Optical fiber end face integrated dual-band achromatic metalens and manufacturing method therefor

Abstract

An optical fiber end face integrated dual-band achromatic metalens and a manufacturing method therefor. The optical fiber end face integrated dual-band achromatic metalens comprises an optical fiber (3) and a double-layer structure arranged on an end face of the optical fiber (3), wherein the double-layer structure comprises an upper focusing metasurface (1) and a lower polarization conversion metasurface (2), wherein the upper focusing metasurface (1) and the lower polarization conversion metasurface (2) are each formed by a sub-wavelength microstructure array; pump light and Stokes light emitted from the end face of the optical fiber (3) are incident to the lower polarization conversion metasurface (2) and are converted into dual-band circularly polarized light via the lower polarization conversion metasurface (2), the dual-band circularly polarized light is incident to the upper focusing metasurface (1) and is subjected to phase modulation via the upper focusing metasurface (1), and the upper focusing metasurface (1) achieves confocusing of the modulated dual-band circularly polarized light. The present application achieves efficient CARS signal excitation, achieving CARS signal detection at the deep part of a biological tissue by an integrated optical fiber end face.

Description

Fiber end-face integrated dual-band achromatic meta-lens and its manufacturing method

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. 202410290994.9, filed on March 14, 2024, entitled “A fiber end-face integrated dual-band achromatic meta-lens and its manufacturing method,” the entire text of which is hereby incorporated by reference. Technical Field

[0003] The present invention relates to the fields of resonance spectrum microspectroscopy detection and optical fiber integrated devices, and in particular to an optical fiber end-face integrated dual-band achromatic meta-lens for optical fiber CARS excitation and a manufacturing method thereof. Background Art

[0004] Coherent anti-Stokes Raman scattering (CARS) microscopy is a nonlinear microscopy technique that exploits the resonant energy levels of molecules. It is commonly used in imaging cells, tissues, and polymers. Its label-free, non-invasive imaging, high spatial resolution, and chemical specificity hold broad application prospects. During CARS microscopy, pump light and Stokes light beams of different wavelengths must be tightly spatially focused onto the same region to maximize CARS signal excitation. Conventional desktop CARS microscope objectives often utilize achromatic lens assemblies or doublets to achieve confocal alignment of the pump and Stokes beams. Limited by lens materials and processing techniques, these achromatic lens assemblies and doublets have centimeter-scale dimensions, making them impractical for integration onto optical fiber probes for deep-tissue CARS imaging. Furthermore, focusing across different wavelengths is accompanied by submillimeter focus drift, which reduces CARS excitation efficiency. In recent years, metalenses, composed of arrays of subwavelength microstructures, have shown promise in replacing traditional achromatic lenses with their micron-scale dimensions. Integration on optical fiber end faces has enabled CARS imaging deep within tissues, a crucial advancement for in vivo detection. Furthermore, by designing the microstructure arrangement, the focus drift of the metalens can be controlled across different wavelength bands, improving CARS excitation efficiency. Therefore, using metalenses to achieve fiber-optic CARS excitation is of great significance for practical applications.

[0005] Summary of the Invention

[0006] In response to the problems existing in the above-mentioned prior art achromatic lenses used for CARS excitation, the purpose of the present invention is to propose a fiber-end-integrated dual-band achromatic metalens and a manufacturing method thereof. The dual-band achromatic metalens is integrated on the fiber end face to control the focus drift between the two bands of pump light and Stokes light to achieve fiber-optic CARS signal excitation.

[0007] In order to achieve the above-mentioned object of the invention, the present invention proposes the following technical solutions:

[0008] A fiber end-face integrated dual-band achromatic meta-lens comprises an optical fiber and a double-layer structure arranged on the end face of the optical fiber, the double-layer structure comprising an upper focusing metasurface and a lower polarization conversion metasurface; wherein the upper focusing metasurface and the lower polarization conversion metasurface are both composed of a subwavelength microstructure array; the subwavelength microstructure array is composed of a plurality of microstructure units arranged in a periodic array on the same substrate, and the microstructure unit further comprises nanopillars and a substrate; pump light and Stokes light emitted from the optical fiber end face are incident on the lower polarization conversion metasurface, and are converted into dual-band circularly polarized light by the lower polarization conversion metasurface, the dual-band circularly polarized light further comprising polarized light circularly polarized pump light and circularly polarized Stokes light, the dual-band circularly polarized light is incident on the upper focusing metasurface, and is phase modulated by the upper focusing metasurface, and the modulated circularly polarized pump light and circularly polarized Stokes light achieve dual-band achromatic confocalization.

