Lithium niobate tunable metasurface sensor based on quasi-continuous domain bound state

Abstract

The utility model belongs to the technical field of optical metasurfaces, and particularly relates to a lithium niobate tunable metasurface sensor based on a quasi-continuous domain bound state. The metasurface is composed of a dicluster periodic array arranged by square lattices and comprises a cylinder made of lithium niobate (LiNbO3) and a substrate made of quartz, and each square lattice comprises two lithium niobate cylinders with the same radius. The high-Q-factor quasi-continuous domain bound state (qui-BIC) resonance is realized by utilizing the excellent electro-optical characteristic of lithium niobate, adjusting the refractive index of the lithium niobate through external voltage and destroying the dielectric constant symmetry of the structure, thereby realizing the high-Q-factor quasi-continuous domain bound state (qui-BIC) resonance. The sensitivity of the sensor to the change of the refractive index of a surrounding medium is up to 209.6 GHz / RIU, and the high sensitivity is kept while the dynamic tuning of the frequency is realized.

Description

TECHNICAL FIELD

[0001] The utility model relates to terahertz metamaterial sensor technical field, concretely relates to a kind of lithium niobate tunable super surface sensor based on quasi-continuum bound state. BACKGROUND

[0002] In the terahertz waveband, super surface sensors are widely used in molecular fingerprinting and biomedical imaging due to their ability to detect molecular vibration information. However, many existing terahertz super surface sensors use metal materials as structural elements, and the intrinsic ohmic loss of metal can cause energy loss of high-frequency electromagnetic signals during propagation, thereby limiting the sensitivity and stability of the sensor. This loss also limits the improvement of the resonance quality factor (Q value), and the Q value of many metal-based terahertz sensors is usually in the range of 10 3 , which cannot meet the demand for high-sensitivity detection.

[0003] Lithium niobate (LiNbO3) is a dielectric material with high dielectric constant, and is widely used in optical fiber communication, quantum communication and microwave photonics due to its excellent electro-optic, acousto-optic and nonlinear optical properties. As an "optical silicon", lithium niobate has significant advantages in electro-optic modulation range, response speed (nanosecond level) and stability. In addition, the high dielectric constant and low loss characteristics of lithium niobate make it perform well in the terahertz waveband, which can significantly reduce the propagation loss of electromagnetic waves, making it an ideal choice for achieving high quality factor and high sensitivity. The super surface sensor designed using the dielectric properties of lithium niobate can overcome the energy loss problem in metal super surface, thereby effectively improving the performance of the sensor.

[0004] Continuum bound state (BIC) has attracted much attention due to its extremely high Q factor, low loss and high selectivity. By adjusting the symmetry of the structure, BIC can be realized in super surface. However, ideal BIC state is difficult to realize in practice, and quasi-continuum bound state (quasi-BIC) is usually used as a substitute. By exciting quasi-continuum bound state through symmetry breaking and combining with Fano resonance mechanism, high Q factor resonance can be achieved, thereby significantly improving the sensitivity and nonlinear conversion efficiency of the sensor.

[0005] Many terahertz super surface sensors usually excite quasi-continuum bound state by breaking the symmetry of the structure, thereby introducing limited radiation loss while maintaining high Q factor. However, this method requires high precision adjustment of physical structure parameters, which increases the manufacturing complexity and precision requirement, thereby increasing the production cost and limiting the widespread application of quasi-continuum bound state structure in sensors. SUMMARY

[0006] In view of the problems in the prior art, the application provides a lithium niobate tunable metasurface sensor based on quasi-continuous domain bound state, and innovatively introduces the quasi-continuous domain bound state into the sensing application of the lithium niobate metasurface. By utilizing the high refractive index and stable chemical and physical properties of the lithium niobate material, a strategy is proposed to destroy the refractive index symmetry of the material, thereby exciting a high-Q factor resonance dominated by quasi-BIC in the terahertz range.

