Tunable metasurface absorber based on gallium oxide and graphene

Abstract

The application relates to a tunable metasurface absorber based on gallium oxide and graphene, which is composed of a plurality of unit structure period arrays, the unit structure comprises, from bottom to top, a substrate, a metal reflection layer, an intermediate dielectric layer, a metal array and a graphene layer, the material of the intermediate dielectric layer is gallium oxide, the metal arrays have the same thickness, and the structural parameters are set according to different functions. The device can realize single-wavelength absorption, multi-wavelength absorption and bandwidth absorption.

Description

Technical Field

[0001] The present invention belongs to the field of microelectronic materials and micro-nano optical devices, and in particular relates to a tunable metasurface absorber based on gallium oxide and graphene. Background Art

[0002] Maxwell's equations tell us that light is an electromagnetic wave. Maxwell's equations for media also reveal that, in principle, electromagnetic waves can be manipulated by arbitrarily controlling the dielectric constant and magnetic permeability of a material. These two parameters describe the response of different materials to electromagnetic waves. However, due to the limited range of atoms and their arrangement in natural materials, the range of variation in their dielectric constant and magnetic permeability is limited, limiting the control of electromagnetic waves. Therefore, metasurfaces have been created through artificial structures to overcome the limitations of limited electromagnetic wave control. Metasurfaces not only offer the advantages of arbitrary dielectric constants and magnetic permeabilities, but their two-dimensional form overcomes the limitations of three-dimensional systems, making them easy to prepare, low-loss, and allowing for a wide range of control.

[0003] Metasurface absorbers are typically composed of a resonant unit and a dielectric substrate. They absorb electromagnetic waves by attenuating the structure itself and converting them into other forms of energy. Conventional absorbers often suffer from drawbacks such as thickness, weight, and poor stability. Metamaterial absorbers offer significant advantages over traditional absorbers, including thinner thickness, stronger absorption, and lighter weight.

[0004] However, one of the limitations of traditional metasurface absorbers is that once the structural parameters are determined, their functionality cannot be modified. Graphene, a two-dimensional material with modulatable conductivity, addresses this limitation by enabling functional adjustability through its combination with metasurfaces. However, due to the material properties of graphene, the operating wavelengths of most graphene metasurfaces are limited to the terahertz and mid-infrared bands, with limited modulation effects in the near-infrared band. Therefore, device structure design is needed to enhance its interaction with the near-infrared band.

[0005] Gallium oxide is mostly used in ultraviolet detection in optics due to its 4.8eV band gap. At the same time, gallium oxide is a transparent oxide dielectric material with low loss, high refractive index, and good chemical and thermal stability. It should also have great application potential in other bands, but there are few reports on it. Summary of the Invention

[0006] The object of the present invention is to provide a tunable metasurface absorber based on gallium oxide and graphene, which can achieve single-wavelength absorption, multi-wavelength absorption and broadband absorption.

[0007] In order to achieve the above objectives, the technical solution adopted by the present invention is: a tunable metasurface absorber based on gallium oxide and graphene, the device is composed of a periodic array of multiple unit structures, the unit structure includes a substrate, a metal reflective layer, an intermediate dielectric layer, a metal array and a graphene layer arranged in sequence from bottom to top, the material of the intermediate dielectric layer is gallium oxide, the metal array has the same thickness, and the structural parameters are set according to different functions.

[0008] Furthermore, the material of the substrate is silicon; the material of the metal reflective layer is metal aluminum, with a thickness of 40nm; the thickness of the intermediate dielectric layer is 30nm; the material of the metal array is gold, with a thickness of 30nm; and the thickness of the graphene layer is 0.34nm.

[0009] Furthermore, when the device is to achieve single-wavelength absorption, the metal array of the device's unit structure is composed of two identical gold rectangles arranged in a row. The width of the unit structure is 450nm and the length is 420nm. The width of the gold rectangle is 40nm and the length is 140nm. The distance between the two gold rectangles is 70nm.

[0010] Furthermore, under a light source of 800nm-1600nm, the device can achieve an absorption rate of 99% at 1123nm; by changing the length of the gold cuboid, the absorption peak frequency of the device will change, while the absorption rate remains unchanged.

