THz-band TM polarization double-band tunable spectrum selective absorber and preparation method thereof

Abstract

A THz-band TM polarization double-band tunable spectrum selective absorber comprises a graphene single layer, a square nanopore array, a dielectric medium thin film layer, an antireflection thin film layer and a dielectric medium substrate from top to bottom, the square nanopore array is composed of two-dimensional periodic nanopore arrays, and when TM polarized light is vertically incident, the two-dimensional periodic nanopore arrays form a two-dimensional periodic nanopore array; incident light in two narrow frequency ranges will be absorbed, along with changes of structure parameters and incident angles, small frequency shift of absorption spectrum occurs, small change of peak absorption efficiency occurs accordingly, absorption higher than 50% can be always kept at each peak frequency, and the full width at half maximum of an absorption spectrum line is always smaller than 0.005 THz. Dynamic tuning is carried out in a large frequency range by changing the Fermi level of graphene. The device is processed by combining an electron beam direct writing device with a microelectronic deep etching process, can be produced in batches, and has an important practical prospect.

Description

Technical Field

[0001] The invention relates to a dual-band tunable spectral selective absorber, in particular to a TM polarization dual-band tunable spectral selective absorber in a terahertz band (THz band) and a preparation method thereof. Background Art

[0002] In recent years, metamaterial absorbers have become a rapidly developing research direction due to their wide application prospects in the fields of high-sensitivity detectors, thermal imaging devices, microbolometers and solar energy capture. A typical metamaterial absorber consists of three parts, from top to bottom: a metal microstructure, a dielectric isolation layer and a metal reflector. Since the narrowband absorber was proposed in 2008, metamaterial absorbers have developed into broadband absorption and multi-band absorption. However, the multi-band selective absorbers proposed so far are mostly limited to static. In order to adjust the ideal multi-band absorption performance to other frequencies, the structural parameters of the absorber must be redesigned and optimized. Therefore, realizing dynamic control of multi-band absorption to meet the diverse needs of practical applications has become an important development trend in this field.

[0003] On the other hand, graphene, as a two-dimensional carbon material, has attracted widespread attention since its first experimental discovery in 2004. Due to its special electrical, mechanical and optical properties, graphene has been widely used in photodetectors, modulators, absorbers and waveguides, etc. In the mid- and far-infrared bands, graphene has two important characteristics: one is that its conductivity can be tuned from near-infrared to terahertz bands through chemical doping or electrostatic regulation; the other is that it has a strong plasmon response, which can effectively enhance the interaction between light and graphene. These characteristics make graphene an ideal candidate material for future tunable optoelectronic and plasmonic devices. Based on these characteristics, tunable graphene absorbers with ideal absorption properties have been widely studied, such as periodic doped graphene nanodisk arrays [Prior Art 1: S. Thongrattanasiri et al., Phys. Rev. Lett., 108(4), 047401, 2012], graphene microstrip metamaterials [Prior Art 2: R. Alaee et al., Opt. Express, 20(27), 28017-24, 2012], periodic cross-shaped graphene arrays, etc. However, these absorbers are usually single-band resonant [Prior Art 3: S. Ke et al., Opt. Express, 23, 8888–8900, 2015].

[0004] In addition, two-dimensional square gratings are usually square groove gratings processed on substrates using micro-nano processing technology. The diffraction problem of sub-wavelength two-dimensional square gratings cannot be handled by simple scalar grating diffraction, but must be solved accurately by using Maxwell's equations in vector form and combined with boundary conditions through a coded computer program. Li has given an algorithm for rigorous coupled wave theory [prior art 4: L. Li, J. Opt. Soc. Am. A14, 2758-2767 (1997)], which can solve the diffraction problem of this type of sub-wavelength two-dimensional gratings.