[0007] Specifically, the refractive index of the lithium niobate is adjusted by an external voltage, the resonance frequency can be flexibly tuned without changing the physical structure, and the sensing performance is further optimized. This method breaks through the limitation of the traditional BIC relying on symmetry, and provides higher sensitivity and tunability for the terahertz sensor based on lithium niobate.

[0008] To achieve the above object, the utility model provides the following scheme:

[0009] The lithium niobate metasurface tunable sensor based on quasi-continuous domain bound state, characterized by: the sensor sequentially comprises a quartz substrate layer (1) and a lithium niobate layer (2) from bottom to top; the lithium niobate layer (2) is periodically arranged on the quartz substrate layer (1); there is one left lithium niobate cylinder and one right lithium niobate cylinder in each period, the two cylinder structure parameters are the same, only a voltage is applied at the right lithium niobate cylinder, the refractive index will change with the voltage, the refractive index asymmetry is formed, the quasi-continuous domain bound state is realized; when in different refractive index environments, the transmission resonance peak will drift, so as to realize the refractive index sensing; the refractive index of the sample to be measured covered on the sensor is in linear relationship with the transmission resonance peak frequency of the sensor, and the refractive index of the sample can be measured.

[0010] The lithium niobate metasurface tunable sensor based on quasi-continuous domain bound state, characterized by: the cross section of each sensor unit structure is a rectangle, the length Pz is 60 microns, and the width Py is 45 microns. The refractive index of the quartz substrate layer (1) is 2, and the refractive index of the lithium niobate layer (2) is 5.2.

[0011] The lithium niobate metasurface tunable sensor based on quasi-continuous domain bound state, characterized by: the thickness of the quartz substrate layer (1) is 30 microns; the lithium niobate layer (2) is a center-symmetrical two-cylinder structure, the radius is R=11 microns, the cylinder height is h=15 microns, and the spacing between the two cylinders is Pz / 2=30 microns.

[0012] The present invention discloses a tunable metasurface sensor based on quasi-continuous domain bound states of lithium niobate, characterized in that: the sensor operates at a frequency of 3.8-4.2 THz, and when a voltage of 100V is applied to the right lithium niobate cylinder, the refractive index of the right lithium niobate cylinder is 5.21, and the refractive index sensitivity of the sensor reaches 209.6 GHz / RIU.

[0013] One objective of this invention is to disrupt the refractive index symmetry of materials to achieve a tunable metasurface of lithium niobate based on a quasi-continuous domain bound state. The specific technical solution adopted is as follows:

[0014] The metasurface sensor unit structure utilizes a lithium niobate crystal with an x-section. In the x and y-axis directions, the refractive index corresponds to that of ordinary light, while in the z-axis direction, it corresponds to that of extraordinary light. It should be noted that the change in refractive index is greatest when the applied electric field is aligned with the z-axis of the lithium niobate crystal. Therefore, in the arrangement of the metasurface and metal electrodes, applying a voltage along the z-axis achieves the optimal electro-optic modulation effect.

[0015] The aforementioned metasurface sensor alters the refractive index of the material by applying an external voltage, thereby disrupting the dielectric constant symmetry of the structure and achieving high Q-factor quasi-continuous bound state resonance. Specifically: the refractive index of the lithium niobate material in the left lithium niobate cylinder remains constant at 5.2. A voltage is applied to the right lithium niobate cylinder, varying in 100V increments within the range of 0 to 400V (the refractive index of the right lithium niobate cylinder varies between 5.2 and 5.24). When the refractive index of the left lithium niobate cylinder equals that of the right lithium niobate cylinder, no resonance peak exists in the transmission spectrum; at this point, the resonance in the infinite quality factor transmission spectrum disappears. As the applied voltage increases, the refractive index of the right lithium niobate cylinder increases, thus broadening the linewidth of the transmission peak. When the refractive index of the left lithium niobate cylinder is not equal to that of the right lithium niobate cylinder, the structural symmetry is disrupted, causing the symmetry-protected continuous bound state to transform into a quasi-continuous bound state.

