Loss-Enabled Chirality Inversion in Terahertz Metasurfaces.

Exceptional points (EPs), known as non-Hermitian degeneracies featuring missing eigenspace dimensions, have led to a variety of intriguing wave phenomena in various physical platforms. Chiral EPs collapsing in two orthogonal eigenstates can lead to unique effects and applications, such as loss-induced transparency, EP-enhanced sensing, and chirality-reversal electronics, etc. However, in previous experiments, chiral EPs were typically induced in fixed structures with inherent active gains or nearby nanotips, which are unfavorable for on-chip integrations with low-power elements. Here, we demonstrate the active control of EPs chirality in situ with exceptional-line metasurface. Selective chirality inversion can be well achieved by only light-induced loss without altering the metasurface size. We also perform an ultrafast chirality switch in the picosecond level within transient disturbance. In a broader view, our results provide a platform for the investigation of metasurface-based non-Hermitian physics and active EP modulation, which can stimulate exciting works along this line in near future.