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We propose a black phosphorus-silicon carbide (BP-SiC) metasurface with in-plane structural symmetry that can act as both a nearly perfect anisotropic absorber and tunable polarized source of mid-infrared (MIR) radiation. The metasurface is a periodic array of square SiC patches integrated with a BP flake at the top and separated from a bottom reflector by a BaF2 spacer. We first use analytical calculations and numerical simulations to study the hybridization of the anisotropic plasmons of BP with isotropic phonons of SiC. We also analyze the in-plane characteristics of the resulting hybrid modes of the BP/SiC heterostructure and the BP-SiC metasurface. It is then demonstrated that the proposed metasurface can serve as a nearly perfect anisotropic absorber of MIR radiation with highly selective and omnidirectional features. It is also shown that the metasurface can be used as a polarized MIR source with tunable temperature, which is determined by the thermal equilibrium between the matter and radiation. The suggested design holds promise for artificial coatings that can tune the blackbody thermal signatures, MIR sensing, and highly directional in-plane transportation of the MIR energy.

We theoretically investigate mid-infrared electromagnetic wave propagation in multilayered graphene-hexagonal boron nitride (hBN) metamaterials. Hexagonal boron nitride is a natural hyperbolic material that supports highly dispersive phonon polariton modes in two Reststrahlen bands with different types of hyperbolicity. Due to the hybridization of surface plasmon polaritons of graphene and hyperbolic phonon polaritons of hBN, each isolated unit cell of the graphene-hBN metamaterial supports hybrid plasmon-phonon polaritons (HPPs). Through the investigation of band structure of the metamaterial we find that, due to the coupling between the HPPs supported by each unit cell, the graphene-hBN metamaterial can support HPP bands. The dispersion of these bands can be noticeably modified for different thicknesses of hBN layers, leading to the appearance of bands with considerably flat dispersions. Moreover, analysis of light transmission through the metamaterial reveals that this system is capable of supporting high-k propagating HPPs. This characteristic makes graphene-hBN metamaterials very promising candidates for the modification of the spontaneous emission of a quantum emitter, hyperlensing, negative refraction, and waveguiding. (C) 2017 Optical Society of America

Various methods for controlling the in-plane propagation direction and topological transitions of phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials rely on using twisted vdW bi-layers or the hybridization of anisotropic vdW materials with other functional materials such as graphene, hBN, and SiC. At the same time, visibly transparent SiC- and SiO2-based metastructures have potential to solve the problem of daytime radiative cooling. Here, as a unique method, we design a bifunctional α-MoO3-integrated SiC metasurface for effectively controlling the in-plane propagation direction of α-MoO3 PhPs. The control is enabled by a topological transition in the PhP dispersion, which is achieved by proper arrangements of the SiC meta-atoms. The proposed functionality of the designed metasurface is beneficial for the mid-infrared in-plane coupling between quantum emitters and heat management. Moreover, the α-MoO3–SiC metasurface functions as an efficient visibly transparent daytime radiative cooler.

Polaritons in anisotropic van der Waals materials (AvdWMs), with either hyperbolic or elliptical topologies, have garnered significant attention due to their ability of field confinement and many useful applications in in-plane polariton nanophotonics, including directional guiding, canalization, and hyperlensing. Here, we obtain the dispersion relation of hybrid surface plasmon polaritons (SPPs) supported by a parallel-plate waveguide composed of an AvdWM, as an example tungsten ditelluride, that is coupled with a graphene layer. Through analytical calculations and numerical simulations, we first investigate the impact of losses on the modal characteristics of SPPs supported by the AvdWM. We then show that the coupling of the anisotropic layer to a graphene sheet in a parallel-plate waveguide heterostructure allows one to control the in-plane propagation and dispersion topology of the hybrid SPPs by changing the spacer thickness and the graphene chemical potential. Moreover, it is found that owing to the different coupling regimes, this anisotropic-isotropic SPPs hybridization can enhance the propagation length and spatial localization of the guided modes. We believe this approach can lead to the realization of vdW heterostructures with improved functionalities for in-plane and out-of-plane infrared nanophotonics.

