Study of optical transitions in an individual ZnO tetrapod using two-photon photoluminescence excitation spectrum

Two-photon excited photoluminescence (PL) measurements have been carried out on an individual ZnO tetrapod at low temperature (8 K) using femtosecond laser pulses in the wavelength range of 714–850 nm. Simultaneously PL and second-harmonic generation were observed. The integrated PL intensity excitation spectrum at different two-photon excitation frequencies has eight peaks, which are in good correspondence to the exciton-phonon complexes L_1b , L_1a , and the free exciton lines B _n=3, A _n=3, B _n=2, A _n=2, B _n=1, and A _n=1 seen in ZnO film. This technique can be used to measure the optical transitions in individual nanostructures, which is very difficult to achieve using the traditional transmission/reflection method.