Refractive index sensor based on polarization conversion of dielectric metasurface

Abstract

The present invention discloses a refractive index sensor based on polarization conversion of dielectric metasurface, the refractive index sensor is composed of an intermediate dielectric layer and two layers of identical nano-unit arrays, the nano-units of the dielectric metasurface are arranged periodically; each dielectric metasurface nano-unit is composed of dielectric blocks, which are 45° to the x-axis or y-axis, and the dielectric metasurface can generate Fano resonance for both normally incident linearly polarized light and circularly polarized light, and is extremely sensitive to the refractive index of the surrounding medium; the present invention avoids the ohmic loss caused by metal materials based on the high refractive index dielectric metasurface, has the advantage of low loss, and is conducive to CMOS integration and large-scale preparation. The refractive index sensor of polarization conversion of the dielectric metasurface provides a low-cost, easy-to-process, and integrable refractive index sensor, which has broad application prospects in the fields of detection, biomedicine, and optical device design.

Description

Technical Field

[0001] The present invention belongs to the field of optical sensing technology, and in particular relates to a refractive index sensor based on polarization conversion of a dielectric metasurface, which is mainly used in optical devices. Background Art

[0002] In many fields such as biosensing and environmental monitoring, the detection of various parameters such as temperature, refractive index, and stress is often involved, and refractive index sensing technology has received widespread attention from researchers. The sensitivity of a refractive index sensor can determine the performance of the sensor. The sensitivity of a refractive index sensor is defined as the resonant wavelength or frequency change per unit refractive index, that is, S = Δλ(Δf) / Δn. Most of the previous refractive index sensors were based on metals. Due to the ohmic loss of metal materials, the resonant peak bandwidth of metal-based refractive index sensors is relatively wide, and the quality factor is usually low. Metal-based refractive index sensors are limited by large volume, high cost, and difficulty in integration. Further research is needed to reduce loss and improve sensitivity of refractive index sensors. In recent years, with the emergence of metamaterials with artificially designed resonant units, a new design scheme has been added to refractive index sensors. Metasurfaces are two-dimensional forms of metamaterials. We can also achieve refractive index sensing by precisely designing the resonant units of metasurfaces. Compared with traditional refractive index sensors, metasurface sensors have the advantages of high sensitivity, small size, low cost, and easy integration.

[0003] Refractive index sensors based on metasurfaces usually produce resonance peaks due to electromagnetic resonance. When the ambient refractive index changes, the position of the resonance peak moves. The principles of refractive index sensors based on dielectric metasurfaces are mostly Mie resonance and Fano resonance. In recent years, high refractive index materials have been widely used by researchers to realize refractive index sensing because of their low loss, ability to achieve high quality factor and local resonance enhancement, and their unique characteristics of compatibility with CMOS processes and low preparation costs. The biggest feature of Fano resonance is its asymmetric sharp spectral lines. Therefore, when using high refractive index dielectric metasurfaces to realize refractive index sensing, extremely narrow line widths, extremely large modulation depths, and extremely large local field enhancements will be formed, which can further improve the sensitivity of the refractive index sensor.

