Second-harmonic whispering-gallery modes in ZnO nanotetrapod.

We present the observation of second-harmonic (SH) whispering-gallery modes (WGMs) in the hexagonal cross-sections of the tapered ZnO nanotetrapod legs. Because of the continuously changing diameter in the leg, several orders of the WGMs at different SH wavelengths can be on resonance simultaneously at different locations along the leg. The strongest SH WGMs occur when the polarization of the fundamental excitation beam is parallel to the c-axis of the crystal along the leg.