Parity-time symmetric graphene metasurfaces

Abstract

In the past decade, parity-time (PT) symmetric quantum Hamiltonian systems have received a great amount of research attention [1, 2], since it was shown that a non-Hermitian Hamiltonians can exhibit an entirely real eigenspectrum, provided that the system is invariant under combined operations of parity (P) and time-reversal (T) symmetry. Although the PT symmetry concept in quantum systems is still under intensive debate, its optics and acoustics analogues can be readily realized by exploiting the formal similarities between the Schrodinger and Helmholtz equations. Optics has emerged as the most versatile platform for studying the PT-symmetric system, since the spatially-modulated distributions of optical gain (e.g., laser) and loss (i.e., optical absorber) are technologically viable. In this talk, we will present a PT-symmetric system in the terahertz (THz) domain, by using the optically-pumped, active graphene metasurface [3, 4] paired with a resistive metallic filament. This PT-symmetric system can achieve an exotic, unidirectional reflectionless propagation of THz waves. The amplified stimulated THz emission and tailored plasmon resonances in the graphene metasurface may effectively realize an equivalent negative-resistance converter at THz and far-infrared wavelengths. By tailoring the geometry of graphene metasurface, the gain, loss, and reactive power can be perfectly balanced in the vicinity of the exceptional point between PT-symmetric and PT-broken phases. Furthermore, we show that the combination of the spectral singularity in a PT-symmetric system and the chemical sensitivity of graphene [5] may lead to exotic scattering responses at the spontaneous PT symmetry-breaking point. At such critical frequency, even a low-level chemical doping may dramatically modulate scattering properties of a graphene-based PT-symmetric sensor, with much higher sensitivity compared to those passive graphene-metasurface micro/nano-sensors. The propose metasurface device may have broad relevance beyond the extraordinary manipulation of THz waves, as it may also open exciting prospects for detecting gas, chemical, and biological agents with ultrahigh sensitivities.