Obtaining functional devices with tunable features is beneficial to advance terahertz (THz) science and technology. Here, we propose multifunctional gradient metasurfaces that are composed of a periodic array of binary Si microcylinders integrated with V O 2 and graphene. The metasurfaces act as transmittive (reflective) beamsplitters for the dielectric (metallic) phase of V O 2 with a switchable characteristic. Moreover, by integrating the metasurfaces with graphene and modifying its chemical potential, one can tune the intensity of the split beam as well as obtain nearly perfect resonant absorptions. Consequently, the proposed metasurfaces can find potential applications in THz interferometers, multiplexers, and light absorbers.

Light-Matter Interactions at subwavelength-designed nanostructures have been the subject of intensive study in recent years. The realization of an ideal "blackbody" absorber is an emerging topic in nanophotonics and nanoplasmonics. An ideal black absorber is an object that harvests incoming light with near-unity efficiency. Based on their absorption spectral coverage, they are classified as narrow-band or broadband absorbers. This requirement can be achieved in bulky designs that have a thickness much larger than its light penetration depth as well as antireflective surface texturing.

Thinning the active layer’s thickness of the semiconductor down to a level comparable with the carriers’ diffusion length while keeping its absorption high is an ultimate goal to boost the performance of optoelectronic devices. Strong interference in multilayer structures is one of the promising and practical solutions owing to their simple and large-scale compatible fabrication route. These nanocavity designs not only provide near unity absorption, but they can also be designed in a way that a spectrally selective absorption response can be achieved. In this letter, we will demonstrate the functionality of a metal–insulator–semiconductor (MIS) cavity to obtain spectrally selective ultrathin photodetectors. To prove our theoretical and numerical findings, a 4-nm-thick amorphous silicon (a-Si)-based MIS cavity is designed, fabricated, and characterized. The experimental results show that the optimized cavity design can act as an efficient visible blind ultraviolet (UV) photodetector. The proposed design shows the responsivity values of 120 and 2.5 mA/W in the UV ( $\lambda =\textsf {350}$ nm) and visible ( $\lambda =\textsf {500}$ nm) regions, respectively.

In recent years, sub-wavelength metamaterials-based light perfect absorbers have been the subject of many studies. The most frequently utilized absorber configuration is based on nanostructured plasmonic metals. However, two main drawbacks were raised for this design architecture. One is the fabrication complexity and large scale incompatibility of these nano units. The other one is the inherent limitation of these common metals which mostly operate in the visible frequency range. Recently, strong interference effects in lithography-free planar multilayer designs have been proposed as a solution for tackling these drawbacks. In this paper, we reveal the extraordinary potential of bismuth (Bi) metal in achieving light perfect absorption in a planar design through a broad wavelength regime. For this aim, we adopted a modeling approach based on the transfer matrix method (TMM) to find the ideal conditions for light perfect absorption. According to the findings of our modeling and numerical simulations, it was demonstrated that the use of Bi in the metal-insulator-metal-insulator (MIMI) configuration can simultaneously provide two distinct functionalities; a narrow near unity reflection response and an ultra-broadband near perfect absorption. The reflection behavior can be employed to realize additive color filters in the visible range, while the ultra-broadband absorption response of the design can fully harvest solar irradiation in the visible and near infrared (NIR) ranges. The findings of this paper demonstrate the extraordinary potential of Bi metal for the design of deep sub-wavelength optical devices.

Graphene-based metasurface nearly perfect absorbers (MPAs) can be used as an efficient tool for the active control and manipulation of waves in the terahertz (THz) gap. Here, we propose a novel, to the best of our knowledge, graphene-based MPA that is designed based on a simple configuration and is capable of absorbing THz radiation within a broad bandwidth of almost 3 THz with polarization-insensitive and omnidirectional characteristics. The MPA comprises a periodic array of graphene patches with two different dimensions that are separated from a gold bottom reflector with an S i O 2 spacer layer. The broadband spectral response of the MPA, which is also verified by analytical calculations, is due to the support of propagating surface plasmon excitations and can be either actively tuned via changes in the chemical potential of graphene or passively adjusted by the modification of the geometrical parameters of the patches and thickness of the spacer layer. As a complement to the previous studies in the literature, due to the simplicity of its design and broad spectral response, it is believed that the suggested graphene-based MPA will find potential applications in THz spectroscopy and communications.