[0011] Furthermore, when the device is to achieve multi-wavelength absorption, the metal array of the device's unit structure is composed of two different arrangements of gold cuboids. The width of the unit structure is 450nm and the length is 420nm. One of the gold cuboids has a width of 40nm and a length of 120nm, and the other gold cuboid has a width of 40nm and a length of 140nm. The spacing between the two gold cuboids is 70nm, that is, the length of one of the gold cuboids is reduced by 20nm, and the other parameters remain unchanged, forming a metal array with an asymmetric structure.

[0012] Furthermore, under a light source of 800nm-1600nm, the device can reach absorption peaks with absorption rates of 78% and 91% at 996nm and 1109nm, respectively; by changing the length of the gold cuboid, the absorption peak frequency with lower absorption rate of the device will change.

[0013] Furthermore, when the device is to achieve broadband absorption, the metal array of the device's unit structure is composed of four gold rectangles. The width of the unit structure is 400nm and the length is 400nm. The width of each gold rectangle is 180nm. The length of the upper left gold rectangle is 140nm, the length of the lower left gold rectangle is 110nm, the length of the upper right gold rectangle is 120nm, and the length of the lower right gold rectangle is 130nm.

[0014] Furthermore, under a light source of 800nm-1600nm, the device can achieve broadband absorption with an absorption rate of more than 60% from 844nm to 1130nm, and an absorption rate of 78% at 1013nm.

[0015] Furthermore, in order to make the device tunable, a graphene layer is set on top of the metal array of the unit structure. By changing the graphene Fermi level, the graphene conductivity is changed, thereby changing the overall equivalent conductivity of the device and realizing the tuning function. When the graphene Fermi level is 0.7eV, the device can achieve a bandwidth absorption rate of more than 65% from 888nm to 1146nm; when the graphene Fermi level is 1.6eV, the device can achieve an absorption rate of 80% from 1097nm to 1254nm. The device can achieve a broadband absorption with an absorption rate of more than 80% from 1133nm to 1294nm, and an absorption rate of 91% at 1230nm; when the Fermi level of graphene is 2.3eV, the device can achieve a broadband absorption with an absorption rate of more than 80% from 1178nm to 1347nm, and an absorption rate of 94% at 1282nm.

[0016] Compared with existing technologies, the present invention has the following advantages: It provides a tunable metasurface absorber based on gallium oxide and graphene to address the problems of traditional metasurface absorbers' inability to actively modulate, the limited application of gallium oxide at other wavelengths, and the limited modulation effect of graphene devices in the near-infrared. The present invention has a novel structure. By constructing a MIM structure and using different top metal array structures, it achieves single-wavelength absorption, multi-peak absorption, and broadband absorption from 800nm ​​to 1600nm. Furthermore, by integrating graphene, a phase-change material, at the top of the device, the overall conductivity of the device is adjusted by varying the Fermi level, achieving tunable broadband absorption with a modulation range of up to 334.17nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the structure of a tunable metasurface absorber based on gallium oxide and graphene in Example 1 of the present invention (the metal array of the unit structure consists of two identical gold cuboids, and graphene is not yet integrated on the top of the unit structure); wherein: (a) is a schematic diagram of the overall device structure, (b) is a schematic diagram of the unit structure, and (c) is a top view of the unit structure;

[0018] Figure 2This is the absorption spectrum of the tunable metasurface absorber based on gallium oxide and graphene in Example 1 of the present invention (the metal array of the unit structure consists of two identical gold cuboids, and graphene has not yet been integrated on the top of the unit structure) at different gold cuboid lengths;

[0019] Figure 3 Schematic diagram of the structure of a tunable metasurface absorber based on gallium oxide and graphene in Example 2 of the present invention (the metal array of the unit structure is composed of two different gold cuboids, and graphene is not yet integrated on the top of the unit structure); wherein: (a) is a schematic diagram of the unit structure, and (b) is a top view of the unit structure;

[0020] Figure 4 This is the absorption spectrum of the tunable metasurface absorber based on gallium oxide and graphene in Example 2 of the present invention (the metal array of the unit structure is composed of two different gold cuboids, and graphene has not yet been integrated on the top of the unit structure) at different reduced lengths of the gold cuboids;

