A second harmonic spatial sensor based on liquid crystal tunable metastructure

Abstract

A second harmonic spatial sensor based on a liquid crystal tunable metastructure enables sensing of the refractive index of media in space. It comprises two periodically poled nonlinear layers on either side and a detection layer in the middle. The first nonlinear layer consists of two layers of lithium niobate sandwiched between an E7 liquid crystal layer. The periodically poled lithium niobate generates second harmonics, and the temperature tunability of the liquid crystal enables temperature-dependent tuning of the measurement range. The second nonlinear layer, consisting of two layers of silicon dioxide sandwiched between the target material layer, acts as a defect mode resonance, creating a localized electromagnetic field and generating a strong transmission-type resonant peak. The nonlinear frequency doubling effect generated in this invention generates second harmonics, and the temperature tunability of the liquid crystal enables sensing of the refractive index of media in different spatial ranges at 20°C and 50°C. The sensing ranges are 1.1 to 1.6 refractive index units (RIU) and 1.5 to 1.8 RIU, respectively. The sensor offers numerous advantages, including non-contact operation, high sensitivity, and an adjustable measurement range.

Description

Technical Field

[0001] The present invention belongs to the technical field of nonlinear second harmonic sensing, and in particular relates to a second harmonic spatial sensor based on a liquid crystal tunable metamer. Background Art

[0002] The nonlinear second harmonic perception effect, resulting from the interaction between electromagnetic waves and nonlinear materials, is a physical phenomenon that exhibits unique properties in specific nonlinear optical systems. When an intense laser beam irradiates a nonlinear optical material with a specific structure and properties, the polarization mechanism no longer follows the simple laws of linear optics due to the nonlinear response characteristics of the atoms or molecules within the material. The electromagnetic wave, originally at the fundamental frequency, undergoes frequency conversion during its interaction with the nonlinear material due to the nonlinear polarization of the material, generating a second harmonic with a frequency twice that of the fundamental frequency. Second harmonic generation holds great potential and promise in fields such as optical detection and biosensing.

[0003] Nonlinear second harmonic generation has extensive and critical practical applications in many fields, and its application in the field of sensors has opened up new solutions for high-precision and high-sensitivity detection. The phenomenon of nonlinear second harmonic generation can be cleverly applied to the detection of refractive index and defects, and is particularly outstanding in the structural integrity assessment and quality control of monitoring devices. In specific applications, by deeply analyzing the transmission characteristics of materials under electromagnetic fields, sensors can accurately identify materials with different refractive indices, and can effectively distinguish between various sensor elements and biological cells. This characteristic makes nonlinear second harmonic generation occupy an extremely important position in the field of high-precision sensor technology, providing indispensable technical support and guarantee for many fields such as biomedical testing, materials science research, and quality monitoring in industrial production processes.

[0004] A search revealed that Chinese patent publication number CN112432925B, published on March 15, 2024, discloses a D-type photonic crystal fiber refractive index sensor device and method based on SPR. This device achieves a resonance peak through the surface plasmon effect and enables the sensing of the refractive index of liquid analytes. This patent improves the dynamic tunability of the sensing process, offering high sensitivity, multiple ranges, a compact structure, and label-free analysis capabilities.

[0005] This shows that current optical sensors mainly use resonance effects such as surface plasmons to excite resonance peaks for detection, but their fixed structure lacks adjustability. Tunable, multi-range measurements are of great significance to the development of sensors. Summary of the Invention

[0006] To address these issues, the present invention proposes a second-harmonic spatial sensor based on a liquid crystal tunable metastructure. This metastructure is designed based on second-harmonic generation (SHG), utilizing defect mode resonance to achieve a strong transmission peak. Furthermore, leveraging the temperature tunability of liquid crystal, the metastructure can detect refractive index at different temperatures using both the fundamental and second-harmonic transmission peaks. Furthermore, the peaks excited by this SHG exhibit excellent sensitivity and precision, making it suitable for sensor research.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] A second harmonic spatial sensor based on a liquid crystal tunable metamer, characterized in that the overall dielectric arrangement order is (lithium niobate 1-E7 liquid crystal-lithium niobate 2) 15 -(Silicon dioxide-Test layer-Silicon dioxide)-(Lithium niobate 1-E7 liquid crystal-Lithium niobate 2) 15 The two sides of the superstructure are defined as structure 1, which is (lithium niobate 1-E7 liquid crystal-lithium niobate 2) 15 , which is composed of a periodically polarized nonlinear lithium niobate layer sandwiched between an E7 liquid crystal layer, wherein lithium niobate 1 and lithium niobate 2 represent different polarization directions. The middle of the superstructure is defined as structure 2, which is (silicon dioxide-test layer-silicon dioxide), and is composed of an E7 liquid crystal layer embedded in the middle of two silicon dioxide layers. Light is incident vertically on structure 1 along the direction of the arrow. In structure 1, lithium niobate 1 and lithium niobate 2 belong to the same medium but have different polarization directions. The dotted arrows represent opposite polarization directions.

