Geometric phase metasurface high-throughput laser direct writing system and method

Abstract

The invention discloses a geometric phase metasurface high-flux laser direct writing system and method, and the system comprises a femtosecond laser, a beam expander, a angular dispersion compensation module, a digital micromirror array, a second 4F system, a spatial light modulator, a fifth lens and an objective lens according to the light advancing direction. Wherein the spatial light modulator is used for performing phase modulation on each unit light spot from the digital micromirror array based on a loaded holographic phase diagram to form a rod-shaped light spot array, and the rod-shaped light spot array forms parallel light transmitted at different angles through the fifth lens and converges at an entrance pupil of the objective lens together. And finally, a parallel direct-writing light spot array is formed in the two-photon photoresist, and geometric phase metasurface unit structure array printing is completed. According to the system, high-flux and high-flexibility direct-writing manufacturing of the geometric phase type metasurface is finally realized.

Description

Technical Field

[0001] This invention belongs to the field of laser direct writing technology, specifically relating to a high-throughput laser direct writing system and method for geometric phase metasurfaces. Background Technology

[0002] Metasurfaces are two-dimensional planar optical devices composed of subwavelength-scale artificial atoms arranged in a specific sequence. By precisely designing the shape, size, and arrangement of these nanostructures, flexible control over the amplitude, phase, polarization, and other properties of light waves can be achieved within subwavelength thickness. Compared to traditional optical elements based on propagation phase accumulation, metasurfaces offer significant advantages such as ultrathinness, ultralight weight, and ease of integration, opening up entirely new pathways for the miniaturization and integration of optical systems. In recent years, metasurfaces have demonstrated enormous application potential in many cutting-edge fields, including superlenses, holographic displays, structured light field generation, quantum optics, and optical stealth.

[0003] In this type of metasurface, anisotropic nanostructures (such as nanorods) are used as half-wave plates. When circularly polarized incident light interacts with it, the outgoing light carries an additional phase (φ = ±2θ) directly related to the azimuth angle of the nanostructure, where θ is the rotation angle of the nanostructure, and the "±" sign corresponds to the left and right rotation of the incident light. This mechanism, which achieves 0 to 2π phase coverage simply by rotating the nanostructure, provides extremely simple and efficient design freedom for wavefront manipulation.

[0004] While metasurfaces hold great promise in theory, a key bottleneck in their transition from laboratory to commercial application lies in the large-scale, high-throughput, and low-cost nanofabrication technology. A practical metasurface device typically comprises millions to billions of nanounits, each requiring precise geometry and orientation. This necessitates fabrication techniques that simultaneously possess high resolution and high efficiency.

[0005] Patent application CN117031891A discloses a laser direct writing system and method, including an excitation source, a suppression source, a micromirror array, and a displacement stage. The micromirror array includes at least two micromirrors arranged in an array, each having a first angle state and a second angle state. The excitation source illuminates the displacement stage through the micromirrors in the first angle state to form a writing pattern, and the suppression source illuminates the displacement stage through the micromirrors in the second angle state to form a suppression spot located at the boundary of the writing pattern. However, the laser direct writing system disclosed in this patent application has low efficiency in fabricating metasurfaces.

[0006] Traditional projection lithography is ill-suited for fabricating complex three-dimensional micro / nano structures. Each fabrication requires the design and fabrication of a specific mask, resulting in high costs and low efficiency. While electron / ion beam lithography offers high resolution, its relatively low writing speed limits its ability to efficiently fabricate large-area micro / nano structures. Laser direct writing offers advantages such as low cost, high writing flexibility, and minimal environmental requirements, but its single-point writing capability still hinders the efficient fabrication of large-area micro / nano structures. Therefore, developing a direct writing technology that balances high resolution, high flexibility, and high processing efficiency has become crucial for advancing metasurface technology towards practical applications. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a high-throughput laser direct writing system for geometric phase metasurfaces. This system uses an SLM to load a static holographic phase map to generate a rod-shaped light spot array, enabling independent and precise control of the rotation direction of the light spot for each geometric shape. It also uses a DMD as a high-speed dynamic optical switch to achieve independent and rapid switching control of each exposure point. Thus, it can quickly write complex geometric phase patterns without frequently refreshing the SLM, ultimately realizing high-throughput and highly flexible direct writing manufacturing of geometric phase metasurfaces.

