Generating and Weaving Topological Event Wavepackets in Photonic Spacetime Crystals with Fully Energy-Momentum Gapped

Abstract: We propose a novel type of topological excitation topological event wavepackets (TEWs) emerging in photonic spacetime crystals (STCs) with spacetime modulated dielectric constants. These TEWs exhibit strong spatiotemporal localization and are topologically protected by a fully opened energy momentum ({\omega}k) gap, within which conventional steady states are absent. We further demonstrate that TEWs are spectrally confined within the {\omega}k-gap, providing a combined measurement for probing the emergence of TEW and the {\omega}k-gap size. Furthermore, we construct a spacetime winding number to elucidate the protection of these events. Unlike previously reported nolinearity-induced event solitons, TEWs originate from topological configuration for linear media, thereby more accessible and versatile for experimental realization. Moreover, we show that TEWs can be periodically woven to form an event lattice, enabling to suppress unwanted noise amplification. Our findings open a new pathway toward topological control in photonic spacetime-modulated systems, enabling the {\omega}k-gap band enginering for wave manipulation ranging from microwave to optical regimes.