[0021] Figure 5 Schematic diagram of the structure of a tunable metasurface absorber based on gallium oxide and graphene in Example 3 of the present invention (the metal array of the unit structure consists of four gold cuboids, and graphene has not yet been integrated on the top of the unit structure); wherein: (a) is a schematic diagram of the unit structure, and (b) is a top view of the unit structure;

[0022] Figure 6 This is the broadband absorption spectrum of the tunable metasurface absorber based on gallium oxide and graphene in Example 3 of the present invention (the metal array of the unit structure consists of four gold cuboids, and graphene has not yet been integrated on the top of the unit structure);

[0023] Figure 7 Schematic diagram of the structure of a tunable metasurface absorber based on gallium oxide and graphene in Example 4 of the present invention (the metal array of the unit structure consists of four gold cuboids, and graphene is integrated on the top of the unit structure);

[0024] Figure 8 This is the absorption spectrum of the tunable metasurface absorber based on gallium oxide and graphene in Example 4 of the present invention (the metal array of the unit structure consists of four gold cuboids, and graphene is integrated on the top of the unit structure) at different graphene Fermi levels.

[0025] In the figure: W: width of the gold cuboid; L: length of the gold cuboid; g: spacing between the gold cuboids; Px: period width of the unit structure; Py: period length of the unit structure; td: thickness of the intermediate dielectric layer; tAl: thickness of the metal reflective layer; tAu: thickness of the metal array; d: reduced length of the gold cuboid. DETAILED DESCRIPTION

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0027] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0028] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0029] like Figure 1 As shown, this embodiment provides a tunable metasurface absorber based on gallium oxide and graphene. The device consists of a periodic array of multiple unit structures. The unit structure includes a substrate, a metal reflective layer, an intermediate dielectric layer, a metal array and a graphene layer arranged in sequence from bottom to top. The material of the intermediate dielectric layer is gallium oxide. The metal array has the same thickness, and the structural parameters are changed and set according to different functions.

[0030] Among them, the material of the substrate is silicon; the material of the metal reflective layer is metal aluminum, with a thickness tAl of 40nm; the material of the intermediate dielectric layer is gallium oxide, with a thickness td of 30nm; the material of the metal array is gold, with a thickness tAu of 30nm; and the thickness of the graphene layer is 0.34nm.

[0031] like Figure 1As shown in the figure, when the device needs to achieve single-wavelength absorption, the metal array of the device's periodic unit structure consists of two identical gold cuboids arranged in a row, with a unit structure width of 450nm and a length of 420nm. The metal array is symmetrical, with the gold cuboids being 40nm wide and 140nm long, and the spacing between the two cuboids being 70nm. Under an 800nm-1600nm light source, the device exhibits an absorption peak at 1123nm with an absorptivity of 99%, achieving near-perfect absorption. This is due to the formation of an electric dipole resonance between the top metal array and the bottom metal reflective layer. These two layers generate a pair of antiparallel induced currents, which in turn generate a strong magnetic response in the intermediate dielectric layer due to the phase difference of π. The interference of the two electric dipoles cancels out reflections in the far field. Therefore, the strong absorption is attributed to these localized magnetic and electric dipole resonances. The energy absorbed by the absorber is primarily concentrated in the metal portion, where it is converted into heat through ohmic losses generated by the metal. By changing the length of the gold cuboid (120nm-180nm), the absorption peak frequency of the device can be changed in the range of 1009nm to 1349nm (the longer the length, the greater the absorption peak frequency), while the absorption rate remains unchanged, such as Figure 2 As shown in the figure, this is because under the action of the incident electromagnetic wave, the metal on the metasurface forms an induced current under dipole resonance, so they can be equivalent to inductors. The gap between the metal structures will accumulate a large amount of charge on both sides due to the action of the current, which can be equivalent to a capacitor. The entire metasurface device can be equivalent to an LC circuit. As the length of the Au rod increases, the equivalent inductance of the top metal array in the equivalent circuit model gradually increases. Since the resonant frequency is proportional to the inductance, the resonant frequency increases, the absorption curve gradually redshifts, and the increase in the equivalent inductance L leads to an increase in the electric dipole moment, a longer electron propagation path, an enhanced free oscillation effect of the Au rod, and a redshift in the spectrum.

