Thickness Dependent Optical Properties of WO 3 Thin Film using Surface Plasmon Resonance
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Thickness Dependent Optical Properties of WO3 Thin Film using Surface Plasmon Resonance Ayushi Paliwal1, Monika Tomar2 and Vinay Gupta1 Department of Physics and Astrophysics, University of Delhi, Delhi 110007, INDIA 2 Department of Physics, Miranda House, University of Delhi, Delhi 110007, INDIA 1 Email id: [email protected]; [email protected] Contact no: +91 9811563101 1
ABSTRACT The effect of tungsten oxide (WO3) thin film thickness on the surface plasmon resonance (SPR) properties have been investigated. WO3 films of varying the thickness (36 nm, 60 nm, 80 nm, 100 nm, 150 nm and 200nm) have been deposited onto Au coated prism (Au/prism) by radio frequency (RF) magnetron sputtering technique. The SPR responses of bilayer films were fitted with the Fresnel’s equations in order to calculate the dielectric constant of WO3 thin film. The variation of complex dielectric constant and refractive index with the thickness of WO3 thin film was studied. INTRODUCTION Surface plasmon resonance (SPR) technique has been widely recognized as a valuable tools for investigating surface interactions [1]. Since the surface plasmon (SP) waves are excited at metal-dielectric interface, thus any change in the vicinity of the interface can be monitored. SP wave at the metal-dielectric interface can be excited in a resonant manner by a visible or infrared light beam. In the Kretschmann configuration a light wave is totally reflected at the interface between the prism coupler and the thin metal layer (deposited on the prism surface) and excites a SP wave at the outer boundary of the metal by evanescently tunnelling through the thin metal layer. Thin films of noble metal such as Gold (Au) and Silver (Ag) are capable of supporting SP waves [2]. Any physical factor that causes variations in the refractive indices, surface inhomogeneity and thickness of the sensing film forming the SPR configurations will generate significant changes in the plasmon dispersion relations and hence change the SPR curve [3]. The changes in the SPR curve can be used to calculate the dielectric constant of the sensing film. Tungsten trioxide is a wide band gap semiconductor having monoclinic structure that has good electrochromic properties. WO3 thin films have been extensively used for the semiconductor gas sensors which require high operating temperatures. SPR technique on the other hand, is best suited at room temperature as well. Dielectric properties of the WO3 film change appreciably on interaction with target even at room temperature , however, detection of dielectric properties using conventional electrical measurement techniques is not easy for WO3 because of high losses. SPR is a very efficient tool to measure the dielectric properties of rich layers. Hence, in the present work an effort has been made to study the dielectric properties of WO3 thin film of varying thickness using SPR technique for their subsequent use in gas sensing applications.
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EXPERIMENT The SP modes have been excited at the WO3-gold interface in the Kretschmann confi
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