Linear and nonlinear optical properties of nanostructured Zn(1−x)SrxO–PVA composite thin films

Abstract

We investigate the effect of strontium doping on the linear and third order nonlinear optical properties of ZnO–polyvinyl alcohol (PVA) nano-composite thin films. Strontium doped ZnO nanoparticles capped with L-arginine were synthesized by low cost soft chemical route. These nanoparticles were characterized by X-ray powder diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy for its crystal structure and surface morphology studies. Linear optical responses of these samples were studied by using ultraviolet–visible (UV–vis) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Presence of excitonic peaks for doped and undoped ZnO was revealed by UV–vis data and shift of excitonic peaks towards lower energy with increase in dopant concentration was observed. Rotational and vibrational signatures of capping agent and ZnO were confirmed by FT-IR spectroscopy. Third order nonlinearity (nonlinear refraction and absorption) of Zn(1−x)SrxO–PVA thin films, deposited on the good optical quality glass substrate, were studied by z-scan technique using He–Ne laser (wavelength 632.8nm) in continuous wavelength regime which shows negative nonlinearity with self-defocusing effect. The large value of n2 (10−4cm2/W) is obtained for 5wt% strontium doped ZnO–PVA thin film and is attributed to the thermal effect. Enhanced nonlinear absorption due to reverse saturable absorption and weak free carrier absorption is observed for all undoped and doped ZnO–PVA thin films and is prominent for 5wt% doping of strontium. Third order nonlinear susceptibility χeff(3) is calculated for all samples.