Mid-infrared (MIR) plasmon-phonon features of heterostructures composing of a plasmonic anisotropic two-dimensional material (A2DM) on a hexagonal boron nitride (hBN) film are analyzed. We derive the exact dispersion relations of plasmon-phonons supported by the heterostructures and demonstrate the possibility of topological transitions of these modes within the second Reststrahlen band of hBN. The topological transitions lead to enhanced local density of plasmon-phonon states, which intensifies the spontaneous emission rate, if the thickness of the hBN layer is appropriately chosen. We also investigate a lateral junction formed by A2DM/hBN and A2DM, demonstrating that one can realize asymmetric guiding, beaming, and unidirectionality of the hybrid guided modes. Our findings demonstrate potential capabilities of the A2DM/hBN heterostructures for active tunable light–matter interactions and asymmetric in-plane polariton nanophotonics in the MIR range.

Black phosphorous (BP) is a well-known two-dimensional van der Waals (vdW) material with in-plane anisotropy and remarkable electronic and optical properties. Here, we comprehensively analyze the near-field radiative heat transfer (NFRHT) between a pair of parallel non-rotated BP flakes that occurs due to the tunneling of the coupled anisotropic surface plasmon polaritons (SPPs) supported by the flakes. It is demonstrated that the covering of the BP flakes with hexagonal boron nitride (hBN) films leads to the hybridization of the BP's SPPs with the hBN's hyperbolic phonon polaritons and to the significant enhancement of the NFRHT at the hBN's epsilon-near-zero frequencies. It is also shown that the NFRHT in the BP/hBN parallel-plate structure can be actively switched between the ON and OFF states by changing the chemical potential of the BPs and that the NFRHT can be modified by altering the number of the BP layers. Finally, we replace hBN with α-MoO3 and explore how the NFRHT is spectrally and strongly modified in the BP/α-MoO3 parallel-plate structure. We believe that the proposed BP/polar-vdW-material parallel-plate structures can prove useful in the thermal management of optoelectronic devices. Following the pioneering work of Polder and Hove,1 the near-field radiative heat transfer (NFRHT) has attracted considerable attention in the last two decades due to its promising applications in thermophotovoltaics,2,3 thermal rectification,4 electroluminescent cooling,5 thermal diodes,6 and transistors.7 While the propagating waves contribute to the far-field radiative heat transfer,8 the evanescent waves are responsible for the heat flux in the NFRHT—also referred to as photon tunneling. It is known that the NFRHT is considerably enhanced due the excitation of surface polaritons.9,10 The efficiency of the NFRHT can exceed the blackbody limit by several orders in magnitude via the resonant coupling of surface plasmon polaritons (SPPs) in structures based on metals,11,12 doped Si,1–14 and surface phonon polaritons in heat transfer devices composed of SiO2,15 Al2O3,16 and SiC.17,18 Moreover, it was shown that due to the thermal excitation of isotropic graphene SPPs, NFRHT between two closely spaced parallel-plates of graphene can be strongly mediated, enhanced, and tuned via the modification of the chemical potential of graphene in the infrared range.19–22 Black phosphorous (BP) is often used as an anisotropic plasmonic van der Waals (vdW) material for the realization of enhanced NFRHT.23,24 It has a tunable bandgap, ranging from 1.51 eV for a monolayer BP to 0.59 eV for a five-layer BP, and a thickness-dependent anisotropic absorption coefficient.25 The latter feature implies that, unlike graphene, the SPPs of BP exhibit anisotropic behavior.26 Moreover, it has been reported that the thermal conductivities of BP along the zigzag and armchair directions are three orders of magnitude lower than that of graphene at 300 K.27 This makes BP a better candidate than graphene for the heat management via NFRHT. Recent studies have revealed that the NFRHT is greatly enhanced by the out-of-plane hyperbolic plasmon polaritons or phonon polaritons (HPPs) of metamaterials28–32 as well as by the in-plane modes of the graphene-based33,34 or BP-based35 metasurfaces. However, the dependence of the maximum wavenumber of HPPs on the size of the unit cell and the associated fabrication complexity (due to electron beam lithography or several film deposition processes) of the hyperbolic metamaterials or metasurfaces challenge their practical realization for the NFRHT purposes. Natural hyperbolic vdW materials, such as hexagonal boron nitride (hBN)36–38 and α-MoO3,39,40 have also been used to enhance the NFRHT. It has been demonstrated that the NFRHT can be mechanically tuned in twisted hBN films37 or actively modulated in graphene-hBN heterostructures.41–43 Because α-MoO3 has different optical responses along its three crystallographic directions, a similar mechanical modulation of the NFRHT is observed for the twisted films of α-MoO3 with differently aligned surfaces.40 Despite a great deal of recent studies on this topic, the active modulation of enhanced NFRHT in non-rotated epsilon-near-zero BP/hBN and BP/α-MoO3 parallel-plate structures has not been analyzed so far to the best of our knowledge. In the present paper, we comprehensively analyze the NFRHT between two parallel non-rotated BP flakes covered with hBN films [Fig. 1(b)]. It is found that the coupling of the anisotropic plasmons of BP with the hyperbolic phonons of hBN considerably enhances the NFRHT near the edges of the Reststrahlen bands (RBs) of hBN, where hBN exhibits the epsilon-near-zero (ENZ) feature. We demonstrate the possibilities of the active and passive tunings of the NFRHT through changing the chemical potential and altering the number of layers of the BP flakes. It is also shown that the replacement of hBN with α-MoO3 allows one to strongly manipulate the spectrum of the NFRHT in the structure.