[0016] Specifically, when there is an electric field E between the two electrodes, the refractive index changes along the z-direction as follows:

[0017]

[0018] Where γ 33 Here are the components of the LiNbO3 electro-optic tensor matrix, V is the driving voltage, G is the gap between electrodes, V / G is the electric field strength, and n is the electric field strength. e The refractive index of lithium niobate is constant at 5.2. Therefore, based on this formula, the refractive index of the lithium niobate cylinder on the right can be calculated under different voltages.

[0019] The beneficial effects of this utility model are as follows:

[0020] The unit structure of this invention is arranged periodically. When a light beam is incident on the sensor, it will cause ring dipole resonance and magnetic dipole resonance, with the magnetic dipole being dominant and forming a distinct peak. When in different refractive index environments, the transmission resonance peak will drift, thereby realizing refractive index sensing.

[0021] This invention discloses a tunable metasurface sensor based on quasi-continuous bound states of lithium niobate. By applying an external voltage to adjust the refractive index of lithium niobate, the symmetry of the dielectric constant is disrupted without altering the physical structure, thus allowing for flexible tuning of the quasi-continuous bound states. This method achieves dynamic adjustment of the resonant frequency through electro-optic modulation without affecting structural integrity, enhancing the sensor's tunability and adaptability. The highest Q-factor is 1.6466 × 10⁻⁶. 4 It can maintain high sensitivity in terahertz sensing, and simplifies the manufacturing process and reduces costs, thus improving the feasibility of practical applications.

[0022] The lithium niobate tunable metasurface sensor based on quasi-continuous domain bound states described in this invention can be used as an on-chip sensor and does not require optical fiber connection with the light source and detector, greatly increasing its application range.

[0023] The lithium niobate tunable metasurface sensor based on quasi-continuous domain bound states described in this invention can be mass-produced using existing semiconductor processing technologies, such as CCP-RIE or ICP-RIE processes. Attached Figure Description

[0024] Figure 1 This is a three-dimensional structural diagram of the present invention.

[0025] Figure 2 This is a three-dimensional diagram of the unit structure of this utility model.

[0026] Figure 3 This is the transmission spectrum of this invention when the refractive index changes.

[0027] Figure 4 This is the transmission spectrum of the sensor of this invention when a terahertz wave is incident perpendicularly.

[0028] Figure 5 This is a linear fitting diagram of refractive index and frequency shift when the sample size is 40 μm. Detailed Implementation

[0029] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present utility model.

[0030] To make the above-mentioned objectives, features and advantages of this utility model more apparent and understandable, the utility model will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Reference Figure 1 The quasi-continuous domain bound state-based tunable metasurface sensor for lithium niobate consists of two parts: a quartz substrate (1) and a lithium niobate layer (2). The lithium niobate layer (2) is periodically arranged on the quartz substrate (1). The quartz substrate (1) has a thickness of 30 μm. The lithium niobate layer (2) consists of two centrally symmetrical cylindrical structures, with the unit cells arranged periodically in a square lattice. The entire sensor array has at least four rows and at least ten columns to ensure sufficient detection sensitivity and performance.

[0032] Reference Figure 2 Each sensor unit has a rectangular cross-section with a length Pz = 60 μm and a width Py = 45 μm. The cylinders within the units are all identical in size, and the two cylindrical structures within each sensor unit are centrally symmetrical. The radius of each cylinder is R = 11 μm, the height of each cylinder is h = 15 μm, and the distance between the two cylinders is Pz / 2 = 30 μm.

[0033] refer to Figure 3 An external voltage is applied to the outer wall of the right lithium niobate cylinder along the z-axis. This voltage varies in 100 V increments within the range of 0-400 V. The refractive index of the right lithium niobate cylinder varies between 5.2 and 5.24, while the refractive index of the left lithium niobate cylinder remains constant. When both the left and right lithium niobate cylinders have a refractive index of 5.2, there is no resonance peak in the transmission spectrum; the resonance at an infinite quality factor disappears. As the applied voltage increases, the refractive index of the right lithium niobate cylinder increases, thus broadening the linewidth of the transmission peak. The resonance peak is sharpest when the refractive index of the right lithium niobate cylinder is 5.21.

