Negative Slope for Second Harmonic Generation Observed at High Excitation Intensities in ZnO Nanorods

The nonlinear optical properties of ZnO nanorods (NRs) synthesized by coprecipitation method were investigated using a focused femtosecond laser light. The excitation wavelength was tuned from 750 to 795 nm so that excitons could be selectively generated via two-photon absorption above or below the exciton ground state of ZnO NRs. Both second harmonic generation (SHG) and two-photon-induced luminescence (TPL) were observed in the nonlinear response spectrum of ZnO NRs. The relative intensities of SHG and TPL were found to depend not only on excitation wavelength, but also on excitation intensity. At high excitation intensities, the nonlinear response spectrum became dominated by TPL for excitation wavelengths shorter than 770 nm, whereas it was still governed by SHG for excitation wavelengths longer than 770 nm. In addition, the intensities of SHG and TPL did not scale quadratically with excitation intensity but exhibited different slopes in different excitation intensity regimes, implying the existence of competition between them. More interestingly, a negative slope, which indicates a reduction of SHG with increasing excitation intensity, was observed at high excitation intensities for excitation wavelengths longer than 770 nm, implying the energy redistribution or energy transfer between SHG and TPL. Meanwhile, a slope much was identified for TPL at high excitation intensities. It is suggested that the reduction in the bandgap energy resulting from the effects of bandgap renormalization and temperature rise were responsible for the rapid increase of TPL. A weak exciton emission was resolved for excitation wavelengths longer than 770 nm and it was explained by the existence of Rabi oscillation.