Computational and Experimental Study of Optical properties of TiO2 Arrays Fabricated via Soft Nanoimprint Lithography

Abstract

In this thesis the optical properties of large 1 mm2 titanium oxide (TiO2) nanoarrays and the interactions with Rhodamine 6G are studied computationally and experimentally. Three TiO2 arrays with different particle size and periodicity are studied, which are labelled A600, A550 and A480. The arrays used in this work were fabricated by the group of Marco Abbarchi from Aix-Marseille Université, Marseille, France using soft-nano imprint lithography, a novel fabrication method for TiO2 that allows for the fast large area arrays to be fabricated. A residual TiO2 layer is left underneath the array from the fabrication process, allowing for the possibility of quasi-guided modes (QGMs) to supported by the array. The conditions of the layer and manipulation of the QGMs are studied finite-difference time domain (FDTD) simulations. QGMs require a layer that is sufficiently optically and physically thick and are coupled to the Rayleigh anomalies. QGMs wavelength is controlled by the properties of the substrate and residual layer. The properties of the individual pillars of the arrays have little to no effect on the QGMs. TiO2 arrays were experimentally characterized and the effect of varying concentrations of Rhodamine 6G (R6G) on the optical properties of the arrays is reported. Normal transmission and reflection measurements of the three arrays show no signs of QGMs. The arrays were not able to support QGMs because of the low refractive index of the residual. The low refractive index of the residual layer is due to discontinuities and porosity of the layer lowering the effective refractive index. QGMs were observed on the arrays as sharp features in the reflection and transmission measurements after the addition of R6G. R6G increased the effective refractive index of the layer. The addition of R6G also significantly changes the darkfield scattering spectra of the arrays as a function of the concentration. These results make it clear that it is possible to modify the properties of the layer by introducing different materials. The modification of the photoluminescence (PL) of R6G coupled to the TiO2 arrays was investigated. The emission of R6G on the arrays and off the arrays is measured using three different focused laser excitations at 375 nm, 405 nm, and 466 nm. The A600 showed a modification of R6G photoluminescence with excitation of the 375 and 405 nm lasers, while the other arrays showed non. Time-resolved photoluminescence spectroscopy, photobleaching, power dependence measurements and FDTD simulations were performed to identify the mechanism responsible for this modification. These measurements point toward excitation enhancement of R6G on the A600 array 405 nm and 375 nm excitation due to higher electric fields regions, although these results are not conclusive.