[0034] Reference Figure 4The transmission spectrum was obtained when a terahertz wave was incident perpendicularly, with a voltage of 100 V applied to the outer wall of the right lithium niobate cylinder (at which point the refractive index of the right lithium niobate cylinder is 5.21). This simulation result was calculated using CST electromagnetic simulation software. As can be seen from the figure, when no material is attached to the surface of the metamaterial sensor, the resonant frequency is 4.146 THz, and the transmission peak value is 0.99. Q is the quality factor. Fitting the transmission peak using the Fano equation, the Q value is 1.6466 × 10⁻⁶. 4 .

[0035] refer to Figure 5 Under the condition that the thickness of the analyte is 40 µm and a voltage of 100 V is applied to the outer wall of the right lithium niobate cylinder (at which point the refractive index of the right lithium niobate cylinder is 5.21), the effect of different refractive indices of the analyte on the resonant frequency shift was studied. When the refractive index of the analyte increases from 1.1 to 1.5, the resonant frequency shift of the sensor increases to 83.2 GHz. The dashed line represents the linear fitting result of the data, and the resonant frequency of the metamaterial and the refractive index of the analyte show a good linear relationship. The sensing performance of the sensor is characterized by the refractive index sensitivity (S), which represents the change in resonant peak frequency per unit refractive index. Its value is S = Δf / Δn, and the unit is GHz / RIU, where Δf is the change in frequency, Δn is the change in refractive index, and RIU is the Refractive Index Unit. According to the linear fitting formula of the sensor, y = 209.6x - 234.62, its refractive index sensitivity is 209.6 GHz / RIU.

[0036] The embodiments described above are merely preferred embodiments of the present utility model and are not intended to limit the scope of the present utility model. Various modifications and improvements made to the technical solutions of the present utility model by those skilled in the art without departing from the spirit of the present utility model should fall within the protection scope defined by the claims of the present utility model.

Claims

1. A tunable metasurface sensor based on quasi-continuous domain bound states of lithium niobate, characterized in that: The structure consists of a quartz substrate layer (1) and a lithium niobate layer (2) arranged sequentially from bottom to top. The lithium niobate layer (2) is periodically arranged on the quartz substrate layer (1). Each period contains one left lithium niobate cylinder and one right lithium niobate cylinder. The two cylinders have the same structural parameters. A voltage is applied only to the right lithium niobate cylinder, and its refractive index changes with the voltage, forming a refractive index asymmetry and realizing a quasi-continuous bound state. When in different refractive index environments, the transmission resonance peak will drift, thereby realizing refractive index sensing. The refractive index of the sample to be measured covered by the sensor is linearly related to the transmission resonance peak frequency of the sensor, which can realize the determination of the refractive index of the sample.

2. The lithium niobate tunable metasurface sensor based on quasi-continuous domain bound states according to claim 1, characterized in that: The cross-section of the sensor unit structure is rectangular, with a length Pz=60μm and a width Py=45μm; the refractive index of the quartz substrate layer (1) is 2, and the refractive index of the lithium niobate layer (2) is 5.

2.

3. The lithium niobate tunable metasurface sensor based on quasi-continuous domain bound states according to claim 1, characterized in that: The quartz substrate layer (1) has a thickness of 30 μm; the lithium niobate layer (2) consists of two centrally symmetrical cylindrical structures, each with a radius of R = 11 μm and a height of h = 15 μm, and the distance between the two cylinders is Pz / 2 = 30 μm.

4. A tunable metasurface sensor based on quasi-continuous domain bound states of lithium niobate according to claim 1, characterized in that: The sensor operates at a frequency of 3.8-4.2 THz. When a voltage of 100V is applied to the right lithium niobate cylinder, the refractive index of the right lithium niobate cylinder is 5.21, and the refractive index sensitivity of the sensor reaches 209.6 GHz / RIU.

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