A Visible-to-Mid-Infrared Tunable Ultra-Narrowband Absorber

Abstract

The present invention discloses an ultra-narrowband absorber tunable from visible to mid-infrared, and its structure includes a substrate, a bottom Bragg reflector, a resonant cavity layer, a lower electrode, a dielectric spacer layer, an upper electrode, a bilayer graphene structure capable of mutual electrostatic gating, and a top Bragg reflector. The absorber is based on Fabry-Perot cavity resonance to form a resonance mode within the resonant cavity layer between the bottom and top Bragg reflectors. The radiation loss rate of this resonance mode is mainly determined by the constituent components, the number of periods of the Bragg reflector, and the Fermi level of graphene together, while the absorption loss rate is determined by the Fermi level of graphene. The bandwidth of the resonance mode is jointly determined by the magnitudes of the radiation loss rate and the absorption loss rate. By constructing a Bragg reflector with appropriate components and number of periods and regulating the Fermi level of graphene to make the system reach critical coupling, an ultra-narrowband perfect absorption tunable from visible to mid-infrared can be achieved.

Description

Technical Field

[0001] The present invention relates to the fields of nanophotonics, light absorption of two-dimensional materials, etc., and specifically relates to a tunable ultra-narrowband perfect absorber from visible to mid-infrared based on a distributed Bragg mirror microcavity and electrostatically tunable graphene. Background Art

[0002] Ultra-narrowband absorbers are highly desired for many important applications, such as optical filters, highly selective thermal emitters, optical sensors, and photodetectors. Recently, dynamically tuning optical properties based on photonic structures is a promising approach to achieve active control in highly integrated optoelectronic devices and has become a research hotspot. Among them, tunable ultra-narrowband perfect absorbers will play an important role in the next generation of active photonic devices. Graphene has attracted extensive interest in photonics and optoelectronic applications due to its extraordinary electronic and optical properties, such as broadband optical response from ultraviolet to terahertz and ultrafast response speed reaching dozens of GHz. In particular, the complex conductivity of graphene can be effectively tuned by an electrostatic gate voltage, showing the potential as an active medium for tunable optical absorbers. However, many currently proposed dynamically tunable ultra-narrowband absorption devices based on graphene and photonic structures suffer from problems such as incompatibility between optical and electrical functions, small working wavelength range, and limited spectral accuracy and modulation depth due to the absorption of the photonic structure itself. Therefore, how to achieve perfect absorption while taking into account characteristics such as ultra-narrowband tunability is a problem that needs to be solved by new absorbers.

[0003] The regulation of optical absorption based on the coupled-mode theory provides us with new ideas. The coupled-mode theory describes the general laws of the coupling between two or more electromagnetic wave modes. Coupling can occur between different electromagnetic wave modes in the same device or between electromagnetic wave modes in different devices. The optical absorber essentially depends on the coupling characteristics between the incident light in free space and a local photon mode. By tuning and matching the radiation loss rate and absorption loss rate of the photon mode, the system can reach the critical coupling state with an absorption rate of 100%. This mode can effectively regulate the optical field at the sub-wavelength scale. Based on the coupled-mode theory, many novel functional photonic devices have been proposed, which show great application potential in many fields such as all-optical integrated chips, optical sensing, optical filters, and selective thermal emission devices. In the present invention, by designing a composite structure of a distributed Bragg mirror microcavity and graphene, based on Fabry-Perot resonance, local field enhancement, and critical coupling regulation, the optical field is concentrated at the graphene and interacts fully with the graphene, greatly improving the optical absorption of the graphene. The target wavelength of the absorber is jointly determined by the cavity length of the resonant cavity and the Fermi level of the graphene. The radiation loss rate of this resonant mode is mainly jointly determined by the constituent components, the number of periods of the distributed Bragg reflector layer, and the Fermi level of the graphene, while the absorption loss rate is mainly determined by the complex surface conductivity of the graphene, that is, by the Fermi level. The Fermi level of the graphene can be regulated by applying voltages to the upper and lower electrodes. When the Fermi energy of the graphene is less than half of the photon energy, the incident light can excite interband transitions, and the regulation of the Fermi level is mainly manifested as the regulation of the imaginary part of the equivalent refractive index of the graphene, that is, the regulation of the absorption rate of the graphene; when the Fermi energy of the graphene is greater than half of the photon energy, the incident light cannot excite interband transitions, and the regulation of the Fermi level is mainly manifested as the regulation of the real part of the equivalent refractive index of the graphene, that is, the regulation of the transmission and reflection phases, and ultimately manifested as the regulation of the absorption peak position. Through these regulations, the radiation loss rate of the microcavity and the absorption loss rate of the graphene are matched with each other at a specific wavelength, thereby realizing a visible-to-middle infrared absorber with perfect absorption, high integration, ultra-narrow band, and wavelength tunability. Summary of the Invention

