Topological Event Wavepackets in Energy‐Momentum‐Gapped Photonic Spacetime Crystals

We propose a new type of topological excitations—topological event wavepackets (TEWs)—that emerge in photonic spacetime crystals (STCs) with spatiotemporally modulated permittivity. TEWs exhibit strong spatiotemporal localization and are topologically protected by a fully opened energy‐momentum () gap, in which steady propagating states are absent. Spectrally confined within this gap, TEWs can serve as a probe for identifying the gap extent. To reveal the underlying topological mechanism, we define a spacetime winding number. In contrast to previously reported nonlinearity‐induced event solitons in STCs, TEWs originate in linear media from the topological configuration, making them more accessible and versatile for experimental realization. Finally, by periodically weaving TEWs into an event lattice, we demonstrate a controllable delay and mitigation of noise‐driven amplification within the ‐gap. Our findings open a new pathway toward topological control in photonic spacetime‐modulated systems, enabling ‐gap band engineering for wave manipulation ranging from microwave to optical regimes.