[0005] In summary, traditional metal and dielectric-based multi-band selective absorbers are usually static structures and cannot be dynamically controlled, while the present invention can achieve dynamic tuning by tuning the Fermi level of graphene. In addition, the currently designed graphene-based tunable THz absorbers usually have wide bandwidths, and the recently reported smaller bandwidth has reached 0.024 THz [Prior Art 5: J. Wu, J. Phys. D: Appl. Phys. 49 (2016) 215108], and are single-band structures. The present invention not only achieves dual-band tunable THz absorption, but also has a bandwidth of only 0.005 THz. Furthermore, previous designs are mostly graphene nanostructures, which are relatively difficult to manufacture. The present invention uses a composite structure of a graphene monolayer and a dielectric nanoarray, which can be manufactured through a simple photolithography process and has a low manufacturing cost. Summary of the invention

[0006] In order to solve the shortcomings of the above-mentioned prior art, the present invention provides a THz-band TM-polarized dual-band tunable spectrally selective absorber. The absorber is based on the guided mode resonance effect and is designed for TM (transverse magnetic) polarized incident light. It can efficiently absorb TM polarized incident light within two narrow frequency ranges and has the ability of dynamic tuning.

[0007] The technical principle of the present invention is as follows:

[0008] When TM polarized light is incident vertically on the absorber, the incident light in two narrow frequency ranges will be absorbed. As the structural parameters and the incident angle change, the absorption spectrum will undergo a small frequency shift, and the peak absorption efficiency will change slightly accordingly. However, the absorption can always be maintained above 50% at each peak frequency, and the half-width of the absorption spectrum is always less than 0.005 THz. In addition, the absorption performance can be dynamically tuned over a larger frequency range by changing the Fermi level of graphene.

[0009] The technical solution of the present invention is as follows:

[0010] A THz-band TM polarization dual-band tunable spectral selective absorber, characterized in that it comprises a graphene monolayer, a square nanohole array layer, a dielectric film layer, an anti-reflection film layer and a dielectric substrate from top to bottom; wherein the square nanohole array layer is composed of two-dimensionally periodically arranged square nanoholes, the periods of the nanoholes in the x and y directions are 27.9-28.1 microns and 24.8-25.0 microns, respectively, the side length and thickness are 6.9-7.1 microns and 11.9-12.1 microns, respectively, the thickness of the dielectric film layer is 4.9-5.1 microns, and the thickness of the anti-reflection film layer is 6.0-6.2 microns;

[0011] The absorber can realize dual-band selective absorption of TM polarized light in the THz frequency range based on the guided mode resonance effect, and the frequency range of the dual-band selective absorption can be dynamically tuned by adjusting the Fermi level of the graphene monolayer.

[0012] Furthermore, the thickness of the graphene monolayer is h0=0.34 nm, the surface conductivity σ(ω) of the graphene monolayer is at room temperature T=300K, and satisfies hω<2E in the mid- and far-infrared frequency range. f When , it is expressed as:

[0013]

[0014] Where ω is the angular frequency, h0 is the thickness of graphene, and h0 = 0.34 nm, h is the reduced Planck constant, E f is the Fermi level, e is the elementary charge, τ is the carrier relaxation time, where i represents the imaginary unit and π represents pi.

[0015] Furthermore, the periods of the square nanopore in the x and y directions are 28 and 24.92 micrometers, respectively, and the side length and thickness of the square nanopore are 7.0 micrometers and 12 micrometers, respectively.

[0016] Furthermore, the thickness of the dielectric thin film layer is 5 micrometers, and the thickness of the anti-reflection thin film layer is 6.1 micrometers.

