Graphene-based Hyperbolic Metamaterials Research Articles

Optical properties of 1D quasiperiodic structures containing graphene-based hyperbolic metamaterials

Optical properties of 1D quasiperiodic structures containing graphene-based hyperbolic metamaterials

Active control of dielectric singularities in indium-tin-oxides hyperbolic metamaterials

Dielectric singularities (DSs) constitute one of the most exotic features occurring in the effective permittivity of artificial multilayers called hyperbolic metamaterials (HMMs). Associated to DSs, a rich phenomenology arises that justifies the ever-increasing interest profuse by the photonic community in achieving an active control of their properties. As an example, the possibility to “canalize” light down to the nanoscale as well as the capability of HMMs to interact with quantum emitters, placed in their proximity, enhancing their emission rate (Purcell effect), are worth mentioning. HMMs, however, suffer of an intrinsic lack of tunability of its DSs. Several architectures have been proposed to overcome this limit and, among them, the use of graphene outstands. Graphene-based HMMs recently shown outstanding canalization capabilities achieving λ/1660 light collimation. Despite the exceptional performances promised by these structures, stacking graphene/oxide multilayers is still an experimental challenge, especially envisioning electrical gating of all the graphene layers. In this paper, we propose a valid alternative in which indium-tin-oxide (ITO) is used as an electrically tunable metal. Here we have numerically designed and analyzed an ITO/SiO2 based HMM with a tunable canalization wavelength within the range between 1.57 and 2.74 μm. The structure feature light confinement of λ/8.8 (resolution of about 178 nm), self-focusing of the light down to 0.26 μm and Purcell factor of approximately 700. The proposed HMM nanoarchitecture could be potentially used in many applications, such as ultra-fast signal processing, high harmonic generation, lab-on-a-chip nanodevices, bulk plasmonic waveguides in integrated photonic circuits and laser diode collimators.

Graphene hyperbolic metamaterials: Fundamentals and applications

Metamaterials have shown potential for next-generation optical materials since they have special electromagnetic responses which cannot be obtained in natural media. Among various metamaterials, hyperbolic metamaterials (HMMs) with highly anisotropic hyperbolic dispersion provide new ways to manipulate electromagnetic waves. Besides, graphene has attracted lots of attention since it possesses excellent optoelectronic properties. Graphene HMMs combine the extraordinary properties of graphene and the strong light modulation capability of HMMs. The experimental fabrication of graphene HMMs recently proved that graphene HMMs are a good platform for terahertz optical devices. The flexible tunability is a hallmark of graphene-based HMMs devices by external gate voltage, electrostatic biasing, or magnetic field, etc. This review provides an overview of up-to-now studies of graphene HMMs and an outlook for the future of this field.

Tunable optical topological transition of Cherenkov radiation

Approaches to generate and manipulate Cherenkov radiation (CR) are challenging yet meaningful. Optical topological transition (OTT) in novel materials and metamaterials is also promising for modern photonics. We study the OTT of CR in graphene-based hyperbolic metamaterials (GHMs) for the first time. In GHMs, conventional and hyperbolic CR can be switched when crossing the topological transition frequency. This frequency can be altered by metamaterial components and external optical elements. For instance, external ultrafast optical pumps cause an ultrafast OTT from the elliptical to the hyperbolic state. Then, hyperbolic CR can be excited by low-energy electrons by leveraging the excellent photothermal properties of graphene. Hyperbolic CR vanishes when the GHM returns to its original state. Furthermore, graphene nonlocality occurs when the electron velocity is low enough, corresponding to a large wave vector. Concretely, when the electron velocity approaches the Fermi velocity of graphene, a nonlocality-induced OTT modifies the plasmonic properties of the GHM and brings a new lower velocity threshold of hyperbolic CR. Therefore, hyperbolic CR can only be induced in a limited velocity range. These findings pave the way for understanding CR properties in active plasmonic metamaterials and may be applied to complex photonic and polaritonic systems.

Using High-k VPP Modes in Grating-Coupled Graphene-Based Hyperbolic Metamaterial for Tunable Sensor Design

Volume plasmon polariton (VPP), a high- k mode that arises due to the coupling between two even modes of adjacent layers of an hyperbolic metamaterial (HMM) configuration, is very difficult to be excited by using prism coupling technique due to huge wave-vector mismatch. In this work, we present a graphene-based HMM structure integrated with metal grating to facilitate excitation of VPP modes. A graphene HMM is composed of multilayer graphene super-lattice similar to metal-dielectric super-lattice structure. We report the analytical formulation of the dispersion relation and numerical analysis of the characteristics of the excited VPP modes for the proposed structure in Terahertz frequency region. The best achieved imaging resolution (spatial) of our proposed structure is 15 nm when used as an infra-red imaging platform. As a sensing platform, a maximum sensitivity of 11,050 nm/RIU is achieved for this configuration. The tunability of the resonance wavelength with respect to the structural parameters of the device is also studied and confirmed. Such promising findings are expected to make the proposed structure with integrated excitation coupler a potential candidate for tunable sensor design for different nanophotonic applications, including imaging, and, biomedical and chemical sensing applications.

