Rf Sputtered Na0.5K0.5NbO3 Films on Oxide Substrates as Optical Waveguiding Material

Abstract

Highly crystalline Na0.5K0.5NbO3 (NKN) thin films of 1–2 μm thickness were deposited by rf-magnetron sputtering of a stoichiometric, ceramic target on single crystal LaAlO3(001) and Al2O3(0112) substrates. X-ray diffraction measurements revealed epitaxial quality of NKN/LaAlO3 film structures, whereas NKN films on sapphire substrates were found to be preferentially c-axis oriented.A prism-coupling technique was used to characterize optical and waveguiding properties. A bright-line spectrum at λ = 632.8 nm, revealed sharp peaks, corresponding to transverse magnetic (TM) and electric (TE) waveguide propagation modes in NKN/LaAlO3 and NKN/Al2O3 thin films. Using a least mean square fit the refractive index for the films and film thickness were calculated. The extraordinary and ordinary refractive indices were determined to n e = 2.207 ± 0.002 and n o = 2.261 ± 0.002, and n e = 2.216 ± 0.002 and n o = 2.247 ± 0.002 at λ = 632.8 nm for 2.0 μm thick NKN films on LaAlO3 and Al2O3, respectively. This corresponds to a birefringence Δn = n e − n o = −0.054 ± 0.003 and Δn = −0.031 ± 0.003 in the films, where the larger Δn for the NKN/LaAlO3 structure can be explained by the superior crystalline quality compared to NKN/Al2O3.Atomic force microscopy images of the film surfaces revealed rms roughnesses of 2.5 nm and 8.0 nm for 1.0-μm thick NKN/LaAlO3 and NKN/Al2O3 films, respectively. We believe surface scattering is one of the main sources of waveguide losses in the thin films.