[0017] Second, the present invention also provides a method for preparing a TM polarization dual-band tunable spectrally selective absorber, which is characterized in that it comprises the steps of:

[0018] S1. Select a dielectric substrate as a base;

[0019] S2. depositing an anti-reflection thin film layer on the dielectric substrate, wherein the thickness of the anti-reflection thin film layer is 6.0 to 6.2 microns;

[0020] S3. A dielectric thin film layer (4) is uniformly deposited on the anti-reflection thin film layer by spin coating, sputtering or evaporation, wherein the thickness of the dielectric thin film layer is 4.9 to 5.1 microns;

[0021] S4. On the dielectric film layer, a two-dimensional periodically arranged square nanohole array layer is fabricated by focused ion beam etching (FIB), electron beam lithography (EBL) or nanoimprinting technology, wherein the periods of the nanoholes in the x and y directions are between 27.9 and 28.1 microns and 24.8 and 25.0 microns, respectively, and the side length and thickness of each nanohole are 6.9 to 7.1 microns and 11.9 to 12.1 microns, respectively;

[0022] S5. On the square nanohole array layer (3), a graphene monolayer is deposited, and the thickness of the graphene monolayer is h0 = 0.34 nm;

[0023] S6. The Fermi level of the graphene monolayer is adjusted by electrochemical methods or gate control technology to achieve the adjustability of the frequency range. Compared with the prior art, the technical effects of the present invention are as follows:

[0024] When the periods of the square nanohole array in the x and y directions are 28 and 24.92 microns respectively, the side length and thickness of the square nanoholes are 7.0 microns and 12 microns respectively, the thickness of the dielectric thin film layer is 5 microns, and the thickness of the anti-reflection thin film layer is 6.1 microns, when the TM polarized light in the THz band is vertically incident, the incident light in two ultra-narrow frequency ranges near the frequencies of 6.26THz and 6.3195THz will be absorbed. With the changes in the structural parameters and the incident angle, the absorption spectrum will undergo a small frequency shift, and the peak absorption efficiency will change slightly accordingly. However, the absorption can always be maintained above 50% at each peak frequency, and the half-height full width of the absorption spectrum line is always less than 0.005THz; in addition, the absorption performance can be dynamically tuned over a large frequency range by changing the Fermi level of graphene.

[0025] The present invention has the advantages of flexible and convenient use, high peak absorption efficiency, small half-height full width of the absorption spectrum, and especially the ability to achieve dynamic regulation of dual-band absorption. It is a very ideal absorption device and can be produced in large quantities and at low cost using an electron beam direct writing device combined with a microelectronic etching process. The performance of the absorber after etching is stable and reliable, and it has important practical prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure of the TM polarization dual-band tunable spectrally selective absorber in the THz band of the present invention, wherein (a) is a schematic diagram of the three-dimensional structure, and (b) is a top view of the square nanohole array layer shown in (a).

[0027] In the figure, 1 represents region 1 (refractive index n1), 2 represents a graphene monolayer, 3 represents a dielectric square nanohole array layer, and the grating ridge refractive index is n h =1.5, the nanohole array is air medium with a refractive index of 1, and 4 is a dielectric film with a refractive index of n f =2.5, 5 is the anti-reflection film layer, the refractive index is n AR =sqrt(n f *n s ), 6 is a dielectric substrate with a refractive index of n s =1.47. x and d y are the periods of the nanohole array in the x and y directions, l and h are the side length and thickness of the nanohole array, respectively. f is the thickness of the dielectric film layer, h AR is the thickness of the anti-reflection film layer.

[0028] Figure 2 is a curve showing the variation of the absorption efficiency of TM polarized light with frequency according to an embodiment of the present invention.

[0029] Figure 3 yes Figure 2 The curves showing the variation of the absorption rate with the incident angle when TM polarized light of three discrete frequencies is incident in the embodiment.

[0030] Figure 4 yes Figure 2 The absorption efficiency of the embodiment in FIG. 1 is a curve showing how the Fermi level changes. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with embodiments and drawings, but the protection scope of the present invention shall not be limited thereto.

[0032] Please refer to Figure 1 , Figure 1 This is the geometric structure of the TM polarization dual-band tunable spectral selective absorber in the THz band of the present invention. In the figure, region 1 is uniform, which is air (refractive index n1=1), and TM polarized light (corresponding to the vibration direction of the magnetic field vector perpendicular to the incident plane) is incident on the device at a certain angle θ.

