Wavelength‐Programmable High‐Q Perfect All‐Dielectric Absorption in a Graphene‐Loaded GaP Dimer Metasurface

Ultra-narrow perfect graphene absorber based on critical coupling

Recently, based on the selective excitation of the guided mode, researchers realized quasi-bound states in the continuum (quasi-BICs) in all-dielectric compound grating waveguide structures. In this paper, we introduce a graphene layer into an all-dielectric compound grating waveguide layer supporting quasi-BIC to achieve near-infrared perfect absorption of graphene. The underlying physical mechanism of perfect absorption can be clearly explained by the critical coupling theory derived from temporal coupled-mode theory in a single-mode, one-port system. By changing the Fermi level and the layer number of the graphene, the absorption rate of the system can be flexibly tuned. In addition, by changing the geometric parameter of the compound grating waveguide structure, the radiation coupling rate of the quasi-BIC can also be flexibly tuned. Therefore, the critical coupling condition can be maintained in a broad range of the Fermi level and the layer number of the graphene. The full width at half maximum of the near-infrared perfect absorption peak can be flexibly tuned from 5.7 to 187.1 nm. This bandwidth-tunable perfect absorber would possess potential applications in the design of 2D material-based optical sensors, electrical switchers, and solar thermophotovoltaic devices.

Using critical coupling to achieve monolayer graphene perfect absorber with high-sensitivity and polarization-independence

Carbon nanotube-based electrically tunable broadband terahertz absorbers

The topological edge mode, which exists at the interface of a one-dimensional (1D) topological photonic crystal (PhC) heterostructure, provides the possibility to realize perfect absorption for its strong field localization effects. In this Letter, it is found that a huge absorption enhancement appears because of the excitation of topological edge mode, while the graphene is sandwiched between two 1D PhCs. The single peak perfect absorption is realized by means of the strong coupling of incident light and Tamm plasmon polaritons (TPPs) which is excited with Ag-PhC structure. Moreover, we use a heterostructure constructed by two PhCs, a monolayer graphene and Ag mirror to theoretically demonstrate that multi-channel perfect absorption can be achieved based on the effect of topological edge mode, TPPs and critical coupling. The angular selectivity of the proposed absorber is also investigated. Both of the absorption peaks are extremely narrow, and the absorption can be maintained more than 97% with the incident angle varying from 0° to 50°. Hence, our results may have potential applications in optical switches, thermal emissions, and narrowband selective filters.

We investigate the electrically tunable Electromagnetic induced transparency (EIT)-like effect of active metamaterial structures composed of a wire and a split ring resonator by the simulation, experiment, and temporal coupled-mode theory. It is illustrated that an EIT-like effect appears as a result of weak coupling between bright and dark resonators. Around the EIT-like peak frequency, the superradiant resonance mode of the bright resonator is highly suppressed by the subradiant resonance mode of the dark resonator, and high transmittance as well as large group delay is manifested. By integrating a varactor diode into the EIT structure and altering the bias voltage, the EIT-like effect can be dynamically tuned. As the bias voltage ranges from 0 V to 8 V, the EIT-like peak frequency exhibits a prominent blueshift of 0.22 GHz and the transmittance experiences a modulation with a modulation depth up to 98%. Using the temporal coupled-mode theory, the transmission spectrum of the EIT structure is predicted and the parameters of the resonator system are retrieved.

