Fig. 1: Topological boundary state in the k gap and the experimental setup.
From: Observation of momentum-gap topology of light at temporal interfaces in a time-synthetic lattice
a Topological boundary states in a frequency bandgap. Topological boundary states are localised at a spatial interface between two insulators with topologically distinct frequency bandgaps, with the frequency conserved. Topological number w switches in the spatial dimension. b Topological boundary states in a k gap. Topological boundary states are localised at a temporal interface between two media with topologically distinct k gaps, with the momentum conserved. Topological number w switches in the temporal dimension. c Schematic diagram of the experimental setup. The system consists of two fibre loops with different lengths. PM: phase modulator, MZM: Mach–Zehnder modulators, OC: optical coupler, VOC: variable optical coupler, SMF: single-mode fibre, PD: photodetector, EDFA: erbium-doped fibre amplifier. d Temporal synthetic lattice mapped from the setup in (c). The orange and green shadows represent gain (+γ0) and loss (−γ0), respectively, controlled by MZM. e Complex bandstructure of the lattice when γ0 = 0.3. A complete k gap opens near Q = π.