[0004] The objective of the present invention is to achieve an optical absorber that simultaneously exhibits perfect absorption, high integration, and ultra-narrow bandwidth from the visible to the mid-infrared range. Specifically, a composite structure of a Bragg reflector mirror microcavity composed of a transparent dielectric material with periodically alternating refractive indices and graphene is employed to realize this device. The Bragg reflector mirror microcavity formed by the transparent dielectric material with periodically alternating refractive indices constitutes a Fabry-Perot cavity structure, which enhances the optical field within the cavity. Graphene is placed at the middle position of the microcavity, enabling it to fully interact with the optical field. By constructing a distributed Bragg reflector layer with appropriate components and number of periods, and by regulating the Fermi level of the graphene bilayer, the radiation loss rate of the distributed Bragg reflector microcavity is matched with the absorption loss rate of graphene at the target wavelength, achieving critical coupling in the system, thereby realizing tunable ultra-narrowband perfect absorption from the visible to the mid-infrared range.

[0005] The absorber structure described above includes a substrate 1, a bottom Bragg reflector layer 2 on the substrate 1, a first mutually electrostatically gateable graphene layer 3-1, a dielectric spacer layer 3-2, a second mutually electrostatically gateable graphene layer 3-3 that forms a resonant cavity 3 on the bottom Bragg reflector layer 2, a lower electrode 4 on the first mutually electrostatically gateable graphene layer 3-1, an upper electrode 5 on the second mutually electrostatically gateable graphene layer 3-3, and a top Bragg reflector layer 6 on the resonant cavity 3.

[0006] The substrate 1 described above is a Si substrate with a 300 nm thick SiO 2 film on its surface.

[0007] The bottom distributed Bragg reflector layer 2 is a multi-layer transparent dielectric material with periodically alternating refractive indices, and its material is a transparent optical dielectric operating in the visible to mid-infrared wavelength range; for different target wavelength bands, two transparent dielectric materials are periodically stacked alternately to form the Bragg reflector layer, generally including two dielectric materials with different refractive indices. Among them, the refractive index of the low-refractive-index material is n 3 , and its thickness is h 3 , the refractive index of the high-refractive-index material is n 4 , and its thickness is h 4 . The method for determining its thickness is based on the thin-film interference effect. When the target wavelength and the thicknesses h 3 , h 4 of the transparent optical dielectric material satisfy the following formula, the distributed Bragg reflector layer can totally reflect the incident light:

[0008]

[0009]

[0010] where λ 0 is the target wavelength in free space.

[0011] The described resonant cavity layer 3 is composed of 3-1, 3-2, and 3-3. For different target wavelength bands, different transparent dielectric materials are used to form the resonant cavity, and its thickness h 1 is determined by the Fabry-Perot cavity length formula:

[0012]

[0013] The described first mutually electrostatically gateable graphene layer 3-1 is a single-atom-layer graphene grown by mechanical exfoliation or by chemical vapor deposition (CVD) method. Graphene exhibits p-type doping characteristics in air, and the Fermi level is usually 200 - 400 meV lower than the Dirac point. Considering the quality degradation of graphene grown by chemical vapor deposition (CVD) method transferred by wet chemical method, the mobility is usually 200 - 400 cm 2 / (V s), which is located in the middle of the resonant cavity and contacts the lower electrode, and realizes mutual electrostatic gating by applying voltages on the upper and lower electrodes.

