Second harmonic space sensor based on liquid crystal tunable superstructure

Abstract

A second harmonic space sensor based on a liquid crystal tunable super-structure realizes sensing of the refractive index of a medium in a space, and comprises periodically polarized nonlinear layers on two sides and a to-be-detected layer in the middle. The device is divided into two parts, a nonlinear layer of the first part is formed by mixing an E7 liquid crystal layer between two layers of lithium niobate, second harmonics are generated by utilizing periodically poled lithium niobate, and tuning of the temperature to the measurement range is realized by utilizing the temperature tunability of liquid crystal, and a layer of an object to be measured is mixed between two layers of silicon dioxide to serve as a defect mode resonance and local electromagnetic field. And a strong transmission type harmonic peak is generated. The non-linear frequency doubling effect formed in the invention can generate second harmonics, the temperature adjustability of the liquid crystal is utilized, the refractive indexes of media in different ranges in the space are sensed at the temperature of 20 DEG C and 50 DEG C of the liquid crystal, the sensing ranges of the refractive indexes are 1.1-1.6 refractive index units (RIU) and 1.5-1.8 RIU respectively, and the liquid crystal refractive index sensing device has the characteristics of non-contact, non-contact, non-contact, non-contact, non-contact, non-contact, non-contact, non-contact, non-contact, non-contact and non-contact functions. The sensitivity is high; the measurement range is adjustable, and the like.

Description

Technical Field

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

[0002] The nonlinear second harmonic perception effect, which is the interaction between electromagnetic waves and nonlinear materials, is a physical phenomenon that exhibits unique properties in specific nonlinear optical systems. When a strong laser beam is irradiated onto a nonlinear optical material with a specific structure and properties, its polarization mechanism no longer follows the simple laws of linear optics due to the nonlinear response characteristics of the atoms or molecules inside the material. The electromagnetic wave, which originally had an incident frequency of the fundamental frequency, undergoes frequency conversion during the interaction with the nonlinear material due to the nonlinear polarization of the material, thereby generating a second harmonic with a frequency twice that of the fundamental frequency. Second harmonic generation has great potential and prospects in the fields of optical detection, biosensing, etc.

[0003] Nonlinear second harmonics have extensive and critical practical applications in many fields, and their 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, especially in the structural integrity assessment and quality control of monitoring devices. In the specific application process, by deeply analyzing the transmission characteristics of materials under electromagnetic fields, the sensor can accurately identify materials with different refractive indices, and can effectively distinguish various sensor elements and biological cells. This feature makes nonlinear second harmonics occupy an extremely important position in the field of high-precision sensor technology, and provides indispensable technical support and guarantee for many fields such as biomedical testing, material science research, and quality monitoring in industrial production processes.

[0004] After searching, it was found that the Chinese patent with publication number CN112432925B disclosed a SPR-based D-type photonic crystal fiber refractive index sensor device and method on March 15, 2024. It realizes a resonance peak through the surface plasmon effect and realizes the perception function of the refractive index of the liquid analyte. This patent improves the dynamic tunability problem in the perception process and has the ability of high sensitivity, multi-range, compact structure and label-free analysis.

[0005] It can be seen that the current optical sensors mainly detect by exciting the resonance peak through resonance effects such as surface plasmons, but the structure is fixed and lacks adjustability. Tunable and multi-range measurement is of great significance to the development of sensors. Summary of the invention

[0006] In view of the above problems, the present invention proposes a second harmonic spatial sensor based on a liquid crystal tunable metamer. The liquid crystal tunable metamer is designed based on second harmonic generation and uses defect mode resonance to achieve a strong transmission peak. At the same time, the temperature tunability of liquid crystal is used to enable the liquid crystal tunable metamer to detect the refractive index at different temperatures using the transmission peaks of the fundamental wave and the second harmonic. At the same time, the peak excited by this liquid crystal tunable metamer has good sensitivity and accuracy and can be used in sensor research.

[0007] To achieve the above object, the technical solution adopted by the present invention is: A second harmonic spatial sensor based on a liquid crystal tunable superstructure, characterized in that the overall medium 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, and structure 1 is (lithium niobate 1-E7 liquid crystal-lithium niobate 2) 15 , which is formed by periodically polarized nonlinear lithium niobate layers sandwiched with 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, structure 2 is (silicon dioxide-test layer-silicon dioxide), which is formed by two silicon dioxide layers with an E7 liquid crystal layer embedded in the middle, and light is vertically incident 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, and the dotted arrows represent opposite polarization directions.

