Design, preparation method and circular dichroism spectrum test system of optical metasurface

Abstract

The present invention discloses a design and preparation method of an optical metasurface and a circular dichroism spectrum test system, and relates to the field of spectral analysis of optically active substances. The circular dichroism spectrum test system in the present invention includes a broadband light source, an optical metasurface, an objective lens, a lens, and a spectrometer. The optical metasurface is used to realize beam splitting and focusing of the left-handed circularly polarized light components and the right-handed circularly polarized light components in the incident light. After passing through the sample to be tested, the two beams of light simultaneously enter the slit of the spectrometer, and the circular dichroism spectrum of the micro-area sample can be directly obtained after a simple calculation of the output signal. The present invention can simultaneously record the spectral signals corresponding to the left-handed polarized light and the right-handed polarized light of the light, and realize the rapid measurement of the circular dichroism spectrum of the sample.

Description

Technical Field

[0001] The present invention relates to the field of spectral analysis of optically active substances, and in particular to a design and preparation method of an optical metasurface and a circular dichroism spectral testing system. Background Art

[0002] When light passes through an optically active substance, the substance has different absorption rates for left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP). The difference between the two is called circular dichroism (CD). Circular dichroism spectroscopy (CD Spectroscopy) is a spectroscopy technique that is widely used to study various types of chiral molecules, especially large biomolecules. For example, the secondary structure of a protein is very sensitive to the environment, temperature or pH value. CD spectroscopy can be used to observe changes in the secondary structure of a protein as the environment changes or as it interacts with other molecules.

[0003] The traditional circular dichroism spectroscopy test system uses a photoelastic modulator to alternately generate left-handed circularly polarized light and right-handed circularly polarized light under the drive of an AC voltage of about 50kHz. The modulated left-handed circularly polarized light and right-handed circularly polarized light pass through the sample to be tested, are detected by the detector, and then input into the amplifier and phase-sensitive detector, and the measured signal is then calculated to obtain CD. Therefore, the equipment is often more complicated, and the test results are not real-time. It can be seen that the existing technology cannot simultaneously achieve the splitting and focusing of left-handed and right-handed circularly polarized light and the simultaneous incidence of the spectrometer, and the test system is more complicated and the detection efficiency is low. Summary of the invention

[0004] The main purpose of the present invention is to provide a design and preparation method of an optical metasurface and a circular dichroism spectroscopy testing system, which can solve the problems in the related art that it is impossible to simultaneously realize the splitting and focusing of left- and right-handed circularly polarized light and simultaneously incident on the spectrometer, and the testing system is relatively complex and the detection efficiency is low.

[0005] The optical metasurface of the present invention has the function of splitting and focusing left-handed and right-handed circularly polarized light, wherein the basic phase distribution required for the focusing function is: Where f represents the focal length of the optical metasurface, λ represents the central wavelength of the incident light when the optical metasurface works normally, x, y represent the coordinate positions of the coordinate system established with the geometric center point of the optical metasurface as the origin; the basic phase distribution required for beam splitting and deflection is where θ i and θ t They represent the incident angle and transmission angle of the incident light respectively. The y direction is consistent with the slit direction of the spectrometer. Δy represents the distance that the focus is deflected along the y direction. is a constant phase factor. When the incident light is incident perpendicular to the optical metasurface, Δy = f·sinθ t , Therefore, the optical metasurface needs to introduce phase distribution The incident light can be focused and deflected along the y direction at the same time. To achieve the splitting and focusing of left-handed and right-handed circularly polarized light, the transmission angle θ of the left-handed circularly polarized light is t,LCP and the transmission angle θ of right circularly polarized light t,RCP On the contrary, satisfying θ t,LCP =-θ t,RCP .

[0006] The optical metasurface of the present invention is composed of a transparent substrate within the working band and a nanoscale anisotropic subwavelength structure. When the incident light interacts with the anisotropic subwavelength structure, the circular polarization state of the incident light is reversed and a geometric phase factor e is generated. 2iσα , where σ = ±1 represents the circular polarization state of the incident light, -1 represents left-handed circular polarization, and +1 represents right-handed circular polarization; α represents the azimuth angle of the anisotropic subwavelength structure on the plane. Therefore, the directional angle distribution of the nanoscale anisotropic structure is in represents the phase distribution of the optical metasurface, and α(x,y) represents the azimuth angle of the anisotropic subwavelength structure. Then, by designing the azimuth angle of the anisotropic wavelength structure, the phase of the incident light can be continuously controlled from 0 to 2π, and the phase changes caused by the incident light in the left-handed and right-handed circularly polarized states have opposite signs.

