Metasurface processing method with phase modulation beam deflection fused with chemical etching

Abstract

The invention discloses a metasurface machining method with phase modulation beam deflection fused with chemical etching, and belongs to the field of ultrafast laser machining and micro-nano machining. The implementation method comprises the following steps: carrying out pixelated coordinate transformation on a metasurface pattern needing to be processed, calculating a coordinate path as a corresponding blazed grating phase, loading the blazed grating phase to a spatial light modulator (SLM) to carry out ultrafast laser high-precision deflection control, carrying out blazed grating phase regulation and control on femtosecond laser through different coordinate transformation, and carrying out high-precision deflection control on the femtosecond laser. Fine processing of the metasurface pattern path is realized; spatial-domain shaping of Bessel-like beams is carried out through the SLM to generate deflection beams which are long in focal depth and free of diffraction, and stability of the metasurface in long-time large-area patterning processing is achieved. The precision of a focusing light spot and the precision of a light beam deflection angle are improved through frequency domain shaping; frequency domain shaping and chemical auxiliary etching are combined, and efficient manufacturing of the phase change material metasurface is achieved. The method has the advantages that the technological process is easy to implement, the machining stability is good, and the surface quality is excellent.

Description

Technical Field

[0001] The present invention belongs to the fields of ultrafast laser processing and micro-nano processing, and relates to a method for combining spatial and frequency-domain collaborative shaping of ultrafast lasers with phase-controlled beam deflection, which is applicable to the precision manufacturing of optoelectronic devices and high-performance materials. Background Art

[0002] As a series of artificial structural units at the sub-wavelength scale, metasurfaces can flexibly manipulate electromagnetic waves and have important application values in photonic devices such as optical imaging and information storage. Most traditional metasurface processing and manufacturing methods adopt methods such as electron beam lithography, ion beam etching, and nanoimprinting. However, these manufacturing methods have problems such as high cost, complex processes, insufficient flexibility, and limited material selection, seriously restricting the research and application values of metasurface optics. In addition, once a traditional metasurface photonic device is manufactured, its unit structure and performance cannot be changed, and its function is fixed. However, more and more scenarios require the function of the device to be dynamically controlled.

[0003] To achieve the dynamic control function of metasurfaces, active materials are usually introduced in the design process of metasurfaces, and the characteristics of the active materials are controlled to achieve the dynamic control of the device function. Phase change material Ge 2 Sb 2 Te 5 is an active material that can undergo reversible phase changes between different phases. Phase changes are usually accompanied by drastic changes in the optical properties of materials. This unique tunable property makes phase change materials have important application values in the field of dynamically reconfigurable metasurfaces.

[0004] Due to its ultrashort pulse duration and ultra-high peak value, ultrafast lasers can precisely and rapidly perform local crystallization or amorphization on phase change materials, having unique advantages in manufacturing and modulation. However, the characteristic sizes of micro-nano resonant units in metasurfaces are usually at the sub-micron or even nano scale. Traditional laser direct writing technology is limited by the accuracy of mechanical components (such as translation stages and mechanical shutters), making it difficult to ensure manufacturing accuracy. Compared with the laser direct writing processing method, the beam deflection manufacturing method using a scanning system can improve the moving speed and moving accuracy of the laser focus. However, due to hardware system limitations, it is impossible to use a high numerical aperture objective lens or difficult to shape Gaussian light, and it is impossible to achieve a smaller focused spot. Through the spatial shaping of the beam by a spatial light modulator, various complex-shaped microstructures can be directly manufactured under single or multiple pulse exposures, significantly improving the processing efficiency and resolution of metasurfaces. However, the energy fluctuations caused by the noise between the shaped light fields and the difficult-to-precisely-control processing size limit the flexibility and consistency during the processing.

[0005] In addition, in the manufacturing of optoelectronic devices and the large-area processing of micro-nano structures, due to the short focal depth of Gaussian light under the processing of high numerical aperture objectives, it is difficult to maintain uniform and consistent processing effects within a large depth range. During long-time large-area processing, the small defocus amount caused by environmental factors or uneven sample surfaces will prevent the focus from stably focusing on the surface of the processed sample, seriously affecting the processing consistency and thus limiting the stability of large-area deep processing.

