We report an indium phosphide nanowire (NW)-induced cavity in a silicon planar photonic crystal (PPC) waveguide to improve the light–NW coupling. The integration of NW shifts the transmission band of the PPC waveguide into the mode gap of the bare waveguide, which gives rise to a microcavity located on the NW section. Resonant modes with Q factors exceeding 103 are obtained. Leveraging on the high density of the electric field in the microcavity, the light–NW interaction is enhanced strongly for efficient nonlinear frequency conversion. Second-harmonic generation and sum-frequency generation in the NW are realized with a continuous-wave pump laser in a power level of tens of microwatts, showing a cavity-enhancement factor of 112. The hybrid integration structure of NW-PPC waveguide and the self-formed microcavity not only opens a simple strategy to effectively enhance light–NW interactions, but also provides a compact platform to construct NW-based on-chip active devices.

A new approach is proposed to accurately determine the thickness of films, especially for ultra-thin films, through spectrum-fitting with the assistance of an interference layer. The determination limit can reach even less than 1 nm. Its accuracy is far better than that of the traditional methods. This determination method is verified by experiments, and the determination limit is at least 3.5 nm compared with the results of atomic force microscope (AFM). Furthermore, a double interference-aided spectra fitting method is proposed to reduce the requirements of the determination instruments, which thus allows one to determine the film’s thickness with a low-precision common spectrometer and to greatly lower the cost. It is a very high-precision determination method for on-site and in-situ applications, especially for ultra-thin films.