[0014] The described dielectric spacer layer 3-2 is SiO 2 、Al 2 O 3 or HfO 2 thin film layer, and its thickness h 2 = 20 nm.

[0015] The described second mutually electrostatically gateable graphene layer 3-3 is a single-atom-layer graphene grown by mechanical exfoliation or by chemical vapor deposition (CVD) method. Graphene exhibits p-type doping characteristics in air, and the Fermi level is usually 200 - 400 meV lower than the Dirac point. Considering the quality degradation of graphene grown by chemical vapor deposition (CVD) method transferred by wet chemical method, the mobility is usually 200 - 400 cm 2 / (V s), which is located in the middle of the resonant cavity and contacts the upper electrode, and realizes mutual electrostatic gating by applying voltages on the upper and lower electrodes.

[0016] The described lower electrode 4 is an Au electrode.

[0017] The described upper electrode 5 is an Au electrode.

[0018] The described top Bragg reflector layer 6 is a multi-layer transparent dielectric material with periodically alternating refractive indices, and its material is a transparent optical medium working in the visible to mid-infrared wavelength bands; for different target wavelength bands, two transparent dielectric materials are used to be periodically stacked alternately to form a distributed Bragg reflector layer, generally including two dielectric materials with different refractive indices, where the refractive index of the low-refractive-index material is n 3 , and the thickness is h 3, the refractive index of the high refractive index material is n 4 , the thickness is h 4 , the method for determining its thickness is based on the thin film interference effect. When the target wavelength and the thickness h of the transparent optical medium material 3 、h 4 satisfy the following formula, the distributed Bragg reflector can totally reflect the incident light:

[0019]

[0020]

[0021] where λ 0 is the wavelength in free space.

[0022] Advantages of the present invention:

[0023] 1 In this structure, the bottom and top Bragg reflectors form a Fabry - Perot cavity. The graphene bilayer and the dielectric spacer that can be electrostatically gated with each other are located in the middle of the optical field concentration in the resonant cavity. By setting the thickness of the Fabry - Perot cavity, the target absorption wavelength is controlled. By constructing a distributed Bragg reflector with appropriate components and number of periods, the radiation loss rate of the microcavity is controlled. By regulating the Fermi level of the graphene bilayer, the effective thickness and absorption loss rate of the microcavity are regulated, so that the radiation loss rate and absorption loss rate of the system match at the target wavelength to achieve critical coupling and realize 100% absorption.

[0024] 2 By changing the components and the number of periods of the transparent dielectric materials forming the bottom and top Bragg reflectors, the radiation loss rate of the resonance mode will be significantly reduced, that is, an extremely high radiation loss quality factor can be obtained. At the same time, by regulating the Fermi level of graphene, the absorption loss rate is reduced to match the radiation loss rate, thereby realizing 100% absorption in an ultra - narrow band.

[0025] 3 By applying a voltage to the graphene bilayer that can be electrostatically gated with each other, the regulation of the Fermi level of the graphene bilayer can be realized. By selecting a dielectric spacer layer with a higher electrostatic strength, the regulation range of the graphene Fermi surface is increased.

[0026] 4 When the Fermi energy of graphene is greater than half of the photon energy, the incident light cannot excite inter - band transitions. The regulation of the Fermi level is mainly manifested as the regulation of the real part of the equivalent refractive index of graphene, that is, the regulation of the transmission - reflection phase, thereby realizing the regulation of the absorption peak wavelength in the target band. Description of the drawings

[0027] Figure 1 is a schematic structural diagram of a tunable ultra - narrow band absorber from visible to mid - infrared;

[0028] Figure 2This is a spectrum of graphene absorptivity at a target wavelength of 550nm in the visible band calculated by the finite-difference time-domain algorithm in this patent, which varies with wavelength and graphene Fermi level;

[0029] Figure 3 The graph is a spectrum of graphene absorption rate at the target wavelength of 1550nm in the near-infrared band calculated by using the finite-difference time-domain algorithm, which shows the change of wavelength and graphene Fermi level;

