Two-photon absorption and resonance enhancement of second harmonic in ZnO nanowires

ZnO nanowires are prepared using vapor-liquid-solid process. Two-photon absorption measurement has been carried out using femtosecond laser pulses ranging from 700-800 nm. In addition to the deep-level green emission around 530 nm due to the surface defect and the near band-edge ultraviolet emission around 360 nm due to the exciton, a second harmonic peak has been observed. The strength of the frequency-doubled component is enhancing while tuning the two-photon absorption wavelength towards the exciton wavelength of the nanowires, due to resonant exciton absorption.