[0007] In order to achieve the above object, one aspect of the present invention provides a method for designing a metasurface in a circular dichroism spectrum testing system, comprising the following steps:

[0008] Obtain the focal length f of the optical metasurface and the central wavelength λ of the incident light;

[0009] Determining the phase distributions required for the circular polarization focusing function and the circular polarization deflection function respectively, and obtaining the phase distribution required to be introduced into the optical metasurface;

[0010] Based on the phase distribution required to be introduced by the optical metasurface, the directional angle distribution of the structural unit working for left-handed circularly polarized light and the directional angle distribution of the structural unit working for right-handed circularly polarized light are obtained respectively by using the optical geometric phase principle;

[0011] The corresponding specific implementation structure is designed according to the directional angle distribution of each structural unit, and finally the optical metasurface is formed.

[0012] Furthermore, the phase distribution required for the circular polarization focusing function is:

[0013] Among them, x and y represent the coordinate position.

[0014] Furthermore, the phase distribution required for the circular polarization deflection function is:

[0015]

[0016] Among them, x, y represent the coordinate position, θ i and θ t denote the incident angle and the transmission angle, respectively. is a constant phase factor.

[0017] The metasurface designed through the above five steps can achieve beam splitting and focusing of the left-handed circularly polarized light and the right-handed circularly polarized light components in the incident light.

[0018] In an exemplary embodiment, the metasurface is designed by dividing the metasurface into two macro regions, wherein the two macro regions are respectively for left-handed circularly polarized light and right-handed circularly polarized light.

[0019] In an exemplary embodiment, the design method of the metasurface is implemented by space multiplexing, and each structural unit in the space includes structural units that work for left-handed circularly polarized light and right-handed circularly polarized light.

[0020] Another aspect of the present invention provides an optical metasurface, which is designed according to the above design method. The metasurface includes a transparent substrate and dielectric nanorods.

[0021] In an exemplary embodiment, the dielectric nanorods are made of silicon, titanium dioxide or silicon nitride.

[0022] Another aspect of the present invention provides a method for preparing the above optical metasurface, comprising the following steps:

[0023] preparing a dielectric layer on a transparent glass substrate;

[0024] Spin-coating an electron beam photolithography positive resist on the dielectric layer, then writing a designed pattern and developing it;

[0025] Using electron beam evaporation technology, a metal protective layer is prepared on the dielectric layer and the electron beam photolithography positive resist;

[0026] Removing the electron beam lithography positive photoresist and the metal protective layer on the electron beam lithography positive photoresist, and retaining the metal protective layer on the dielectric layer;

[0027] Using a plasma etching process to write the metasurface pattern in the dielectric layer;

[0028] The metal protection layer on the dielectric layer is removed.

[0029] In an exemplary embodiment, the circular dichroism spectrum test system adopts the metasurface designed by the above-mentioned design method. The circular dichroism spectrum test system includes a broadband light source, a first lens, the metasurface, an objective lens, a second lens and a spectrometer;

[0030] The incident light emitted by the broadband light source is passed through the first lens and the metasurface to realize circular polarization beam splitting and focusing. The left-handed circularly polarized light and the right-handed circularly polarized light after beam splitting and focusing are irradiated onto the sample to be tested. The objective lens and the second lens cooperate to collect the light beam after irradiating the sample to be tested. The light beam enters the slit of the spectrometer, and the spectrometer simultaneously records the absorption spectrum information A(λ) of the sample to be tested for the left-handed circularly polarized light and the right-handed circularly polarized light. LCP and A(λ) RCP , by programming the spectrometer to calculate CD = A (λ) LCP -A(λ) RCP The circular dichroism spectrum of the sample to be tested is measured in real time.

[0031] Compared with the prior art, in the circular dichroism spectrum test system of the present invention, the light emitted by the light source can achieve beam splitting and focusing of left-handed and right-handed circularly polarized light through a single optical metasurface, that is, left-handed circularly polarized light and right-handed circularly polarized light are generated simultaneously. These two beams of circularly polarized light can pass through the sample to be tested at the same time and be incident on the spectrometer at the same time, thereby achieving rapid real-time testing. This solves the problem of alternating generation of left-handed polarized light and right-handed polarized light in the prior art. In addition, due to the simplification of the photoelastic modulator, the circular dichroism spectrum test system using the metasurface will be more integrated than the traditional circular dichroism spectrum test system, and rapid real-time testing can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for describing the embodiments of the present invention are briefly introduced below.

