spot radius of the incident light so that the spot of the incident light covers the sample. The incident light excites the two harmonic targets to generate multiple high-order harmonics respectively.

[0026] 5) The filtering device filters out the higher harmonics of the order to be measured. The phase lock of the higher harmonics of the same order generated by the two harmonic targets occurs in the far field. The spatial difference of the phase response of the solid higher harmonics is converted into the change of the higher harmonic interference fringes.

[0027] 6) The camera collects interference fringes and transmits them to the computer; the computer obtains the phase difference of the higher harmonics generated by the two harmonic targets through the interference fringes.

[0028] For two harmonic targets that are the same material with different field enhancement factors, the intensity of the driving laser is changed by the power control module, and the interference fringes shift, thus obtaining the response of the solid high-order harmonic phase to the excitation light intensity.

[0029] Using a harmonic target material with a known solid higher harmonic phase or a solid target material with a solid higher harmonic phase that is independent of the excitation light intensity, the phase difference of the higher harmonics is obtained by measuring the interference fringes, and the solid higher harmonic phase of another harmonic target material is calculated.

[0030] Furthermore, by adjusting the light intensity of the driving laser through the power control module, the far-field interference of the same order higher harmonics under different light intensities is measured sequentially to obtain the dependence of the phase of the solid higher harmonics on the light intensity.

[0031] Furthermore, by replacing different filters, the far-field interference of different orders of higher harmonics under different light intensities was measured sequentially to obtain the dependence of the phase of different orders of solid-state higher harmonics on light intensity.

[0032] By changing the wavelength of the driving laser and repeating the above operation, the far-field interference under different driving laser wavelengths is measured to obtain the dependence of the phase of the solid-state higher harmonics on the wavelength.

[0033] Advantages of this invention:

[0034] 1) The system is simple: It adopts a single-beam driven laser and forms two spatially separated harmonic light sources through the high-order harmonic response of two harmonic targets, without the need for complex beam splitting optical path design;

[0035] 2) Wavelength tunable: By changing the laser wavelength, the influence of the laser wavelength on the interference fringes under a fixed interference spacing can be observed without making other changes to the optical path;

[0036] 3) High stability: The mutually phase-locked high-order harmonic light source is formed by two spatially separated harmonic targets within the phase plane of the single-beam driven laser focal spot. The spatial position of the high-order harmonic light source is fixed, and the spacing between them and their respective relative phases of the driving laser remain fixed. It has strong robustness to the pointing jitter of the single-beam driven laser and the optical path fluctuation of the front optical path. The interference fringes formed are highly stable and no additional stability feedback device is required.

[0037] 4) Simple data processing: Single-order high-harmonic waves are filtered out by a filter, and their far-field interference is directly measured on the camera;

[0038] 5) High customizability: The spacing, material type, and micro / nano structure of the harmonic target can be customized using modern and mature sample transfer and micro / nano fabrication techniques. These customizable parameters are closely related to properties such as the period, resolution, and light intensity response of the interference fringe measurement. The method of this invention is particularly suitable for measuring two-dimensional layered materials that are difficult to prepare as bulk single crystals.

[0039] 6) High resolution: The period of interference fringes is inversely proportional to the target spacing. By reducing the spacing between the two harmonic targets, the angular period of far-field interference is inversely amplified. Under the condition that the pixel unit size of the camera is fixed, reducing the spacing between the harmonic targets will significantly improve the resolution of measuring fringe movement. Attached Figure Description

[0040] Figure 1This is a schematic diagram of one embodiment of the apparatus for measuring the phase of higher harmonics in a solid according to the present invention;

[0041] Figure 2 The image shows an interference fringe pattern obtained according to an embodiment of the method for measuring the phase of higher harmonics in a solid according to the present invention. Detailed Implementation