[0006] As a non-diffracting beam, Bessel beam has significant long focal depth characteristics and can maintain a stable focusing effect within a large depth range, making it very suitable for large-area and deep precision processing. Therefore, by using a spatial light modulator (SLM) to flexibly control the beam phase, loading a blazed grating to superimpose the Bessel phase, and combining with a high numerical aperture objective, the refined deflection control of small spots and high-stability processing can be achieved simultaneously. Combining frequency domain shaping, after doubling the frequency of the laser from 1030 nm to 515 nm, the focusing accuracy and deflection accuracy of the beam are significantly improved, making it more suitable for high-quality large-area metasurface structure processing. After laser processing, combined with chemical-assisted etching, high-quality metasurfaces can be finally fabricated. Summary of the Invention

[0007] In order to solve the problems of limited focal depth, insufficient accuracy, and complex process flow in metasurface processing, the purpose of the present invention is to provide a metasurface processing method that combines phase modulation beam deflection and chemical etching. By adopting the non-diffracting characteristics of Bessel-like beams, the focal depth can be significantly extended, and the processing stability and consistency within a large area can be improved. At the same time, by dynamically loading deflection modulation phases on the SLM, precise deflection and focusing control of the beam can be achieved.

[0008] The purpose of the present invention is achieved through the following technical solutions:

[0009] A metasurface processing method that combines phase modulation beam deflection and chemical etching disclosed by the present invention performs pixelated coordinate transformation on the metasurface pattern to be processed, calculates the coordinate path as the corresponding blazed grating phase, and loads the blazed grating phase onto a spatial light modulator SLM for high-precision deflection control of ultrafast lasers. By performing different coordinate transformations to control the blazed grating phase of femtosecond lasers, refined processing of the metasurface pattern path is achieved; through the SLM, spatial domain shaping of Bessel-like beams is performed to generate a long focal depth non-diffracting deflected beam to achieve the stability of the metasurface during long-time large-area patterning processing; the focusing spot accuracy and beam deflection angle accuracy are improved through frequency domain shaping; combining frequency domain shaping and chemical-assisted etching, efficient manufacturing of phase change material metasurfaces is realized.

[0010] A method for fabricating a metasurface by phase-modulating beam deflection and integrating chemical etching, disclosed by the present invention, comprises the following steps:

[0011] Step 1: Adjust the optical path, turn on a femtosecond laser with a central wavelength of 1030 nm, and convert the femtosecond laser into 515 nm through a frequency doubling crystal BBO to improve the spot focusing accuracy and beam deflection accuracy.

[0012] Step 2: The laser beam is introduced into a two-layer optical path through beam expansion, a filter, and several mirrors to ensure the stability of the beam quality.

[0013] Step 3: Convert the processing pattern into pixel coordinates, and set the pixel pitch according to the size of the processed pattern; calculate the deflection angle of the pixel beam corresponding to each coordinate based on the objective lens focal length and the pixel pitch, and generate the corresponding blazed grating deflection phase; set the external trigger mode of the laser, and the number of output pulses N each time it is triggered; load the blazed grating phase corresponding to each pixel onto the spatial light modulator SLM to perform deflection control of the femtosecond laser. Each time the laser is triggered, the SLM loads the deflection phase corresponding to the next pixel according to the processing path; perform phase modulation on the femtosecond laser by sequentially loading the blazed grating phases corresponding to each coordinate in the processing path to perform pathwise deflection of the metasurface pattern.

[0014] Step 4: Superimpose a conical lens phase on the deflection phase corresponding to each pixel in Step 3, and set the Bessel phase parameter r 0 , perform spatial shaping of the Gaussian beam to generate a Bessel-like beam with a depth of focus dozens of times that of the Gaussian beam, thereby improving the stability and consistency of processing.

[0015] Step 5: Transport the phase-regulated and spatially shaped deflection Bessel-like beam to the entrance pupil plane of an objective lens with a high numerical aperture through a 4f system, and form a Bessel-like beam with a long depth of focus after focusing by the objective lens; move the displacement stage to move the sample processing surface to the stable depth-of-focus region of the Bessel-like beam, and adjust the energy E so that the phase change material crystallizes in the central action region of the Bessel-like beam pulse, and crystallization cannot occur in the sidelobe region; after the above steps are completed, automatically load the deflection phase sequentially through software control to perform patterned deflection processing of the metasurface.

