Nonlinear frequency conversion is controlled by vacuum fluctuations.

Ever since the advent of nonlinear optics, the generation of light by frequency-conversion is drawing continued attention, and leading to emerging applications such as supercontinuum sources for ultra stable clocks and advanced microscopy. A modern approach to frequency-conversion is to switch light confined in micro- and nanocavity resonances to enable on-chip operation. Supposedly, nonlinear frequency conversion in such confined media differs from traditional non-linear optics in three key features regarding output spectrum, frequency shift, and critical time scale. Therefore, we switch GaAs-AlAs microcavities by the electronic Kerr effect, and study a range of quality factors to bridge the confined and the traditional non-linear regimes. We uncover the key role of the density of vacuum fluctuations, i.e., the local density of optical states (LDOS) for newly generated frequencies, a concept inspired by cavity quantum electrodynamics (cQED). As a result, we succeed to establish a framework, which not only describes nonlinear optics both in traditional bulk and in confined media but also opens a new control dimension in changing the color of light.