Ultrathin multi-band coherent perfect absorber in graphene with high-contrast gratings

Trapping wide-angle circularly polarized waves with giant circular dichroism using a single-channel coherent perfect graphene meta-surface absorber

We propose a single-channel coherent perfect graphene meta-surface absorber to trap circularly polarized (CP) waves perfectly through cascading a polarization converting meta-array and a monolayer graphene sheet. To be more specific, the incident CP waves will be reflected into dual sets of orthogonally linearly polarized (LP) waves, where each group of the same polarized reflecting components can achieve identical amplitudes and a 180^° phase difference at a certain frequency to have coherent absorption. In particular, we show that such a single-channel coherent perfect graphene meta-surface absorber possesses the merits of the wide-angular chiral absorbing capacity of CP waves with giant circular dichroism, thus should pave the way for building up more advanced coherent absorbers and offer more freedom to manipulate CP waves.

Independently tunable multi-band terahertz absorber based on graphene sheet and nanoribbons

A multi-band terahertz (THz) absorber based on graphene sheet and nanoribbons is proposed and investigated. In the studied frequency range, five absorption peaks are observed, with four originate from lateral Fabry-Perot resonance (LFPR) and one originates from guided-mode resonance (GMR). The LFPR and GMR peaks behave differently when geometric parameters are adjusted, which makes independent tuning possible. When period increases, the GMR peak red shifts and the frequencies of LFPR peaks remain almost unchanged. On the contrary, as nanoribbon width increases, the frequency of GMR remains almost unchanged while that of LFPRs decrease significantly. With increasing top dielectric layer thickness, the LFPR peaks blue shift while the GMR peak red shifts. In addition, the absorber has the merit of multi-band high absorptivity and frequency stability under large angle oblique incidence. The proposed terahertz absorber may benefit the areas of medical imaging, sensing, non-destructive testing, THz communications and other applications.

Transforming zero-index media into geometry-invariant coherent perfect absorbers via embedded conductive films

In this work, we demonstrate an approach to realize geometry-invariant multi-channel coherent perfect absorbers by embedding ultrathin conductive films in zero-index media. Coherent perfect absorption can be achieved for waves incidents from an arbitrary number of input channels as long as the total width of the channels equals to a critical value that is only determined by the length and material parameters of the conductive films instead of their shapes and positions. The absorption attributes to induced currents in the conductive films by the electric fields of incidence, and the shape- and position-independent characteristics originate from the uniformly distributed electric fields inside the zero-index media. By using dielectric photonic crystals and photonic-doped zero-index media, we numerically demonstrate such an interesting transformation from zero-index media to coherent perfect absorbers. Furthermore, ultrathin coherent perfect absorbers based on zero-index media are also demonstrated in waveguides. Our work reveals a unique mechanism to change the material responses between zero-index media and coherent perfect absorbers.

Ultra-high quality graphene perfect absorbers for high performance switching manipulation

Wavelength-selective light absorption and the related switching operations are highly desired in optical devices. Herein, we report the approach for ultra-high quality (Q) graphene perfect optical absorption, which possesses impressive performance in switching manipulation. A record-breaking Q-factor (up to 10⁵) is observed, suggesting one or two orders of magnitude larger than that of the conventional graphene absorbers. The ultra-low external leakage loss rate of quasi-bound states in the continuum (BIC) resonator and the ultra-low intrinsic absorption loss rate in the resonant mode volume are the main contributions for the ultra-high Q perfect absorption. By introducing a Kerr nonlinear medium, spectral relative intensity can be changed from 0 to 100% when an ultra-low pump light with the intensity of only 5 kW cm^-2 is used. After a rather slight tuning of the refractive index (Δn = 5×10^-4) for the resonators, the absorption contrast ratio reaches 31 dB. The switching related spectral wavelength shift sensitivity is up to 915 nm/RIU and the figure of merit (FOM) is 50 833. These features confirm the ultra-high tunability and switching manipulation. It is believed that the ultra-high Q-factor absorption offered by all-dielectric configuration provides plentiful potential applications for graphene-based devices in the all-optical switch, modulator, notch filter, etc.

Anisotropic Photonics Topological Transition in Hyperbolic Metamaterials Based on Black Phosphorus

Based on in-plane anisotropy of black phosphorus (BP), anisotropic photonics topological transition (PTT) can be achieved by the proposed hyperbolic metamaterials structure, which is composed of alternating BP/SiO₂ multilayer. Through effective medium theory and calculated iso-frequency contour, PTT can be found by carefully choosing the incident plane and other parameters. With the finite element method and transfer matrix method, a narrow angular optical transparency window with angular full width at half maximum of 1.32° exists at PTT. By changing the working wavelength, thickness of SiO₂, or electron doping of black phosphorus, the incident plane of realizing PTT can be modulated, and anisotropic PTT is achieved.