[0033] FIG1 is a schematic diagram of a structure of a metasurface structure unit provided by an embodiment of the present invention; wherein: Figure 1a A three-dimensional schematic diagram of a metasurface structure unit provided by an embodiment of the present invention; Figure 1b A top view of a metasurface structure unit provided by an embodiment of the present invention;

[0034] Figure 2 A top view of a metasurface provided by an embodiment of the present invention;

[0035] Figure 3 A top view of another metasurface provided by an embodiment of the present invention;

[0036] FIG4 is a schematic diagram of a method for preparing a metasurface provided by an embodiment of the present invention; wherein: Figure 4a-4g Corresponding to steps 210-260;

[0037] Figure 5 A schematic diagram of a circular dichroism spectroscopy testing system based on a metasurface provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application, and cannot be interpreted as limiting the present invention.

[0039] like Figure 1a As shown in FIG. 1 , a three-dimensional schematic diagram of a metasurface structure unit of an embodiment is shown. Figure 1a As shown in FIG. 1 , the most basic structural unit in the metasurface includes an anisotropic subwavelength structure 1 and a transparent substrate 2. The length of the transparent substrate 2 is P x , width is P y The length of the anisotropic sub-wavelength structure 1 is L, the width is W, and the height is H.

[0040] like Figure 1b FIG. 1 is a top view of a metasurface structure unit of an embodiment. The direction angle of the anisotropic sub-wavelength structure of the metasurface on the plane is α.

[0041] In one embodiment, a method for designing a metasurface is provided to design a direction angle at which anisotropic sub-wavelength structures are distributed on a transparent substrate. The method for designing a metasurface includes the following five steps:

[0042] Step 110, determining design parameters f and λ of the optical metasurface, wherein f represents the focal length of the optical metasurface, and λ represents the central wavelength of the incident light when the optical metasurface works normally.

[0043] Step 120, using the design parameters f and λ to calculate the phase distribution required for the focusing function Where x and y represent the coordinate positions of a coordinate system established with the geometric center point of the optical metasurface as the origin.

[0044] Step 130, using the design parameter λ to calculate the phase distribution required for the deflection function where θ t Represents the transmission angle of the incident light.

[0045] Step 140, using the design parameters f and λ to calculate the phase distribution of the metasurface

[0046] Step 150, using the design parameters f and λ to calculate the directional angle distribution of the anisotropic sub-wavelength structure working for left-handed circularly polarized light and the directional angle distribution of anisotropic subwavelength structures working for right-handed circularly polarized light

[0047] like Figure 2 The figure shows a top view of another embodiment of the metasurface. The metasurface in this embodiment is divided into two macroscopic regions to design deflection and focusing functions for left-handed circularly polarized light and right-handed circularly polarized light respectively. Figure 2 In the embodiment shown, the optical metasurface is divided into two regions of equal area, the upper macro region for left-handed circularly polarized light, and the lower macro region for right-handed circularly polarized light. Since the deflection function of left-handed and right-handed circularly polarized light has been divided by macro regions, only the phase distribution required for the focusing function is considered during the design of the metasurface. The phase distribution required for the deflection function is not considered

[0048] Those skilled in the art will appreciate that the specific division method of the region is not limited to being divided into upper and lower parts, but may also be divided into left and right halves, or any other method of equally dividing the region into two areas. The purpose of the division is to separate the working areas for left-hand rotation and right-hand rotation in space, thereby deflecting the required basic phase distribution.

[0049] Specifically, the size of the metasurface is 2mm×2mm, the central wavelength of the incident light when the optical metasurface is working normally is λ=550nm, and the focal length of the optical metasurface is f=5mm. The material of the anisotropic subwavelength structure of the optical metasurface is silicon nitride nanorods, P x =P y =400nm, H=300nm, W=100nm, H=1400nm, and the substrate is transparent glass.

[0050] The metasurface is divided into two parts of equal area, the first area and the second area, which correspond to left-handed circularly polarized light and right-handed circularly polarized light respectively. The phase distribution required for focusing is: The rotation angle distribution of the anisotropic subwavelength structure in the first region is: The coordinate system (x1, y1) is a coordinate system established with the center position of the first region as the origin, the y1 direction is parallel to the slit direction of the spectrometer, and the x1 direction is perpendicular to the y1 direction; the rotation angle distribution of the anisotropic subwavelength structure in the second region is: The coordinate system (x2, y2) is a coordinate system established with the center position of the second region as the origin, the y2 direction is parallel to the slit direction of the spectrometer, and the x2 direction is perpendicular to the y2 direction.

[0051] like Figure 3 FIG. 1 is a top view of another metasurface of an embodiment. The metasurface in this embodiment adopts a spatial multiplexing method, and each structural cell contains structural units that work for both left-handed and right-handed circularly polarized light.

