Shape-dependent two-photon absorption in semiconductor nanocrystals.

We report our theoretical investigation onto the shape dependence of two-photon absorption (TPA) in semiconductor nanocrystals (NCs). Based on a four-band model under effective mass approximation, we have developed a simple analytical theory capable of providing a quantitative explanation of the recent TPA measurement on CdS nanorods [Appl. Phys. Lett. 94, 103117 (2009)]. With this theory, we have systematically revealed the characteristics of TPA in CdSe and ZnO NCs with four different shapes: sphere, cube, cylinder and cuboid. Due to the splitting of degenerate energy levels caused by the decreased degree of symmetry, nanocuboids and nanocubes exhibit greater TPA cross-sections than nanocylinders and nanospheres of similar sizes, respectively. Similarly, nanocuboids and nanocylinders possess larger TPA cross-sections than nanocubes and nanospheres of similar lateral dimension, respectively. Given TPA-allowed transitions, nanocuboids show stronger size dependence than nanocylinders. The size dependence of TPA cross-section is more sensitive to the lateral size than the longitudinal size in the cases of nanocylinders and nanocuboids.