[0033] As can be seen from the figure, a THz-band TM polarization dual-band tunable spectrally selective absorber includes a top-down graphene monolayer 2, a square nanohole array layer 3, a dielectric thin film layer 4, an anti-reflection thin film layer 5 and a dielectric substrate 6.

[0034] Graphene monolayer 2: As a tunable element, its thickness h0 is extremely thin, but it can affect the spectral characteristics of the absorber by changing the Fermi level (Ef).

[0035] Square nanohole array layer 3: It is composed of two-dimensional periodically arranged square nanoholes, with periods of 27.9-28.1 μm and 24.8-25.0 μm in the x and y directions, and side lengths and thicknesses of 6.9-7.1 μm and 11.9-12.1 μm, respectively. It can induce a guided mode resonance effect, thereby enhancing light absorption within a specific frequency range.

[0036] Dielectric film layer 4: with a thickness of 4.9 to 5.1 μm, used to support the propagation of guided modes and provide necessary phase matching conditions;

[0037] Anti-reflection film layer 5: with a thickness of 6.0 to 6.2 micrometers, used to reduce the reflection of incident light on the absorber surface, thereby improving the light absorption efficiency.

[0038] Dielectric substrate 6: serves as the supporting layer of the entire structure.

[0039] The thickness of a graphene monolayer is h0 = 0.34 nm, and its equivalent dielectric constant is:

[0040]

[0041] Where ε0 is a simple vacuum constant, ω is the angular frequency, and σ(ω) is the surface conductivity of graphene, which satisfies hω<2E at room temperature T=300K and in the mid- and far-infrared frequency range. f When , according to the local random phase approximation, it can be expressed as:

[0042]

[0043] Where h is the reduced Planck constant, E f is the Fermi level, e is the elementary charge, τ is the carrier relaxation time. In this embodiment, the carrier relaxation time τ=0.25ps. The Fermi level E of graphene f =0.3ev.

[0044] In such Figure 1 Under the geometric structure shown, the present invention uses rigorous coupled wave theory [prior art 4] to calculate the absorption efficiency of the TM polarization dual-band spectrally selective absorber in the THz frequency range. Optimization is performed using rigorous coupled wave theory [prior art 5] and simulated annealing rule [prior art 5: W. Goffe et al., J. Econometrics 60, 65-99 (1994)].

[0045] A specific embodiment of the present invention is given below. In this case, the refractive indices of the grating ridges and grating grooves of the nanohole array are n h =1.5 and n l=1, and its periods in the x and y directions are d x =28 and d y =0.89*d x = 24.92 microns, the side length and thickness are l = 7 and h = 12 microns respectively, and the thickness of the dielectric film layer is h f =5 microns, the thickness of the anti-reflection film layer is h AR =6.1 microns.

[0046] Table 1 shows a series of embodiments of the present invention. max1 and f max2 Represents the two peak frequencies of the dual-band selective absorption spectrum, A min is the corresponding minimum value of peak absorption efficiency, and FWHM represents the maximum value of the full width at half maximum of the transmission spectrum.

[0047] Table 1 Resonant frequency and corresponding absorption efficiency when TM polarized light is incident

[0048]

[0049]

[0050] It can be seen from Table 1 that under different structural parameters, the absorber can achieve high-efficiency absorption (peak absorption efficiency is higher than 50%) in two specific frequency ranges, and the full width at half maximum (FWHM) of the absorption spectrum is extremely narrow (less than 0.005 THz), such as Figure 2-3 In addition, by changing the Fermi level of graphene (such as Figure 4 The absorption characteristics can be further dynamically adjusted over a larger frequency range.

[0051] By adjusting the period of the nanohole array in the x and y directions, the side length and thickness of the square nanoholes, and the thickness of the dielectric film layer and the anti-reflection film layer, an ultra-narrowband TM polarization dual-band tunable spectrally selective absorber with narrow absorption spectral linewidth and good spatial directionality can be obtained.

