Infrared polarization insensitive tunable band elimination metasurface filter

Abstract

The invention provides an infrared polarization insensitive tunable band-stop metasurface filter, which is used for infrared spectrum filtering and infrared wave regulation and control application, and comprises an aluminum substrate, a silicon dioxide layer and a mixed metasurface which is formed by single-layer graphene and a gold structure and covers 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 back-and-forth phase accumulation, so that electromagnetic wave absorption is realized under a specific wavelength, and a band elimination filtering effect is realized. According to the invention, the working range can be moved to the near-infrared or terahertz wave band by adjusting the geometric dimension of the unit structure, and the device can be widely applied to the fields of spectral filtering and spectral imaging.

Description

Technical Field

[0001] The invention belongs to the technical field of optical engineering, and in particular relates to an infrared polarization-insensitive tunable band-stop metasurface filter. Background Art

[0002] Filters are important components for signal acquisition and processing, and play an important role in fields such as communications and sensing. With the emergence and widespread application of metasurfaces, the performance of metasurface filters has been greatly improved, and they have also shown important applications in the optical band, such as target detection, signal enhancement, and spectral imaging. In the far-infrared band, due to its demand in fields such as target detection and imaging, the design and application of filters have aroused great interest. However, after the filter is designed and manufactured, its working wavelength is also determined, which brings inconvenience to the application of the filter. Therefore, the realization of tunable filtering is an important research direction. Due to the lack of suitable materials in the far-infrared band, it is still difficult to realize flexible and efficient tunable filtering. In recent years, the emergence of tunable materials combined with functional metasurfaces is expected to realize large-scale, high-efficiency tunable filtering. The use of external excitations such as light, electricity, and heat to change the optical properties of materials combined with the periodically arranged microstructures artificially designed by the metasurface can manipulate the wavefront of light on a subwavelength scale, thus realizing dynamic tunable filtering within a certain band. The combination of tunable materials and metasurface devices with specific functions plays an important role in improving modulation speed, expanding modulation range, and lightweighting. In 2011, Ju et al. first regulated the optical properties of graphene by applying voltage to graphene

[12] . In recent years, active tuning devices based on graphene have been proposed, such as amplitude modulation[13-15], tunable filtering[16-21] and phase modulation[22,23]. These devices have been greatly improved compared with previous ones, but they also have certain disadvantages, such as small working range, polarization sensitivity, high requirements on the incident angle, difficulty in manufacturing, and complex processes. Summary of the invention

[0003] The purpose of the present invention is to provide an infrared polarization-insensitive tunable band-stop metasurface filter to solve the problems of small working range, polarization sensitivity, high requirements on incident angle and difficult manufacturing of the current infrared tunable metasurface, so as to improve the measurement accuracy.

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

[0005] An infrared polarization-insensitive tunable band-stop metasurface filter is characterized in that it comprises an aluminum substrate, a silicon dioxide layer, and a hybrid metasurface consisting of a single-layer graphene and a gold structure covered on the silicon dioxide layer; wherein the gold structure is arranged in an I-shaped orthogonal form, and the aluminum substrate and the single-layer graphene constitute a Fabry-Pero cavity, and a critical coupling condition is achieved by round-trip phase accumulation, thereby achieving absorption of electromagnetic waves at a specific wavelength and achieving a band-stop filtering effect.

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

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

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

[0009] Preferably, the specific wavelength means that the round-trip phase accumulation is not close to π to achieve a critical coupling condition 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 is regulated, and then the optical properties of the filter are regulated, to achieve dynamically tunable band-stop filtering and achieve a modulation depth of more than 85%.

