CN113552072B - Optical sensor based on total reflection enhancement mechanism - Google Patents

Abstract

The invention discloses an optical sensor based on a total reflection enhancement mechanism, which comprises a substrate, a double-layer super-structure surface and a medium spacing layer, wherein the super-structure surface is a metal nano-structure body, the two metal nano-structure bodies are respectively positioned on the upper surfaces of two opposite planes of the medium spacing layer, the two metal nano-structure bodies can rotate relatively, and the medium spacing layer is arranged above the substrate; when the circularly polarized light is incident from the substrate direction, the incident angle is larger than the critical angle and is not 0 degrees and not 90 degrees, the incident angle is the included angle between the incident light direction and the substrate vertical direction, and the circularly polarized light is totally reflected on the interface between the double-layer super-structure surface and the air. The invention has simple structure and greatly reduces the processing difficulty; the change of the characteristics such as the refractive index, the concentration, the chiral characteristic and the like of the external environment can be judged by utilizing the change of the resonant frequency of the external chiral response and the circular dichroism amplitude, and the method can be expanded to the fields of online optical detection, biological sensing and the like.

Description

An Optical Sensor Based on Total Reflection Enhancement Mechanism

technical field

The invention belongs to the field of optical sensors, and relates to an optical sensor based on a total reflection enhancement mechanism, which is mainly used for sensing and measuring the refractive index, concentration and chiral characteristics of the external environment.

Background technique

A sensor is a detection device that can sense the measured information and convert the sensed information into electrical signals or other required forms of information output according to certain rules to meet the requirements of information transmission, processing, storage, display, recording and control requirements. Optical sensors measure based on optical principles. It has many advantages such as non-contact and non-destructive measurement, almost no interference, high-speed transmission, and telemetry and remote control.

Metamaterials are artificial materials composed of structural units whose characteristic size is much smaller than the working wavelength, and possess extraordinary physical properties that natural materials do not possess. Optical metamaterials are one of the important branches of metamaterials. They have freely designable electromagnetic properties and have great potential in the application of devices with special functions such as holography, super-diffraction limit resolution, and polarization conversion.

Chiral materials can control circularly polarized electromagnetic waves, but the cross-coupling efficiency between electric and magnetic fields in natural chiral materials is very weak, while chiral metamaterials can improve the cross-coupling efficiency and greatly improve the polarization of materials Control properties such as circular dichroism and optical rotation. Active optical chiral metamaterials can be applied to new optical sensors, modulators, and optical switches. Circular dichroism refers to the characteristic that metamaterials have different absorption properties for left-handed circularly polarized light and right-handed circularly polarized light, resulting in different transmittances. This characteristic can be used to determine the asymmetric structure of the molecule, etc.

Contents of the invention

In view of the above prior art, the technical problem to be solved by the present invention is to provide a novel optical sensor based on a total reflection enhancement mechanism with a simple structure and convenient manufacture.

In order to solve the above-mentioned technical problems, an optical sensor based on a total reflection enhancement mechanism of the present invention includes a substrate, a double-layer metasurface and a dielectric spacer layer. The metasurface is a metal nanostructure, and the two metal nanostructures are respectively located at On the upper surfaces of the two planes opposite to the dielectric spacer layer, the two metal nanostructures can be rotated relative to each other, and the dielectric spacer layer is arranged above the substrate; when circularly polarized light is incident from the direction of the substrate, the incident angle is greater than the critical angle and the incident angle is not 0° And not 90°, the incident angle is the angle between the incident light direction and the vertical direction of the substrate, and the circularly polarized light will be totally reflected at the interface between the double-layer metasurface and air.

The present invention also includes:

1. The coupling strength of the sensor can be changed by adjusting the rotation angle and separation distance between the two metal nanostructures.

2. The optical sensor uses the resonant frequency of the externally induced chiral response or the change of the circular dichroic amplitude to judge the change of the refractive index, concentration, and chiral characteristics.

3. The material of the dielectric spacer layer is silicon dioxide, magnesium fluoride or zinc oxide.

4. The base material is quartz glass or silicon nitride.

Beneficial effects of the present invention: the present invention is a novel optical sensor that utilizes a total reflection enhancement mechanism to sense and measure changes in the external environment. Due to the simple use of the evanescent wave generated by total reflection to sense the sensitivity of the air interface environment change, the resonant frequency of the externally induced chiral response and the change of the circular dichroic amplitude can determine the external environment refractive index, concentration , chiral characteristics and other characteristics of the changes. This new type of optical sensor based on the total reflection enhancement mechanism can easily adjust the incident angle and azimuth angle of the incident circularly polarized light to optimize the system performance and obtain the best externally induced chiral response enhancement effect. Because this new type of optical sensor based on the enhanced total reflection mechanism can change the system performance parameters through simple adjustments, it has a simple structure and is easy to manufacture. It can be extended to online optical detection, biosensing and other fields.