[0016] Step 6: After the laser processing is completed, combine with a chemical-assisted etching process, immerse the sample in a 15 wt% - 30 wt% tetramethylammonium hydroxide (TMAH) solution with an etching rate of 30 nm / h - 60 nm / h, and etch away the unprocessed area to leave the laser-processed patterned area, realizing high-precision and high-consistency large-area processing of the phase change material metasurface structure.

[0017] Further, in step six, the sample is immersed in a 15 wt% - 30 wt% tetramethylammonium hydroxide (TMAH) solution with an etching rate of 50 nm / h to etch away the unprocessed area and leave the laser-processed patterned area.

[0018] Further, in step four, the Bessel phase parameter r 0 has a value range of 5 - 100 μm.

[0019] Further, the phase change material is germanium antimony telluride alloy Ge 2 Sb 2 Te 5 .

[0020] A metasurface processing device for phase modulation beam deflection fusion chemical etching is used to implement the metasurface processing method of phase modulation beam deflection fusion chemical etching. A metasurface processing device for phase modulation beam deflection fusion chemical etching includes a femtosecond laser, a half-wave plate, a polarization beam splitter PBS, a beam expander, a frequency doubling crystal BBO, a filter, a spatial light modulator SLM, a plano-convex lens, a high numerical aperture objective lens, a sample, and a CCD camera.

[0021] The femtosecond laser generates laser pulses. After passing through the electric energy adjustment module composed of the first half-wave plate and the polarization beam splitter PBS, laser pulses with a specified energy are output. Then, after passing through the beam expander, the laser beam expander expands the diameter of the collimated input beam to a larger collimated output beam. After that, the laser enters the frequency domain shaping module composed of the frequency doubling crystal BBO and the filter. The frequency doubling crystal BBO converts the laser wavelength from 1030 nm to 515 nm, and the filter filters out the 1030 nm laser. Then, the 515 nm laser passes through the second half-wave plate and the first climbing optical path to reach the two-layer optical path. Among them, the laser is phase-modulated by the beam splitter and the spatial light modulator SLM, and then the phase on the SLM is transported to the objective pupil plane through the 4f system composed of the first lens, the second climbing optical path, and the second lens. The laser enters the objective lens and is focused on the sample surface for sample processing.

[0022] Further, the laser is a femtosecond laser with a central wavelength of 1030 nm, a pulse width of 290 ps, and a maximum repetition frequency of 200 kHz; the femtosecond laser is frequency-doubled to a wavelength of 515 nm by BBO, thereby improving the focusing accuracy and deflection accuracy of the beam; the phase map loaded on the SLM is a blazed grating superimposed with a conical lens phase, which is used to generate a Bessel-like beam for refined deflection control.

[0023] Beneficial effects:

[0024] 1. A method for fabricating a metasurface by integrating phase-modulated beam deflection and chemical etching disclosed by the present invention is a high-precision large-area fabrication method based on the collaborative shaping of the spatial and frequency domains of an ultrafast laser and chemical-assisted etching. By combining the spatial shaping, frequency-domain shaping of a Bessel beam, and phase modulation of beam deflection, and cooperating with chemical-assisted etching, it can efficiently achieve high-precision large-area fabrication of a phase-change material metasurface, effectively improving the processing consistency, processing precision, and processing efficiency.

[0025] 2. A method for fabricating a metasurface by integrating phase-modulated beam deflection and chemical etching disclosed by the present invention realizes flexible and precise deflection control of the beam through phase modulation, making the patterning process of the metasurface more flexible and precise. In addition, by superimposing the Bessel phase to shape the Gaussian light into a Bessel-like beam, the depth of focus of the beam is significantly extended, enhancing the stability within a large area and depth range, thereby achieving high-precision and high-stability patterning modification of the phase-change material metasurface.