[0009] The subwavelength microstructure array on the upper focusing metasurface includes elliptical microstructure units, rectangular microstructure units and double rectangular microstructure units, wherein the nanocolumns in each microstructure unit are non-rotationally symmetric structures.

[0010] The angle θ between the long axis or long side direction of the elliptical, rectangular or double rectangular nanopillars located at each location of the subwavelength microstructure array of the upper focusing metasurface and the long axis or long side direction of the elliptical, rectangular or double rectangular nanopillar located at the center of the array satisfies the following formula:

[0011] Among them, φ(r,λ s ) represents the phase distribution function in the pump band, represents the additional phase delay of the nanopillar to the pump light and Stokes dual bands, and △φ(r) represents the difference between the phase distribution functions required by the upper focusing metasurface in the two bands.

[0012] The phase distribution function of the metalens is shown below:

[0013] Where r and f represent the radius and focal length of the upper focusing metasurface, and λ represents the wavelength of the selected pump light and Stokes light.

[0014] The subwavelength microstructure array on the lower polarization conversion metasurface includes elliptical microstructure units and rectangular microstructure units, wherein the nanocolumns in each microstructure unit are a double-symmetrical structure.

[0015] The elliptical or rectangular nanopillars located at various locations in the subwavelength microstructure array of the lower polarization conversion metasurface produce an additional phase difference of π / 2 when modulating the incident light along the major and minor axes or the long and wide sides of the polarization direction, and the transmission efficiency is greater than 50%. The length and width dimensions of the rectangular nanopillars are 75 to 750 μm, and the major and minor axis dimensions of the elliptical ones are 75 to 750 μm.

[0016] The size of the metalens is 10 to 50 μm in diameter, 10 to 20 μm in thickness, and has a focal length of 40 to 200 μm.

[0017] The pump light wavelength range is 1015-1070nm, the Stokes light wavelength is 740-1015nm, and the focus drift in the two bands is less than 15μm.

[0018] The angle θ between the long axis or long side direction of the elliptical or rectangular nanocolumns at each location of the subwavelength microstructure array of the lower polarization conversion metasurface and the linear polarization direction of the optical fiber output light at that location is π / 4.

[0019] A method for manufacturing a dual-band achromatic metalens integrated on an optical fiber end face, wherein the metalens processing method includes two-photon printing and micromachining lithography, and the upper focusing metasurface and the lower polarization conversion metasurface are processed separately; the upper focusing metasurface and the lower polarization conversion metasurface are bonded together with ultraviolet glue to form a dual-band achromatic metalens, and then the metalens is fixed to the end face of the optical fiber with ultraviolet glue.

[0020] Compared with the prior art, the present invention has the following characteristics and advantages:

[0021] 1) Can improve the CARS signal excitation efficiency;

[0022] 2) Compared with the traditional spatial block-structure CARS microscopic imaging achromatic lens, the structure size is greatly reduced;

[0023] 3) Compared with the fiber-optic tapered CARS excitation probe, it has higher mechanical strength and can penetrate deeper into tissues, thus enabling CARS signal detection deep within biological tissues. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG1 is a schematic diagram of the structure of a fiber end-face integrated dual-band achromatic meta-lens according to the present invention;

[0025] FIG2 is a schematic diagram of the three-dimensional structure of microstructure units of different cross-sectional shapes used in the present invention and the angle θ with the long axis direction of the array center (the linear polarization direction of the optical fiber output light), (2a) a schematic diagram of the three-dimensional structure of an elliptical microstructure unit, (2b) a schematic diagram of the three-dimensional structure of a rectangular microstructure unit, and (2c) a schematic diagram of the three-dimensional structure of a double rectangular microstructure unit;