[0032] like Figure 3 As shown in the figure, when the device needs to achieve multi-wavelength absorption, the metal array of the device's unit structure is composed of two different gold cuboids. The width of the unit structure is 450nm and the length is 420nm. One of the gold cuboids has a width of 40nm and a length of 120nm, and the other has a width of 40nm and a length of 140nm. The distance between the two gold cuboids is 70nm. That is, the length of one of the gold cuboids in the unit structure is reduced (reduced by 20nm), and other parameters remain unchanged, forming an asymmetric metal array. Figure 4As shown in the figure, under a light source of 800nm-1600nm, the device exhibits absorption peaks with absorption rates of 78% and 91% at 996nm and 1109nm, respectively. This is because the symmetry of the structure is broken, resulting in the emergence of an inhomogeneous electromagnetic field, which creates an effective coupling path for the bright film that can directly couple with the incident light field and the dark film that cannot couple with the incident light field. Fano resonance is excited, and the asymmetric structure produces strong coupling. From the perspective of the equivalent circuit model, the destruction of symmetry makes the distribution of capacitance and inductance in the equivalent LC circuit of the resonant cavity uneven, resulting in the appearance of two resonant peaks with different frequencies. As the length d increases, the asymmetry becomes stronger, the spectral line broadens, and the modulation depth of the middle U-shaped spectral line gradually increases, indicating that the absorption of spurious frequencies is reduced. The resonant frequency blue-shifts with the increase of asymmetry. This is because the reduction in material size reduces the capacitance and inductance that determine the peak, and the resonant frequency decreases.

[0033] like Figure 5 As shown in the figure, when the device needs to achieve broadband absorption, the metal array of the device periodic unit structure is composed of four gold cuboids arranged in an array. The width of the unit structure is 400nm, the length is 400nm, the width of the gold cuboid is 180nm, the length of the upper left cuboid is 140nm, the length of the lower left cuboid is 110nm, the length of the upper right cuboid is 120nm, and the length of the lower right cuboid is 130nm, thus constructing an asymmetric array structure. Figure 6 As shown in the figure, under a light source of 800nm-1600nm, the device can achieve an absorption rate of over 60% from 844nm to 1130nm, forming broadband absorption, with an absorption rate of 78% at 1013nm. This is due to the asymmetric structure generating multi-peak absorption. If the frequencies of these peaks are close enough, they can overlap, thus broadening the absorption spectrum and achieving broadband absorption.

[0034] like Figure 7 As shown in Figure 1, when the device needs to achieve tunable bandwidth absorption, graphene is added to the top of the device based on the periodic unit structure of the bandwidth absorption device. By changing the Fermi level of graphene, the conductivity of graphene is changed, resulting in changes in the overall equivalent impedance of the device and the resonant frequency. The real part of the graphene conductivity is in μ c =0.7-2.3eV, while the imaginary part increases significantly with the increase of Fermi level; Figure 8As shown in the figure, when the Fermi level of graphene is 0.7eV, the device achieves a broadband absorption rate of more than 65% from 888nm to 1146nm; when the Fermi level of graphene is 1.6eV, the device achieves a broadband absorption rate of more than 80% from 1097nm to 1254nm, of which 88% can be achieved at 1202nm; when the Fermi level of graphene is 1.9eV, the device can achieve a broadband absorption rate of more than 80% from 1133nm to 1294nm, of which 91% can be achieved at 1230nm; when the Fermi level of graphene is 2.3eV, the device can achieve a broadband absorption rate of more than 80% from 1178nm to 1347nm, of which 94% can be achieved at 1282nm. Compared with the absorption spectrum without graphene layer, the bandwidth frequency is red-shifted by 334nm; similar to the structure without graphene, the enhanced magnetic field is confined to the dielectric, while the electric field vectors of the top and bottom metals form a closed current loop, causing strong dipole resonance. Not only does the device metal participate in the energy absorption of the asymmetric structure device containing graphene, but the energy absorption of the top graphene layer is also very significant, where the metal absorption energy is lower than that of the asymmetric structure device without graphene.

