A polarization insensitive dual-band tunable graphene absorber at the THz frequency

A dual-band and polarization insensitive tunable graphene absorber for THz frequency has been proposed and investigated. The absorber consists of a square graphene ring with a slit at the middle of each side separated from a metallic mirror by a dielectric spacer. Two distinct absorption peaks of 99.87% and 97.82% are observed at 3.92 THz and 6.96 THz, respectively. In addition, the distributions of electric field intensity are also presented for well-recognized the physical origin of such perfect absorption phenomenon. Furthermore, the influence of geometric parameters on the dual-band absorption properties is studied in detail to provide a useful guidance for practical fabrication. The perfect dual-band absorption properties can also be dynamically tuned through the change of the Fermi energy along with large angle insensitivity, both are interesting for real application. The proposed tunable graphene absorber should found potential applications in areas including sensors, modulators, and detectors.     

Enhancement of THz absorption in monolayer graphene for light at Brewster angle incidence

The enhancement of absorption in graphene for light at Brewster angle incidence is investigated. It is achieved by placing a graphene on a resonant Brewster filter that incorporating a spacer layer. The absorber presents above 50% absorption at resonance, which is attributed to the excitation of guided mode resonances. The electromagnetic field intensity distributions are illustrated to intuitively confirm the physical mechanism of such phenomenon. Moreover, the influence of geometric parameters on absorption is investigated to provide a useful guidance for practical fabrication. Besides, it is found that the absorption properties not only can be controlled by adjusting the incident angle but also can be dynamically tuned by changing the Fermi level. Last, the graphene absorption can be easily extended to multichannel by only an increase in the thickness of spacer. The results open new avenues for combining graphene with general guided mode resonance structure to enable novel optoelectronics device applications.     

Narrow and Dual-Band Tunable Absorption of a Composite Structure with a Graphene Metasurface

A tunable absorber, composed of a graphene ribbon on two layers of TiO_2-Au between two slabs of dielectric material all on a metal substrate, is designed and numerically investigated. The absorption of the composite structure varies with the geometrical parameters of the structure and the physical parameters of graphene at mid-infrared frequencies. The numerical simulation shows that a near-perfect absorption with single and dual bands can be achieved in a certain frequency range. We also analyze the electric and surface current distributions to study the dual-band absorber. The results show that the absorber can be tuned by the chemical potential and electron-phonon relaxation time of graphene, and electromagnetically induced transparency phenomenon can be obtained. The results of this study may be beneficial in the fields of infrared communication, perfect absorbers,sensors and filters.     

Tunable multi-band terahertz absorber based on graphene nano-ribbon metamaterial

The tunable multi-band selective absorption effect with graphene nano-ribbon metamaterial is investigated. It is achieved by depositing a set of graphene nano-ribbons on a dielectric spacer backed with a metallic mirror. A dual-band and tri-band absorbers are designed to illustrate such multi-band selective absorption behavior. The designed graphene absorbers exhibit near-unity absorption at multiple resonance frequencies, which is attributed to the plasmonic resonance of graphene nano-ribbons with different widths. Moreover, the multiple resonance wavelengths can be tuned in a wide frequency range by changing the Fermi energy and the absorbers possess large angle insensitivity. Last, a Fabry-Perot model is employed to give a physical understanding of such multi-band selective absorption effect. It is believed that the conclusions may be useful for designing of next-generation graphene-based optoelectronic devices.     

一种光激发可切换的双频太赫兹超材料吸收器

提出了一种基于方环-金属线结构的光激发动态可切换双频太赫兹超材料吸收器.通过调节嵌入在结构间隙中的光敏硅和锗的绝缘/导通状态,该吸收器可在不改变结构的前提下在三个双频完美吸收态之间自由切换.结果表明:当没有光泵浦时,该吸收器工作在0.987 THz和1.767 THz双频吸收态;当采用800 nm激光泵浦时,该吸收器可...     