Being one-atom thick and tunable simultaneously, graphene plays the revolutionizing role in many areas. The focus of this paper is to investigate the modal characteristics of surface waves in structures with graphene in the far-infrared (far-IR) region. We discuss the effects exerted by substrate permittivity on propagation and localization characteristics of surface-plasmon-polaritons (SPPs) in single-layer graphene and theoretically investigate characteristics of the hybridized surface-phonon-plasmon-polaritons (SPPPs) in graphene/LiF/glass heterostructures. First, it is shown how high permittivity of substrate may improve characteristics of graphene SPPs. Next, the possibility of optimization for surface-phonon-polaritons (SPhPs) in waveguides based on LiF, a polar dielectric with a wide polaritonic gap (Reststrahlen band) and a wide range of permittivity variation, is demonstrated. Combining graphene and LiF in one heterostructure allows to keep the advantages of both, yielding tunable hybridized SPPPs which can be either forwardly or backwardly propagating. Owing to high permittivity of LiF below the gap, an almost 3.2-fold enhancement in the figure of merit (FoM), ratio of normalized propagation length to localization length of the modes, can be obtained for SPPPs at 5-9 THz, as compared with SPPs of graphene on conventional glass substrate. The enhancement is efficiently tunable by varying the chemical potential of graphene. SPPPs with characteristics which strongly differ inside and around the polaritonic gap are found. ispartof: Scientific Reports vol:8 issue:1 ispartof: location:England status: published

In this paper, we study transmission through the embedded arrays of subwavelength annular apertures at near-infrared. Single, i.e. non-embedded arrays of annular holes are known for their capability for high-efficiency transmission even through rather thick apertures in a wide frequency range, extending from microwaves to the visible. In the suggested structures, which contain up to four embedded arrays, multiband operation can be obtained, so that each array is mainly responsible for one of four transmission bands. In such a way, a demultiplexing-like functionality can be realized, i.e. the desired parts of the incident-wave spectrum are distributed between several transmission channels. In the studied structures, we obtain (nearly) zero phase advancement and that indicates near-zero-index behavior at the expected propagation thresholds of plasmonic modes in the frequency domain. Therefore, the earlier developed concept of supercoupling and squeezing into very narrow waveguide channels is applicable to the studied structures. The number of near-zero-index bands is determined by the number of the embedded arrays. The effects of thickness, width of the slits, and permittivity of the filling material are numerically studied and discussed in detail. It is shown that multiband transmission may exist in the near-zero-index regime in a very wide range of parameter variations.