[0026] FIG3 is a schematic diagram of a phase distribution function required for a fiber end-face integrated dual-band achromatic meta-lens to achieve a focusing function according to the present invention;

[0027] in:

[0028] 1. Upper focusing metasurface, 2. Lower polarization conversion metasurface, 3. Optical fiber, 4. Substrate, 5. Nanopillars, 6. Subwavelength microstructure array, 7. Pump light and Stokes light. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] Example 1:

[0031] As shown in FIG1 , the present invention discloses a fiber end-face integrated dual-band achromatic meta-lens. The meta-lens is composed of a double-layer metasurface, including an upper focusing metasurface 1 and a lower polarization conversion metasurface 2. The pump light and Stokes light emitted from the end face of the optical fiber 3 first pass through the lower polarization conversion metasurface 2, and their polarization states are converted into circularly polarized light. Then, the pump light and Stokes light emitted from the end face of the optical fiber 3 pass through the lower polarization conversion metasurface 2, and their polarization states are converted into circularly polarized light. Then, the circularly polarized pump light and the circularly polarized Stokes light are phase-modulated by the upper focusing metasurface 1. The modulated circularly polarized pump light and the circularly polarized Stokes light achieve dual-band achromatic confocalization, thereby improving the CARS signal excitation efficiency. Wherein, the upper focusing metasurface 1 and the lower polarization conversion metasurface 2 are both composed of subwavelength microstructure arrays. The subwavelength microstructure array is an array composed of a plurality of microstructure units composed of nano-pillars arranged in a periodic array on the same substrate 4. The nano-pillars of the subwavelength microstructure arrays of the upper focusing metasurface and the lower polarization conversion metasurface are arranged in a non-rotationally symmetric manner.

[0032] Specifically, the metalens of the present invention has dimensions of 10 to 50 μm in diameter, 10 to 20 μm in thickness, and a focal length of 40 to 200 μm. It supports pump light wavelengths ranging from 1015 to 1070 nm and Stokes light wavelengths from 740 to 1015 nm, with focal shifts of less than 15 μm in both wavelength bands.

[0033] As shown in FIG2 , schematic diagrams of the three-dimensional structures of microstructure units of different cross-sectional shapes used in the present invention are shown: (2a) a schematic diagram of the three-dimensional structure of an elliptical microstructure unit, (2b) a schematic diagram of the three-dimensional structure of a rectangular microstructure unit, and (2c) a schematic diagram of the three-dimensional structure of a double-rectangular microstructure unit. The substrate 4 and the nanopillars 5 constitute the microstructure unit.

[0034] Specifically, the substrate is made of transparent dielectric materials, including quartz glass. The nanorods are made of dielectric materials, including silicon, titanium dioxide, and silicon nitride. Nanorods with different geometric parameters have different additional phase delays for pump light and Stokes light bands. That is, different additional phases are applied to the pump light and Stokes light bands. And the difference between the two The major and minor axes of the elliptical nanopillars are 75 to 750 μm. The length and width of the rectangular nanopillars are 75 to 750 μm. The double rectangular nanopillars are 250 to 750 μm long and 75 to 300 μm wide, with the two rectangles 50 to 150 μm apart.

[0035] For the upper focusing metasurface 1:

[0036] The cross-sectional shapes of the nanocolumns include rectangular, elliptical, and double rectangular. Therefore, the subwavelength microstructure array constituting the upper focusing metasurface includes elliptical microstructure units, rectangular microstructure units, and double rectangular microstructure units. The nanocolumns constituting the subwavelength microstructure array of the upper focusing metasurface are non-rotationally symmetric structures. The angle θ between the long axis (or long side) direction of the elliptical (or rectangular, double rectangular) nanocolumns located at various locations of the subwavelength microstructure array of the upper focusing metasurface and the long axis (or long side) direction of the elliptical (or rectangular, double rectangular) nanocolumns located at the center of the array is equal to the phase distribution function φ(r,λ) of the pump band that the upper focusing metasurface needs to satisfy in order to achieve focusing. s ) is half of the value at this location; the additional phase delay of the nanopillars selected from various locations in the array for the pump light and the Stokes dual band The difference △φ(r) between the phase distribution functions required by the upper focusing metasurface in the two bands is equal, that is, the following two equations are satisfied:

[0037] Among them, φ(r,λ s ) represents the phase distribution function in the pump band, represents the additional phase delay of the nanopillar to the pump light and Stokes dual bands, and △φ(r) represents the difference between the phase distribution functions required by the upper focusing metasurface in the two bands.

