Observation of optical precursors in free space

Abstract

When an electromagnetic pulse is incident into a dispersive medium, a kind of precursor can be excited. However, precursors in free space have not yet been investigated so far, because free space is nondispersive for propagating waves. Here, we theoretically reveal that, by launching evanescent waves into free space, optical precursors can be excited and observed as an optical beat in the time domain in the far field, with a one-to-one mapping relationship between the beating frequencies and the transverse wavenumbers of evanescent waves, which can be used to retrieve, in principle, a resolution far beyond the diffraction limit carried by the evanescent waves. The structural dispersion of free space to evanescent waves contributes to the precursors. Numerical simulations in the optical band and proof-of-principle experiments in the terahertz regime confirm the theoretical predictions.