Wavelength Sensitivity of Single Nanowire Excitation Polarization Anisotropies Explained through a Generalized Treatment of Their Linear Absorption

Abstract

We investigate the excitation polarization anisotropy of individual semiconductor nanowires (NWs) by monitoring their band edge emission above 680 nm in order to clarify the origin of their strong polarization response. Samples studied include both CdSe and CdSe/CdS core/shell nanowires grown using solution chemistry as well as analogous wires made via chemical-vapor-deposition (CVD). In the limit of optically thick wires, with radii above ∼25 nm, we find NW optical responses consistent with the interaction between strong dielectric contrast influences and the onset of bulk-like behavior. Namely, a sizable wavelength dependence of the excitation polarization anisotropy (ρ(exc)) exists when NW diameters become comparable to the wavelength of light inside the wire. As a consequence, pronounced ρ(exc) rolloffs occur at short wavelengths. By contrast, thinner wires do not exhibit such wavelength dependencies, in agreement with earlier studies. We quantitatively explain observed wavelength sensitivities by modeling the NW as an absorbing dielectric cylinder under plane wave excitation. A comparison of predicted ρ(exc)-values to experimental numbers shows good agreement and confirms the existence of wavelength-dependent ρ(exc)-values in optically thick wires. Additional results of the model include generalized expressions for NW linear absorption cross-sections under parallel, perpendicular, and circularly polarized excitation. This study therefore adds to a growing body of knowledge about NW polarization anisotropies, specifically, their response in a size regime where dielectric contrast effects compete with the onset of bulk-like behavior.