[0026] 3. A method for fabricating a metasurface by integrating phase-modulated beam deflection and chemical etching disclosed by the present invention utilizes the non-diffracting property of the Bessel-like beam to keep the beam energy concentrated within a large depth range, ensuring the uniformity and high quality of large-area fabrication. At the same time, by frequency-domain shaping, the 1030 nm laser is converted into 515 nm, significantly improving the focusing precision and deflection control of the beam, and further enhancing the resolution and surface quality of the processing.

[0027] 4. A method for fabricating a metasurface by integrating phase-modulated beam deflection and chemical etching disclosed by the present invention, based on the above three beneficial effects, makes the metasurface processing device integrating phase-modulated beam deflection and chemical etching have the advantages of easy implementation of the structure and flexible operation, and is suitable for micro-nano processing, the precision manufacturing of high-performance optoelectronic devices and functional materials. Description of the Drawings

[0028] Figure 1 It is a schematic diagram of a metasurface processing device integrating phase-modulated beam deflection and chemical etching;

[0029] Figure 2 It is a schematic diagram of the high-precision deflection of an ultrafast laser phase-modulated Bessel-like beam;

[0030] Figure 3 It is a schematic diagram of a high-precision large-area phase-change material metasurface processing method by combining the deflection of an ultrafast laser Bessel-like beam and chemical-assisted etching;

[0031] Figure 4 It is a flowchart of a method for fabricating a metasurface by integrating phase-modulated beam deflection and chemical etching;

[0032] Wherein: 1 - femtosecond laser, 2 - first half-wave plate, 3 - polarization beam splitter (PBS), 4 - laser beam expander, 5 - second harmonic generation crystal (BBO), 6 - filter, 7 - second half-wave plate, 8 - first ultrafast mirror, 9 - first climbing optical path, 10 - first aperture, 11 - beam splitting, 12 - second aperture, 13 - second ultrafast mirror, 14 - spatial light modulator (SLM), 15 - third aperture, 16 - fourth aperture, 17 - first lens, 18 - second climbing optical path, 19 - second lens, 20 - descending optical path, 21 - CCD camera, 22 - third ultrafast mirror, 23 - fourth ultrafast mirror, 24 - NA0.8 objective lens, 25 - displacement stage, 26 - light source. Detailed implementation mode

[0033] To better illustrate the purpose and advantages of the present invention, the following further describes the content of the invention with reference to the drawings and examples.

[0034] Example 1:

[0035] This example discloses

[0036] In order to achieve high-precision large-area phase change material metasurface processing based on the deflection of ultrafast laser-like Bessel beams and chemical-assisted etching, a metasurface processing method combining phase modulation beam deflection and chemical etching disclosed in this example can perform high-precision processing of the metasurface of phase change materials in a large area by combining chemical-assisted etching technology. As Figure 1As shown in the figure, a metasurface processing device that combines phase modulation beam deflection and chemical etching disclosed in this embodiment includes a femtosecond laser 1, a first half-wave plate 2, a polarization beam splitter PBS 3, a laser beam expander 4, a frequency doubling crystal BBO 5, a filter 6, a second half-wave plate 7, a first ultrafast mirror 8, a first climbing optical path 9, a first aperture 10, a beam splitter 11, a second aperture 12, a second ultrafast mirror 13, a spatial light modulator SLM 14, a third aperture 15, a fourth aperture 16, a first lens 17, a second climbing optical path 18, a second lens 19, a descending optical path 20, a CCD camera 21, a third ultrafast mirror 22, a fourth ultrafast mirror 23, a NA0.8 objective lens 24, a displacement stage 25, and a light source 26. The femtosecond laser 1 generates laser pulses, and after passing through the electric energy adjustment module composed of the first half-wave plate 2 and PBS 3, laser pulses with a specified energy are output. Then, after passing through the beam expander 4, the laser beam expander can expand the diameter of the collimated input beam to a larger collimated output beam. After that, the laser enters the frequency domain shaping module composed of BBO 5 and the filter 6. BBO can change the laser wavelength from 1030 nm to 515 nm, and the filter can filter out the 1030 nm laser. Then, the 515 nm laser passes through the second half-wave plate 7 and the first climbing optical path 9 to reach the two-layer optical path. Among them, the laser is phase-modulated by the beam splitter 11 and SLM 14, and then the phase on the SLM is transported to the entrance pupil plane of the objective lens through the 4f system composed of the first lens 17, the second climbing optical path 18, and the second lens 19. Finally, the laser enters the objective lens 21 and is focused on the sample surface to complete the sample processing.