The invention has a simple structure and greatly reduces the difficulty of processing; the change of the resonant frequency of the externally induced chiral response and the amplitude of the circular dichroism can be used to judge the changes in the characteristics of the external environment such as refractive index, concentration, and chiral characteristics, and can Expand to online optical detection, biosensing and other fields. The interlayer coupling strength of the metal nanostructure can be adjusted simply by changing the thickness of the dielectric spacer layer.

Description of drawings

Figure 1(a) is the cell geometry of the novel optical sensor;

Figure 1(b) is the front view of the cell geometry of the novel optical sensor;

Figure 1(c) is a top view of the cell geometry of the novel optical sensor;

Fig. 2 is a schematic diagram of circularly polarized light incidence;

Figure 3 is the simulated reflection spectrum of the new optical sensor;

Figure 4 is the circular dichroism spectrum of the novel optical sensor;

Fig. 5 is a schematic diagram of covering the dielectric layer on the sensor;

Fig. 6 is a graph of simulation results of measuring the refractive index of different medium layers.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

The novel optical sensor based on the total reflection enhancement mechanism of the present invention can utilize its strong externally induced circular dichroism for sensing applications, and the novel optical sensor is composed of a substrate, a double-layer metasurface and a dielectric spacer layer. The double-layer metasurface is composed of two metal nanostructures distributed periodically. The two metal nanostructures are respectively located on the upper and lower sides of the dielectric spacer layer, and there is a rotation angle between them to form a rotating structure. When the circularly polarized light is incident from the substrate direction at an azimuth angle other than 0° and not 90°, and the incident angle is larger than the critical angle, the circularly polarized light will be totally reflected at the interface between the double-layer metasurface and air, and there will be no transmission signal. At this time, a very strong reflective circular dichroism is generated, and the change of the external environment can be detected by using the change of the amplitude of the circular dichroism.

The total reflection enhancement mechanism refers to the total reflection that occurs at the interface between the double-layer metasurface and the air, and the optical sensor has no transmission signal in the case of a large angle of incidence, thus forming a chiral response of the total reflection enhancement mechanism.

The double-layer metasurface is composed of two metal nanostructures, and there is a structural rotation angle between them. The coupling strength of the sensor can be changed by adjusting the rotation angle and spacing distance between the upper and lower two metal nanostructures.

The dielectric spacer layer mainly carries the metal nanostructure, and its thickness can adjust the interlayer coupling strength of the metal nanostructure. Its material can be silicon dioxide, magnesium fluoride, zinc oxide and other media.

The substrate is used to support the overall structure, and its material can be selected from materials such as quartz glass and silicon nitride.

The incident circularly polarized light is incident at a non-0° or non-90° azimuth angle, which means that the direction of the incident circularly polarized light cannot coincide with the direction of the two metal nanostructures. At this time, the system composed of the double-layer metasurface and the incident wave vector cannot coincides with its own mirror image, so the system exhibits a strong externally induced chiral response.

The exogenous chiral optical response depends on the incident angle and azimuth of the incident circularly polarized light. Optimizing system performance can obtain the best exogenous chiral response enhancement effect. Adjusting the incident angle and azimuth of the incident circularly polarized light and optimizing the system performance can obtain the best externally induced chiral response enhancement effect.

Sensitivity to environmental changes at the air interface will be sensed using evanescent waves generated by total reflection.

Changes in the resonant frequency and circular dichroism amplitude of the externally induced chiral response can be used to determine changes in the refractive index, concentration, and chiral characteristics of the external environment. The sensor can be extended to online optical detection, biosensing, etc. field.

Since this novel optical sensor is a subwavelength structure, the thickness of the structure is negligible in practical applications.

