Chucai Guo

Coherent perfect absorption and transparency in a nanostructured graphene film

We show numerically that both coherent perfect absorption and transparency can be realized in a monolayer graphene. The graphene film, doped and patterned with a periodical array of holes, can support plasmonic resonances in the Mid-infrared range. Under the illumination of two counter-propagating coherent optical beams, resonant optical absorption may be tuned continuously from 99.93% to less than 0.01% by controlling their relative phase which gives a modulation contrast of 40 dB (about 30 dB for transmission). The phenomenon provides a versatile platform for manipulating the interaction between light and graphene and may serve applications in optical modulators, transducers, sensors and coherent detectors.

Unidirectional transmission in non-symmetric gratings made of isotropic material

We achieve a broadband unidirectional transmission or One-way diffraction grating by cascading two parallel gratings made of isotropic material with different periods. In order to significantly reduce the reciprocal transmission of the zero order, one of them is chosen to be a subwavelength grating and designed as a wideband reflector for the incident-wave. It is demonstrated that more than 65 percent of the incident-wave energy can be transmitted unidirectionally with less than 0.22 percent transmission in the opposite direction at normal incidence for TE polarization. And, the relative bandwidth of the unidirectional transmission is greater than 10 percent.

Metallic nanofilm half-wave plate based on magnetic plasmon resonance

We proposed and fabricated a nanofilm half-wave plate consisting of periodic arrays of orthogonally coupled slit-hole resonator structures in Au film. Experimental results reveal that 95.2% of energy of the incident linearly polarized light is converted to the perpendicular polarization direction after reflection from the nanostructure. The wave plate is single layer with only 180 nm thickness, which is much thinner than the operation wavelength. Our method can be expanded to other resonant structures or transmitted case.

Electrically controlling the polarizing direction of a graphene polarizer

We theoretically demonstrate a polarizer with an electrically controllable polarizing direction in the far infrared range using two orthogonal periodic arrays of graphene ribbons, which have different widths and are supported on a dielectric film placed on a thick piece of metal. The operation mechanism originates from the polarization-dependent resonant absorption of the two orthogonal graphene ribbons, which can be respectively controlled with different external bias voltages. The operation wavelength can be expanded to terahertz (THz) radiation.

One-way transmission of linearly polarized light in plasmonic subwavelength metallic grating cascaded with dielectric grating

We show that optical transmission of linearly polarized light through a plasmonic subwavelength metallic grating cascaded with a dielectric grating at a 45° angle to each other is asymmetric in opposite directions. A key characteristic of this asymmetric transmission is that the polarization of the transmitted light is changed. Simulation results reveal that transmission of 0.92 in one direction and 10(-5) in the opposite direction can be obtained at normal incidence at a wavelength of 1550 nm. Because of their high optical performance and loose fabrication requirements, the structures may provide practical applications in the control of light transmission.

Broadband wave absorption in single-layered and nonstructured graphene based on far-field interaction effect

We present a wave absorption design consisting of periodical arrays of dielectric bricks on the dielectric substrate, which is coated with single-layered and nonstructured graphene, supported by a thick piece of metal. The design is demonstrated to broadband near-perfect absorption with 0.82 terahertz (THz) bandwidth of over 90% absorption and with central frequency of 1.68 THz. The broadband absorption mechanism originates from two contributions. Firstly, the periodical arrays of dielectric bricks on the nonstructured graphene can provide both a set of graphene plasmon resonances with large relative frequency interval and relative radiation rate γ/ω in the THz range. Secondly, the linewidth of each resonance can be broadened by the far-field interaction between neighboring resonators to overlap and spread over a wide frequency region in the THz range. The design in this paper is simple, and consequently facilitates the fabrication and promotes the application of broadband graphene absorbers.

Monolayer-graphene-based perfect absorption structures in the near infrared.

Subwavelength perfect optical absorption structures based on monolayer-graphene are analyzed and demonstrated experimentally. The perfect absorption mechanism is a result of critical coupling relating to a guided mode resonance of a low index two-dimensional periodic structure. Peak absorption over 99% at wavelength of 1526.5 nm with full-width at half maximum (FWHM) about 18 nm is demonstrated from a fabricated structure with period of 1230 nm, and the measured results agree well with the simulation results. In addition, the influence of geometrical parameters of the structure and the angular response for oblique incidence are analyzed in detail in the simulation. The demonstrated absorption structure in the presented work has great potential in the design of advanced photo-detectors and modulators.

Electrically tuneable directional coupling and switching based on multimode interference effect in dielectric loaded graphene plasmon waveguides

We have numerically demonstrated that electrically tuneable directional coupling and switching can be realized based on the multimode interference effect in dielectric-loaded graphene plasmon waveguides (DLGPWs) of our own design. The total field profile resulting from a superposition of all guided modes in the multimode dielectric-loaded graphene plasmon waveguide is electrically controllable because the propagation properties of the first three guided modes supported by the DLGPW can be effectively manipulated by electrostatic doping of graphene. The functional size of the device is only several micrometres, which is much smaller than the working wavelength. Such electrically controlled multifunctional devices may find potential applications in high-density integrated active plasmonic circuits.

Broadband terahertz absorber based on multi-band continuous plasmon resonances in geometrically gradient dielectric-loaded graphene plasmon structure

We propose a broadband terahertz absorber consisting of nonstructured graphene loaded with arrays of elliptic dielectric cylinders. The relative bandwidth for the absorption above 90% reaches about 65%. The working mechanism of broad bandwidth mainly comes from two aspects. One is that the nonstructured graphene loaded with elliptic dielectric cylinders provides multiple discrete graphene plasmon resonances with large relative frequency interval. The other is that, for each discrete resonance, there exists a set of continuous plasmon resonances because the width of the dielectric structure varies continuously and gradiently. The broadband terahertz absorber we demonstrate here, based on geometrically gradient dielectric structures and nonstructured graphene, avoids the graphene processing, which shows great potential applications in related devices.

Monolayer-graphene-based broadband and wide-angle perfect absorption structures in the near infrared

Broadband optical absorption structures in the near infrared by coupling monolayer-graphene with periodical metal structures are proposed and demonstrated numerically. Optical absorption of graphene with over-50%-absorption bandwidth up to hundreds of nanometer caused by magnetic dipole resonances and magnetic coupling effect are investigated in detail, and the demonstrated bandwidths are one order higher than those caused by dielectric guiding mode resonances. In addition, the influences of geometrical parameters of structures are fully analyzed and these demonstrated structures show angular-insensitive absorption for oblique incidence in a large angular range. The demonstrated absorption structures in this work provide new design ideas in the realization of advanced graphene-based optoelectronic devices.