[0037] The parameters of the femtosecond laser used in the experiment are as follows: The laser has a central wavelength of 1030 nm, a pulse width of 290 ps, and a repetition frequency of 1 - 200 kHz. The sample is a 200-nanometer GST thin film on a single-crystalline silicon substrate.

[0038] As Figure 4 shown in the figure, a metasurface processing method that combines phase modulation beam deflection and chemical etching disclosed in this embodiment includes the following specific steps:

[0039] Step 1: After the optical path is installed, set the laser wavelength to 1030 nm, the energy to 1.5 nJ, control the laser external trigger mode, the number of output pulses of a single phase diagram is 2, turn on the femtosecond laser, and the laser passes through the half-wave plate, polarization beam splitter PBS, beam expander, BBO (frequency doubling crystal), filter, and several mirrors, beam splitting, spatial light modulator SLM, several lenses, several apertures, and a NA0.8 objective lens in sequence, and finally is focused on the displacement stage carrying the sample.

[0040] Step 2: Select a circular ring pattern with a radius of 1.5 μm in the processing pattern library for processing. Decompose the image into 30 pixels through pixelization, convert them into pixel coordinates, generate a pixel coordinate path through path planning, and then perform deflection phase conversion on each pixel point on the path.

[0041] Step 3: Realize the deflection of each pixel point on the path by loading a blazed grating phase on the spatial light modulator; load a cone lens phase on the spatial light modulator, and its parameter is r 0 = 50 to shape the Gaussian beam into a Bessel beam, thereby extending the focal depth and improving the stability and consistency of the beam.

[0042] Step 4: When the laser passes through the SLM, the deflection phase of the beam is loaded, and then phase transfer is carried out through a 4f lens system, so that the deflection phase acts on the entrance pupil surface of the objective lens. Under the action of the objective lens, the focal point shifts, realizing single-pixel processing of the pattern structure. Through the above steps, operations on all pixel points on the path can be completed, thereby completing the modification processing of the phase change material GST on the sample.

[0043] Step 5: Immerse the processed sample in a 25wt% tetramethylammonium hydroxide solution with an etching rate of 50 nm / h, then immerse it in absolute ethanol for 5 minutes, soak and rinse it in deionized water for 5 minutes, take out the sample and dry it to obtain the finally processed metasurface sample.

[0044] The above specific description further details the purpose, technical solution and beneficial effects of the invention. It should be understood that the above is only a specific embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A super-surface processing method combining phase modulated beam deflection and chemical etching, characterized in that: The metasurface pattern to be processed is transformed into pixel coordinates, the coordinate path is calculated as the corresponding blazed grating phase, and the blazed grating phase is loaded into the spatial light modulator SLM for high-precision deflection control of ultrafast lasers. The blazed grating phase of the femtosecond laser is regulated through different coordinate transformations to achieve refined processing of the metasurface pattern path; the Bessel-like beam is spatially shaped through the SLM to generate a long-focus, deep-diffraction, non-diffraction deflected beam, so as to achieve the stability of the metasurface in long-term, large-area patterning processing; The accuracy of the focused light spot and the accuracy of the beam deflection angle are improved through frequency domain shaping; frequency domain shaping and chemically assisted etching are combined to achieve efficient manufacturing of phase change material metasurfaces.

