  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.