[0008] This invention provides a geometric phase metasurface high-throughput laser direct writing system, which, according to the light propagation direction, includes a femtosecond laser, a beam expander, a angular dispersion compensation module, a digital micromirror array, a second 4F system, a spatial light modulator, a fifth lens, and an objective lens; The spatial light modulator is used to perform phase modulation on each unit spot from the digital micromirror array based on the loaded holographic phase map to form a rod-shaped spot array. The rod-shaped spot array forms parallel light transmitted at different angles through the fifth lens, which converges to the entrance pupil of the objective lens, and finally forms a parallel direct-write spot array on the two-photon photoresist to form a geometric phase metasurface.

[0009] Preferably, the spatial light modulator includes m×n liquid crystal array units, each of which is loaded with a holographic phase map and simultaneously focuses the incident m×n unit light spots, so that the parallel light is focused into an m×n bar-shaped focal array after being modulated by the spatial light modulator.

[0010] More preferably, each liquid crystal array unit contains L×L liquid crystal pixels, and each liquid crystal pixel can generate a phase in the range of 0-2π, and the corresponding holographic phase map is loaded through L×L liquid crystal pixels.

[0011] Preferably, the L value of each liquid crystal array unit is determined by the metasurface control unit period and the objective lens magnification required for actual writing.

[0012] Preferably, the digital micromirror array comprises m×n micromirror arrays, each of which is used to modulate the amplitude of the unit light spot incident on the corresponding liquid crystal array unit, thereby realizing the refresh of the direct-write light spot array.

[0013] More preferably, the micromirror array includes multiple micromirrors, and the intensity and uniformity of the unit light spot are controlled by switching the independent "on" and "off" states of the micromirrors, so as to obtain a unit light spot with uniform intensity.

[0014] Preferably, the character dispersion compensation module includes a blazed grating and a first 4F system according to the direction of light propagation; The blazed grating is used to compensate for the angular dispersion of the light beam emitted from the digital micromirror array; The first 4F system is used to image the light field of a blazed grating onto a digital micromirror array.

[0015] Preferably, the second 4F system includes a third convex lens and a fourth convex lens.

[0016] Preferably, the focal array generated by the spatial light modulator is transformed into parallel light transmitted at different angles after passing through the fifth convex lens, and the light spots overlap at the entrance pupil of the objective lens, and finally focus on the focal plane of the objective lens to form a parallel direct-write light spot array.

[0017] This invention also provides a high-throughput laser direct writing method for geometric phase metasurfaces, which uses the aforementioned high-throughput laser direct writing system to perform the high-throughput laser direct writing method for geometric phase metasurfaces, including: The femtosecond laser generates a femtosecond laser beam, which is then expanded by a beam expander and pre-compensated for angular dispersion by an angular dispersion compensation module before irradiating the digital micromirror array (DMD). The digital micromirror array (DMD) modulates the amplitude of the incident light spot, which is then imaged onto the spatial light modulator (SLM) by the second 4F system and modulated by a holographic phase map to generate a geometrically shaped light spot array. The geometrically shaped light spot array then passes through a fifth convex lens and an objective lens, and is finally imaged onto the focal plane of the objective lens on the displacement stage for high-throughput super-resolution geometric phase metasurface two-photon direct writing.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: Compared with traditional direct-write lithography serial dot-writing technology, this invention utilizes SLM to generate thousands of rod-shaped light spots at once, upgrading the nanorod array processing mode from "point-by-point scanning" to "parallel writing of the array," thereby improving processing efficiency by more than four orders of magnitude. At the same time, DMD is introduced for high-speed dynamic beam splitting and switching control, avoiding the time loss caused by frequently refreshing the SLM hologram when processing nanorods with different rotation orientations, laying a solid foundation for the large-area, practical fabrication of metasurfaces.

[0019] This invention directly generates a rod-shaped light spot array with "rotation specificity," which perfectly matches the physical formation mechanism of geometric phase nanorods. It achieves the most direct and efficient mapping from "design pattern" to "physical structure," solving the core contradiction in the field of metasurface preparation where "efficiency" and "flexibility" are difficult to achieve simultaneously. It provides a high-throughput direct writing solution that is high-speed, flexible, low-cost, and high-precision, which is of great significance for promoting metasurface technology from the laboratory to practical applications. Attached Figure Description

[0020] Figure 1 A schematic diagram of the principle of a high-throughput laser direct writing system for geometric phase metasurfaces provided in a specific embodiment of the present invention; Figure 2 This is a schematic diagram of the generation of a rod-shaped light spot array provided in a specific embodiment of the present invention. Detailed Implementation

[0021] To better understand the purpose, technical solution, and advantages of this application, the application is described and explained below in conjunction with the accompanying drawings and embodiments.