Dual-function terahertz metasurface based on vanadium dioxide and graphene

A dual-function terahertz metasurface based on VO₂ and graphene is proposed in this paper. It consists of a gold layer embedded with VO₂ patches, a SiO₂ spacer layer, a VO₂ layer, graphene and a SiO₂ spacer substrate. When the bottom VO₂ layer is in the metallic state, the designed metasurface can achieve absorption. When the top VO₂ patches are in the metallic state, the proposed metasurface can be used as a single-band absorber with terahertz absorptance of 99.7% at 0.736 THz. When the top VO₂ patches are in the insulating state, the designed structure behaves as a dual-band absorber with an absorptance of 98.9% at 0.894 THz and 99.9% at 1.408 THz. In addition, the absorber is polarization insensitive and keeps good performance at large angles of incidence. When the bottom VO₂ is in an insulating state, the metasurface shows electromagnetically induced transparency. The transparent window can be dynamically regulated by controlling the chemical potential of graphene. The proposed metasurface exhibits the advantages of terahertz absorption, electromagnetically induced transparency and dynamic control, which provides more options for the design of terahertz devices in the future.     

Polarization‐independent absorption enhancement in a graphene square array with a cascaded grating structure

The polarization‐independent enhanced absorption effect of graphene in the near‐infrared range is investigated. This is achieved by placing a graphene square array on top of a dielectric square array backed by a two‐dimensional multilayer grating. Total optical absorption in graphene can be attributed to critical coupling, which is achieved through the combined effect of guided‐mode resonance with the dielectric square array and the photonic band gap with the two‐dimensional multilayer grating. To reveal the physical origin of such a phenomenon, the electromagnetic field distributions for both polarizations are illustrated. The designed graphene absorber exhibits near‐unity polarization‐independent absorption at resonance with an ultra‐narrow spectrum. Moreover, the polarization‐independent absorption can be tuned simply by changing the geometric parameters. The results may have promising potential for the design of graphene‐based optoelectronic devices.     

A tunable dual-band THz absorber based on graphene sheet and ribbons

A tunable terahertz dual band absorber consisted of graphene ribbon arrays and a graphene sheet with near-unity absorption is proposed and studied in this letter. With the coupling enhancement of graphene ribbon and graphene sheet, a perfect dual band absorber with absorption over 99% at 3.67 THz and 5.88 THz is achieved. Moreover, the principal of the coupling enhancement is analyzed. By optimizing the parameters of the structure, not only the absorption can be enhanced, but also the center of absorbing frequency could be shifted. Another approach to adjust the absorber–tuning the chemical potential of graphene which is more convenient and timely is investigated. Furthermore, the sensing of the refractive index is emulated by changing the refractive index of the medium on the top. Results show a broad application prospect of the absorber proposed in this paper.     

Numerical investigation of a tunable metamaterial perfect absorber consisting of two-intersecting graphene nanoring arrays

In this paper, we report a numerical investigation of tunable dual-band metamaterial perfect absorber (MMPA) consisting of two-intersecting graphene nanorings arrays. The optical absorption performance of absorber is dominated by the inner/outer radius of nanorings and center distance between the two nanorings. For the resonance modes ${\lambda }_1$ and ${\lambda }_2$, the sensitivity of probing the surrounding refractive index can reach 7.97 μm/RIU and 13.27 μm/RIU respectively. The absorption performance of the under-study graphene nanorings can be flexibly adjusted by modulating the Fermi level of graphene with voltage. Due to excellent wavelength selectivity and high refractive-index sensitivity, the presented tunable dual-band MMPA has great application prospects as sensors in biopharmaceutical, environmental monitoring.     

Perfect absorption in a monolayer graphene at the near-infrared using a compound waveguide grating by robust critical coupling

We present a perfect graphene absorber with a compound waveguide grating at the near-infrared. The analytical approach is mainly based on the coupled leaky mode theory, which turns the design of the absorber to finding out the required leaky modes supported by the grating structure. Perfect absorption occurs only when the radiative loss of the leaky mode matches the intrinsic absorption loss, which is also named the critical coupling condition.Furthermore, we also demonstrate that the critical coupling of the system can be robustly controlled, and the perfect absorption wavelength can be easily tuned by adjusting the parameters of the compound waveguide grating.     

