Microwave Vortex Beam Lasing via Photonic Time Crystals

Microwave lasing carrying orbital angular momentum (OAM) holds significant potential for advanced applications in fields such as high-capacity communications, precision sensing, and radar imaging. However, conventional approaches to masers fail to produce emission with embedded OAM. The recent emergence of photonic time crystals (PTCs)—artificially structured media with periodically varying electromagnetic properties in time—offers a paradigm shift toward resonance-free lasing without the need for gain media. Yet, pioneering PTC designs have been based on three-dimensional bulk structures, which lack a surface-emitting configuration, and do not possess the capability to modulate OAM, thus hindering the realization of surface-emitted PTC masing that carries OAM. Here, we report the first experimental demonstration of surface-emitted microwave vortex beam lasing using ring-shaped PTCs, without the need for either a gain medium or a high-Q cavity. By developing a multiplier-driven time-varying metamaterial that achieves over 100% equivalent permittivity modulation depth, we establish momentum band gaps (k gaps) with sufficient bandwidth to overcome intrinsic losses and enable self-sustained coherent microwave amplification. Furthermore, space-time modulation induces nonreciprocity between clockwise and counterclockwise k-gap modes within the circularly symmetric PTC structure, facilitating the selective generation of microwave lasing carrying OAM, a functionality that is not readily accessible in conventional maser architectures. Our Letter bridges PTC physics with coherent OAM-carrying microwave emission, establishing a transformative platform for next-generation wireless communications, advanced sensing systems, and OAM-based technologies.