Graphene-based tunable hyperbolic microcavity

Graphene-based hyperbolic metamaterials provide a unique scaffold for designing nanophotonic devices with active functionalities. In this work, we have theoretically demonstrated that the characteristics of a polarization-dependent tunable hyperbolic microcavity in the mid-infrared frequencies could be realized by modulating the thickness of the dielectric layers, and thus breaking periodicity in a graphene-based hyperbolic metamaterial stack. Transmission of the tunable microcavity shows a Fabry–Perot resonant mode with a Q-factor > 20, and a sixfold local enhancement of electric field intensity. It was found that by varying the gating voltage of graphene from 2 to 8 V, the device could be self-regulated with respect to both the intensity (up to 30%) and spectrum (up to 2.1 µm). In addition, the switching of the device was considered over a wide range of incident angles for both the transverse electric and transverse magnetic modes. Finally, numerical analysis indicated that a topological transition between elliptic and type II hyperbolic dispersion could be actively switched. The proposed scheme represents a remarkably versatile platform for the mid-infrared wave manipulation and may find applications in many multi-functional architectures, including ultra-sensitive filters, low-threshold lasers, and photonic chips.

The tailored saturated-unsaturateddefect modes obtained by nonlinear threshold light in graphene-based hyperbolicmetamaterial

The tailored saturated-unsaturated defect modes are realized in the proposed graphene-based hyperbolic metamaterials (HMs) in the mid-infrared and near-infrared bands by using the threshold light. In the case of saturated absorption, graphene is identified as a transparent and lossless medium. The effects of the incident angle, the number of periods, and the chemical potential of graphene are also taken into consideration. The computed results show that the greater saturated defect mode (SDM) can be observed at a smaller incident angle and a larger period. The unsaturated defect mode (USDM) cannot be affected by the incident angle while it can be controlled by the chemical potential. In addition, two novel threshold lights (0.2 MW/cm2 and 60 MW/cm2) producing bistable graphene absorption are analyzed by adopting nonlinear Kerr (NLK) materials. These results can be applied to nonlinear optical switches and logic devices.

Magnetic-field control of near-field radiative heat transfer between graphene-based hyperbolic metamaterials

Due to the optical transitions between non-equidistant quantized Landau levels in an external magnetic field, graphene can be employed to dynamically tune the near-field radiative heat transfer (NFRHT). In this paper, we investigate the magnetic-field control of NFRHT between two graphene-based hyperbolic metamaterials (GHMs). We find that the magnetic field significantly regulates the NFRHT between two GHMs via modifying the intrinsic hyperbolic modes. Specifically, the radiative heat transfer in the low-frequency range is remarkably suppressed for chemical potential of graphene of 0.05 eV accompanied by the splitting of the heat flux peak with the increase in the magnetic field intensity. We also analyze the magnetoresistance effect related to the heat flux, which reaches 78.23% when H = 7 T. Moreover, we find that the effect of the magnetic field on the hyperbolic modes of GHMs is much stronger for lower chemical potentials. We look forward to the applications of our findings in dynamical thermal management at the nanoscale.

Dynamically tunable angular optical transparency induced by photonic topological transition in graphene-based hyperbolic metamaterials

Based on photonic topological transition (PTT), the dynamically tunable angular optical transparency of graphene-based hyperbolic metamaterials is theoretically and numerically investigated in mid-infrared region. The results indicate that an ultra-narrow angular optical transparency window with angular full width at half maximum of 0.8° appears near the transition point of PTT for TM waves, which is achieved by tailoring the metamaterial's equi-frequency surface from closed ellipsoid to open hyperboloid. In addition, we further propose the application of TM-pass/TE-reflect polarization beam splitter in the case of normal incidence. Besides, the broadband of angular optical transparency can be realized by changing the Fermi level of graphene.

Perfect Narrowband Absorber Based on Patterned Graphene-Silica Multilayer Hyperbolic Metamaterials

Graphene-based hyperbolic metamaterials are well known for their optical anisotropy, high absorption of electromagnetic radiation, and low energy loss. We proposed a novel multilayer graphene-silica hyperbolic metamaterial designed as a grating structure providing narrowband near-perfect absorption of radiation in a mid-infrared band. The absorber is designed for insensitivity of the absorptance peak positioning on the angle of the incident radiation and distinct polarization selectivity of absorption for the TM mode only. A new way of handling the absorption is achieved by varying the thickness of its combined graphene-silica layers. Possibly, the type of absorber will be applicable in the fields of optical sensing, photoelectric detection, and other potential fields.

