An infrared polarization-insensitive tunable band-stop metasurface filter

Abstract

The application provides an infrared polarization-insensitive tunable band-stop metasurface filter, which is used for infrared spectrum filtering and infrared wave regulation and control applications and comprises an aluminum substrate, a silicon dioxide layer and a mixed metasurface composed of a single-layer graphene and a gold structure covering the silicon dioxide layer; wherein the gold structure is arranged in an I-shaped orthogonal form, the aluminum substrate and the single-layer graphene form a Fabry-Pero cavity, and a critical coupling condition is realized by utilizing round-trip phase accumulation, so that the absorption of electromagnetic waves at a specific wavelength is realized, and a band-stop filtering effect is realized. By adjusting the geometric size of the unit structure, the working range can be moved to the near-infrared or terahertz wave band, and the application has great application in the field of spectrum filtering and spectrum imaging.

Description

Technical Field

[0001] This invention belongs to the field of optical engineering technology, specifically an infrared polarization-insensitive tunable bandstop metasurface filter. Background Technology

[0002] Filters are crucial components in signal acquisition and processing, playing a vital role in fields such as communication and sensing. With the emergence and widespread application of metasurfaces, the performance of metasurface filters has been significantly improved, demonstrating important applications in the optical band, such as target detection, signal enhancement, and spectral imaging. In the far-infrared band, the design and application of filters have attracted considerable interest due to their demand in target detection and imaging. However, once a filter is designed and manufactured, its operating wavelength is fixed, which brings inconvenience to its application. Therefore, achieving tunable filtering is an important research direction. The lack of suitable materials in the far-infrared band makes achieving flexible and efficient tunable filtering challenging. In recent years, the emergence of tunable materials combined with functional metasurfaces promises to achieve wide-range, high-efficiency tunable filtering. By utilizing external stimuli such as light, electricity, and heat to alter the optical properties of materials, combined with the ability of artificially designed periodically arranged microstructures on metasurfaces to manipulate the wavefront of light at the subwavelength scale, dynamic tunable filtering within a specific wavelength range can be achieved. The combination of tunable materials with functional metasurface devices plays a significant role in improving modulation speed, expanding modulation range, and reducing weight. In 2011, Ju et al. first used voltage to modulate the optical properties of graphene

[12] . In recent years, active tuning devices based on graphene have been proposed one after another, such as amplitude modulation[13-15], tunable filtering[16-21] and phase modulation[22,23]. These devices have made great improvements compared with the previous ones, but they also have certain disadvantages, such as small operating range, polarization sensitivity, high requirements for incident angle, difficulty in manufacturing, and complex process. Summary of the Invention

[0003] The purpose of this invention is to provide an infrared polarization-insensitive tunable bandstop metasurface filter to solve the problems of small working range, polarization sensitivity, high requirements for incident angle, and difficulty in manufacturing of current infrared tunable metasurfaces, thereby improving measurement accuracy.

[0004] The technical solution of the present invention is as follows:

[0005] An infrared polarization-insensitive tunable bandstop metasurface filter is characterized by comprising an aluminum substrate, a silicon dioxide layer, and a hybrid metasurface composed of a monolayer of graphene and a gold structure covering the silicon dioxide layer; wherein the gold structure is arranged in an orthogonal I-shape, and the aluminum substrate and the monolayer of graphene constitute a Fabry-Pero cavity, utilizing round-trip phase accumulation to achieve critical coupling conditions, thereby absorbing electromagnetic waves at a specific wavelength and achieving a bandstop filtering effect.

[0006] Preferably, the period of the gold structure is 2μm, including structural portions with length L1 of 1900nm and length L2 of 1000nm, the width W_Au of the gold is 100nm, and the thickness d_Au is 60nm.

[0007] Preferably, the gold unit structure is isotropic, making the filter insensitive to the polarization direction of the incident light.

[0008] Preferably, the silicon dioxide layer serves as a dielectric layer with a thickness d of 400 nm and a predetermined refractive index in the far-infrared band.

[0009] Preferably, the specific wavelength refers to the critical coupling condition where the round-trip phase accumulation is not close to π at a dielectric layer thickness d that is much smaller than the wavelength.

[0010] By changing the voltage applied to graphene, the Fermi level of graphene can be modulated, thereby controlling the optical properties of the filter, achieving dynamically tunable bandstop filtering with a modulation depth of over 85%.

[0011] By adjusting the geometry of the gold cell structure, including the length and width of the gold structure and the thickness of the silicon dioxide layer, the operating range of the filter can be shifted to the near-infrared or terahertz band.

