Tunable light absorption of graphene using topological interface states

Bandwidth tunability of graphene absorption enhancement by hybridization of delocalized surface plasmon polaritons and localized magnetic plasmons

The bandwidth-tunable absorption enhancement of monolayer graphene is theoretically studied in the near-infrared wavelengths. The monolayer graphene is placed on the silver substrate surface with a periodic array of one-dimensional slits. Two absorption peaks are found to result from the hybridization of delocalized surface plasmon polaritons and localized magnetic plasmons. The positions of absorption peaks are accurately predicted by a coupling model of double oscillators. The full width at half maximum of absorption peaks is largely tuned from about 1-200 nm by changing the array period of slits. The effect of the slit size on absorption peaks is also investigated in detail. Our work is promising in applications for photoelectric devices.

Spin-dependent and tunable perfect absorption in a Fabry-Perot cavity containing a multi-Weyl semimetal

Spin-dependent absorption has been widely studied in metamaterials and metasurfaces with chirality since it develops significant applications in multiplexed holograms, photodection, and filtering. Here, the one-dimensional photonic crystal Fabry-Perot (FP) cavity containing a multi-Weyl semimetal (mWSM) defect is proposed to investigate the spin-dependent perfect absorption. Results denote that the distinct refractive indices of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves are present due to the nonzero off-diagonal term of mWSM, thus supporting the perfect absorption of RCP and LCP waves at distinct resonant wavelengths. The different perfect absorption wavelengths of RCP and LCP waves reveal the spin-dependent perfect absorption. By altering the Fermi energy, tilt degree of Weyl cones, Weyl nodes separation, topological charge, and thickness of the mWSM layer, the perfect absorption wavelength of RCP and LCP waves can be regulated conveniently. Particularly, the linear tunable perfect absorption wavelength with thickness of the mWSM layer supports the accurate determination of perfect absorption wavelength at distinct mWSM thicknesses. Our studies develop simple and effective approaches to acquire the spin-dependent and adjustable perfect absorption without the external magnetic field, and can find practical applications in spin-dependent photonic devices.

Strong coupling of multiple optical interface modes with ultra-narrow linewidth in one-dimensional topological photonic heterostructures

Coherent coupling of optical modes with a high Q-factor underpins realization of efficient light-matter interaction with multi-channels in resonant nanostructures. Here we theoretically studied the strong longitudinal coupling of three topological photonic states (TPSs) in a one-dimensional topological photonic crystal heterostructure embedded with a graphene monolayer in the visible frequencies. It is found that the three TPSs can strongly interplay with one another in the longitudinal direction, enabling a large Rabi splitting (∼ 48 meV) in spectral response. The triple-band perfect absorption and selective longitudinal field confinement have been demonstrated, where the linewidth of hybrid modes can reach 0.2 nm with Q-factor up to 2.6 × 10³. Mode hybridization of dual- and triple-TPSs were investigated by calculation of the field profiles and Hopfield coefficients of the hybrid modes. Moreover, simulation results further show that resonant frequencies of the three hybrid TPSs can be actively controlled by simply changing the incident angle or structural parameters, which are nearly polarization independent in this strong coupling system. With the multichannel, narrow-band light trapping and selectively strong field localization in this simple multilayer regime, one can envision new possibilities for developing the practical topological photonic devices for on-chip optical detection, sensing, filtering, and light-emitting.

Topological plasmons in stacked graphene nanoribbons

In this Letter, we theoretically study the topological plasmons in Su-Schrieffer-Heeger (SSH) model-based graphene nanoribbon (GNR) layers. We find that for the one-dimensional (1D) stacked case, only two topological modes with the field localized in the top or bottom layer are predicted to exist by the Zak phase. When we further expand the stacked 1D GNR layers to two-dimensional (2D) arrays in the in-plane direction, the topology is then characterized by the 2D Zak phase, which predicts the emergence of three kinds of topological modes: topological edge, surface, and corner modes. For a 2D ribbon array with Nx × Ny units, there are 4(Ny - 1), 4(Nx - 1), and 4 topological edge, surface, and corner modes, and the field is highly localized at the edge/surface/corner ribbons. This work offers a platform to realize topological modes in GNRs and could be important for the design of topological photonic devices such as lasers and sensors.

Ultra-broadband and completely modulated absorption enhancement of monolayer graphene in a near-infrared region

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

Examining the optical model of graphene via the photonic spin Hall effect

In modern optics, there are two general models to describe the behavior of light in graphene: the zero-thickness model and the slab model. The difference in physical phenomena predicted by the two models is very small, which is hardly distinguished by traditional measurement methods. Therefore, which model can describe the light-matter interaction in graphene more exactly is still a challenging issue. In this work, based on the sensitive optical phenomenon called the photonic spin Hall effect, the small difference can be magnified to a detectable level by the weak-value amplification. The experimental results show that the zero-thickness model can more accurately describe the interaction between light and monolayer or bilayer graphene, while the case of more than two layers, which can no longer be regarded as two-dimensional thickness, should be described by the slab model. Our results may provide information on light interacting with graphene for future investigation in photonics and optoelectronics.

Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry-Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry-Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry-Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry-Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

Correlation between the optical absorption and twisted angle of bilayer graphene observed by high-resolution reflectance confocal laser microscopy

We report a systematic study of the optical absorption of twisted bilayer graphene (tBLG) across a large range of twist angles from 0° to 30° using a high-resolution reflectance confocal laser microscopy (RCLM) system. The high-quality single crystalline tBLG was synthesized via the efficient plasma enhanced chemical vapor deposition techniques without the need of active heating. The sensitivity of acquired images from the RCLM were better than conventional optical microscopes. Although the highest spatial resolution of RCLM is still lower than scanning electron microscopes, it possesses the advantages of beam-damage and vacuum free. Moreover, the high intensity-resolution (sensitivity) images firstly allowed us to distinguish the slight absorption differences and analyze the correlation between the optical absorption and twisted angle of tBLG after data processing procedures. A maximum absorption (minimum transmission) was observed at the stacking angle of tBLG from 10° to 20°, indicating the interplay between the laser and the electron/hole van-Hove singularities when tBLG oriented around the critical angle (θ_c∼13°). The twisted angle correlated optical absorption paves an alternative way not only to visibly identify the interlayer orientation of tBLG but also to reflect the characterization of the interlayer coupling via its band structure.

Conjugated topological interface-states in coupled ring resonators

The optical properties of topological photonics have attracted much interest recently because its potential applications for robust unidirectional transmission that are immune to scattering at disorder. However, researches on topological series coupled ring resonators (T-SCRR) have been much less discussed. The existence of topological interface-states (TIS) in the T-SCRR is described for the first time in this article. An approach has been developed to achieve this goal via the band structure of dielectric binary ring resonators and the Zak phase of each bandgap. It is found that an ultra-high-Q with complete transmission is obtained by the conjugated topological series coupled ring resonators due to the excitation of conjugated topological interface-states, which is different from those in conventional TIS. Furthermore, the problem of transmission decreases resulting from high-Q increases in the traditional photonic system is significantly improved by this approach. These findings could pave a novel path for developing advanced high-Q filters, optical sensors, switches, resonators, communications and quantum information processors.