Dynamically tunable directional subwavelength beam propagation based on photonic spin Hall effect in graphene-based hyperbolic metamaterials.

Photonic spin Hall effect (PSHE) of type II hyperbolic metamaterials is achieved due to near filed interference, which provides a way to decide the propagation direction of subwavelength beam. In this paper, we propose graphene-based hyperbolic metamaterials (GHMMs), which is composed of the alternating graphene/SiO2 multilayer. The numerical results show that when a dipole emitter is placed at the boundary of the GHMMs, the subwavelength beam with λ/40 full-with half maximum can be excited and propagates along the left or right channel, which is dependent on polarization handedness. In addition, we further demonstrate that the unidirectional propagation angle can be dynamically tuned by changing the external electric field bias applied to graphene.

Graphene-based hyperbolic metamaterial as a switchable reflection modulator.

A tunable graphene-based hyperbolic metamaterial is designed and numerically investigated in the mid-infrared frequencies. Theoretical analysis proves that by adjusting the chemical potential of graphene from 0.2 eV to 0.8 eV, the reflectance can be blue-shifted up to 2.3 µm. Furthermore, by modifying the number of graphene monolayers in the hyperbolic metamaterial stack, we are able to shift the plasmonic resonance up to 3.6 µm. Elliptic and type II hyperbolic dispersions are shown for three considered structures. Importantly, a blue/red-shift and switching of the reflectance are reported at different incident angles in TE/TM modes. The obtained results clearly show that graphene-based hyperbolic metamaterials with reversibly controlled tunability may be used in the next generation of nonlinear tunable and reversibly switchable devices operating in the mid-IR range.

Enhancement and modulation of spontaneous emission near graphene-based hyperbolic metamaterials

We theoretically investigate the spontaneous emission of a quantum emitter near the surface of a hyperbolic metamaterial (HMM) which is alternately stacked by graphene and silicon (Si). Compared with that near the surface of ordinary material, spontaneous emission decay rate near HMM has an giant enhancement in a very wide frequency range because of the excitation of highly localized bulk plasmon polariton in HMM. Even comparing with that near graphene under the same conditions, the spontaneous emission decay rate near HMM can be enhanced by several orders of magnitude. Furthermore, the spontaneous emission decay rate determined by HLBPPs can be flexibly modulated by chemical potential which is tuned by gate voltage. In addition, the interaction between two quantum emitters nearby graphene-based HMM in the reflection configuration is also considered, and the super- and subradiative regimes can be obtained. More importantly, the superradiative state to the subradiative state of the system can be switched freely by external field. The results in this study offer another platform to enhance spontaneous emission and tune the interaction between two emitters.

Graphene-based hyperbolic metamaterials for a tunable subwavelength dark hollow beam.

Hyperbolic metamaterials have recently been widely investigated in nanophotonics systems. Here, we propose an alternating graphene/${{\rm SiO}_2}$SiO2 multilayer structure as an anisotropic medium with hyperbolic dispersion. When in-plane and out-of-plane effective permittivity is negative and positive, respectively, the incident beam (transverse magnetic polarization wave) can be split into two subwavelength beams, and a dark hollow beam can be achieved for circularly polarized incidence. Also, the size of the dark hollow beam can be tuned by changing the Fermi level. Our method is believed to be used as a tunable optical tweezer for controlling molecules.

Tunable terahertz structure based on graphene hyperbolic metamaterials

Photoexcited graphene can behave as the gain medium aiming to come up with the coherent radiation at low THz spectral region. However, its response is very weak because of its atomic dimensions. Herein, a different design aiming to obtain effective tunable THz amplifiers, having small dimensions and lasers with broadband operation based on active THz hyperbolic metamaterials (HMM) is demonstrated. HMMs are considered by employing multiple stacked photoexcited graphene sheets divided by dielectric spacers. Herein, we study and characterize the hyperbolic THz regime of the studied atomic active HMM. A broadband slow-wave propagation regime takes place if the graphene-based HMM system is periodically patterned. This occurs due to the hyperbolic dispersion. Doing so, reconfigurable amplification of THz waves in a broad-spectrum region is attained. This might be engineered by tuning the quasi-Fermi level of graphene. Moreover, the mechanisms leading to the increase of the frequency region of bound surface wave have been proposed in the frame of the current study.

