Loss Compensation and Coherent Virtual Absorption via Virtual Complex Frequency Wave Excitation

ABSTRACT Complex frequency waves (CFWs), exhibiting extraordinary propagation properties including virtual gain/loss, have attracted increasing research attention. However, the experimental excitation of CFWs at high frequencies, especially in microwave and optical regimes, remains a big challenge. Here, by establishing a mathematical bridge between real and complex frequencies, this work presents a general method for CFW excitation, termed virtual CFW excitation, enabling us to obtain all electromagnetic responses for complex frequencies, including wavevector, electric/magnetic field, and transmission/reflection coefficient, even in the presence of intrinsic noise. Specifically, using virtual CFW excitation can offset the effect caused by material losses and the lost field information of both a point source and surface plasmon polaritons can be successfully recovered. Moreover, coherent virtual absorption is verified by proof‐of‐principle microwave transmission‐line experiments. Our finding provides an effective tool to explore phenomena of complex frequencies, mitigating experimental difficulties and promoting the future development of optical devices.