[0012] Preferably, the thickness of the aluminum substrate is 500 nm.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] 1) This invention employs a hybrid metasurface composed of an aluminum substrate, a silicon dioxide layer, a single-layer graphene, and a gold structure. This not only realizes the function of an FP resonator but also achieves dynamically tunable filtering through the voltage tuning characteristics of graphene. Furthermore, the orthogonal I-beam design of the gold structure ensures that the filter is insensitive to the polarization direction of the incident light.

[0015] 2) In the FP resonator, a resonant cavity capable of absorbing electromagnetic waves of specific wavelengths is formed by using a metal substrate and graphene as mirrors. Unlike traditional FP resonators, the round-trip phase accumulation in the resonator of this invention does not need to approach π, and the critical coupling condition can be achieved with a dielectric layer thickness much smaller than the wavelength. This design allows the filter to maintain high filtering efficiency while having a smaller size and more flexible design space.

[0016] 3) By adjusting the geometry of the unit structure, the operating range of the filter can be shifted to the near-infrared or terahertz bands. This tunability makes the filter more widely applicable in fields such as spectral filtering and spectral imaging. Furthermore, the filter maintains over 85% filtering efficiency and modulation depth under both perpendicular and 120° oblique incidence conditions, further enhancing its practicality. A tuning range of 1.7 μm, from 12.4 μm to 14.1 μm, is achieved with a voltage adjustment of 5-30 V. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of an infrared polarization-insensitive tunable bandstop metasurface filter according to an embodiment of the present invention;

[0018] Figure 2 This is a schematic diagram of the gold structure in this invention;

[0019] Figure 3 A tunable spectrum;

[0020] Figure 4 This is a polarization-insensitive spectrum;

[0021] Figure 5 Spectra at different incident angles. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments and accompanying drawings, but these should not be construed as limiting the scope of protection of the present invention.

[0023] Please see Figure 1 , Figure 1 This is a schematic diagram of an infrared polarization-insensitive tunable bandstop metasurface filter according to an embodiment of the present invention. As shown in the figure, the tunable filter consists of an aluminum substrate (d), a silicon dioxide layer (c), and a hybrid metasurface of monolayer graphene (b) and gold (a) covered on SiO2. The aluminum substrate has a thickness of 500 nm, the SiO2 dielectric layer has a thickness of 400 nm, the refractive index of SiO2 in the far-infrared band is 1.45, the period of the unit cell structure is 2 μm, and the gold structure is an orthogonal I-beam structure, as shown below. Figure 2As shown, the gold structure has lengths L1 = 1900 nm, L2 = 1000 nm, a width W_Au = 100 nm, and a thickness d_Au = 60 nm. The metal substrate and graphene act as two mirrors, forming a Fabry-Perot (FP) cavity. In this resonator, the round-trip phase accumulation does not need to approach π, and critical coupling conditions can be achieved at a dielectric layer thickness d much smaller than the wavelength, thus enabling the absorption of electromagnetic waves of specific wavelengths. The surface conductivity of graphene can be represented using the Kubo model.

[0024]

[0025] f here d =1 / (1+exp[(ε-μ) c ) / (k B T)]) is a Fermi-Dirac distribution, k B ε is the Boltzmann constant, ε is the energy, and μ is the energy. c Here, represents the chemical potential (Fermi level) of graphene, e is the electron charge, and T is the temperature. It is the reduced Planck constant, and τ is the carrier relaxation time, which is a function of carrier mobility τ = μE F / (ev f 2 ),

[0026] μ = 200000cm 2 / (V·s) is the carrier mobility. The first term in Equation 1.1 corresponds to the in-band transition process, which can be expressed as:

[0027]

[0028] The second term corresponds to inter-band transitions:

[0029]

[0030] The two transition processes together determine the surface conductivity of graphene. For this parallel-plate capacitor structure, the carrier concentration in graphene is n0 = ε0ε d V g / (d·e), Fermi level Where v f =1×10 6 m / s is the Fermi velocity, ε d V is the relative permittivity of the dielectric layer, d is the thickness of the dielectric layer, and V is the relative permittivity of the dielectric layer. gThis is the voltage between the two plates of the capacitor. The formula shows that the carrier concentration on graphene is related to the applied voltage, the thickness of the dielectric layer, and the dielectric constant. The change in carrier concentration also affects the size of the Fermi level. Therefore, by applying a voltage across a parallel capacitor, the Fermi level of graphene can be changed, thereby controlling the optical properties of the device.

