Tunable Nonlinear Response and Hyperbolicity in Two-dimensional Materials through Band Nesting and Potential Metasurface Applications

Abstract

Metasurfaces are artificially designed planar devices that can manipulate the scattering of electromagnetic (EM) waves, including its phase, polarization, amplitude etc. The \textit{holy grail} of metasurface research is to achieve active tuning capabilities, as passive metasurfaces have limited applications and frequency bandwidth. To address this challenge, different techniques have been explored such as electrical, mechanical, thermal, and optical tuning to achieve tunability/reconfigurability. The fundamental properties of EM waves are phase, polarization, amplitude, and frequency. To harness the full advantage of metasurface-based integrated photonics, complete manipulation of EM waves is desired, which means simultaneous control of more than one property. Through arbitrary control of EM wave proeprties, we can achieve flat optics based plenoptic, polarimetric, and hyperspectral imaging systems. The key advantage of two-dimensional (2D) materials as a promising platform for active metasurfaces is the inherent tunability of optical properties with external stimuli. 2D materials-based plasmonics (e.g graphene) offers a promising avenue of realizing dynamically tunable metasurfaces as the carrier concentration can be controlled by electric gating/doping. Most importantly, the electronic bandstructure of 2D materials can be tuned by an external electric field, which affects both linear and nonlinear optical properties. In this work, we have explored tuning of linear birefringence and nonlinear optical suscpetibilities in anisotropic 2D materials by introducing \textit{band nesting}. Band nesting enables enhancement of the resonant terms in both linear and nonlinear optical susceptibilities by maximizing the joint density-of-states. We proposed a mechanism to generate giant anisotropic second harmonic nonlinear response via double resonance effect, where the ideal band setup would be a triplet of nested bands separated by the fundamental resonance energy, $\hbar \omega$ . We demonstrated theoretically that the proposed phenomenon can be realized in bilayer SnS by band tuning and the second harmonic susceptibility is enhanced by several orders of magnitude. Moreover, the tunability of the polarization anisotropy can be useful for realizing novel polarization-sensitive devices. In addition, we showed that the extreme in-plane birefringence in naturally hyperbolic material WTe$_\text{2}$ can be actively tuned by introducing nested pairs of bands via bandstructure engineering with external electric field, and shifting the Fermi level by electronic doping. Both the hyperbolic frequency range and the hyperbolicity type can be actively tuned. The toggling of the hyperbolicity type is done by controlling the interplay of two nested interband transitions along $\Gamma-X$ and $\Gamma-Y$. We expect this scheme of dynamic electrical tuning of material parameters will open new windows towards achieving metasurface-based active manipulations of optical waves.