[0004] In 2009, the University of Stuttgart in Germany proposed an I-shaped metal metasurface that can induce Fano resonance and realize refractive index sensing of near-infrared liquids (Nano Letters, 2010, 10 (4): 1103-1107). The group proposed a disk structure combining metal and dielectric in the following year, realizing perfect absorption metasurface biosensing (Nano Letters, 2010, 10 (7): 2342-2348). In 2011, Wuhan University proposed an elliptical ring-column metasurface combining silver and silicon dioxide, which can realize refractive index sensing of reflective spectra in the mid-infrared band (Optics Letters, 2011, 36 (17): 3326-3328). In 2016, Zhejiang University proposed a plasma disk structure absorber (Sci Rep, 2016, 6: 24063), which can also realize refractive index sensing. In 2018, Zabol University of Iran proposed a plasma resonant cavity to realize refractive index sensing (Applied Optics, 2018, 57(27): 7798-7804). The above refractive index sensing is based on the metasurface of metal materials. Due to the inherent loss and processing difficulty of metals, metal refractive index sensing has certain limitations. Dielectric materials are more promising for refractive index sensing. In 2017, China University of Metrology proposed a sensor that uses a resonant unit of a silicon block to realize dual parameters of refractive index and temperature. The design requires the resonant unit to simultaneously excite electric resonance and magnetic resonance (Optics Express, 2017, 25(13): 15241-15251). The simultaneous excitation of electric resonance and magnetic resonance by silicon blocks requires high size of the resonant unit, which is not conducive to processing, and the article only realizes sensing of linearly polarized light. Hunan University also proposed an open resonant ring structure in 2017, but only realized refractive index sensing of linearly polarized light (Plasmonics, 2017). In 2018, Nankai University in my country proposed an “all-dielectric metasurface refractive index sensor based on polarization-insensitive Fano resonance” with application number 201811611888.7, which includes three V-shaped nanostructures with 120° rotational symmetry. Since the three antenna arrangement has more stringent processing requirements, it is not suitable for large-scale production. In addition, a “double C” shaped silicon nanoantenna proposed by Nankai University in 2019 also achieved refractive index sensing in the near-infrared, but the design structure is relatively complex and inconvenient to prepare (Semiconductor Optoelectronics, 2019, 40(4)). Wuhan University of Technology proposed a 120° rotationally symmetrical three-silicon block refractive index sensor with patent number 202010545061.1, but its sensing sensitivity is relatively low.These latest research results show that although some progress has been made in the research of metasurface refractive index sensors, it is still a difficult problem to obtain a refractive index sensor that has polarization insensitivity, polarization conversion and refractive index sensing, and is easy to implement, low-cost and miniaturized. Summary of the invention

[0005] The object of the present invention is to provide a refractive index sensor based on polarization conversion of a dielectric metasurface.

[0006] The present invention is achieved in that:

[0007] The dielectric metasurface polarization conversion refractive index sensor is composed of a dielectric metasurface composed of an intermediate dielectric layer and two layers of identical nanounit arrays, and the dielectric metasurface nanounit arrays are arranged periodically; each dielectric metasurface nanounit is composed of dielectric blocks and is 45° to the x-axis or the y-axis. The metasurface proposed in the present invention can generate Fano resonance, is not sensitive to normally incident linearly polarized light and circularly polarized light, and is extremely sensitive to the refractive index of the surrounding medium.

[0008] The present invention may also include:

[0009] The refractive index sensor consists of a dielectric metasurface composed of an intermediate dielectric layer and two layers of identical nanounit arrays, wherein the dielectric metasurface nanounits are arranged periodically; each dielectric metasurface nanounit is composed of dielectric blocks and is 45 degrees to the x-axis or the y-axis. The metasurface can generate Fano resonance, is insensitive to normally incident linearly polarized light and circularly polarized light, and is extremely sensitive to the refractive index of the surrounding dielectric environment.

[0010] The medium is one of silicon, germanium, silicon carbide and gallium arsenide, and different media can achieve polarization conversion with different efficiencies.

[0011] The intermediate medium layer is one of the optical materials silicon, silicon dioxide, magnesium fluoride and calcium fluoride.

[0012] The incident light can be either linearly polarized light or circularly polarized light, and both can undergo polarization conversion. Y-polarized light incident on the above-mentioned dielectric metasurface is converted into x-polarized light, and left-handed circularly polarized light incident on the above-mentioned dielectric metasurface is converted into right-handed circularly polarized light.

[0013] The dielectric metasurface is composed of a dielectric array and an intermediate dielectric layer. The geometric shape of the dielectric array unit is one of a rectangle, a circle and a triangle, and the spatial distribution is a periodic structure. The dielectric metasurface ensures polarization conversion and high transmission.

[0014] The dielectric metasurface can operate in visible light, near-infrared light and microwave bands.

[0015] The described dielectric metasurface can be extended to optical fibers and silicon-based optical platforms to achieve on-chip integrated polarization conversion and sensing.

[0016] The dielectric metasurface has two layers, and when one layer of the dielectric metasurface is embedded in a substrate, high-efficiency polarization conversion and refractive index sensing can also be achieved.

[0017] The dielectric metasurface polarization conversion refractive index sensor can be integrated into communication optical devices using current micro-nano processing methods, which is conducive to large-scale production.