An Infrared Metamaterial Broadband Absorber Based on a Simple Titanium Disk with High Absorption and a Tunable Spectral Absorption Band

A metamaterial absorber is proposed that functions in the medium- (3–5 µm) and long-wavelength (8–12 µm) infrared (medium-wavelength infrared, MWIR, and long-wavelength infrared, LWIR, respectively) regions. The proposed design, which consists of periodic cells, can be tuned to achieve single-band or dual-band light absorption by changing the periodicity of the structure. Each cell forming the metamaterial absorber consists of a bottom metal plate (Al), a top metal disk (Ti), and an intermediate dielectric medium (Si or ZnS) in which a metal disk (Ti) is embedded. For a period of 0.85 µm, the absorber achieves broadband absorption in the LWIR region, with an average absorption of 92.1%. Further, the absorber shows acceptable tolerance to irradiation at oblique incidence. For a period of 2 µm, a peak absorption of 99.05% is achieved in the MWIR region, thereby providing dual-band absorption. Tuning the periodicity of the structure enhances the localized surface plasmon resonance, with the absorption mechanism explained by establishing an equivalent parallel LC circuit. The absorption properties demonstrated by the proposed metamaterial absorber are promising for thermal imaging and infrared spectroscopy.     

A dual-band polarization insensitive metamaterial absorber with split ring resonator

A dual-band polarization insensitive absorber has been proposed . Unlike the previous dual band absorber composed of composite structures, only one square metal ring with a slit at the middle of each side can be used to achieve the dual-band absorption. The calculated results show two distinct absorption peaks of 0.96 at 10 GHz and 0.99 at 20 GHz. In addition, the positions of the two peaks are strongly influenced by the width of the slit (g). More importantly, the absorptions of the two peaks keep higher than 0.9 while g changing. The dual-band absorber may have many potential applications in scientific and technological areas because of its excellent absorption characteristics and concise structure.     

Doping-induced changes in the saturable absorption of monolayer graphene

Graphene is a broadband, fast saturable absorber well suited for passive mode-locking of lasers. The broadband absorption, ultra-short recovery time, and low cost of graphene absorbers compare favorably with traditional semiconductor saturable absorber mirrors (SESAMs). However, it remains difficult to tailor the parameters of a monolayer graphene absorber such as the modulation depth and the insertion loss; this limits the absorber??s design freedom, which is often required for mode-locking without Q-switching instability. We demonstrate in this work that, by hole-doping graphene chemically to various Fermi levels, the modulation depth and insertion loss are modified. Further control of graphene??s saturable absorption by electric-field gating and its application to active suppression of Q-switching in lasers is discussed.     

Realization of THz dualband absorber with periodic cross-shaped graphene metamaterials

A dualband terahertz(THz) absorber including periodically distributed cross-shaped graphene arrays and a gold layer spaced by a thin dielectric layer is investigated.Numerical results reveal that the THz absorber displays two perfect absorption peaks.To elucidate the resonant behavior, the LC model is introduced to fit the spectra.Moreover, the strength and linewidth of the absorption peak can be effectively tuned with structural parameters and the relaxation time of graphene.Owing to its rotational symmetry, this THz absorber exhibits polarization insensitivity.Our designed absorber is a promising candidate in applications of tunable optical sensors and optical filters.     

基于石墨烯互补超表面的可调谐太赫兹吸波体

通过在石墨烯超表面设计周期性切条,实现了基于石墨烯互补超表面的可调谐太赫兹吸波体.通过改变外加电压来改变石墨烯的费米能级,吸波体实现频率可调谐特性.研究了石墨烯费米能级、结构尺寸对超材料吸波体吸收特性的影响,并利用多重反射理论研究了其物理机理并且证明了模拟方法的可行性.研究结果表明:当石墨烯费米能级取0.6 eV,基底厚度13μm,石墨烯上切条长宽分别为2.9μm,0.1μm时,吸波体在1.865 THz可以实现99.9%的完美吸收;石墨烯费米能级从0.4 eV增大到0.9 eV,吸波体共振频率从1.596 THz蓝移到2.168 THz,且伴随共振吸收率的改变,吸收率在0.6 eV时达到最大;通过改变费米能级实现的最大吸收率调制度达84.55%.     