[0022] A specific embodiment of the present invention provides a two-photon high-throughput direct writing device based on controllable multi-dot grayscale writing, such as... Figure 1 As shown, the details are as follows: The geometric phase metasurface high-throughput laser direct writing system provided in the specific embodiment of the present invention comprises, in sequence according to the light propagation direction, a femtosecond laser 1, a beam expander 2, a blazed grating 3, a first convex lens 4, a second convex lens 5, a digital micromirror array (DMD) 6, a third lens 7, a fourth lens 8, a spatial light modulator (SLM) 9, a fifth lens 10, an objective lens 11, and a displacement stage 12.

[0023] In a specific embodiment of the present invention, a blazed grating 3, a first lens 4, and a second lens 5 constitute a angular astigmatism compensation module. The first lens 4 and the second lens 5 form a first 4F system, which is used to image the light field of the blazed grating 3 onto a digital micromirror array (DMD6) to compensate for the angular astigmatism generated by the DMD6 as a grating element itself. The focal lengths F1 and F2 of the first lens 4 and the second lens 5 satisfy F1 / F2=dθ2 / dθ1, where dθ1 and dθ2 are the angular astigmatisms of the blazed grating 3 and the DMD6, respectively. By adjusting the position and orientation of the blazed grating 3, the angular astigmatism of the light beam is suppressed after it exits the DMD6 through the 4F system.

[0024] The digital micromirror array (DMD6) provided in this specific embodiment modulates the amplitude of the incident light spot. The DMD6 divides the effective pixel area into m×n micromirror array units, each corresponding to one light spot. Each micromirror array unit includes k×k micromirrors. The DMD6 independently switches the "on" and "off" states of the k×k micromirrors within each micromirror array unit, thereby independently controlling the intensity and uniformity of each light spot. Because it can control the intensity and uniformity of each light spot, the photolithography accuracy of the photoresist is improved. The micromirror array provided in this specific embodiment performs high-speed amplitude modulation of the light spot, enabling rapid refresh and exposure time control of the direct-write rod-shaped light spot array.

[0025] The second 4F system provided in a specific embodiment of the present invention includes a third convex lens 7 and a fourth convex lens 8. This second 4F system is used to image the light field on the digital micromirror array (DMD) onto the spatial light modulator (SLM) plane, enabling each micromirror array unit of the DMD to independently and correspondingly control a set of liquid crystal array units on the SLM.

[0026] The spatial light modulator SLM9 provided in this specific embodiment of the invention comprises m×n liquid crystal array units for focusing m×n incident unit light spots. One unit light spot corresponds to one liquid crystal array unit, and a maximum of m×n bar-shaped focal arrays can be generated after the SLM9. Each liquid crystal array unit contains L×L liquid crystal pixels, and each liquid crystal pixel can generate a phase in the range of 0-2π, which together form a phase hologram. The selection of the L value provided in this embodiment is determined by parameters such as the metasurface control unit period and objective lens magnification required for actual writing.

[0027] The focal array generated by the SLM9 provided in the specific embodiment of the present invention is transformed into parallel light transmitted at different angles after passing through the fifth convex lens 11, and the light spots overlap at the entrance pupil surface of the objective lens 11, and finally focused on the focal plane of the objective lens 11 to form a parallel straight-write rod-shaped light spot array.

[0028] Conventional laser direct-write scanning, to ensure higher resolution, generally uses point scanning, with its scanning units being converging circular pixels. Geometric phase-type metasurface control units are typically elongated, and scanning point-by-point using conventional circular spots would waste a significant amount of time. Therefore, compared to the scanning processing using multiple point spots disclosed in existing technologies, the rod-shaped spot provided in this application can be processed in a single step, resulting in a significant improvement in efficiency.