[0042] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0043] like Figure 1 As shown, the apparatus for measuring the phase of higher harmonics in solids in this embodiment includes: a laser driving module, a power control module, a focusing lens, a sample holder, an adjustment platform, a mechanical rotating wheel, multiple filters, a folding mirror, an illumination source, an imaging lens, a camera, and a computer. The sample holder is mounted on the adjustment platform and can be translated and adjusted in three dimensions. The position of the sample is adjusted by adjusting the platform and the sample holder. The sample holder is rotatable around the optical axis and uses a transparent substrate. The sample includes two different solid harmonic targets, which are mounted on the sample holder. The two harmonic targets have different micro / nano structures or are thin films of different materials. The sample surface is perpendicular to the optical axis; the camera is an ICCD area array camera, with the center of the laser coinciding with the center of the camera's field of view; the filter is a bandpass filter, with each filter corresponding to a different order of higher harmonics; multiple filters and imaging lenses are mounted on a mechanical rotating wheel, the axis of which is parallel to the optical axis; when imaging and positioning the sample, the illumination light is guided onto the sample for bright-field illumination via a folding mirror, and the mechanical rotating wheel inserts the imaging lens into the optical path; when measuring the phase of solid-state higher harmonics, the folding mirror is moved out of the optical path, and the mechanical rotating wheel inserts the filter corresponding to the order of the higher harmonic to be measured into the optical path.

[0044] The illumination source uses a white light source to emit white light. The driving laser module generates mid-infrared laser light with a tunable wavelength range of 3μm to 5μm. The focusing lens has high transmittance (≥90%) in the mid-infrared band, focusing the mid-infrared laser or illumination light onto the harmonic target. The focusing lens adjusts the spot radius of the incident light to tens of micrometers, covering two harmonic targets.

[0045] Example 1

[0046] The method for measuring the phase of higher harmonics in a solid in this embodiment includes the following steps:

[0047] 1) Using semiconductor coating technology, harmonic targets are prepared on a fused silica substrate. Two harmonic targets are spatially separated with a spacing of 10~30μm. In this embodiment, one harmonic target uses a ZnO thin film and the other uses a GaAs thin film. Both materials are commonly used semiconductor materials with rich and mature preparation processes. The two different harmonic targets are used as samples, and the fused silica substrate is used as a sample holder. The sample holder is placed on an adjustment platform with the sample surface perpendicular to the optical axis and the center of the sample located on the optical axis.

[0048] 2) Before measuring the phase of higher harmonics in solids, the sample is first imaged and positioned: the illumination source emits white light, which is guided onto the optical axis by a refracting mirror and focused onto the sample by the imaging lens. The surface morphology of the sample is imaged onto the camera. The position of the sample holder is adjusted by adjusting the platform so that the sample is located in the center of the camera's field of view. The imaging surfaces of the imaging lens, the sample, and the camera meet the imaging conditions.

[0049] 3) Solid-state high harmonic phase measurement: The folding mirror is moved out of the optical path; the driving laser module emits a mid-infrared laser with a wavelength of 4.35μm as the driving laser, and the intensity of the driving laser is adjusted by the power control module;

[0050] 4) The driving laser is focused onto the sample by a focusing lens. The focusing lens adjusts the spot radius of the incident light so that the focused spot of the incident light covers the sample. The incident light excites the two harmonic targets to generate multiple high-order harmonics respectively.

[0051] 5) The filter device selects the higher harmonics of the set order to be measured. The phase lock of the higher harmonics of the same order generated by the two harmonic targets is achieved. The two higher harmonics of the same order are used as two coherent higher harmonic light sources and interfere in the far field. The spatial difference of the phase response of the solid higher harmonics is transformed into the change of the higher harmonic far field interference.

[0052] 6) The camera collects far-field interference and transmits it to the computer; the computer obtains the phase difference of the higher harmonics generated by the two harmonic targets through the interference fringes; Figure 2 The image shows interference fringes of the 11th harmonics from two harmonic targets spaced 15 μm apart, with phase differences of 0, π / 2, and π, respectively. The center coordinates of the interference fringes are 0 mrad, 7 mrad, and 14 mrad, respectively. Figure 2 As can be seen, the interference fringes shift as the phase difference changes; the amount of shift in the interference fringes corresponds to the amount of phase shift in the higher harmonics of the solid.