[0035] In this embodiment, the method for preparing the tunable metasurface absorber based on gallium oxide and graphene includes the following steps:

[0036] S1: Electron beam evaporation is used to deposit an aluminum (Al) layer on a silicon substrate as a metal reflective layer.

[0037] S2: A gallium oxide (Ga2O3) layer is deposited on the aluminum layer using atomic layer deposition as an intermediate dielectric layer.

[0038] S3: Spin-coat two layers of electron beam resist on the gallium oxide layer, one layer of polymethyl methacrylate and another layer of methyl methacrylate on top.

[0039] S4: On top of the resist, a very thin layer of chromium (Cr) is deposited as a charge dissipation layer.

[0040] S5: Patterning the double layer resist using electron beam lithography.

[0041] S6: After patterning, the sample is developed in a solution containing 1 part methyl isobutyl ketone and 3 parts isopropyl alcohol at low temperature to remove the exposed resist.

[0042] S7: Deposit a gold (Au) layer using thermal evaporation.

[0043] S8: Stripping was performed by soaking the sample in hot acetone for 12 h, followed by rinsing with acetone and IPA.

[0044] S9: Using a CVD graphene sample on copper foil (covering both sides), cut a small piece large enough to cover the entire device fabrication area and place it on a glass slide. A drop of water is then placed on the glass slide to secure the piece in place. We then spin-coat a layer of 950 K A4 PMMA on top of the graphene sheet (at 2500 rpm for 1 minute) to protect the graphene layer on this side from the following processing steps. The sheet is then flipped over to expose the bare graphene side of the copper foil. This small piece is secured to the glass slide by taping it on all four sides with Kapton tape.

[0045] S10: The glass slide is placed in an oxygen (O2) plasma chamber for 15 minutes in order to etch the graphene away from that side of the copper foil.

[0046] S11: Remove the graphene-covered copper foil (now on one side) and cut off the four edges of the foil. Place the graphene-covered copper foil in copper etchant to separate the graphene and copper foil.

[0047] S12: Use a small wafer to scoop out the floating graphene sheet from the etchant and place it in deionized water to remove all the copper etchant.

[0048] S13: The graphene sample was scooped out of the deionized water and transferred to the device. We then gently blew it with a nitrogen (N2) gun to remove any water droplets underneath the graphene and allowed it to dry for 24 hours. Afterward, the sample was placed in acetone for 5 minutes and then rinsed with acetone and IPA to remove any PMMA on top. The sample was then blown dry using a nitrogen gun.

[0049] S14: Ti (10 nm) and Au (40 nm) were evaporated and deposited onto the graphene sample in vacuum through a template mask for source, drain, and gate electrodes.

[0050] S15: Preparation of PEO electrolyte. Polyethylene oxide (PEO, Mw = 600,000, ACROSORGANICS) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Aladdin 99%) were placed in a blue-capped bottle in a glove box at an oxygen-to-lithium ratio of EO:Li = 18:1. An appropriate amount of acetonitrile (Innochem, 99.9%) was added to achieve a polymer concentration of 50 mg / mL. The mixture was sealed in the blue-capped bottle and heated at 55°C with stirring for 12 hours until a homogeneous solution was formed. The resulting slurry was then transferred to a mixing tank and stirred in an orbiting mixer at 1500 rpm in defoaming mode for 10 minutes to obtain a bubble-free solution. The resulting solution was then cast onto a device in the glove box and spin-dried for 12 hours to form a film. The film was then dried under vacuum at 55°C for 24 hours to completely remove the acetonitrile solvent.

[0051] S16: Apply different voltage V to electrolyte gate eg Immediately cool the sample (<1 minute) to below 250 K to avoid electrochemically induced sample degradation. At this temperature, Li+ and ions are "frozen out" and no longer mobile in the electrolyte, pinning the accumulated charge in the Debye layer to the graphene surface. The induced charge carrier density does not change significantly with time and temperature until the sample is again warmed up.

[0052] The methods of electron beam evaporation, atomic layer deposition, electron beam lithography, thermal evaporation deposition, etc. in the present embodiment are all prior art and will not be described here.

