Frequency-Selective Terahertz Wave Amplification by a Time Boundary in Huygens' Metasurface

Ultrafast manipulation of optical resonance can establish the time-boundary effect in time-variant media leading to a new degree of freedom for coherent control of electromagnetic waves. Here, it is demonstrated that a free-standing all-dielectric Huygens' metasurface of degenerate electric and magnetic resonances can prompt broadband near-unity transmission in its static state, whereas it enables wave amplification in the presence of time boundaries. The time boundary is realized by femtosecond laser excitations that transiently inject free carriers into the constituent meta-atoms for dynamic removal of a pre-established twofold degeneracy. It is showed that the transmittance in the photo-excited Huygens' metasurface can exceed unity, corresponding to the terahertz (THz) wave amplification at a record high amplification rate of up to 20% in intensity. Remarkably, the associated operating frequency can be tuned by fine control over the interval between the arrival of the time boundary and that of the seed THz pulse. By numerical simulations and analysis with time-dependent coupled mode theory, the wave amplification results are shown from the ultrafast Q-switching and shift in resonant frequencies. This work demonstrates a new approach to achieving tunable amplification in an optical microcavity by exploiting the concept of time-variant media and the unique electromagnetic properties of Huygens' metasurface.