Fig. 1. Schematic illustration of the self-frequency conversion NW laser and its structural properties.

a The schematic of NW laser with fundamental lasing (red light) and self-frequency-conversion lasing (cyan light) signals pumped by an 800 nm femto-second pulsed laser (yellow light). b Tilted 30° SEM image of the as-grown NW array. c–e HAADF-STEM (c), In (d) and Ga (e) mapping images of the NW cross-section. Scale bar is 100 nm. f HRTEM and FFT (inset) images taken along $⟨1\overline{1}0⟩$ zone axis of the NW showing a zincblende crystal structure. Scale bar is 1 nm. Scale bar of inset is 2 nm⁻¹. g SEM image of a NW transferred onto a SiO₂/Si substrate. Upper-left inset and bottom-right inset show the zoomed images of the end facet of the transferred NW and end facet of a standing NW, respectively, indicating the high facet quality of the NW as an optical cavity. All scale bars are 500 nm