Active Control of Topological Exceptional Points in Non-Hermitian Metasurfaces

TL;DR

This paper demonstrates ultrafast active control of exceptional points in non-Hermitian metasurfaces using pump-probe delay in the THz regime, enabling rapid eigenmode switching and topological phase manipulation.

Contribution

It introduces pump-probe delay as a new dynamic parameter for real-time control of EP topology in non-Hermitian photonic systems.

Findings

Achieved sub-picosecond eigenmode switching (~0.5 ps).

Realized complete EP encirclement within ~2 ps.

Demonstrated >99% cross-polarization modulation depth.

Abstract

Active control and ultrafast switching of non-Hermitian photonic systems are essential for next-generation reconfigurable optical technologies. Here, we demonstrate dynamic temporal manipulation of EPs in the terahertz (THz) regime using optically excited germanium (Ge) as an active medium. By exploiting pump-probe delay as a continuous tuning parameter, we achieve sub-picosecond eigenmode switching (~0.5 ps) and realize a complete time-resolved EP encirclement within ~2 ps, enabling direct observation of topological phase accumulation. At EP, the metasurface exhibits highly asymmetric transmission for circularly polarized light, characteristic of chiral mode response. Furthermore, we observe ultrafast eigenmode switching and topological phase evolution within ~1 ps, achieving >99% cross-polarization modulation depth. The measured results show strong agreement with theoretical modeling, with a high Petermann factor of approximately 10^3, confirming the effectiveness of the design. Our work establishes pump-probe delay time as a dynamical control parameter for EP topology, introducing a new regime of ultrafast non-Hermitian photonics for high-speed switching, enhanced sensitivity, and tunable polarization control in the THz domain.