Two photon absorption associated anisotropic on ultrafast carrier dynamics in a-plane ZnO thin film
Chia-Hui Lu1*, Ja-Hon Lin1, Hsing-Jung Su1, Wei-Rein Liu2, Wen-Feng Hsieh3
1Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei, Taiwan
2National Synchrotron Radiation Reserach Center, Hsinchu, Taiwan
3Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
* presenting author:Chia-Hui Lu,
Ultrafast carrier dynamics by different sample thickness and pump polarization at state above the bandgap were investigated. 97 nm and 291 nm a-plane ZnO epi-film grown on r-plane sapphire substrate were subjected to the degenerated reflection-type pump probe technique with a frequency doubling femtosecond Ti:sapphire laser as the light source. From the polarized reflection spectrum using the white light source with the electric field perpendicular and parallel to c-axis of the sample, respectively, the two valence bandedge energy levels of ZnO film were measured that corresponds to the E1 and E2 bands. Two photon absorption (TPA), band filling (BF) and bandgap renormalization (BGR) effects were observed while polarization of the pump beam was perpendicular to c-axis. However, only the band BF effect was observed in the transient differential reflectance (TDR) trace while the polarization of the pump beam was parallel to c-axis of the ZnO film. The similar experiment results were observed in the thick ZnO epifilm as well. The results indicated the dynamic behavior of the carriers in non-polar a-plane ZnO. The theoretical and experimental results were discussed.

Keywords: a-ZnO, Two photon absorption, Band filling, Bandgap renormalization