Reconfigurable ultrathin optical devices via modulation of graphene

Creating an ultrathin optical device has always been a goal for the optics engineers to pursue in order to realize miniaturized optics systems. To achieve this goal, ultrathin optical devices with different kinds of structures and functionalities have been extensively studied, but most of them are not reconfigurable, meaning that their optical properties cannot been tuned once fabricated. Multiple tuning methods have been proposed to overcome this issue. Among them, the electrical tuning of graphene stands out for its ultrafast tuning speed, broad tuning spectrum and atomically thin thickness.In this dissertation, I will first introduce metasurfaces and two-dimensional (2D) materials, which are the two major platforms to build ultrathin optical devices, as well as the working principles of tunable optical devices based on graphene. Next, I will use three chapters to discuss the major results of my related research works: (1) A reflective-type tunable metasurface that combines metallic plasmonic structures with graphene operating in the near-infrared spectrum (≤3.0µm) has been proposed and experimentally demonstrated. Specifically, the applied voltage to tune this device can be rather small due to the 'step-like' change of graphene's conductivity at this spectral range. Moreover, since graphene's interband transition is less sensitive to its carrier mobility, this device can work well with wet-transferred Chemical Vapor Deposition (CVD) graphene that has low mobility. In addition, by carefully designing the structure parameters, the metasurface can be tunable in both near-infrared and mid-infrared spectra under different polarizations of light. (2) A bilayer structure made of graphene and α-phase molybdenum trioxide (α-MoO3) is studied on its tunability of optical forces. The simulation results show that under plane-wave illumination, both the amplitude and direction of the optical forces exerted on a nanoparticle above this bilayer structure can be tuned in the mid-infrared spectral range. The dynamics of the optical forces can be understood by studying the corresponding isofrequency contours of the hybrid plasmon-phonon polaritons supported by the graphene/α-MoO3 bilayer. We also analytically reproduced the numerical results using the dipole approximation method, revealing the origin of the optical forces. (3) An electrically tunable graphene/BIC metasurface has been demonstrated at a working wavelength of around 3 µm. We have improved the coupling strength between graphene and the BIC metasurface by tuning the asymmetric factor, which is confirmed from absorption spectra that were measured using FTIR spectrometer. Furthermore, I have tested the electrical tuning of the metasurface utilizing the "Cut and Stick" method of ion gel. Owing to the high Q-factor of BIC metasurface, a remarkable tuning of the optical response was achieved, even though the resonant wavelength shift was only 0.01 µm. Measured modulation depth ΔT\/T_max=26.8 % at 3.03 µm for reflection and ΔR\/R_max=28.9 % at 3.00 µm for transmission with a small bias voltage of ±2 V, which can be even higher if the beam divergence can be further suppressed.--Author's abstract