Tailoring optical properties of WO3 films in the visible to infrared range by ion bombardment and its description by an oscillator model

Evaporated and sputtered tungstentrioxide (WO 3 ) films were bombarded with 300-keV He + and Ar + ions, respectively, and the resulting changes of the real [n(λ)] and imaginary [k(λ)] parts of the refractive index in the visible to infrared range of the wavelength λ were determined in situ. The primary effect of an ion-induced blue coloration is directly reflected by an absorption band centered at 1100 nm, which is growing for increasing ion fluences and exponentially approaching saturation. This monotonous increase of absorption is accompanied by changes of n(λ) with a characteristic fixed point n(λ*) independent of the ion bombardment, while for λ>λ* an ion-induced increase and for λ<λ* a decrease of n are observed as compared to the unbombarded as-prepared samples. The experimental results can be quantitatively explained by assuming a superposition of a standard Cauchy dispersion representing the unbombarded WO, matrix and ion-induced Lorentz oscillators. While the density of these oscillators as well as their damping coefficients exhibit an exponential saturation behavior for increasing ion fluences, their eigenfrequencies as well as the matrix contribution are practically independent of the bombardment.