[0052] The TM polarization dual-band tunable spectrally selective absorber of the present invention has the advantages of flexible and convenient use, high peak absorption efficiency, extremely narrow half-height full width of the absorption spectrum, and dynamic regulation of dual-band absorption. It is a very ideal absorption device. It can be produced in large quantities and at low cost by combining an electron beam direct writing device with a microelectronic etching process. The performance of the absorber after etching is stable and reliable, and it has important practical prospects.

Claims

1. A THz-band TM polarization dual-band tunable spectrally selective absorber, characterized in that: The invention comprises a graphene monolayer (2), a square nanohole array layer (3), a dielectric thin film layer (4), an anti-reflection thin film layer (5) and a dielectric substrate (6) from top to bottom; wherein the square nanohole array layer is composed of two-dimensionally periodically arranged square nanoholes, the periods of the nanoholes in the x and y directions are 27.9 to 28.1 microns and 24.8 to 25.0 microns respectively, the side length and thickness are 6.9 to 7.1 microns and 11.9 to 12.1 microns respectively, the thickness of the dielectric thin film layer is 4.9 to 5.1 microns, and the thickness of the anti-reflection thin film layer is 6.0 to 6.2 microns; The absorber can realize dual-band selective absorption of TM polarized light in the THz frequency range based on the guided mode resonance effect, and the frequency range of the dual-band selective absorption can be dynamically tuned by adjusting the Fermi level of the graphene monolayer.

2. The TM polarization dual-band tunable spectrally selective absorber according to claim 1, characterized in that: The thickness of the graphene single layer h0=0.34 nm.

3. The TM polarization dual-band tunable spectrally selective absorber according to claim 2, characterized in that: The surface conductivity σ(ω) of the graphene monolayer is 300K at room temperature T, and satisfies hω<2E in the mid-infrared frequency range. f When , it is expressed as: Where ω is the angular frequency, h0 is the thickness of graphene, and h0 = 0.34 nm, h is the reduced Planck constant, E f is the Fermi level, e is the elementary charge, τ is the carrier relaxation time, where i represents the imaginary unit and π represents pi.

4. The TM polarization dual-band tunable spectrally selective absorber according to claim 1, characterized in that: The periods of the square nanopore in the x and y directions are 28 and 24.92 micrometers, respectively, and the side length and thickness of the square nanopore are 7.0 micrometers and 12 micrometers, respectively.

5. The TM polarization dual-band tunable spectrally selective absorber according to claim 1, characterized in that: The thickness of the dielectric thin film layer is 5 micrometers, and the thickness of the anti-reflection thin film layer is 6.1 micrometers.

6. A method for preparing a THz-band TM polarization dual-band tunable spectrally selective absorber, characterized in that: Includes steps: S1. Selecting a dielectric substrate (6) as a base; S2. depositing an anti-reflection thin film layer (5) on the dielectric substrate (6), wherein the thickness of the anti-reflection thin film layer (5) is 6.0 to 6.2 microns; S3. uniformly depositing a dielectric thin film layer (4) on the anti-reflection thin film layer (5) by spin coating, sputtering or evaporation, wherein the thickness of the dielectric thin film layer (4) is 4.9 to 5.1 microns; S4. On the dielectric film layer (4), a two-dimensional periodically arranged square nanohole array layer (3) is fabricated by focused ion beam etching (FIB), electron beam lithography (EBL) or nanoimprinting technology, wherein the periods of the nanoholes in the x and y directions are between 27.9 and 28.1 microns and 24.8 and 25.0 microns, respectively, and the side length and thickness of each nanohole are 6.9 to 7.1 microns and 11.9 to 12.1 microns, respectively; S5. On the square nanohole array layer (3), a graphene monolayer (2) is deposited, and the thickness of the graphene monolayer (2) is h0 = 0.34 nm; S6. The Fermi level of the graphene monolayer (2) is adjusted by electrochemical methods or gate control technology to achieve frequency range adjustability.

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