[0053] 7) Adjust the intensity of the driving laser using the power control module, and repeat steps 3) to 6). Measure the far-field interference of the same higher harmonics at different light intensities. The shift in far-field interference reflects the dependence of the phase of the solid-state higher harmonics on the light intensity. Taking a sample with a 15μm gap between two harmonic targets as an example, the distance from the sample to the camera is 20cm, and the light intensity starts from 0.2TW / cm. 2 Increased to 0.5 TW / cm 2 The resulting fringe displacement on the camera imaging surface is 1.84 mm, and the fringe shift exceeds 400 camera pixels (common pixel size 4.54 μm), exhibiting extremely high signal-to-noise ratio in fringe shift measurement. Since the phase of the 11th harmonic generated by the ZnO thin film does not change with the incident light intensity, while the phase of the 11th harmonic generated by the GaAs thin film is related to the incident light intensity, the 1.84 mm fringe displacement measured in the example corresponds to a 0.69π phase shift of the 11th harmonic generated by the GaAs thin film.

[0054] 8) Rotate the mechanical wheel and insert the filter corresponding to the higher harmonic of the order to be measured into the optical path. Repeat steps 3) to 7) to measure the far-field interference of different orders of higher harmonics under different light intensities to obtain the dependence of the phase of different orders of solid higher harmonics on light intensity. Select the filter corresponding to the 5th to 11th order harmonics. The harmonics generated by ZnO are dominated by the intra-band mechanism, and the phase of the generated solid higher harmonics does not depend on the driving light intensity. The harmonics generated by GaAs are dominated by the inter-band mechanism. The movement of the interference fringes directly reflects the change of the phase of the harmonics generated by GaAs with the driving light intensity. Select the filter corresponding to the 11th order and above harmonics. The harmonics generated by ZnO and GaAs are both dominated by the inter-band mechanism. The harmonic phase changes with the driving light intensity. The movement of the fringes reflects the difference in the response of the harmonic phase of the two materials to the driving light intensity. The amount of this movement of the interference fringes is closely related to the band structure of the material. This invention provides a new method for band structure measurement.

[0055] 9) Change the wavelength of the driving laser and repeat steps 3) to 8) to measure the far-field interference at different driving laser wavelengths in turn to obtain the dependence of the solid higher harmonic phase on the wavelength of the driving laser.

[0056] Example 2

[0057] In this embodiment, two spatially separated micro / nano structures with different field enhancement factors, made of the same material, are fabricated on a fused silica substrate using focused ion beam milling micro / nano fabrication technology. These structures serve as harmonic targets, enabling the measurement of the high-order harmonic phase of the material itself. Taking a flat ZnO thin film (10 μm horizontal dimension, 10 μm thickness) as one harmonic target and a ZnO frustum (5 μm upper surface diameter, 15 μm lower surface diameter, 5 μm height) as another example, the structural parameters of the frustum are designed so that the light intensity at the exit end of the frustum is four times that of the thin film under internal focusing. The distance between the two microstructures is 15 μm. The relationship between the movement of the 13th harmonic fringe generated by a 4.35 μm laser and the driving light intensity is measured. The phase of the 13th harmonic generated by the ZnO material depends on the incident light intensity. Following steps 2) to 9) in Example 1, the incident laser intensity is adjusted from 0.2 TW / cm². 2 Increased to 0.5 TW / cm 2 The measured fringe displacement was 0.9 mm (the distance between the sample and the camera was 20 cm), corresponding to a phase difference change of 0.4π. This design allows for the determination of the relationship between the phase of the solid-state higher harmonics generated by the material itself and the driving light intensity, utilizing the differences in driving light intensity caused by different structural field enhancement factors, thus achieving self-reference measurement.

[0058] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. An apparatus for measuring the phase of a solid high harmonic, characterized by, The device comprises a driving laser module, a power control module, a focusing lens, a sample holder, a filtering device, an illumination light source, an imaging lens, a camera and a computer; wherein the sample comprises two different harmonic targets arranged on the sample holder, the two harmonic targets have different micro-nano structures or are thin films of different materials, and the surface of the sample is perpendicular to the optical axis; The illumination light source emits illumination light which is focused by the focusing lens to illuminate the sample, and the surface topography of the sample is imaged by the imaging lens to the camera, the position of the sample holder is adjusted so that the sample is located at the center of the field of view of the camera, and the center of the sample is located on the optical axis; The driving laser module emits femtosecond laser as driving laser, and the power control module adjusts the light intensity of the driving laser; the driving laser is incident on the sample through the focusing lens, the focusing lens adjusts the spot radius of the incident light, and the spot of the incident light covers the sample; the incident light excites the two harmonic targets to generate multi-order high harmonics, respectively; the filtering device selects the high harmonics to be measured, the high harmonics of the same order generated by the two harmonic targets are phase-locked, interference occurs in the far field, and the spatial difference of the phase response of the solid high harmonic is converted into the change of the high harmonic interference fringe; the camera collects the interference fringe and transmits it to the computer; the computer obtains the phase difference of the high harmonics generated by the two harmonic targets according to the interference fringe.

