Coherent perfect absorption and asymmetric interferometric light-light control in graphene with resonant dielectric nanostructures

CPA-induced Hz to THz broadband absorber with switchable perfect absorption between radio-microwave and THz frequency spectrum

The coherent perfect absorption (CPA) occurring in the graphene sheet suspended in air can be utilized to develop an ultrathin, ultra-broadband absorber working in the frequency range from a few hertz (Hz) to terahertz (THz) with perfect absorption. A graphene sheet is studied to induce the CPA to cover radio, microwave and lower THz frequency ranges. A graphene resonator able to provide the surface plasmon resonance (SPR) is combined with the graphene sheet to provide CPA at either side of a thin dielectric layer forms metamaterial structure with the cavity and enhances the absorption bandwidth in the THz region by creating a resonance near quasi-CPA frequency. A dielectric silicon resonator is embedded in the structure, which creates dipolar resonances between the resonances obtained by the formed cavity between the graphene sheet and resonator. This enhances the absorption level in the THz region. The absorption bandwidth is further enhanced to 7 THz by including a graphene disc at the top of the silicon resonator. Thus, the multiple multi-order resonances occurring in the silicon dielectric and SPR of graphene resonators are merged with the phenomena of CPA occurring in the graphene sheets to extend the CPA bandwidth in the THz regime. The doping level of graphene or its tunable Fermi energy based on the applied DC electric field provides the tunability in the total obtained absorption bandwidth. The symmetric structure provides polarization-insensitive behavior with an allowed incident angle of more than 45° with more than 90% absorption.

Broadband coherent perfect absorption employing an inverse-designed metasurface via genetic algorithm

Coherent perfect absorption (CPA) possesses the unique characteristics of flexibly and actively molding the flow of light. However, restricted by the low design efficiency and limited geometry variety of metamaterial structures, the common CPA metamaterial absorbers based on artificial design show poor performance in bandwidth operation. Here, we proposed a tungsten-based metamaterial absorber to achieve broadband CPA via employing genetic algorithm inverse design. Under the irradiation of two coherent beams, the high coherent absorption (>90%) can be achieved within a wide range from 1.32 to 3.28 µm. By simply adjusting the relative intensity or phase difference of the two coherent beams, the absorption intensity can be continuously modulated to realize the transition between coherent perfect absorption and coherent perfect transparency. Moreover, the coherent absorption maintains greater than 90% over a broad range of incident angles for both TM and TE polarizations. The scattering matrix theorem is applied to explain the physical mechanism of CPA, and the analytical results exhibit good consistency with the numerical calculations. Such a tungsten-based CPA metamaterial absorber with broadband tunability and exceptional angular stability is expected to be utilized in optical signal processing chips, all-optical modulators, and optical switchers.

Dynamically tunable coherent perfect absorption in topological insulators at oblique incidence

The effective engineering of light absorption has been the focus of intensive research to realize the novel optoelectronic devices based on a topological insulator, a unique topologically protected surface Dirac-state quantum material with excellent prospects in electronics and photonics. Here, we theoretically proposed a versatile platform for manipulating the light-matter interaction employing the dynamically tunable coherent perfect absorption (CPA) in the topological insulator Bi_1.5Sb_0.5Te_1.8Se_1.2(BSTS). By simply varying the phase difference between two coherent counter-propagating beams, the BSTS-based CPA device can be continuously switched from the high transparency state to the strong absorption state, leading to the modulation of absorption ranging from 0.2% to 99.998%. Under the illumination of TE-polarized wave, the high absorption (>90%) can be implemented within a broad range from 0.47 to 1.51 μm through a proper incident angle alteration. In addition, the quasi-CPA wavelength can be flexibly selected by tuning the bulk thickness of BSTS film while maintaining high modulation depth of 10⁴. Such BSTS-based CPA device with flexible tunability, wide absorption modulation range, and high modulation depth is expected to be utilized in a wide range of potential applications such as in next-generation coherent detectors, coherent modulators, all-optical switches, and signal processors.

Infrared all-dielectric Kerker metasurfaces

The unidirectional scattering of electromagnetic waves in the backward and forward direction, termed Kerkers' first and second conditions, respectively, is a prominent feature of sub-wavelength particles, which also has been found recently in all-dielectric metasurfaces. Here we formulate the exact polarizability requirements necessary to achieve both Kerker conditions simultaneously with dipole terms only and demonstrate its equivalence to so-called "invisible metasurfaces". We further describe the perfect absorption mechanism in all-dielectric metasurfaces through development of an extended Kerker formalism. The phenomena of both invisibility and perfect absorption is shown in a 2D hexagonal array of cylindrical resonators, where only the resonator height is modified to switch between the two states. The developed framework provides critical insight into the range of scattering response possible with all-dielectric metasurfaces, providing a methodology for studying exotic electromagnetic phenomena.