[0038] The phase distribution function of the metalens is shown below:

[0039] Wherein, r and f represent the preset radius and focal length of the upper focusing metasurface 1, and λ represents the wavelength of the selected pump light and Stokes light.

[0040] For the lower polarization conversion metasurface 2:

[0041] The cross-sectional shapes of the nanocolumns include rectangular and elliptical. Therefore, the subwavelength microstructure array on the lower polarization conversion metasurface includes elliptical microstructure units and rectangular microstructure units, wherein the nanocolumns in each microstructure unit are, and the nanocolumns constituting the subwavelength microstructure array of the lower polarization conversion metasurface are of a dual symmetrical structure, that is, light will produce different light modulation phenomena in the directions of the two different symmetry axes of the nanocolumns. The elliptical (or rectangular) nanocolumns located at various locations in the subwavelength microstructure array of the lower polarization conversion metasurface modulate the incident light in the direction of the major and minor axes (or the long and wide sides) of the polarization direction, respectively, and the transmission efficiency is greater than 50%. The length and width dimensions of the rectangular nanocolumns are 75 to 750 μm, and the major and minor axis dimensions of the elliptical ones are 75 to 750 μm.

[0042] The angle between the long axis (or long side) of the elliptical (or rectangular) nanopillars at each location in the subwavelength microstructure array of the lower polarization conversion metasurface and the linear polarization direction of the light emitted from the optical fiber at that location is π / 4. In this case, each nanopillar is equivalent to a quarter-wave plate. When the angle between the long axis (or long side) of the elliptical (or rectangular) nanopillars at each location and the linear polarization direction of the light emitted from the optical fiber at that location is π / 4, the linearly polarized light at that location can be converted into circularly polarized light.

[0043] Circularly polarized pump light and Stokes light from the lower polarization conversion metasurface 2 pass through the upper focusing metasurface 1. When the light passes through a microstructured nanopillar, a wavelet source is generated at that location. For different wavelet sources generated by nanopillars with different long axis orientations, according to the principle of geometric phase modulation, the additional phase difference between the wavelet sources is half the angle between the long axes of the two corresponding nanopillars. When the angle between the long axis of a nanopillar at each location in the subwavelength microstructure array and the long axis of the nanopillar at the center of the array is equal to half the value of the phase distribution function required by the upper focusing metasurface to achieve focusing, the upper focusing metasurface can achieve focusing of circularly polarized light. The same metasurface requires different phase distribution functions for focusing light in different wavelength bands. When the dual-band additional phase delay of the nanopillars selected at each location in the array is equal to the difference between the metasurface's phase distribution functions for pump and Stokes light, the metasurface can simultaneously meet the phase distribution functions for both bands, achieving dual-band achromatic confocal focusing of pump and Stokes light, thereby improving the efficiency of CARS signal excitation.

[0044] Example 2:

[0045] The fabrication and processing methods for a fiber-end-integrated dual-band achromatic metalens of the present invention include two-photon printing and micromachining lithography. The upper focusing metasurface 1 and the lower polarization conversion metasurface 2 are fabricated separately. The upper focusing metasurface 1 and the lower polarization conversion metasurface 2 are bonded together with UV adhesive to form the dual-band achromatic metalens, which is then affixed to the end face of an optical fiber 3 using UV adhesive.