[0031] To verify this process, the proposed filter model was validated using COMSOL Mutiphysics software. Perpendicularly incident TE-polarized light was irradiated onto a periodically arranged unit structure. Under voltages ranging from 5 to 30 V, the reflectivity and absorptivity of the device were as follows: Figure 3 As shown, due to the critical coupling condition of the resonator, electromagnetic waves of a certain wavelength are absorbed, resulting in zero reflectivity and a band-stop filtering effect. To achieve active modulation, considering that graphene can be chemically doped and its Fermi level can be tuned using an applied voltage, and that graphene conductivity is closely related to carrier concentration, we use a simpler external electric field to achieve active modulation of the filter. Due to graphene's high carrier mobility, ultrafast modulation of graphene can be achieved. When the voltage increases from 5V to 30V, the band-stop center wavelength of the model undergoes a blue shift due to the change in the device surface conductivity, corresponding to a change in the Fermi level of graphene. Through the voltage change, the band-stop center wavelength shifts from 14.1μm to 12.4μm, with a tuning range of 1.7μm. At the band-stop center wavelength, resonant absorption is strongest, transmittance is zero, and reflectivity in other bands is basically unaffected, with a bandwidth of approximately 1.5μm.

[0032] Beyond filtering performance, the filter's behavior under different linear polarization directions of incident light and under oblique incidence is also an important aspect of evaluating filter performance. Filters that are polarization insensitive and maintain high efficiency over a large incident angle range have greater applicability. Our proposed filter model, due to the isotropic nature of its unit cell structure, can achieve the same filtering effect under all different polarization directions of incident light, such as... Figure 4 As shown, when the polarization angle of the incident light gradually changes from parallel to the x-axis to perpendicular to the x-axis, the reflection spectrum of the device does not change. Therefore, the device has the same resonance intensity for electromagnetic waves of any polarization direction, exhibiting polarization insensitivity.

[0033] When the incident wave changes from perpendicular incidence to oblique incidence at ±30° angles, the reflectivity remains almost identical to that of perpendicular incidence, with a filtering efficiency decrease of only 1.3%. Furthermore, even with oblique incidence at ±60° angles, the modulation depth can still reach over 85%. Figure 5As shown, the filtering capability is well maintained. The electric field distribution diagram also reveals that when the incident angle is within a 30° cone around the xy-plane normal, the electric field strength is almost equal to that under perpendicular incidence. In a larger 60° cone, the electric field strength is only slightly lower than under perpendicular incidence, reaching 85% of that under perpendicular incidence. Therefore, our proposed tunable filter model exhibits excellent performance in terms of insensitivity to incident wave polarization and adaptability to wide-angle incidence, making it suitable for various operating conditions and possessing great application potential in spectral filtering.

Claims

1. An infrared polarization-insensitive tunable bandstop metasurface filter, characterized in that, The metasurface comprises an aluminum substrate, a silicon dioxide layer, and a monolayer of graphene and a gold structure covering the silicon dioxide layer. The gold structure is arranged in an orthogonal I-shape. The aluminum substrate and the monolayer of graphene form a Fabry-Pero cavity. By accumulating the round-trip phase of electromagnetic waves within the Fabry-Pero cavity at a specific wavelength, a critical coupling condition is met, thereby achieving absorption and band-stop filtering of the electromagnetic wave at that specific wavelength. The thickness of the silicon dioxide layer is much smaller than the specific wavelength, and the round-trip phase accumulation is not close to π.

2. The infrared polarization-insensitive tunable bandstop metasurface filter according to claim 1, characterized in that, The period of the gold structure is 2 μm. Each orthogonal I-shaped structure includes a main arm with a length L1 of 1900 nm extending along a first direction and a side arm with a length L2 of 1000 nm extending along a second direction orthogonal to the first direction. The width W_Au of the gold is 100 nm and the thickness d_Au is 60 nm.

3. The infrared polarization-insensitive tunable bandstop metasurface filter according to claim 2, characterized in that, The gold structure is isotropic, making the filter insensitive to the polarization direction of the incident light.

4. The infrared polarization-insensitive tunable bandstop metasurface filter according to claim 1, characterized in that, The silicon dioxide layer serves as a dielectric layer with a thickness d of 400 nm and a predetermined refractive index in the far-infrared band.

5. The infrared polarization-insensitive tunable bandstop metasurface filter according to claim 4, characterized in that, The specific wavelength refers to the critical coupling condition where the round-trip phase accumulation is not close to π at a dielectric layer thickness d that is much smaller than the wavelength.

6. The infrared polarization-insensitive tunable bandstop metasurface filter according to any one of claims 1-5, characterized in that, By changing the voltage applied to graphene, the Fermi level of graphene can be modulated, thereby controlling the optical properties of the filter, achieving dynamically tunable bandstop filtering with a modulation depth of over 85%.

7. The infrared polarization-insensitive tunable bandstop metasurface filter according to any one of claims 1-5, characterized in that, By adjusting the geometry of the gold cell structure, including the length and width of the gold structure and the thickness of the silicon dioxide layer, the operating range of the filter can be shifted to the near-infrared or terahertz band.

8. The infrared polarization-insensitive tunable bandstop metasurface filter according to any one of claims 1-5, characterized in that, The thickness of the aluminum substrate is 500 nm.

Images