极化控制的双波段宽带红外吸收器研究

红外吸收器在红外隐身、辐射制冷、红外探测、传感器等方面有重要的应用前景.一维光栅型吸收器由于其结构简单、易于加工的优势备受关注,然而其不足之处是频带很窄,且只对一种极化有效.本文提出了一种基于简单一维周期结构的双波段宽带吸收器,对横磁波和横电波都有效,且吸波频段随入射波的极化方式而改变.该结构的基本单元由八个梯度排列的子单元构成,每个子单元由两层金属-介质双层膜垂直层叠组成.全波仿真结果表明,在1.68—2μm波段,该结构对横磁波吸收超过90%,而对横电波吸收很小(小于6%);在3.8-3.9μm波段,该结构对横电波吸收超过90%,而对横磁波吸收很小(小于5%).另外,该结构具有宽角度吸收特性,当入射角增大到60°时仍然能够保持较高的吸收率和较宽的吸收频带.     

An ultra-thin dual-band wide-angle polarization-insensitive metamaterial absorber with near-unity absorbance

In this paper, a dual-band metamaterial absorber (MMA) with wide-angle and polarization-insensitivity is proposed. The MMA consists of two copper layers with a layer of FR-4 between them. And its top layer consists of a cross-shaped resonator and a square ring resonator. The calculation result demonstrates that there are two distinct absorption peaks, whose absorptivity are 99.933% at 3.8441 GHz and 99.99% at 9.1094 GHz. And its thickness is only 1.34% of the wavelength of the lowest absorption frequency. The dual-band MMA shows polarization-insensitivity for normal incident wave and shows high absorptivity in a wide incident angle for both TE and TM polarization wave. In addition, we discuss the working mechanism. The influences of main parameters on the dual-band MMA's absorption are also analyzed. The proposed ultra-thin MMA has simple structure and high absorptivity, which has many potential applications, such as thermal radiometer, detection sensor, stealth technology.     

Surface plasmon-enhanced dual-band infrared absorber for VO_x-based microbolometer application

We propose a periodic structure as an extra absorption layer(i.e., absorber) based on surface plasmon resonance effects, enhancing dual-band absorption in both middle wavelength infrared(MWIR) and long wavelength infrared(LWIR)regions. Periodic gold disks are selectively patterned onto the top layer of suspended SiN/VO_2/SiN sandwich-structure.We employ the finite element method to model this structure in COMSOL Multiphysics including a proposed method of modulating the absorption peak. Simulation results show that the absorber has two absorption peaks at wavelengths λ =4.8 μm and λ = 9 μm with the absorption magnitudes more than 0.98 and 0.94 in MWIR and LWIR regions, respectively. In addition, the absorber achieves broad spectrum absorption in LWIR region, in the meanwhile, tunable dual-band absorption peaks can be achieved by variable heights of cavity as well as diameters and periodicity of disk. Thus, this designed absorber can be a good candidate for enhancing the performance of dual band uncooled infrared detector, furthermore, the manufacturing process of cavity can be easily simplified so that the reliability of such devices can be improved.     

超薄柔性透射型超构材料吸收器

设计并加工了一种超薄柔性透射型吸收器,总体厚度为0.288 mm,可实现柔性弯曲,容易做到与曲面目标共形.该吸收器由三层结构组成,底层是金属光栅,中间为介质层,表面单元由两条平行放置的尺寸不同的金属线组成.仿真和实验结果表明,对横电波在5和7 GHz的吸收分别达到97.5%和96.0%,对横磁波在3.0—6.5 GHz都能保持90%以上的透射率.两个吸收频点可分别独立调节,增加了设计的灵活性.另外,当入射角增大到60°时,该吸收器的性能基本不受影响,表现出良好的广角特性.     

基于石墨烯和二氧化钒的太赫兹宽带可调谐超材料吸收器

太赫兹超材料吸收器作为一种重要的太赫兹功能器件,被广泛应用于生物医学传感、电磁隐身、军用雷达等多个领域.但这种传统的超材料吸收器结构具有可调谐性差、功能单一、性能指标不足等缺点,已经无法满足复杂多变的电磁环境的要求,因此可调谐超材料吸收器逐渐成为了太赫兹功能器件领域的研究热点.为实现超材料吸收器吸收特性的调谐,通常从调...