The absorption of light based on quasi-BICs is a significant factor influencing the performance of solar cells and photodetectors. Nevertheless, the development of multiple narrowband perfect absorbers remains a significant challenge. In this study, three distinct types of BIC were first discovered to coexist within a metasurface structure. This paper proposes a dual grating metasurface (DGM) structure based on three classes of BICs supported by near-infrared spectroscopy. It achieves perfect absorption in four narrow bands dominated by quasi-BICs, with each of the peaks exceeding 99.5%. The physical mechanism of each resonance has been analysed using temporal coupled mode theory, which has revealed the existence of the Symmetry-Protected BIC, Friedrich-Wintgen BIC and Fabry-Pérot BIC. Moreover, the underlying mechanisms of the distinct resonance modes are revealed through the multipolar decomposition of these resonances. The metasurface has significant potential for utilisation as an optical switch, which is capable of achieving an optimal modulation depth, switching contrast, extinction ratio, and insertion loss of 99.9%, 127 932%, -31.1 dB, and 0.0007 dB, respectively. The DGM structure offers a superior quad-frequency synchronised optical switch in comparison to conventional optical switches. And it also exhibited a maximum sensitivity of 328.6 nm RIU-1 and a maximum FOM of 93.9 RIU-1 when used as a sensor. The work presented herein will facilitate the exploration of a novel avenue for the study of ultra-high performance multifunctional devices based on a multitude of types of BICs.

We propose a highly compact structure for near total light absorption in a monolayer of graphene in the visible. The structure consists of a grating slab covered with the graphene monolayer. The grating slab is separated from a metallic back reflector by a dielectric spacer. The structure supports a guided resonance in the visible. We show that such a structure enhances light–matter interactions in graphene via critical coupling by matching the external leakage rate of the guided resonance and the intrinsic loss rate in the system. We also show that, by using the dielectric spacer between the grating and the metallic mirror, near total absorption in the graphene monolayer can be achieved in the visible without the need for thick multilayer dielectric mirrors. The proposed structure could find applications in the design of efficient nanoscale visible-light photodetectors and modulators.

Whispering-gallery-mode optical microresonators have found impactful applications in various areas due to their remarkable properties such as ultra-high quality factor (Q-factor), small mode volume, and strong evanescent field. Among these applications, controllable tuning of the optical Q-factor is vital for on-chip optical modulation and various opto-electronic devices. Here, we report an experimental demonstration with a hybrid structure formed by an ultra-high-Q microtoroid cavity and a graphene monolayer. Thanks to the strong interaction of the evanescent wave with the graphene, the structure allows the Q-factor to be controllably varied in the range of 3.9 × 105 ∼ 6.2 × 107 by engineering optical absorption via changing the gap distance in between. At the same time, a resonant wavelength shift of 32 pm was also observed. Besides, the scheme enables us to approach the critical coupling with a coupling depth of 99.6%. As potential applications in integrated opto-electronic devices, we further use the system to realize a tunable optical filter with tunable bandwidth from 116.5 MHz to 2.2 GHz as well as an optical switch with a maximal extinction ratio of 31 dB and response time of 21 ms.

Dual-band, polarization-insensitive metamaterial perfect absorber based on monolayer graphene in the mid-infrared range

Mirror-coupled toroidal dipole bound states in the continuum for tunable narrowband perfect absorption

Multi-band and high-sensitivity perfect absorber based on monolayer graphene metamaterial

We numerically demonstrate 100% light absorption at near-infrared frequencies via a subwavelength multilayer dielectric grating (SMDG) structure covered by a graphene monolayer. Without using plasmonic response, the total absorption is associated with critical coupling, which is enabled by the synergetic effect of the guided resonance and photonic bandgap of the system. Moreover, calculation results demonstrate that critical coupling can be tuned by varying the SMDG structural parameters or simply modulating the gate voltage on the graphene monolayer, as a result to flexibly control the absorption of the system. Furthermore, the enhanced resonance modes are not local, which can be longitudinally coupled deep into the SMDG, effectively facilitating the light–matter interaction. This control over total absorption offered by our design may hold potential in engineering many ultra-compact and high-performance optoelectronic devices.

Sub-nanometer linewidth perfect absorption in visible band induced by Bloch surface wave

We demonstrate experimentally close to total absorption in monolayer graphene based on critical coupling with guided resonances in transfer printed photonic crystal Fano resonance filters at near infrared. Measured peak absorptions of 35% and 85% were obtained from cavity coupled monolayer graphene for the structures without and with back reflectors, respectively. These measured values agree very well with the theoretical values predicted with the coupled mode theory based critical coupling design. Such strong light-matter interactions can lead to extremely compact and high performance photonic devices based on large area monolayer graphene and other two–dimensional materials.