Topological Temporal Boundary States in a Non-Hermitian Spatial Crystal

Area of Science:

• Topological physics
• Non-Hermitian systems
• Spatiotemporal phenomena

Background:

• Periodic modulation of material index creates momentum gaps, analogous to spatial crystals.
• Theoretical studies predicted topological temporal boundary states (TTBSs) in these momentum gaps.
• Existing TTBS research primarily relies on temporal modulation.

Purpose of the Study:

• To experimentally realize a new type of TTBS in a non-Hermitian system.
• To investigate TTBS emergence without temporal modulation.
• To explore topological transitions induced by sign flips in loss/gain profiles.

Main Methods:

• Utilized a non-Hermitian spatial crystal with spatially periodic loss and gain.
• Induced a sudden sign flip in the loss and gain profile.
• Observed the emergent mode in the Bloch momentum gap using a 1D active mechanical lattice.

Main Results:

• Discovered a novel TTBS associated with a parity-time broken phase, not temporal modulation.
• Observed a mode emerging in the Bloch momentum gap at the sign-flipping instant.
• Demonstrated that the temporal flip induces a topological transition in time, creating a TTBS analogous to the Jackiw-Rebbi state.

Conclusions:

• The study experimentally realized a new TTBS in non-Hermitian systems, linked to parity-time symmetry breaking.
• This TTBS emerges from a temporal boundary created by flipping loss/gain profiles.
• The findings deepen the understanding of temporal topological phases and offer new methods for controlling transient waves using topological concepts.