Tunable thermoresponsive TiO₂/SiO₂ Bragg stacks based on sol-gel fabrication methods

Thermoresponsive TiO2/SiO2 one-dimensional photonic crystals (Bragg stacks) fabricated via sol-gel processing methods represent a promising class of environmentally responsive nanostructures featuring optically encoded temperature and humidity detection. The thermo-optic response of the layer materials is amplified by their inherent porosity owing to adsorption/desorption of ambient humidity into the mesoporous multilayer structure. Based on a comprehensive analysis of the impact of layer thickness, refractive index and thermo-optic coefficient on the stop band position, and width of various Bragg stack architectures, design criteria for thermoresponsive Bragg stacks operating in the visible range of the optical spectrum are put forward. A large and well-defined thermo-optic signature is expected for material combinations featuring individually high thermo-optic coefficients with the same sign or allowing for large changes in the effective refractive indices due to water adsorption in the porous layers reinforcing the thermo-optic response, as observed in the TiO2/SiO2 couple. Important practical aspects of the performance of thermoresponsive Bragg stacks are addressed, including the hysteresis properties of TiO2/SiO2 Bragg stacks during multiple heating/cooling cycles, as well as response and recovery times (∼2-4 s) of the multilayer system during external changes in ambient humidity.