Bi-tunable asymmetric light transmission (AT) and nearly perfect resonant absorption functionalities are achieved by a Lorentz-reciprocal metamaterial for the operation at the mid-infrared (MIR) wavelengths and transverse magnetic polarization. The bi-tunable metamaterial with bi-functional features and a total thickness of 1.8 mu m is based on an hBN/graphene/hBN heterostructure that is bounded by a Ge grating on the upper side and a hybrid VO2/Au grating on the lower side. Through analytical calculations, we first investigate how the dispersion characteristics of the high-beta hyperbolic phonon polaritons of hBN can be controlled and hybridized through the insulator (i-VO2) to metal (m-VO2) transition of VO2 in a bare hBN/VO2 heterostructure. Then, at the absence of graphene and owing to the support of the hybridized high-beta modes, a broad and efficient AT with forward-to-backward contrast exceeding 40% is obtained by numerical calculations for the i-VO2 case, as the first functionality of the structure. Moreover, it is found that for the m-VO2 case, the device is no longer transmittive and a nearly perfect resonant absorption response, as the second functionality, is observed for backward illumination. Finally, by introducing multilayer graphene into the structure and considering the intermediate states of VO2 in the calculations, the bi-tunable transmission and absorption characteristics of the device are investigated. We believe the designed metamaterial is well-suited for MIR optical diodes, sensors, and thermal emitters. (c) 2019 Optical Society of America

We theoretically prove in this paper that using planar multilayer graphene-hexagonal boron nitride (hBN) metamaterials (GhMMs) can yield ultrabroadband and high-contrast asymmetric transmission (AT) and asymmetric absorption (AA) of light. The AA and AT features are obtained in the far-infrared (FIR) and mid-infrared (MIR) regions for normally incident light with transverse magnetic polarization. Here, the GhMMs are integrated with two asymmetric gratings of Ge and are composed of alternating multilayers of graphene (11 multilayers) and hBN layers (10 layers). Moreover, the total subwavelength thickness of the hybrid structures is about 3 mu m, being less than half of the free-space wavelength up to nearly 50 THz. This approach-which is similar to the one introduced by Xu and Lezec [Nat. Commun. 5, 4141 (2014)] for a passive hyperbolic metamaterial operating in the visible range-is based on the excitation of high-beta modes of the GhMM with different transmission characteristics. In addition to being ultrabroadband and high-contrast, AT and AA features of the proposed GhMMs can be actively tuned by varying the chemical potential of graphene. Furthermore, it is shown that an on-off switching of AT factor at FIR and selective tunability at MIR frequencies can be obtained via varying mu. Due to its subwavelength and planar configuration and active operation, these multilayer graphene-hBN metamaterials with AT and AA characteristics hold promise for integration with compact optical systems operating in the MIR and FIR ranges and are suitable for applications such as optical diodes, sensors, and thermal emitters. Published under license by AIP Publishing.

Achieving nearly perfect light absorption from the microwave to optical region utilizing metamaterials has begun to play a significant role in photonics and optoelectronics due to their vital applications in thermal emitters, thermal photovoltaics, photovoltaics, sensing, filtering, and photodetection. However, employing passive components in designing perfect absorbers based on metamaterials and photonic crystals imposes some limits on their spectral operation. In order to overcome those limits, extensive research has been conducted on utilizing different materials and mechanisms to obtain active metamaterial light absorbers. In this review paper, we investigate the recent progress in tunable and reconfigurable metamaterial light absorbers through reviewing different active materials and mechanisms, and we provide a perspective for their future development and applications. (C) 2019 Optical Society of America

