Linear and nonlinear optical absorption of excitonic states in a wurtzite ZnO nanowire: Quantum size effect

Abstract

Taking into account the anisotropic confined situations of quasi-one-dimensional (Q1D) nanowires (NWs) in free axial- and confined radial directions and the anisotropic wurtzite ZnO crystal, a two-parameter variational approach is brought forward and applied to investigate the luminous properties of excitonic states and their optical absorption features in wurtzite ZnO NW systems. The quantum size effects on the excitonic binding energies of the ground state and the first excited excitons as well as the linear and nonlinear absorption coefficients are analyzed in detail. Numerical calculations on a freestanding ZnO NW are performed. The calculated excitonic binding energies in the wurtzite ZnO NWs are far larger than those in cubic GaAs-based quantum wires and NWs with the same radius. This is mainly ascribed to the large effective masses of electron and hole and the relatively small dielectric constants in ZnO semiconductor. The calculated results show that the nonlinear and the total absorption coefficients take their maximum as the NW radius is smaller than one Bohr radius of exciton (2.0 nm) in ZnO bulk semiconductor. Moreover, the narrower the radius of the ZnO NWs (2.5 nm), the greater the blue-shift of the resonant photon energies of the absorption coefficients. The numerical results also show that the two-parameter variational approach is reasonable and necessary for the description of excitonic states and their optical absorptions in Q1D wurtzite ZnO NWs.