Observing the localization of light in space and time by ultrafast second-harmonic microscopy

Multiple coherent scattering and the constructive interference of certain scattering paths form the common scheme of several remarkable localization phenomena of classical and quantum waves in randomly disordered media(1). Prominent examples are electron transport in disordered conductors(2,3), the localization of excitons in semiconductor nanostructures(4,5), surface plasmon polaritons at rough metallic films(6,7) or light in disordered dielectrics(8-11) and amplifying media(1,12-14). However, direct observation of the fundamental spatiotemporal dynamics of the localization process remains challenging(15). This holds true, in particular, for the localization of light occurring on exceedingly short femtosecond timescales and nanometre length scales. Here, we combine second harmonic microscopy with few-cycle time resolution to probe the spatiotemporal localization of light waves in a random dielectric medium. We find lifetimes of the photon modes of several femtoseconds and a broad distribution of the local optical density of states, revealing central hallmarks of the localization of light.