In this article, a lithography-free multilayer based color filter is realized using a proper series connection of two cavities that shows relatively high efficiency, high color purity, and a wide view angle. The proposed structure is a metal-insulator-metal-insulator-semiconductor (MIMIS) design. To optimize the device performance, at the first step, transfer matrix method (TMM) modeling is utilized to find the right choices of materials for each layer. Simulations are carried out later on to optimize the geometries of the layers to obtain our desired colors. Finally, the optimized devices are fabricated and experimentally characterized to evaluate our modelling findings. The characterization results of the fabricated samples prove the successful formation of efficient and wide view angle color filters. Unlike previously reported FP based designs that act as a band-stop filter in reflection mode (absorbing a narrow frequency range and reflecting the rest of the spectrum), this design generates a specific color by reflecting a narrow spectral range and absorbing the rest of the spectrum. The findings of this work can be extended to other multilayer structures where an efficient connection of cavities in a tandem scheme can propose functionalities that cannot be realized with conventional FP resonators.

In this paper, we propose an electrically tunable mid-infrared plasmonic-phononic absoiber with omnidirectional and polarization insensitive nearly perfect resonant absorption characteristics. The absorber consists of a graphene/hexagonal boron nitride (hBN)/graphene multilayer on top of a gold bottom reflector separated by a dielectric spacer. The graphene/hBN/graphene multilayer is patterned as a hole array in square lattice. We analytically and numerically prove that, due to the support of hybrid plasmon-phononpolaritons, nearly perfect multi-resonant absorption peaks with high quality factors are obtained both inside and outside of the Reststrahlen band of hBN. As a result of the hybridization of graphene plasmons with the hyperbolic phonon polaritons of hBN, the high quality resonant absorptions of the metamaterial are almost unaffected by decreasing the phenomenological electron relaxation time of graphene. Moreover, the obtained resonances can be effectively tuned in practice due to the continuity of the graphene layers in the hole array metamatenal. These features make the graphene-hBN metamaterial a skeptical design for practical purposes and mid-infrared multi-functional operations such as sensing. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

In this paper, we propose a methodology to maximize the absorption bandwidth of a metal-insulator-metal (MIM) based absorber. The proposed structure is made of a Cr-Al2O3-Cr multilayer design. At the initial step, the optimum MIM planar design is fabricated and optically characterized. The results show absorption above 0.9 from 400 nm to 850 nm. Afterward, the transfer matrix method is used to find the optimal condition for the perfect light absorption in an ultra-broadband frequency range. This modeling approach predicts that changing the filling fraction of the top Cr layer can extend light absorption toward longer wavelengths. We experimentally proved that the use of proper top Cr thickness and annealing temperature leads to a nearly perfect light absorption from 400 nm to 1150 nm, which is much broader than that of a planar design. Therefore, while keeping the overall process lithography-free, the absorption functionality of the design can be significantly improved. The results presented here can serve as a beacon for future performance-enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity. (c) 2018 Chinese Laser Press

The phase change behavior of vanadium dioxide (VO2) has been widely explored in a variety of optical and photonic applications. Commonly, its optical parameters have been studied in two extreme regimes: hot (metallic) and cold (insulating) states. However, in the transition temperatures, VO(2)acts like an inherent metamaterial with mixed metallic-insulating character. In this range, the portions of metallic and insulating inclusions are tuned by temperature, and therefore a gradual change of optical parameters can be achieved. In this paper, a universal hybrid modeling approach is developed to model VO(2)in the intermediate region. For this aim, the measured reflectivity data, is analyzed and matched through the transfer matrix method (TMM) simulations where an effective medium theory (EMT) is employed. Based on the findings of this approach, not only the relative portions of inclusions are tailored but also their grain shapes are significantly altered in the transition range. Finally, the modeling approach is testified by experimental findings through dynamic device applications operating at short and mid infrared wavelengths. In addition, the hysteretic behaviors on electrical, optical, and structural parameters of the VO(2)film along the heating and cooling cycles are demonstrated by the experiments and scrutinized by the simulations.