Creating Anti-Chiral Exceptional Points in Non-Hermitian Metasurfaces for Efficient Terahertz Switching.

Non-Hermitian degeneracies, also known as exceptional points (EPs), have presented remarkable singular characteristics such as the degeneracy of eigenvalues and eigenstates and enable limitless opportunities for achieving fascinating phenomena in EP photonic systems. Here, the general theoretical framework and experimental verification of a non-Hermitian metasurface that holds a pair of anti-chiral EPs are proposed as a novel approach for efficient terahertz (THz) switching. First, based on the Pancharatnam-Berry (PB) phase and unitary transformation, it is discovered that the coupling variation of ±1 spin eigenstates will lead to asymmetric modulation in two orthogonal linear polarizations (LP). Through loss-induced merging of a pair of anti-chiral EPs, the decoupling of ±1 spin eigenstates are then successfully realized in a non-Hermitian metasurface. Final, the efficient THz modulation is experimentally demonstrated, which exhibits modulation depth exceeding 70% and Off-On-Off switching cycle less than 9 ps in one LP while remains unaffected in another one. Compared with conventional THz modulation devices, the metadevice shows several figures of merits, such as a single frequency operation, high modulation depth, and ultrafast switching speed. The proposed theory and loss-induced non-Hermitian device are general and can be extended to numerous photonic systems varying from microwave, THz, infrared, to visible light.