Combining Fig. 1(a), Fig. 1(b) and Fig. 1(c), Fig. 1(a) is the unit geometry structure of the sensor, Fig. 1(b) is the front view of the unit geometry structure, Fig. 1(c ) is the top view of the geometric structure of the unit. 1 is a rotating structure composed of two metal nanostructures, one of which is located at the top of the medium 2, and the other is located at the bottom of the medium 2, and gold is used as the material of the metal nanostructure. The length of the Au nanostructure is set to 240nm, the width is set to 50nm, and the thickness is set to 30nm, the rotation angle of the two nanostructures is 90°, and the distance between the two metal nanostructures is 150nm. 2 is a dielectric layer, using MgF ₂ as the dielectric layer, the length and width of the dielectric layer are set to 300nm, and the thickness is set to 180nm. The role of the dielectric layer is mainly to carry the metal nanostructure and fix the metal nanostructure. 3 is the substrate of the structure, using glass as the substrate, the length and width of the glass substrate are set to 300nm, the dielectric layer and the substrate form a cube, and the substrate is equivalent to the base of the structure. Make the incident light incident from the direction of the substrate, and the angle between the incident light direction and the vertical direction of the sensor is 70°, as shown in Figure 2, we can observe that the light is totally reflected, and the incident light is right-handed polarized light, and the reflected light for left-handed polarized light. Using CST for simulation, the simulated reflection spectrum is shown in Figure 3, where R ₊₊ represents the reflection coefficient of right-handed polarized waves, R _+- represents the circular cross-polarization reflection coefficient, and R- ₊ represents the circular cross-polarization reflection Coefficient, R _-- represents the left-handed circular polarization reflection coefficient. Observing Figure 3, it can be found that the reflection curves of left-handed polarized waves and right-handed polarized waves always coincide. There are two obvious resonance peaks in the circular cross-polarization reflection curve, and the two resonance peaks appear at 221THz and 341THz respectively. In order to study the circular dichroism of the material more intuitively, we obtained the circular dichroism spectrum of this material according to the calculation formula of circular dichroism ΔR=R _-+ -R _+- , as shown in Figure 4. Observing Figure 4, it can be found that there are two obvious extreme values of circular dichroism. At 221 THz, the circular dichroism reaches negative 0.5498. At 341 THz, the circular dichroism has a value of 0.4301. It shows that at these two frequencies, the polarization conversion effect of the structure is the best.

Looking at Figure 5, we add the analyzed medium layer 4 to the sensor unit structure, and by simulating medium layers with different refractive indices, we get the simulation results shown in Figure 6. From Fig. 6, we can see that the maximum value of the amplitude of the circular dichroism curve of the medium layer with different refractive index is different, and the frequency value point at the extreme value of the circular dichroism is also different. Therefore, we can measure the refractive index of the medium by detecting the change of resonant frequency and the change of circular dichroism amplitude.

Claims (5)

1. An optical sensor based on a total reflection enhancement mechanism, characterized in that: comprise a substrate, a double-layer metasurface and a medium spacer layer, the metasurface includes two metal nanostructures perpendicular to each other, the first metal nanostructure Located on the top of the medium spacer layer, the second metal nanostructure is located at the bottom of the medium spacer layer, and the medium spacer layer is arranged above the substrate; when circularly polarized light is incident from the direction of the substrate, the incident angle is greater than the critical angle and the incident angle is not 0° and not 90°, the incident angle is the angle between the direction of the incident light and the vertical direction of the substrate, the circularly polarized light will be totally reflected at the interface between the double-layer metasurface and the air; the height of the first metal nanostructure and the second metal nanostructure The separation distance is 150nm; the first metal nanostructure and the second metal nanostructure are elongated; the length is 240nm, the width is 50nm, and the thickness is 30nm; the length and width of the dielectric spacer layer are 300nm, and the thickness is 180 nm; the length and width of the substrate are 300 nm. 2 . An optical sensor based on a total reflection enhancement mechanism according to claim 1 , wherein the coupling strength of the sensor can be changed by adjusting the rotation angle and separation distance between two metal nanostructures. 3. An optical sensor based on a total reflection enhancement mechanism according to claim 1 or 2, characterized in that: the optical sensor uses the resonant frequency of the externally induced chiral response or the change of the circular dichroic amplitude to judge Changes in refractive index, concentration, chiral character. 4. An optical sensor based on a total reflection enhancement mechanism according to claim 1 or 2, wherein the material of the dielectric spacer layer is silicon dioxide, magnesium fluoride or zinc oxide. 5. An optical sensor based on a total reflection enhancement mechanism according to claim 1 or 2, characterized in that: the base material is quartz glass or silicon nitride.

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