Tunability of the Brewster angle and dispersion type of the asymmetric graphene-based hyperbolic metamaterials

In this paper, the reflection and transmission properties of the plane electromagnetic waves are investigated at the interface of an asymmetric graphene-based hyperbolic metamaterial composed of alternating stacks of graphene monolayers and dielectric layers. It is shown that the Brewster angle can be changed by tailoring the orientation of the optical axis of the considered structure as well as the chemical potential of the graphene. Furthermore, the propagation of the wave vectors and Poynting vectors are studied in the given structure. Such structures have singlet dispersion behavior for the transverse electric wave and triplet dispersion behavior for the transverse magnetic wave. These types of dispersion can be switched by modifying the orientation of the optical axis, the chemical potential of the graphene, and the frequency of the impinging wave. Such tunability characteristics of the graphene-based metamaterials may be practical to design optical devices such as hyperlenses and tunable optical filters.

Nonlocal quantum gain facilitates loss compensation and plasmon amplification in graphene hyperbolic metamaterials

Graphene-based hyperbolic metamaterials have been predicted to transport evanescent fields with extraordinarily large vacuum wave vectors. It is particularly at much higher wave vector values that the commonly employed descriptional models involving structure homogenization and assumptions of an approximatively local graphene conductivity start breaking down. Here, we combine a nonlocal quantum conductivity model of graphene with an exact mathematical treatment of the periodic structure to develop a toolset for determining the hyperbolic behavior of these graphene-based hyperbolic metamaterials. The quantum conductivity model of graphene facilitates us to predict the plasmonic amplification in graphene sheets of the considered structures. This allows us to reverse the problem of Ohmic and temperature losses, making this simple yet powerful arrangement practically applicable. We analyze the electric field distribution inside the finite structures, concluding that Bloch boundary solutions can be used to predict their behavior. With the transfer-matrix method, we show that at finite temperature and collision loss, we can compensate for losses, restoring imaging qualities of the finite structure via an introduction of chemical imbalance.

Tunable terahertz amplification based on photoexcited active graphene hyperbolic metamaterials [Invited

The efficient amplification and lasing of electromagnetic radiation at terahertz (THz) frequencies is a non-trivial task achieved mainly by quantum cascade laser configurations with limited tunability and narrowband functionality. There is a strong need of compact and efficient THz electromagnetic sources with reconfigurable operation in a broad frequency range. Photoexcited graphene can act as the gain medium to produce coherent radiation at low THz frequencies but its response is very weak due to its ultrathin thickness. In this work, we demonstrate an alternative design to achieve efficient tunable and compact THz amplifiers and lasers with broadband operation based on active THz hyperbolic metamaterials (HMM) designed by multiple stacked photoexcited graphene layers separated by thin dielectric sheets. The hyperbolic THz response of the proposed ultrathin active HMM is analytically and numerically studied and characterized. When the graphene-based HMM structure is periodically patterned, a broadband slow-wave propagation regime is identified, thanks to the hyperbolic dispersion. In this scenario, reconfigurable amplification of THz waves in a broad frequency range is obtained, which can be made tunable by varying the quasi-Fermi level of graphene. We demonstrate that the THz response of the presented tunable THz amplifiers or lasers is controlled by the incident optical pumping (photodoping) and the loaded dielectric materials in the HMM waveguide array, an interesting property that can have great potential for THz amplification, emission, and sensing applications.

Contribution of terahertz waves to near-field radiative heat transfer between graphene-based hyperbolic metamaterials**Project supported by the National Natural Science Foundation of China (Grant Nos. 11704175, 11664024, and 61367006).

Hyperbolic metamaterials alternately stacked by graphene and silicon (Si) are proposed and theoretically studied to investigate the contribution of terahertz (THz) waves to near-field radiative transfer. The results show that the heat transfer coefficient can be enhanced several times in a certain THz frequency range compared with that between graphene-covered Si bulks because of the presence of a continuum of hyperbolic modes. Moreover, the radiative heat transfer can also be enhanced remarkably for the proposed structure even in the whole THz range. The hyperbolic dispersion of the graphene-based hyperbolic metamaterial can be tuned by varying the chemical potential or the thickness of Si, with the tunability of optical conductivity and the chemical potential of graphene fixed. We also demonstrate that the radiative heat transfer can be actively controlled in the THz frequency range.

Analysis of tunable and highly confined surface wave in the photonic hypercrystals containing graphene-based hyperbolic metamaterial

The property of surface wave at the interface between a dielectric half-space and a photonic hypercrystals (PHC) containing graphene-based hyperbolic metamaterial (GHMM) is investigated. It is observed that the propagation length, figure-of-merit (FOM), absorption resonance, nanoscale mode confinement and penetration depth of surface wave can be tuned by varying the Fermi energy of graphene sheets via electrostatic biasing. It is also found that the surface states in the hypercrystal containing GHMM allow for both high wave numbers and long propagation lengths at the same time. It is shown that the surface wave in the PHC has extremely low group velocity and the magnitude of group velocity can be well regulated. Here we also analyse the optical forces from the surface wave in the case of Rayleigh particles, and large and tunable radiation force component has been observed.