Hybrid Standing Wave and Whispering Gallery Modes in Needle-Shaped ZnO Rods: Simulation of Emission Microscopy Images Using Finite Difference Frequency Domain Methods with a Focused Gaussian Source

Two-photon emission microscopy is used to investigate the photoluminescence properties of individual ZnO rods. The rods are 10–20 μm in length with a tapered cross section that varies from 1 to 2 μm at the midpoint to several hundred nanometers at the ends. The tapered shape and hexagonal cross section result in complex optical resonator modes that lead to periodic patterns in the two-photon emission image. Finite-difference frequency domain methods using a series of excitation sources, including focused Gaussian, point dipole, and plane wave, suggest that resonator modes have both standing wave (Fabry-Pérot) and whispering gallery mode character, whose relative contributions vary along the rod axis.