[0011] By adjusting the geometric dimensions of the gold unit 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 moved 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 present invention has the following beneficial effects:

[0014] 1) The present invention adopts a hybrid metasurface composed of an aluminum substrate, a silicon dioxide layer, a single-layer graphene and a gold structure, which not only realizes the function of the FP resonator, but also realizes dynamic tunable filtering through the voltage tuning characteristics of graphene. The I-shaped orthogonal 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 a specific wavelength is formed by using a metal substrate and graphene as a reflector. Unlike the conventional FP resonator, the round-trip phase accumulation in the resonator of the present invention does not need to be close to π, and the critical coupling condition can be achieved at a dielectric layer thickness much smaller than the wavelength. This design enables the filter to have a smaller volume and more flexible design space while maintaining a high filtering efficiency.

[0016] 3) By adjusting the geometric dimensions of the unit structure, the working range of the filter can be moved to the near-infrared or terahertz band. This tunability makes the filter have a wider application prospect in the fields of spectral filtering and spectral imaging. In addition, the filter can maintain a filtering efficiency and modulation depth of more than 85% under vertical incidence and 120° oblique incidence, further improving its practicality. Under the voltage adjustment of 5-30V, the central wavelength can be tuned from 12.4μm-14.1μm with a total range of 1.7μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of an infrared polarization-insensitive tunable band-stop metasurface filter according to an embodiment of the present invention;

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

[0019] Figure 3 It is an adjustable spectrum diagram;

[0020] Figure 4 is the polarization-insensitive spectrum;

[0021] Figure 5 Spectra at different incident angles. DETAILED DESCRIPTION

[0022] The present invention is further described below by way of embodiments in conjunction with the accompanying drawings, but the protection scope of the present invention shall not be limited thereto.

[0023] See also Figure 1 , Figure 1 The schematic diagram of the infrared polarization-insensitive tunable band-stop metasurface filter of the embodiment of the present invention is shown in the figure. As shown in the figure, the tunable filter is composed of an aluminum substrate d, a silicon dioxide layer c, and a hybrid metasurface of a single-layer graphene b and gold a covered on SiO2. The thickness of the aluminum substrate is 500nm, the thickness of the dielectric layer SiO2 is 400nm, the refractive index of SiO2 in the far-infrared band is 1.45, the period of the unit structure is 2μm, and the gold structure is an I-shaped orthogonal structure, such as Figure 2As shown, the length of the gold structure is L1 = 1900nm, L2 = 1000nm, the width of gold is W_Au = 100nm, and the thickness is d_Au = 60nm. The metal substrate and graphene form a Fabry-Perot (FP) cavity as two reflectors. In this resonator, the round-trip phase accumulation does not have to be close to π, and the critical coupling condition can be achieved at a dielectric layer thickness d that is much smaller than the wavelength, thereby enabling the absorption of electromagnetic waves of a specific wavelength. The surface conductivity of graphene can be expressed by the Kubo model:

[0024]

[0025] Here f d =1 / (1+exp[(ε-μ c ) / (k B T)]) is the Fermi-Dirac distribution, k B is the Boltzmann constant, ε is the energy, μ c is the chemical potential (Fermi level) of graphene, e is the electron charge, T is the temperature, is the reduced Planck constant, τ is the carrier relaxation time, which is a function of the 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 intraband transition process and can be expressed as:

[0027]

[0028] The second term corresponds to the interband transition:

[0029]

[0030] The two transition processes jointly 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 speed, ε d is the relative dielectric constant of the dielectric layer, d is the thickness of the dielectric layer, V gis the voltage between the two plates of the capacitor. The formula shows that the carrier concentration on graphene is related to the applied voltage and the thickness and dielectric constant of the dielectric layer. The change in carrier concentration is also related to the size of the Fermi level. Therefore, by applying voltage across the two ends of the parallel capacitor, the Fermi level of graphene can be changed, thereby regulating the optical properties of the device.