[0018] Compared with the prior art, the advantages of the present invention are:

[0019] 1. When a layer of the metasurface of the dielectric metasurface polarization conversion refractive index sensor is embedded in the substrate, it still has high polarization conversion and sensing characteristics, which is conducive to integration into the optical fiber end face. This patent provides a feasible polarization conversion refractive index sensing design solution for the communication field;

[0020] 2. Due to the flexible design of the dielectric metasurface nanounits, the working band of the refractive index sensor based on dielectric metasurface polarization conversion can be easily adjusted;

[0021] 3. The refractive index sensing performance of the refractive index sensor for dielectric metasurface polarization conversion is only affected by the dielectric metasurface and is not affected by other factors. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1a -b Schematic diagram of the structure of the refractive index sensor for polarization conversion of dielectric metasurface;

[0023] Figure 2a -b is a schematic diagram of polarization conversion of linear polarized light and circular polarized light on dielectric metasurface;

[0024] Figure 3a It is a schematic diagram of the dielectric metasurface nanounit structure;

[0025] Figure 3b This is a design rendering of a double-layer dielectric metasurface placed at the end of an optical fiber;

[0026] Figure 4a is the cross-polarization transmittance and sensing response characteristics of the dielectric metasurface under linear polarized light incidence;

[0027] Figure 4b is the cross-polarization transmittance and sensing response characteristics of the dielectric metasurface under circularly polarized light incidence;

[0028] Figure 5 It is a schematic diagram of feasible solutions for micro-nano processing of dielectric supersurfaces.

[0029] In the figure: 1 is the middle dielectric layer of the refractive index sensor of the dielectric metasurface polarization conversion, 2-1 is the first dielectric metasurface of the refractive index sensor of the dielectric metasurface polarization conversion, 2-2 is the second dielectric metasurface of the refractive index sensor of the dielectric metasurface polarization conversion DETAILED DESCRIPTION

[0030] The present invention is described in more detail below with reference to the accompanying drawings:

[0031] Embodiment 1:

[0032] In Example 1, a schematic diagram of a dielectric metasurface polarization conversion sensor is listed. Since the dielectric metasurface has the characteristic of polarization conversion, after the x-polarized light passes through the dielectric metasurface, the y-polarized light is output with the help of the polarization conversion characteristics of the dielectric block. Similarly, after the y-polarized light passes through the dielectric metasurface, the x-polarized light is output with the help of the polarization conversion characteristics of the dielectric block. Because the dielectric metasurface has the characteristic of polarization insensitivity, after the left-handed circularly polarized light passes through the dielectric metasurface, the right-handed polarized light is output with the help of the polarization conversion characteristics of the dielectric block. Similarly, after the right-handed circularly polarized light passes through the dielectric metasurface, the left-handed polarized light is output with the help of the polarization conversion characteristics of the dielectric block. Flexible design of the nano-units of the dielectric metasurface can achieve optical band and high-efficiency polarization conversion. Figure 1a -b provides a basic structural schematic diagram and front view of the dielectric metasurface. Figure 2a is a schematic diagram of the polarization conversion of y-polarized light into x-polarized light after passing through the dielectric metasurface, and Figure 2b Schematic diagram of the polarization conversion of left-handed circularly polarized light into right-polarized light after passing through a dielectric metasurface.

[0033] Embodiment 2:

[0034] In Example 2, several basic unit structures of dielectric metasurfaces are provided, such as Figure 3a Here, the dielectric metasurface resonance unit structure is a tetrahedron, a cylinder, a torus cylinder, a triangular prism, a sphere, and a truncated cone. In practical applications, the structure of the dielectric metasurface resonance unit is not limited to the above structures, and the structure can be changed and optimized according to specific requirements. Figure 3b The figure shows a schematic diagram of the application scenario of dielectric metasurface in optical fiber system. Here, a layer of metasurface is embedded in the substrate, and another layer of dielectric metasurface structure is transferred to the end face of the optical fiber.

[0035] Embodiment 3:

[0036] In Example 3, in the optical band of 180-185 THz, the dielectric metasurface refractive index sensor is composed of two layers of silicon blocks, which are 45° to the x-axis or y-axis, with silicon dioxide as the intermediate dielectric layer. Among them, the relative dielectric constant of silicon is 11.9, and the relative dielectric constant of silicon dioxide is 2.31. The dielectric metasurface resonant unit period p is 800nm, the thickness h is 380nm, the resonant unit length a is 650nm, the resonant unit length b is 250nm, and the thickness t of the intermediate dielectric layer is 200nm. Figure 4a The left figure shows the cross-polarization transmittance of y-polarized light after entering the dielectric metasurface when the ambient refractive index changes from 1-1.1. Figure 4a The right figure shows the sensitivity of y-polarized light at different refractive indices. Calculated by S = Δλ(Δf) / Δn, the sensitivity of linearly polarized light is 108nm / RIU (12THz / RIU). Figure 4b The left figure shows the cross-polarization transmittance of left-handed circularly polarized light after it enters the dielectric metasurface when the ambient refractive index changes from 1-1.1. Figure 4b The right figure shows the sensitivity of left-handed circularly polarized light at different refractive indices. Calculated by S = Δλ (Δf) / Δn, the sensitivity of circularly polarized light is 101nm / RIU (11.2THz / RIU). Figure 4a and Figure 4b It can be obtained that the above dielectric metasurface has polarization insensitivity and can ensure refractive index sensing. By designing the shape and size of the dielectric metasurface resonant unit, the dielectric metasurface refractive index sensor can be extended to other bands.

[0037] Embodiment 4:

[0038] In Example 4, a process flow chart for preparing a nano-medium metasurface is provided, which mainly includes the following steps: substrate cleaning, film coating, spin coating of electron beam glue (or photoresist), development and fixing, photolithography (electron beam exposure and ultraviolet lithography), etching, and degumming.

[0039] The structure of the metasurface in the embodiment is only a schematic diagram and does not limit the size, shape and direction of the resonance unit.

[0040] The present invention discloses a refractive index sensor based on polarization conversion of a dielectric metasurface, wherein the refractive index sensor is composed of an intermediate dielectric layer and two layers of identical nanounit arrays, and the nanounits of the dielectric metasurface are arranged periodically; each dielectric metasurface nanounit is composed of dielectric blocks, which are 45° to the x-axis or y-axis. The metasurface proposed by the present invention can generate Fano resonance for both normally incident linearly polarized light and circularly polarized light, and is extremely sensitive to the refractive index of the surrounding medium. This design can be extended to other bands, and refractive index sensing of different working bands can be achieved using high-refractive-index, low-loss media. The dielectric metasurface based on a high-refractive-index avoids the ohmic loss caused by metal materials, has the advantage of low loss, and is conducive to CMOS integration and large-scale preparation. The refractive index sensor of polarization conversion of the dielectric metasurface provides a low-cost, easy-to-process, and integrable refractive index sensor, which has broad application prospects in the fields of detection, biomedicine, and optical device design.

Claims

1. A refractive index sensor based on polarization conversion of dielectric metasurface, characterized in that: The refractive index sensor is composed of a dielectric metasurface composed of an intermediate dielectric layer and two layers of identical nano-unit arrays, wherein the dielectric metasurface nano-units are arranged periodically; each dielectric metasurface nano-unit is composed of dielectric blocks. x Axis and y The axes are all 45°, and the dielectric metasurface can produce Fano resonance. It is insensitive to normally incident linearly polarized light and circularly polarized light, but extremely sensitive to the refractive index of the surrounding dielectric environment, thus realizing polarization conversion and refractive index sensing.

2. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1, characterized in that: The medium is one of silicon, germanium, silicon carbide and gallium arsenide, and different media can achieve polarization conversion with different efficiencies.

3. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1, characterized in that: The intermediate medium layer is one of the optical materials silicon, silicon dioxide, magnesium fluoride and calcium fluoride.

4. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1, characterized in that: Polarization conversion can occur when the incident light is linearly polarized or circularly polarized. y Polarized light incident on the above medium metasurface is converted into x Polarized light, left-handed circularly polarized light incident on the above-mentioned medium metasurface is converted into right-handed circularly polarized light.

5. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1 is characterized in that: The dielectric metasurface is composed of a dielectric array and an intermediate dielectric layer. The geometric shape of the dielectric array unit is one of a rectangle, a circle and a triangle, and the spatial distribution is a periodic structure. The dielectric metasurface ensures polarization conversion and high transmission.

6. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1 is characterized in that: The dielectric metasurface operates in the visible light, near-infrared light and microwave bands.

7. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1 is characterized in that: The described dielectric metasurface is extended to optical fibers and silicon-based optical platforms to achieve on-chip integrated polarization conversion and sensing.

8. The refractive index sensor based on polarization conversion of dielectric metasurface according to claim 1 is characterized in that: The dielectric metasurface has two layers, and when one layer of the dielectric metasurface is embedded in a substrate, high-efficiency polarization conversion and refractive index sensing can be achieved.

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