[0009] As a further improvement of the present invention, the liquid crystal metamer can form a strong fundamental wave transmission peak in the fundamental wave frequency range of 130.5 to 134.1 terahertz, and a strong second harmonic transmission peak in the second harmonic frequency range of 266.8 to 271.5 terahertz. The refractive index of lithium niobate in the structure is described by the dispersion relation, and its refractive index is 2.1, and the refractive index of silicon dioxide is 1.45.

[0010] When an electromagnetic wave enters this device from air, the nonlinear effect of lithium niobate generates a second harmonic, which, combined with the local field enhancement of the defect mode, forms a sharp transmission peak in the spectrum. The incident electromagnetic wave enters the structure perpendicularly, parallel to the normal. A strong fundamental wave transmission peak is formed in the fundamental frequency range of 130.5 to 134.1 terahertz, and a strong second harmonic transmission peak is formed in the second harmonic frequency range of 266.8 to 271.5 terahertz. The refractive index of the lithium niobate in the structure, described by the dispersion relation, is approximately 2.1, while the refractive index of silicon dioxide is approximately 1.45.

[0011] Based on second harmonic generation and defect mode resonance, a strong transmission peak can be formed at a specific frequency point, which has good sensing sensitivity and can be used for refractive index detection.

[0012] As a further improvement of the present invention, the liquid crystal in the liquid crystal metamer is of E7 type, and for electromagnetic waves incident with TM polarization, its refractive index is extraordinary, and its refractive index value in the frequency band is about 1.64.

[0013] As a further improvement of the present invention, the thicknesses of lithium niobate 1 and lithium niobate 2 on both sides are 0.3 microns and 0.4 microns respectively, the thickness of the E7 liquid crystal layer is 1.8 microns, and the thicknesses of the silicon dioxide layer and the layer to be measured in the middle periodic structure are 0.7 microns and 1.6 microns respectively.

[0014] Compared with the prior art, the present invention has the following technical effects:

[0015] The present invention uses second harmonic generation and local defect mode resonance to form a sharp transmission peak, replacing the traditional principle of using plasmon effects to form resonance peaks, achieving the effect of only one transmission peak within the frequency band and having good resolution, so that the refractive index sensor has good recognition.

[0016] The present invention is effective for TM polarized electromagnetic waves.

[0017] The size of the device is at the micron level, achieving a compact and miniaturized structure, and has a good inspiration for industrial integrated applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the overall system structure of a liquid crystal tunable metamer for realizing refractive index sensing based on second harmonic generation according to an embodiment of the present invention;

[0019] Figure 2 Schematic diagram of a second harmonic collection device according to an embodiment of the present invention;

[0020] Figure 3 is the transmission spectrum of the fundamental wave at different refractive indices according to an embodiment of the present invention;

[0021] Figure 4 is a linear fitting curve of the fundamental wave transmission peak refractive index sensor according to an embodiment of the present invention;

[0022] Figure 5 is the transmission spectrum of the second harmonic at different refractive indices of light according to an embodiment of the present invention;

[0023] Figure 6 is a linear fitting curve of the second harmonic transmission peak refractive index sensor according to an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The following is a detailed description of the technical solution of the application in conjunction with the accompanying drawings. The described embodiments are only part of the embodiments involved in this patent. All non-innovative embodiments based on this embodiment by other researchers in this field fall within the scope of protection of this patent.

[0025] The present invention is a liquid crystal tunable metamer based on second harmonic generation to achieve refractive index sensing. The overall structure is as follows Figure 1 The overall order of the medium is (lithium niobate 1-E7 liquid crystal-lithium niobate 2) 15 -(Silicon dioxide-Test layer-Silicon dioxide)-(Lithium niobate 1-E7 liquid crystal-Lithium niobate 2) 15 In the present invention, the frequency shift of the resonance peaks of the fundamental wave and the second harmonic is used to achieve sensing, mainly for sensing the refractive index of the layer to be measured. In a specific implementation method, various substances, such as liquids, can be filled in the layer to be measured. By sensing the refractive index of the layer to be measured, the function of distinguishing different refractive indices is achieved.

