Multifunctional space–time phase modulated graphene metasurface

Abstract

Metasurfaces provide special features for manipulating electromagnetic wavefronts that are not possible with conventional optical devices. A common approach in designing metasurfaces has been the use of spatially varying metallic and/or dielectric nanoantennas separated with a subwavelength distance to obtain the required local phase change yielding the desired optical performance. In this paper, we propose a space–time phase modulation technique based on a graphene metasurface with the possibility of actively manipulating the electromagnetic wavefront. In this technique, we utilize graphene microribbon arrays that exhibit resonant behavior at terahertz (THz) frequencies. By applying an alternating voltage with a particular modulation frequency and phase, the time-dependent changes in the complex refractive indices of the graphene ribbons can be induced. This phenomenon results in the active control of the reflection amplitude and phase and the generation of the harmonic frequencies in the output reflection spectra. Theoretically, by using the Floquet analysis, it is shown that the reflected wave has harmonic frequencies, and the phase of the reflection wave at each harmonic component changes through changing the modulation phase of each graphene ribbon. The performance of the wavefront manipulation technique is evaluated using the finite difference time domain method and the circuit model. The results of the proposed circuit model are in good agreement with those of the full-wave simulation. Additionally, the applications of the proposed space–time phase modulated graphene metasurface for realizing an anomalous reflector and a lens with a tunable focal length are explained in detail.