Lasing-like dynamics with virtual gain driven by complex-frequency excitations.

Complex-frequency excitation controls non-Hermitian light-matter interactions by temporally shaping signals to bypass inherent material gain or loss constraints. While virtual loss has enabled coherent perfect absorption, its time-reversed counterpart: virtual gain, remains less explored in optical systems. Here, we theoretically and experimentally demonstrate lasing-like dynamics in a passive whispering-gallery-mode microcavity using complex-frequency excitations. Virtual gain counteracts intrinsic material and radiation losses, producing an instantaneous transmittance that exceeds unity and saturates at a quasi-steady value. Beyond a critical threshold-like point, the system enters a regime of divergent, exponentially growing response, mimicking the transient buildup of a real laser without requiring population inversion or active media. This linear effect allows for the robust coexistence of lasing-like behavior and perfect absorption, with transitions controlled by virtual gain tuning. These results establish a versatile framework for manipulating non-Hermitian interactions in passive platforms, offering remarkable potential for applications in sensing, optical communications, and energy storage.