[0030] Figure 4 This is a spectrum of graphene absorptivity at a target wavelength of 5000nm in the mid-infrared band calculated using the finite-difference time-domain algorithm, which shows the change of wavelength and graphene Fermi level;

[0031] Figure 5 This is a spectrum of graphene absorptivity at a target wavelength of 10000nm in the mid-infrared band calculated using the finite-difference time-domain algorithm, showing the variation of wavelength and graphene Fermi level. DETAILED DESCRIPTION

[0032] The present invention proposes a method for preparing an ultra-narrowband perfect absorber that is tunable from visible to mid-infrared. For ease of description, the following will take a composite structure working at 550nm as an example and describe the specific implementation of the present invention in detail in conjunction with the accompanying drawings:

[0033] 1 First, on a 300nm thick SiO 2 The TiO films and Si substrates were deposited by chemical vapor deposition (CVD) 2 / SiO 2 Transparent dielectric materials, including TiO 2 Thickness h 3 =64nm, SiO 2 Thickness h 4 =93nm, which constitutes the mirror microcavity of the bottom Bragg reflection layer.

[0034] 2. Deposit a thickness of h on the bottom distributed Bragg reflector microcavity. 1 / 2 = 93nm SiO 2 Constitutes part of the resonant cavity layer.

[0035] 3 The single atomic layer graphene grown by chemical vapor deposition (CVD) was transferred to the resonant cavity layer. With the help of a template mask, Cr (20nm) / Au (90nm) was deposited by electron beam evaporation to form the lower electrode.

[0036] 4 Then atomic layer deposition (ALD) is used to form h 2 = 20nm HfO 2 layer onto single atomic layer graphene.

[0037] 5 The single-atom-layer graphene grown by chemical vapor deposition (CVD) is transferred onto HfO 2 . With the help of a template mask, the upper electrode is deposited by electron beam evaporation with Cr (20 nm) / Au (90 nm).

[0038] 6 SiO with a thickness of h 1 / 2 = 93 nm is deposited on the single-atom-layer graphene to form another part of the resonant cavity layer. 2

[0039] 7 5 pairs of TiO 2 / SiO 2 transparent dielectric materials are deposited on the resonant cavity layer by chemical vapor deposition (CVD), where the thickness of TiO 2 is h 3 = 64 nm, and the thickness of SiO 2 is h 4 = 93 nm, which forms the top Bragg reflector mirror microcavity.

