Structural and light coupling characteristics of patterned silica–titania sol–gel thin films with/without nano gold coatings

Abstract

High fidelity one-dimensional (1D) stripe/two-dimensional (2D) square pillar-like mesoscale patterned silica–titania sol–gel thin films with and without nano gold coatings on silica glass were successfully fabricated using a topographically stripe-like patterned (periodicity, 1.5 μm; peak height, 120 nm) polydimethylsiloxane stamp by a facile soft lithography technique. The material properties of the films were characterized for detailed analyses of their surface morphologies, microstructures, chemical bonding, etc. The films without Au coatings were amorphous in nature and highly transparent (∼80%) over the visible region. The visible absorption peak of nano gold in the Au-coated films confirmed the formation of surface plasmons (SPs) of cubic nano-Au. The films showed different water wetting behaviors as determined by static water contact angle measurements. This change in water wetting behavior can be explained on the basis of the root mean square surface roughnesses of the films. A very low optical loss (∼0.23 dB cm⁻¹) was found for the silica–titania waveguide film at 632.8 nm. Moreover, the 1D/2D patterned films showed above 90% absolute diffraction efficiency with a 632.8 nm red laser source. Finite difference time domain software was used to observe the light confinement, coupling and coupled mode propagation of the fabricated patterned films. These fabricated patterned films could be used as efficient optical grating/SP resonance couplers for application in waveguide sensor devices.