Wire-size-dependent optical activity in electrochemically self-assembled II–VI semiconductor nanowire arrays

We report a non-monotonic dependence on wire diameter of the change in effective refractive index in electrochemically self-assembled nanowire arrays when illuminated with ultraviolet light. The wires are fabricated by electro-depositing CdS, ZnO and ZnSe in anodic porous alumina templates with 50-, 25- and 10-nm diameter pores. A pump-probe setup in an optical interferometer is used to test the thin film arrays. The observed non-monotonic optical behavior can be attributed to the competing effects of decreasing array filling factor and increasing wire optical activity from quantum confinement with reduction in wire diameter. Electromagnetic wave simulations indicate that changes in the effective permittivity of the individual quantum wires in the array must increase rapidly as wire size decreases. Changes in the real part of the permittivity in 10-nm diameter wires are up to an order of magnitude larger than those in 50-nm diameter wires.