[0040] Embodiment

[0041] The resonant wavelength of the graphene-based ultra-narrowband perfect absorber integrated with the Bragg reflector mirror microcavity in this embodiment is 550 nm. The transparent dielectric materials with periodically alternating refractive indices that form the bottom and top Bragg reflector mirror microcavities are TiO 2 and SiO 2 . SiO 2 is used to form the resonant cavity layer, where the dielectric spacer layer is HfO 2 . Single-layer graphene is covered on both the upper and lower surfaces of the dielectric spacer layer, and there are electrodes on the single-layer graphene for convenient electrostatic regulation. Through electromagnetic simulation optimization, the sizes of the transparent dielectric materials of this structure are: h 1 = 186 nm, h 2 = 20 nm, h 3 = 64 nm, h 4 = 93 nm. Figure 2 For the graphene-based ultra-narrowband perfect absorber integrated with the Bragg reflector mirror microcavity, under the illumination of incident light, the bottom and top Bragg reflector layers form a Fabry - Perot resonance mode, which makes the light field concentrate in the exact middle of the resonant cavity, that is, the positions where the two single-layer graphene are located, enhancing the interaction length between graphene and light, and making the light absorption of this structure reach 100% perfect absorption. As Figure 2 shown, the absorption intensity of graphene decreases with the increase of the Fermi level. When the resonant wavelength of the graphene-based ultra-narrowband perfect absorber integrated with the Bragg reflector mirror microcavity moves to the near-infrared 1550 nm, both the bottom and top Bragg reflector layers consist of 10 pairs of transparent dielectric materials TiO 2 and SiO​2 , where the dielectric spacer layer uses HfO 2 , and single-layer graphene is covered on the upper and lower surfaces of the dielectric spacer layer. Through electromagnetic simulation optimization, the sizes of the transparent dielectric materials of this structure are: h 1 = 528nm, h 2 = 20nm, h 3 = 188nm, h 4 = 264nm. Figure 3 It can be seen from that the absorption peak wavelength of graphene changes with the Fermi level of graphene. When the Fermi energy of graphene is greater than half of the photon energy, the incident light cannot excite interband transitions. The regulation of the Fermi level is mainly manifested as the regulation of the real part of the equivalent refractive index of graphene, that is, the regulation of the transmission and reflection phases, so as to realize the regulation of the absorption peak wavelength in the target band. Compared with the visible band, the number of periods of the transparent dielectrics constituting the bottom and top Bragg reflection layers is increased, significantly reducing the radiation loss rate of the resonance mode, that is, a very high radiation loss quality factor can be obtained. At the same time, by regulating the Fermi level of graphene, the absorption loss rate is reduced to match the radiation loss rate, thus realizing perfect absorption in an ultra-narrow band. For the mid-infrared band of 5μm, the transparent dielectric materials constituting the bottom and top Bragg reflection layers are Si and Si 3 N 4 , where there are 9 pairs of Si / Si 3 N 4 at the bottom and 8 pairs of Si / Si 3 N 4 at the top. The dielectric spacer layer uses HfO 2 , and single-layer graphene is covered on the upper and lower surfaces of the dielectric spacer layer. Through electromagnetic simulation optimization, the sizes of the transparent dielectric materials of this structure are: h 1 = 1072nm, h 2 = 20nm, h 3 = 365nm, h 4 = 536nm, as Figure 4 shows, the absorption of graphene reaches perfect absorption in an ultra-narrow band at the target wavelength of 5000nm. For another mid-infrared band of 10μm, the transparent dielectric materials constituting the bottom and top Bragg reflection layers are Si and ZnSe, where there are 8 pairs of Si / ZnSe at the bottom and 7 pairs of Si / ZnSe at the top. The dielectric spacer layer uses HfO 2 , and single-layer graphene is covered on the upper and lower surfaces of the dielectric spacer layer. Through electromagnetic simulation optimization, the sizes of the transparent dielectric materials of this structure are: h 1 = 2090nm, h 2 = 20nm, h 3 = 731nm, h 4 = 1045nm. As Figure 5As shown, the absorption of graphene reaches ultra-narrowband perfect absorption at the target wavelength of 10000 nm.

Claims

1. A super-narrowband absorber tunable from visible to mid-infrared, characterized in that: The structure of the absorber is as follows: on a substrate (1), there is a bottom Bragg reflector (2), a resonator (3) composed of a first electrostatically mutually-gated graphene layer (3-1), a dielectric spacer layer (3-2), and a second electrostatically mutually-gated graphene layer (3-3) is located on the bottom Bragg reflector (2), a lower electrode (4) is on the first electrostatically mutually-gated graphene layer (3-1), an upper electrode (5) is on the second electrostatically mutually-gated graphene layer (3-3), and a top Bragg reflector (6) is on the resonator (3); The substrate (1) described is a Si substrate with a 300-nm-thick SiO 2 film; The described resonant cavity layer (3) includes a first electrostatically gateable graphene layer (3-1), a dielectric spacer layer (3-2), and a second electrostatically gateable graphene layer (3-3). The first electrostatically gateable graphene layer (3-1) is a single-atom-layer graphene located on the bottom Bragg reflector layer (2). The dielectric spacer layer (3-2) is a SiO 2 , Al 2 O 3 or HfO 2 thin film layer located on the first electrostatically gateable graphene layer (3-1), with a thickness h 2 = 20 nm. The second electrostatically gateable graphene layer (3-3) is a single-atom-layer graphene located on the dielectric spacer layer (3-2). The lower electrode (4) is an Au electrode; The upper electrode (5) is an Au electrode.

Images