[0053] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application in other forms. Any skilled person in the art can use the disclosed technical content to make changes or modifications to equivalent embodiments. However, any simple modification, equivalent change and modification made to the above embodiments without departing from the technical solution of the present application, and in accordance with the technical essence of the present application, still falls within the protection scope of the present application.

Claims

1. A tunable metasurface absorber based on gallium oxide and graphene, characterized in that: The device consists of a periodic array of multiple unit structures. The unit structure includes a substrate, a metal reflective layer, an intermediate dielectric layer, a metal array, and a graphene layer arranged in sequence from bottom to top. The intermediate dielectric layer is made of gallium oxide. The metal array has the same thickness, and the structural parameters are set according to different functions. When the device is to achieve single-wavelength absorption, the metal array of the device's unit structure is composed of two identical gold cuboids arranged in a row. The width of the unit structure is 450nm and the length is 420nm. The width of the gold cuboid is 40nm and the length is 140nm. The distance between the two gold cuboids is 70nm. When the device is to achieve multi-wavelength absorption, the metal array of the device's unit structure is composed of two different gold cuboids. The width of the unit structure is 450nm and the length is 420nm. One of the gold cuboids is 40nm wide and 120nm long, and the other is 40nm wide and 140nm long. The distance between the two gold cuboids is 70nm, that is, the length of one of the gold cuboids is reduced by 20nm, and the other parameters remain unchanged, forming an asymmetric metal array. When the device is to achieve broadband absorption, the metal array of the device's unit structure is composed of four gold cuboids. The width of the unit structure is 400nm and the length is 400nm. The width of each gold cuboid is 180nm. The length of the upper left gold cuboid is 140nm, the length of the lower left gold cuboid is 110nm, the length of the upper right gold cuboid is 120nm, and the length of the lower right gold cuboid is 130nm.

2. The tunable metasurface absorber based on gallium oxide and graphene according to claim 1, characterized in that: The material of the substrate is silicon; the material of the metal reflective layer is metal aluminum, with a thickness of 40nm; the thickness of the intermediate dielectric layer is 30nm; the material of the metal array is gold, with a thickness of 30nm; and the thickness of the graphene layer is 0.34nm.

3. The tunable metasurface absorber based on gallium oxide and graphene according to claim 1, characterized in that: When the device is to achieve single-wavelength absorption, under a light source of 800nm-1600nm, the device can achieve an absorption rate of 99% at 1123nm; by changing the length of the gold rectangle, the absorption peak frequency of the device will change, while the absorption rate remains unchanged.

4. The tunable metasurface absorber based on gallium oxide and graphene according to claim 1, characterized in that: When the device is to achieve multi-wavelength absorption, under a light source of 800nm-1600nm, the device can reach absorption peaks with absorption rates of 78% and 91% at 996nm and 1109nm respectively; by changing the length of the gold cuboid, the absorption peak frequency with the lowest absorption rate of the device will change.

5. The tunable metasurface absorber based on gallium oxide and graphene according to claim 1, characterized in that: When the device is to achieve broadband absorption, under a light source of 800nm-1600nm, the device can achieve broadband absorption with an absorption rate of more than 60% from 844nm to 1130nm, of which the absorption rate can reach 78% at 1013nm.

6. The tunable metasurface absorber based on gallium oxide and graphene according to claim 1, characterized in that: In order to make the device tunable, a graphene layer is set on the top of the metal array of the unit structure. By changing the graphene Fermi level, the graphene conductivity is changed, thereby changing the overall equivalent conductivity of the device to achieve tuning function. When the graphene Fermi level is 0.7eV, the device can achieve a bandwidth absorption rate of more than 65% from 888nm to 1146nm; when the graphene Fermi level is 1.6eV, the device can achieve an absorption rate of more than 80% from 1097nm to 1254nm. Broadband absorption, where the absorption rate can reach 88% at 1202nm; when the Fermi level of graphene is 1.9eV, the device can achieve broadband absorption with an absorption rate of more than 80% from 1133nm to 1294nm, where the absorption rate can reach 91% at 1230nm; when the Fermi level of graphene is 2.3eV, the device can achieve broadband absorption with an absorption rate of more than 80% from 1178nm to 1347nm, where the absorption rate can reach 94% at 1282nm.

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