📓 In the last decade, there has been a rapid growth in research on the wavelength-scale design of materials to control optical information. Photonic crystal microcavities allow the trapping of optical fields in near fundamentally-small volumes, with many diverse applications such as filtering and switching. To effect conditional operations in a microcavity-based device, the nonlinear optical properties of multi-mode microcavities must be explored, and this theme forms the focus of this book. A resonant scattering technique is developed to simultaneously excite multiple microcavity modes with a short-pulse laser. Using this method, the second-order nonlinear properties of InP-based microcavities are investigated, revealing sum-frequency mixing between two modes. An ultrafast pump-probe experiment is then described which demonstrates all-optical switching and frequency conversion in a silicon microcavity.This book should be a significant resource for experimentalists in the nanophotonics community, as it contains comprehensive chapters discussing each of the techniques of sample nanofabrication, numerical modeling, and optical spectroscopy that are instrumental to this work.