[0052] Specifically, the size of the metasurface is 2mm×2mm, the central wavelength of the incident light when the optical metasurface is working normally is λ=550nm, the focal length of the optical metasurface is f=10mm, and the lateral deflection Δy=0.5mm. The material of the anisotropic subwavelength structure of the optical metasurface is silicon nitride nanorods, P x =P y =400nm, H=300nm, W=100nm, H=1400nm, and the substrate is transparent glass.

[0053] The basic phase distribution required for the focusing function is: Where x and y represent the coordinate positions of the coordinate system established with the geometric center point of the optical metasurface as the origin; the basic phase distribution required for beam splitting and deflection is where θ i and θ t They represent the incident angle and transmission angle of the incident light respectively, Δy represents the deflection angle of the incident light along the y direction (where the y direction is consistent with the slit direction of the spectrometer), is a constant phase factor. When the incident light is perpendicular to the optical metasurface, Δy = f·sinθ t , Therefore, the optical metasurface needs to introduce phase distribution The functions of focusing the incident light and deflecting the light along the y direction can be achieved simultaneously.

[0054] Now, we design the phase distributions that satisfy the above for left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP). and To achieve the focusing and beam splitting function, the deflection amount for left-handed circularly polarized light and right-handed circularly polarized light is Δy LCP =-Δy RCP .

[0055] Using the spatial multiplexing method, a 2×2 cell is constructed, which contains structural units for both left-handed and right-handed circularly polarized light to achieve the phase distribution required for focusing deflection. The angular distribution of the structural unit for left-handed circularly polarized light is: The angular distribution of the structural unit for right-handed circularly polarized light is: The coordinate system (x, y) is established with the geometric center point of the optical metasurface as the origin, the y direction is parallel to the slit direction of the spectrometer, and the x direction is perpendicular to the y direction.

[0056] It can be seen from the above that the metasurface designed by the metasurface design method proposed in this application can realize the circular deflection, beam splitting and focusing functions of the incident light.

[0057] As shown in FIG. 4 , in a specific embodiment, the preparation process of the metasurface includes the following steps (in the figure, 51 is a transparent substrate, 52 is a medium, 53 is an electronic glue / photoresist, and 54 is an etch-resistant metal):

[0058] Step 210, preparing a dielectric layer ( Figure 4a ).

[0059] Preferably, the dielectric layer material is silicon nitride with a thickness of 1400 nm.

[0060] Preferably, the method for preparing the dielectric layer is chemical vapor deposition, magnetron sputtering, thermal evaporation, etc.

[0061] Step 220, spin-coating an electron beam lithography positive resist on the dielectric layer ( Figure 4b ), then write the designed pattern and develop it ( Figure 4c ).

[0062] Step 230, using electron beam evaporation technology, a metal protective layer is prepared on the dielectric layer and the electron beam photolithography positive resist ( Figure 4d ).

[0063] Preferably, the metal protective layer is made of chromium.

[0064] Step 240, removing the electron beam lithography positive photoresist and the metal protective layer on the electron beam lithography positive photoresist, and retaining the metal protective layer on the dielectric layer ( Figure 4e ).

[0065] Step 250, using a plasma etching process to write the metasurface pattern in the dielectric layer ( Figure 4f ).

[0066] Step 260, removing the metal protective layer on the dielectric layer ( Figure 4g ).

[0067] In some embodiments, the optical metasurface can not only adopt a single-layer transmission type, but also be designed as a double-layer reflective optical metasurface to achieve the same function, wherein the reflective metasurface can utilize a metal-medium-metal three-layer reflective structure. Moreover, by selecting different materials and designing and optimizing the metafunctional unit, the above technical solution can be extended to other bands. For dielectric metasurfaces, silicon, titanium dioxide, silicon nitride and other materials can be selected as the medium in the visible light band, and silicon can be selected in the infrared band; for metal-medium-metal three-layer reflective metasurfaces, gold (or silver, aluminum) and silicon dioxide can be selected in the visible near-infrared band, gold (or silver, aluminum) and CaF2, MgF2, Ge, polytetrafluoroethylene and other media can be selected in the infrared band, and gold (or silver, aluminum) and transparent ceramics and other media can be selected in the microwave band. Different bands can also have different choices for the preparation method, for example, electron beam lithography is mostly used in the visible light band, ultraviolet lithography technology can be selected in the infrared band, and printed circuit board technology can be used in the microwave band.

[0068] like Figure 5 , which is a schematic diagram of a circular dichroism spectroscopy testing system based on a metasurface according to an embodiment.