[0031] In order to verify this process, COMSOL Muitiphysics software was used to verify the proposed filter model. The reflectivity and absorptivity of the device were shown in Figure 2 at a voltage of 5-30 V using vertically incident TE polarized light irradiated on the periodically arranged unit structure. Figure 3 As shown in the figure, due to the critical coupling condition of the resonator, the electromagnetic wave of a certain wavelength is absorbed and the reflectivity is 0, forming the effect of band-stop filtering. In order to achieve the effect of active modulation, considering that graphene can use chemical doping and external voltage to regulate the Fermi level and the conductivity of graphene is closely related to the carrier concentration, we use a simpler external electric field to achieve active modulation of the filter. Due to the high carrier mobility of graphene, ultra-fast regulation of graphene can be achieved. When the voltage increases from 5V to 30V, due to the change in the surface conductivity of the device, the band-stop center wavelength of the model blue-shifts, corresponding to the change in the Fermi level of graphene. Through the change of voltage, the center wavelength of the band-stop moves from 14.1μm to 12.4μm, and the tuning range is 1.7μm. At the center wavelength of the band-stop, the resonance absorption is the strongest and the transmittance is zero. The reflectivity of other bands is basically unaffected, and the bandwidth is about 1.5μm.

[0032] On the basis of filtering performance, the performance of the filter under different linear polarization directions of incident light and under oblique incidence is also an important aspect to measure the performance of the filter. Polarization-insensitive filters that can maintain high efficiency over a wide range of incident angles have greater applicability. Due to the isotropy of the unit structure, the filter model we proposed can achieve the same filtering effect under all different polarization directions of the 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, so the device has the same resonance intensity for electromagnetic waves of any polarization direction, showing polarization-insensitive characteristics.

[0033] When the incident wave changes from vertical incidence to oblique incidence at ±30°, the reflectivity is almost the same as that of vertical incidence, and the filtering efficiency is only reduced by 1.3%. In the case of oblique incidence at ±60°, the modulation depth can still reach more than 85%. Figure 5As shown, the filtering capability can be well maintained. It can also be seen from the electric field distribution diagram that when the incident angle is within the cone range of 30° around the normal of the xy plane, its electric field strength is almost equal to that at vertical incidence, and within another larger cone range of 60°, its electric field strength is only slightly lower than that at vertical incidence, reaching 85% of the electric field strength at vertical incidence. Therefore, the tunable filter model we proposed has the excellent performance of being insensitive to the polarization of the incident wave and adapting to wide-angle incidence, can adapt to different working conditions, and has great application potential in spectral filtering.

Claims

1. An infrared polarization-insensitive tunable band-stop metasurface filter, characterized in that: A hybrid metasurface comprising an aluminum substrate, a silicon dioxide layer, and a single-layer graphene and gold structure covering the silicon dioxide layer; wherein the gold structure is arranged in an orthogonal I-shaped form, and the aluminum substrate and the single-layer graphene constitute a Fabry-Pero cavity, which utilizes round-trip phase accumulation to achieve critical coupling conditions, thereby achieving absorption of electromagnetic waves at a specific wavelength and realizing a band-stop filtering effect.

2. The infrared polarization-insensitive tunable band-stop metasurface filter according to claim 1, characterized in that: The gold structure has a period of 2 μm, including a structure portion with a length L1 of 1900 nm and a length L2 of 1000 nm. The width W_Au of gold is 100 nm and the thickness d_Au is 60 nm.

3. The infrared polarization-insensitive tunable band-stop metasurface filter according to claim 2, characterized in that: The gold unit structure is isotropic, so that the filter is insensitive to the polarization direction of the incident light.

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

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

6. The infrared polarization-insensitive tunable band-stop metasurface filter according to any one of claims 1 to 5, characterized in that: By changing the voltage applied to graphene, the Fermi level of graphene is regulated, and then the optical properties of the filter are regulated, to achieve dynamically tunable band-stop filtering and achieve a modulation depth of more than 85%.

7. The infrared polarization-insensitive tunable band-stop metasurface filter according to any one of claims 1 to 5, characterized in that: By adjusting the geometric dimensions of the gold unit 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 moved to the near-infrared or terahertz band.

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

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