2. The method for processing a super surface by phase modulated beam deflection combined with chemical etching as claimed in claim 1, characterized in that: The following steps are included: Step 1: Adjust the optical path, turn on the femtosecond laser with a central wavelength of 1030nm, and convert the femtosecond laser to 515nm through the frequency doubling crystal BBO to improve the spot focusing accuracy and beam deflection accuracy; Step 2: The laser beam is introduced into the second-layer optical path through a beam expander, a filter and several reflectors to ensure the stability of the beam quality; Step 3: Convert the processed pattern into pixel coordinates and set the pixel spacing according to the size of the processed pattern; The deflection angle of the pixel light beam corresponding to each coordinate is calculated according to the focal length of the objective lens and the pixel spacing, and the corresponding blazed grating deflection phase is generated; Set the laser external trigger mode, and output the number of pulses N each time it is triggered; load the blazed grating phase corresponding to each pixel on the spatial light modulator SLM to perform deflection control of the femtosecond laser. Each time the laser is triggered, the SLM loads the deflection phase corresponding to the next pixel according to the processing path; phase modulate the femtosecond laser by sequentially loading the blazed grating phase corresponding to each coordinate in the processing path to perform path-based deflection of the metasurface pattern; Step 4: Superimpose the conic lens phase on the deflection phase corresponding to each pixel in step 3, and set the Bessel phase parameter r0 to perform spatial shaping of the Gaussian beam to generate a Bessel-like beam with a focal depth of dozens of times that of the Gaussian beam, thereby improving the stability and consistency of processing; Step 5: The phase-controlled and spatially shaped deflected Bessel-like beam is transported to the entrance pupil plane of the objective lens with a high numerical aperture through the 4f system, and is focused by the objective lens to form a Bessel-like beam with a long focal depth; The moving stage moves the sample processing surface to the focal depth region where the Bessel-like beam is stable, and adjusts the energy E so that the phase change material is crystallized in the central action region of the Bessel-like beam pulse, and the side lobe region cannot be crystallized; after the above steps are completed, the deflection phase is loaded in sequence to perform patterned deflection processing of the metasurface; Step 6: After the laser processing is completed, combined with the chemical-assisted etching process, the sample is immersed in a 15wt% to 30wt% tetramethylammonium hydroxide TMAH solution with an etching rate of 30nm / h to 60nm / h, and the unprocessed area is corroded to leave the laser-processed patterned area, thereby achieving high-precision and high-consistency large-area processing of the phase change material super surface structure.

3. The method for processing a super surface by phase modulated beam deflection combined with chemical etching as claimed in claim 2, characterized in that: In step six, the sample is immersed in a 15 wt % to 30 wt % tetramethylammonium hydroxide (TMAH) solution at an etching rate of 50 nm / h, and the unprocessed area is corroded to leave the laser processed patterned area.

4. The method for processing a super surface by phase modulated beam deflection combined with chemical etching as claimed in claim 2, characterized in that: In step 4, the value range of the Bessel phase parameter r0 is set to 5-100 μm.

5. The method for processing a super surface by phase modulated beam deflection combined with chemical etching as claimed in claim 2, characterized in that: The phase change material is germanium antimony tellurium alloy Ge2Sb2Te5.

6. An apparatus for implementing the method according to claim 1, 2, 3, 4 or 5, characterized in that: Including femtosecond laser, half-wave plate, polarization beam splitter PBS, beam expander, frequency doubling crystal BBO, filter, spatial light modulator SLM, plano-convex lens, high numerical aperture objective lens, sample, CCD camera; The femtosecond laser generates laser pulses, which output laser pulses of specified energy after passing through the electric energy regulation module composed of the first half-wave plate and the polarization beam splitter PBS, and then pass through the beam expander. The laser beam expander expands the diameter of the collimated input beam to a larger collimated output beam. The laser then enters the frequency domain shaping module composed of the frequency doubling crystal BBO and the filter. The frequency doubling crystal BBO converts the laser wavelength from 1030nm to 515nm. The filter filters out the 1030nm laser. The 515nm laser then passes through the second half-wave plate and the first climbing optical path to reach the second-layer optical path, where the laser is phase modulated by the beam splitter and the spatial light modulator SLM, and then the phase on the SLM is transported to the entrance pupil plane of the objective lens through the 4f system composed of the first lens, the second climbing optical path, and the second lens. The laser enters the objective lens and focuses on the sample surface for sample processing.

7. The device according to claim 6, characterized in that: The laser is a femtosecond laser with a central wavelength of 1030nm, a pulse width of 290ps, and a repetition frequency of up to 200kHz; the femtosecond laser is frequency-doubled to a wavelength of 515nm through BBO, thereby improving the focusing accuracy and deflection accuracy of the light beam; the phase map loaded on the SLM is a blazed grating superimposed on a conical lens phase, which is used to generate a Bessel-like beam with refined deflection control.

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