[0069] A circular dichroism spectrum testing system in this embodiment includes a broadband light source 601 , a first lens 602 , a metasurface 603 , an objective lens 604 , a second lens 605 and a spectrometer 606 .

[0070] Specifically, the broadband light source 601 emits a stable light beam with a wide frequency band. After the light beam passes through the first lens 602 and the metasurface 603, the left-handed circularly polarized light and the right-handed circularly polarized light are split and focused. The separated left-handed and right-handed circularly polarized light are simultaneously irradiated onto the sample to be detected 607. The objective lens 604 and the second lens 605 cooperate to collect the light beam after irradiating the sample to be detected 607. The light beam enters the slit of the spectrometer 606. The spectrometer 606 simultaneously records the absorption spectrum information A(λ) of the sample to be detected 607 to the left-handed circularly polarized light and the right-handed circularly polarized light. LCP and A(λ) RCP , after calculation CD = A (λ) LCP -A(λ) RCP The circular dichroism spectrum of the sample 607 to be detected is measured in real time.

[0071] The above description is only a partial implementation method of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A method for designing an optical metasurface, characterized in that: The following steps are involved: Obtain the focal length f of the optical metasurface and the central wavelength λ of the incident light; Determining the phase distributions required for the circular polarization focusing function and the circular polarization deflection function respectively, and obtaining the phase distribution required to be introduced into the optical metasurface; Based on the phase distribution required to be introduced by the optical metasurface, the directional angle distribution of the structural unit working for left-handed circularly polarized light and the directional angle distribution of the structural unit working for right-handed circularly polarized light are obtained respectively by using the optical geometric phase principle; Designing a corresponding specific implementation structure according to the directional angle distribution of each structural unit to finally form the optical metasurface; The phase distribution required for the circular polarization focusing function is determined using the following formula: and / or The phase distribution required for the circular polarization deflection function is determined using the following formula: Among them, x, y represent the coordinate position, θ i and θ t denote the incident angle and the transmission angle, respectively. is a constant phase factor; After obtaining the phase distribution required to be introduced into the optical metasurface, the optical metasurface is divided into a first region and a second region that work for left-handed circularly polarized light and right-handed circularly polarized light respectively, and the first region and the second region have the same area; The directional angle distribution of the structural unit working for left-handed circularly polarized light is obtained according to the following formula: The coordinate system (x1, y1) is a coordinate system established with the center position of the first area as the origin; The directional angle distribution of the structural unit working for right-handed circularly polarized light is obtained according to the following formula: The coordinate system (x2, y2) is a coordinate system established with the center position of the second area as the origin.

2. The method for designing an optical metasurface according to claim 1, wherein: After the phase distribution required to be introduced into the optical metasurface is obtained, the structural cells are constructed in a spatial multiplexing manner, so that each structural cell in the optical metasurface includes structural units that work for left-handed circularly polarized light and right-handed circularly polarized light at the same time.

3. The method for designing an optical metasurface according to claim 2, wherein: The directional angle distribution of the structural unit working for left-handed circularly polarized light is obtained according to the following formula: The directional angle distribution of the structural unit working for right-handed circularly polarized light is obtained according to the following formula: The coordinate system (x, y) is a coordinate system established with the geometric center point of the optical metasurface as the origin.

4. An optical metasurface obtained by the design method according to any one of claims 1 to 3, characterized in that: The optical metasurface includes a transparent substrate and dielectric nanorods.

5. The optical metasurface according to claim 4, wherein: The material of the dielectric nanorods is silicon, titanium dioxide or silicon nitride.

6. A method for preparing an optical metasurface as claimed in claim 4 or 5, characterized in that: The following steps are involved: preparing a dielectric layer on a transparent glass substrate; Spin-coating an electron beam photolithography positive resist on the dielectric layer, then writing a designed pattern and developing it; Using electron beam evaporation technology, a metal protective layer is prepared on the dielectric layer and the electron beam photolithography positive resist; Removing the electron beam lithography positive photoresist and the metal protective layer on the electron beam lithography positive photoresist, and retaining the metal protective layer on the dielectric layer; Using a plasma etching process to write the metasurface pattern in the dielectric layer; The metal protection layer on the dielectric layer is removed.

7. A circular dichroism spectrum testing system, characterized in that: It comprises a broadband light source, a first lens, the optical metasurface according to claim 4 or 5, an objective lens, a second lens and a spectrometer; The incident light emitted by the broadband light source is circularly polarized and focused after passing through the first lens and the optical metasurface, and the separated left-handed circularly polarized light and right-handed circularly polarized light are simultaneously irradiated onto the sample, and then collected by the objective lens and the second lens and enter the spectrometer.

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