[0029] The generation of rod-shaped light spot arrays, as follows: Figure 2 As shown. Each liquid crystal array unit of the SLM9 is loaded with a holographic phase map, corresponding to... Figure 2In a sub-region of the holographic phase image, the phase distribution of each holographic phase image is similar to that of a cylindrical lens, allowing parallel light to be focused into rod-shaped spots after SLM9 modulation. Rotating the entire hologram by a specific angle and re-sampling the phase yields rod-shaped spots at different angles. Each liquid crystal array unit generates a rod-shaped spot at a specific angle, forming a set of rod-shaped spot arrays with varying angles. Each liquid crystal array unit is corresponding to a micromirror array via a second 4F system. The micromirror array performs high-speed amplitude modulation on the spot, enabling rapid refresh and exposure time control of the rod-shaped spots, thus achieving high-throughput fabrication of the geometric phase metasurface.

[0030] Because the SLM9 has high phase accuracy in controlling the light field, loading a holographic phase map onto the SLM9 can achieve accurate control of the direction and period of the rod-shaped light spot. The DMD provided in the specific embodiment of the present invention has a high refresh rate and can perform fast switching. Through the synergistic effect of the SLM9 and the DMD, high-throughput and high-precision metasurface unit exposure can be achieved.

Claims

1. A high-throughput laser direct-writing system for geometric phase metasurfaces, characterized in that, According to the direction of light propagation, it includes a femtosecond laser, a beam expander, a spectral dispersion compensation module, a digital micromirror array, a second 4F system, a spatial light modulator, a fifth lens, and an objective lens; The spatial light modulator is used to perform phase modulation on each unit spot from the digital micromirror array based on the loaded holographic phase map to form a rod-shaped spot array. The rod-shaped spot array forms parallel light transmitted at different angles through the fifth lens, which converges to the entrance pupil of the objective lens, and finally forms a parallel direct-write spot array on the two-photon photoresist to form a geometric phase metasurface.

2. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 1, characterized in that, The spatial light modulator includes m×n liquid crystal array units, each of which is loaded with a holographic phase map and simultaneously focuses the incident m×n unit light spots, so that the parallel light is focused into an m×n bar-shaped focal array after being modulated by the spatial light modulator.

3. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 2, characterized in that, Each liquid crystal array unit contains L×L liquid crystal pixels, and each liquid crystal pixel can generate a phase in the range of 0-2π. The corresponding holographic phase map is loaded through L×L liquid crystal pixels.

4. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 3, characterized in that, The L value of each liquid crystal array cell is determined by the metasurface control cell period and objective lens magnification required for actual writing.

5. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 1, characterized in that, The digital micromirror array comprises m×n micromirror arrays, each of which is used to modulate the amplitude of the unit light spot incident on the corresponding liquid crystal array unit, thereby realizing the refresh of the direct-write light spot array.

6. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 5, characterized in that, The micromirror array contains multiple micromirrors. By switching the independent "on" and "off" states of the micromirrors, the intensity and uniformity of the unit light spot can be controlled to obtain a unit light spot with uniform intensity.

7. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 1, characterized in that, The character dispersion compensation module includes a blazing grating and a first 4F system according to the direction of light propagation; The blazed grating is used to compensate for the angular dispersion of the light beam emitted from the digital micromirror array; The first 4F system is used to image the light field of a blazed grating onto a digital micromirror array.

8. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 1, characterized in that, The second 4F system includes a third convex lens and a fourth convex lens.

9. The high-throughput laser direct writing system for geometric phase metasurfaces according to claim 1, characterized in that, The focal array generated by the spatial light modulator is transformed into parallel light transmitted at different angles after passing through the fifth convex lens, and the light spots overlap at the entrance pupil of the objective lens, and finally focus on the focal plane of the objective lens to form a parallel direct-write light spot array.

10. A high-throughput laser direct writing method for geometric phase metasurfaces, characterized in that, The method for high-throughput laser direct writing of geometric phase metasurfaces using the high-throughput laser direct writing system according to any one of claims 1-9 includes: The femtosecond laser generates a femtosecond laser beam, which is then expanded by a beam expander and pre-compensated for angular dispersion by an angular dispersion compensation module before irradiating the digital micromirror array (DMD). The digital micromirror array (DMD) modulates the amplitude of the incident light spot, which is then imaged onto the spatial light modulator (SLM) by the second 4F system and modulated by a holographic phase map to generate a geometrically shaped light spot array. The geometrically shaped light spot array then passes through a fifth convex lens and an objective lens, and is finally imaged onto the focal plane of the objective lens on the displacement stage for high-throughput super-resolution geometric phase metasurface two-photon direct writing.

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