[0046] While the above content illustrates embodiments of the present invention, modifications may be made to the technical solutions described in the above embodiments, such as changes in the material and structural shape of the metalens, without departing from the scope of the invention as set forth in the appended claims. The scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A fiber end-face integrated dual-band achromatic meta-lens, characterized by: The invention comprises an optical fiber and a double-layer structure arranged on the end face of the optical fiber, wherein the double-layer structure comprises an upper focusing metasurface and a lower polarization conversion metasurface; wherein the upper focusing metasurface and the lower polarization conversion metasurface are both composed of a subwavelength microstructure array; the subwavelength microstructure array is composed of a plurality of microstructure units arranged in a periodic array on the same substrate, and the microstructure unit further comprises a nanocolumn and a substrate; the pump light and Stokes light emitted from the end face of the optical fiber are incident on the lower polarization conversion metasurface, and are converted into dual-band circularly polarized light by the lower polarization conversion metasurface, the dual-band circularly polarized light further comprises polarized light circularly polarized pump light and circularly polarized Stokes light, the dual-band circularly polarized light is incident on the upper focusing metasurface, and is phase modulated by the upper focusing metasurface, and the modulated circularly polarized pump light and circularly polarized Stokes light realize dual-band achromatic confocalization.

2. The optical fiber end-face integrated dual-band achromatic metalens according to claim 1, characterized in that: The subwavelength microstructure array on the upper focusing metasurface includes elliptical microstructure units, rectangular microstructure units and double rectangular microstructure units, wherein the nanocolumns in each microstructure unit are non-rotationally symmetric structures.

3. The optical fiber end-face integrated dual-band achromatic meta-lens according to claim 2, characterized in that: The angle θ between the long axis or long side direction of the elliptical, rectangular or double rectangular nanopillars located at each location of the subwavelength microstructure array of the upper focusing metasurface and the long axis or long side direction of the elliptical, rectangular or double rectangular nanopillar located at the center of the array satisfies the following formula: Among them, φ(r,λ s ) represents the phase distribution function in the pump band, represents the additional phase delay of the nanopillar to the pump light and Stokes dual bands, and △φ(r) represents the difference between the phase distribution functions required by the upper focusing metasurface in the two bands.

4. The optical fiber end-face integrated dual-band achromatic meta-lens according to claim 3, characterized in that: The phase distribution function of the metalens is shown below: Where r and f represent the radius and focal length of the upper focusing metasurface, and λ represents the wavelength of the selected pump light and Stokes light.

5. The optical fiber end-face integrated dual-band achromatic metalens according to claim 1, characterized in that: The subwavelength microstructure array on the lower polarization conversion metasurface includes elliptical microstructure units and rectangular microstructure units, wherein the nanocolumns in each microstructure unit are a double-symmetrical structure.

6. The optical fiber end-face integrated dual-band achromatic meta-lens according to claim 1, characterized in that: The elliptical or rectangular nanopillars located at various locations in the subwavelength microstructure array of the lower polarization conversion metasurface produce an additional phase difference of π / 2 when modulating the incident light along the major and minor axes or the long and wide sides of the polarization direction, and the transmission efficiency is greater than 50%. The length and width dimensions of the rectangular nanopillars are 75 to 750 μm, and the major and minor axis dimensions of the elliptical ones are 75 to 750 μm.

7. The optical fiber end-face integrated dual-band achromatic metalens according to claim 1, characterized in that: The size of the metalens is 10 to 50 μm in diameter, 10 to 20 μm in thickness, and has a focal length of 40 to 200 μm.

8. The optical fiber end-face integrated dual-band achromatic meta-lens according to claim 1, characterized in that: The pump light wavelength range is 1015-1070nm, the Stokes light wavelength is 740-1015nm, and the focus drift in the two bands is less than 15μm.

9. The optical fiber end-face integrated dual-band achromatic metalens according to claim 1, characterized in that: The angle θ between the long axis or long side direction of the elliptical or rectangular nanocolumns at each location of the subwavelength microstructure array of the lower polarization conversion metasurface and the linear polarization direction of the optical fiber output light at that location is π / 4.

10. The method for manufacturing a fiber end-face integrated dual-band achromatic metalens according to any one of claims 1 to 9, characterized in that: The meta-lens processing method includes two-photon printing and micro-machining lithography. The upper focusing meta-surface and the lower polarization conversion meta-surface are processed separately. The upper focusing meta-surface and the lower polarization conversion meta-surface are glued together with UV glue to form a dual-band achromatic meta-lens. The meta-lens is then fixed to the end face of the optical fiber with UV glue.

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