[0008] 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.

[0009] When electromagnetic waves are incident on this device from air, the nonlinear effect of lithium niobate generates second harmonics, and the local field enhancement of the defect mode forms a sharp transmission peak in the spectrum. The incident electromagnetic wave enters this structure vertically in a direction parallel to the normal. A strong fundamental wave transmission peak can be formed in the fundamental frequency range of 130.5 to 134.1 terahertz, and a strong second harmonic transmission peak can be formed in the second harmonic frequency range of 266.8 to 271.5 terahertz. The refractive index of lithium niobate in the structure can be described by the dispersion relation, and its refractive index is about 2.1, and the refractive index of silicon dioxide is about 1.45.

[0010] 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.

[0011] As a further improvement of the present invention, the liquid crystal in the liquid crystal metamer is of E7 type, and its refractive index is extraordinary light refractive index for TM polarized incident electromagnetic waves, and its refractive index value in the frequency band is about 1.64.

[0012] As a further improvement of the present invention, the thicknesses of the lithium niobate 1 and the 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.

[0013] Compared with the prior art, the present invention has the following technical effects: The present invention uses second harmonic generation and local defect mode resonance mode 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.

[0014] The present invention is effective for TM polarized electromagnetic waves.

[0015] 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

[0016] Figure 1 A 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; Figure 2 is a schematic diagram of a second harmonic collection device according to an embodiment of the present invention; Figure 3 is the transmission spectrum of the fundamental wave at different refractive indices according to an embodiment of the present invention; Figure 4 is a linear fitting curve of the fundamental wave transmission peak refractive index sensor according to an embodiment of the present invention; Figure 5 is a transmission spectrum of the second harmonic at different refractive indices of light according to an embodiment of the present invention; 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

[0017] 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 of other researchers in this field based on this embodiment are within the scope of protection of this patent.

[0018] The present invention is a liquid crystal tunable metamer based on second harmonic generation to realize refractive index sensing, and 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 realize sensing, mainly for sensing the refractive index of the layer to be measured. The specific implementation method is that various substances, such as liquid, etc., can be filled in the layer to be measured, and the function of distinguishing different refractive indices is realized by sensing the refractive index of the layer to be measured.

[0019] At the same time, the middle structure is composed of silicon dioxide and the layer to be tested, and the periodically polarized 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, and the thicknesses are 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 tested is 1.6 microns.

[0020] like Figure 2 As shown in the figure, a schematic diagram of the second harmonic collection device is shown. First, a tunable laser is used to output laser light in a continuous wavelength range. Then, the direction of the light 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, so that these parallel lights are more focused and can better generate second harmonics after passing through the sample structure. Then, a polarizer is used to detect the polarization state of the light. Finally, an optical spectrum analyzer is used to analyze the spectral characteristics of the second harmonic.

[0021] like Figure 3 As shown, 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.

[0022] like Figure 4 As shown, when the refractive index sensor is working, it can detect substances with a refractive index of 1.1-1.6, including a variety of important solutions. The linear fit R square is 0.996 and the sensitivity is 7.14 THz / RIU.

[0023] like Figure 5As shown, at the fundamental frequency of around 260 terahertz, 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.

[0024] 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 square is 0.997 and the sensitivity is 14.6 THz / RIU.

[0025] 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.

[0026] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A second harmonic spatial sensor based on a liquid crystal tunable metastructure, characterized in that: 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 The two sides of the superstructure are defined as structure 1, and structure 1 is (lithium niobate 1-E7 liquid crystal-lithium niobate 2) 15 , which is formed by periodically polarized nonlinear lithium niobate layers sandwiched with 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, structure 2 is (silicon dioxide-test layer-silicon dioxide), which is formed by two silicon dioxide layers with an E7 liquid crystal layer embedded in the middle, and light is vertically incident 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, and the dotted arrows represent opposite polarization directions.

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 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.

3. The second harmonic spatial sensor based on a liquid crystal tunable metastructure according to claim 1, characterized in that: The liquid crystal in the liquid crystal metamer is of E7 type, and its refractive index is extraordinary light refractive index for TM polarized incident electromagnetic waves, and its refractive index value in the frequency band is about 1.

64.

4. The second harmonic spatial sensor based on a liquid crystal tunable metastructure according to claim 1, characterized in that: 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.

Images