2. The apparatus of claim 1, wherein, The micro-nano structure is prepared on the harmonic target by micro-nano machining, and the size of the micro-nano structure is micron level; the harmonic targets with different micro-nano structures have different field enhancement factors.

3. The apparatus of claim 1, wherein, It also comprises a folding mirror arranged before the focusing lens; before the solid high harmonic phase measurement, the sample is imaged and positioned, and the illumination light is guided to the sample by the folding mirror for bright field illumination; during the solid high harmonic phase measurement, the folding mirror is removed from the optical path.

4. The apparatus of claim 1, wherein, The filtering device comprises a mechanical rotating wheel and a plurality of filter pieces; each filter piece corresponds to high harmonics of different orders; the plurality of filter pieces and the imaging lens are mounted on the mechanical rotating wheel, and the rotating shaft of the mechanical rotating wheel is parallel to the optical axis; during the imaging and positioning of the sample, the mechanical rotating wheel inserts the imaging lens into the optical path; during the solid high harmonic phase measurement, the mechanical rotating wheel inserts the filter piece corresponding to the high harmonics of the order to be measured into the optical path.

5. A method of measuring the phase of a solid high harmonic according to the apparatus of any one of claims 1 to 4, characterized in that, The method comprises the following steps: 1) arranging two different harmonic targets as a sample on a sample holder, the two harmonic targets have different micro-nano structures or are thin films of different materials, and the surface of the sample is perpendicular to the optical axis; 2) the illumination light source emits illumination light which is focused by the focusing lens to illuminate the sample, and the surface topography of the sample is imaged by the imaging lens to the camera, the position of the sample holder is adjusted so that the sample is located at the center of the field of view of the camera, and the center of the sample is located on the optical axis; 3) the driving laser module emits femtosecond laser as driving laser, and the power control module adjusts the light intensity of the driving laser; 4) the driving laser is incident on the sample through the focusing lens, the focusing lens adjusts the spot radius of the incident light, and the spot of the incident light covers the sample, and the incident light excites the two harmonic targets to generate multi-order high harmonics, respectively; 5) The filter device selects the high-order harmonic of the order to be measured. The phase of the high-order harmonic of the same order generated by the two harmonic targets is locked, and interference occurs in the far field. The spatial difference of the phase response of the solid high-order harmonic is converted into the change of the high-order harmonic interference fringes. 6) The camera collects the interference fringes and transmits them to the computer. The computer obtains the phase difference of the high-order harmonic generated by the two harmonic targets through the interference fringes.

6. The method of claim 5, wherein, For two harmonic targets made of the same material with different field enhancement factors, the power control module changes the light intensity of the driving laser, and the interference fringes move. The movement of the interference fringes reflects the change of the phase difference of the high-order harmonic generated by the two harmonic targets, and the response of the solid high-order harmonic phase to the excitation light intensity is obtained.

7. The method of claim 5, wherein, Using a harmonic target with a known solid high-order harmonic phase or a target with a solid high-order harmonic phase independent of the excitation light intensity, the phase difference of the high-order harmonic is obtained by measuring the interference fringes, and the solid high-order harmonic phase of another harmonic target is calculated.

8. The method of claim 5, wherein, Adjust the light intensity of the driving laser through the power control module, and measure the far-field interference of the high-order harmonic of the same order under different light intensities in sequence to obtain the dependence of the solid high-order harmonic phase on the light intensity.

9. The method of claim 8, wherein, Replace different filters and measure the far-field interference of high-order harmonics of different orders under different light intensities in sequence to obtain the dependence of the solid high-order harmonic phase of different orders on the light intensity.

10. The method of claim 9, wherein, Change the wavelength of the driving laser and repeat the above operation to measure the far-field interference under different wavelengths of the driving laser in sequence to obtain the dependence of the solid high-order harmonic phase on the wavelength.

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