Semiconductor nanowires (NWs) are promising for realizing various on-chip nonlinear optical devices, due to their nanoscale lateral confinement and strong light-matter interaction. However, high-intensity pulsed pump lasers are typically needed to exploit their optical nonlinearity because light couples poorly with nanometric-size wires. Here, we demonstrate microwatts continuous-wave light pumped second harmonic generation (SHG) in AlGaAs NWs by integrating them with silicon planar photonic crystal cavities. Light-NW coupling is enhanced effectively by the extremely localized cavity mode at the subwavelength scale. Strong SHG is obtained even with a continuous-wave laser excitation with a pump power down to ~3 uW, and the cavity-enhancement factor is estimated around 150. Additionally, in the integrated device, the NWs SHG is more than two-order of magnitude stronger than third harmonic generations in the silicon slab, though the NW only couple s with less than 1% of the cavity mode. This significantly reduced power-requirement of NWs nonlinear frequency conversion would promote NW-based building blocks for nonlinear optics, specially in chip-integrated coherent light sources, entangled photon-pairs and signal processing devices.
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