Investigation of acoustic plasmons in vertically stacked metal/dielectric/graphene heterostructures for multiband coherent perfect absorption

Coherent absorption, as the time-reversed counterpart to laser, has been widely proposed recently to flexibly modulate light-matter interactions in two-dimensional materials. However, the multiband coherent perfect absorption (CPA) in atomically thin materials still has been elusive. We exploit the multiband CPA in vertically stacked metal/dielectric/graphene heterostructures via ultraconfined acoustic plasmons which can reduce the photon wavelength by a factor of about 70 and thus enable multiple-order resonances on a graphene ribbon of finite width. Under the illumination of two counter-propagating coherent beams, the two-stage coupling scheme is used for exciting multispectral acoustic plasmon resonances on the heterostructure simultaneously, thereby contributing to the ultimate multiband CPA in the mid-infrared region. The strong dependence of the nearly linear dispersion of acoustic plasmons on the chemical potential in graphene and the separation between the metal and the graphene allows the tunability in spectral positions of absorption peaks. Intriguingly, the absorption of each resonant peak is continuously tuned by varying the relative amplitude of two counter-propagating beams, and even their phase difference, respectively. The maximum modulation depth of 4.46*105 is observed. The scattering matrix is employed to demonstrate the principle of CPA and the finite-difference time-domain (FDTD) simulations are used for elucidating the flexible tunability. More importantly, the multiband coherent absorber is robust to the incident angle, and thus undoubtedly benefits extensive applications on optoelectronic and engineering technology areas for modulators and optical switches.

Ultra-narrowband visible light absorption in a monolayer MoS2 based resonant nanostructure

Enhance light absorption in two-dimensional (2D) materials are of great importance for the development of many optoelectronic devices such as photodetectors, modulators and thermal emitters. In this paper, a resonant nanostructure based on subwavelength gratings of monolayer molybdenum disulphide (MoS₂) is proposed. It is shown numerically that the excitation of guided modes in the proposed structure leads to perfect absorption in the visible range. The linewidth of the absorption spectrum can be narrow down to 0.1 nm. The resonance wavelength exhibits an almost linear dependence on the incidence angle. The proposed structure provides a method to design ultra-narrowband absorbers and similar designs can be applied to other 2D materials. It may find applications for optical filters, directional thermal emitters, 2D materials based lasers and others.

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

High-contrast gratings (HCGs) can be designed as a resonator with high-quality factor and surface-normal emission, which are excellent characters for designing optical devices. In this work, we combine HCGs with plasmonic graphene structure to achieve an ultrathin five-band coherent perfect absorber (CPA). The presented CPA can achieve multi- and narrow-band absorption with high intensity under a relatively large incident angle. The good agreement between theoretical analysis and numerical simulated results demonstrates that our proposed HCGs-based structure is feasible to realize CPA. Besides, by dynamically adjusting the Fermi energy of graphene, we realize the active tunability of resonance frequency and absorption intensity simultaneously. Benefitting from the combination of HCGs and the one-atom thickness of graphene, the proposed device possesses an extremely thin feature. Our work proposes a novel method to manipulate coherent perfect absorption and is helpful to design tunable multi-band and ultrathin absorbers.

A Tunable Dual-Band and Polarization-Insensitive Coherent Perfect Absorber Based on Double-Layers Graphene Hybrid Waveguide

A suspended monolayer graphene has only about 2.3% absorption rate in visible and infrared band, which limits its optoelectronic applications. To significantly increase graphene's absorption efficiency, a tunable dual-band and polarization-insensitive coherent perfect absorber (CPA) is proposed in the mid-infrared regime, which contains the silicon array coupled in double-layers graphene waveguide. Based on the FDTD methods, dual-band perfect absorption peaks are achieved in 9611 nm and 9924 nm, respectively. Moreover, due to its center symmetric feature, the proposed absorber also demonstrates polarization-insensitive. Meanwhile, the coherent absorption peaks can be all-optically modulated by altering the relative phase between two reverse incident lights. Furthermore, by manipulating the Fermi energies of two graphene layers, two coherent absorption peaks can move over a wide spectrum range, and our designed CPA can also be changed from dual-band CPA to narrowband CPA. Thus, our results can find some potential applications in the field of developing nanophotonic devices with excellent performance working at the mid-infrared regime.

Graphene-Based Perfect Absorption Structures in the Visible to Terahertz Band and Their Optoelectronics Applications

Graphene has unique properties which make it an ideal material for photonic and optoelectronic devices. However, the low light absorption in monolayer graphene seriously limits its practical applications. In order to greatly enhance the light absorption of graphene, many graphene-based structures have been developed to achieve perfect absorption of incident waves. In this review, we discuss and analyze various types of graphene-based perfect absorption structures in the visible to terahertz band. In particular, we review recent advances and optoelectronic applications of such structures. Indeed, the graphene-based perfect absorption structures offer the promise of solving the key problem which limits the applications of graphene in practical optoelectronic devices.