Biaxial optical anisotropy of self aligned silver nanoparticles and nanowires

In the present study ion beam sputtering has been used for prestructuring of a silicon substrate followed deposition of metal by ebeam evaporation. First a low energy ion beam (Ar+, 500 eV) is incident on the substrate surface at an angle of 67∘ to the surface normal to produce well ordered (20-50 nm) ripple patterns. Then physically vaporized Ag atoms are deposited at grazing angle of 70∘ to the surface normal and normal to the ripples direction. Varying deposition parameters, i.e. ripple periodicity, substrate temperature and atomic flux, we were able to produce well ordered nanoparticles and nanowires. Self-aligned Ag nanoparticles and nanowires deposited on pre-patterned ripple surfaces exhibit strong optical anisotropy. Generalised ellipsometry measurements show that off diagonal Jones matrix elements (Ψ_ps, Δ_ps, Ψ_sp, Δ_sp) are non zero and vary with Eulers angle \phi. This indicates that such a medium is biaxial in nature. A biaxial layer model approach is used to calculate dielectric functions for such a system. Tauc-Lorentz oscillators are used along x and y direction independently and Drude model along z-direction for nanoparticles. This approach provides a very good fitting with the measured Jones matrix element Ψ_pp, Δ_pp, Ψ_ps, Δ_ps, Ψ_sp, Δ_sp. Different cases for ordered nanoparticles and wires will be presented.