[0026] The middle structure is composed of silicon dioxide and the layer to be measured, and the periodically poled lithium niobate and liquid crystal layers are attached to both sides. The refractive index of the medium silicon dioxide is described by the dispersion relation of silicon dioxide, and the thickness is 0.7 microns. The refractive index of lithium niobate 1 and lithium niobate 2 can also be described by the model, with thicknesses of 0.3 microns and 0.4 microns respectively. The thickness of the E7 liquid crystal is 1.8 microns, and the refractive index is 1.64. Finally, the thickness of the layer to be measured is 1.6 microns.

[0027] like Figure 2 As shown in the figure, the schematic diagram of the second harmonic collection device is as follows. First, a tunable laser is used to output laser light within a continuous wavelength range. Then, the direction of the optical path is changed through a reflector so that the fundamental wave is vertically incident on the polarizer, allowing specific polarized light to be transmitted. Then, a collimator is used to convert the divergent light into parallel light, making these parallel lights more focused. After passing through the sample structure, they can better generate second harmonics. The polarization state of the light is then detected by an analyzer. Finally, an optical spectrum analyzer is used to analyze the spectral characteristics of the second harmonic.

[0028] like Figure 3 As shown in FIG, within the fundamental frequency range of 130.5 THz to 134.1 THz, the different refractive indices of the layer to be measured correspond to six independent resonance peaks.

[0029] like Figure 4 As shown in the figure, when the refractive index sensor is working, it can detect substances with a refractive index range of 1.1-1.6, including a variety of important solutions. The linear fit R-squared is 0.996, and the sensitivity is 7.14 THz / RIU.

[0030] like Figure 5As shown, at the fundamental frequency of around 260 terahertz, the different refractive indices of the layer to be measured correspond to their own independent resonance peaks, and the perception of different refractive indices can be achieved based on the frequency of the resonance peaks.

[0031] like Figure 6 As shown in the figure, when the refractive index sensor is working, it can detect substances with a refractive index of 1.5-1.8, including a variety of important solutions. The linear fit R-squared is 0.997, and the sensitivity is 14.6 THz / RIU.

[0032] After specific design, the present invention can realize the function of a refractive index sensor. The present invention has good sensitivity and linearity, and effectively realizes the function of a sensor.

[0033] The above description is merely a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent variation based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A second harmonic spatial sensor based on a liquid crystal tunable metastructure, characterized in that: The arrangement order of the overall medium is: a first periodic structure, an intermediate structure, and a second periodic structure arranged in sequence along the light incident direction; The first periodic structure and the second periodic structure are both formed by sequentially stacking a plurality of periodic units, wherein the structure of the periodic unit is lithium niobate layer-liquid crystal layer-lithium niobate layer, wherein the polarization directions of the two lithium niobate layers in a single periodic unit are opposite; the number of periodic units in the first periodic structure and the second periodic structure is 15; The intermediate structure is silicon dioxide layer-to-be-tested medium layer-silicon dioxide layer; Light is vertically incident on the first periodic structure; The lithium niobate layer is a nonlinear lithium niobate layer for generating second harmonics; the liquid crystal layer is an E7 liquid crystal, which has temperature tuning characteristics, so that the liquid crystal tunable metamer can operate at different temperatures; The above-mentioned spatial sensor is used to realize the refractive index perception of the medium layer to be measured, and the incident light is a TM polarized electromagnetic wave.

2. The second harmonic spatial sensor based on a liquid crystal tunable metastructure according to claim 1, characterized in that: The liquid crystal metamer can form a strong fundamental wave transmission peak in the fundamental wave frequency range of 130.5 to 134.1 terahertz, and a strong second harmonic transmission peak in the second harmonic frequency range of 266.8 to 271.5 terahertz. The refractive index of lithium niobate is described by the dispersion relation, and its refractive index is 2.1, and the refractive index of silicon dioxide is 1.

45.

3. The second harmonic spatial sensor based on a liquid crystal tunable metastructure according to claim 1, characterized in that: For TM polarized incident electromagnetic waves, the extraordinary light refractive index of the E7 liquid crystal within the working frequency band is 1.

64.

4. The second harmonic spatial sensor based on a liquid crystal tunable metastructure according to claim 1, characterized in that: In the periodic unit, the thicknesses of the two lithium niobate layers arranged sequentially along the light incident direction are 0.3 microns and 0.4 microns respectively, and the thickness of the E7 liquid crystal layer is 1.8 microns; in the intermediate structure, the thickness of the silicon dioxide layer is 0.7 microns, and the thickness of the medium layer to be measured is 1.6 microns.

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