Humidity-Enhanced Thermally Tunable TiO₂/SiO₂ Bragg Stacks

Tunable, stimuli-responsive photonic crystals (PCs) have developed into a fast growing, interdisciplinary research field attracting attention from various scientific communities, such as photonics, sensing, and materials chemistry. Here, we propose a thermally tunable and environmentally responsive optical filter derived from nanoparticle-based TiO₂/SiO₂ one-dimensional photonic crystals, christened Bragg stacks (BSs). Photonic crystals with textural mesoporosity were obtained by bottom-up assembly based on sequential spin-coating suspensions of TiO₂ and SiO₂ nanoparticles on glass substrates. The mechanism of the BS thermal tunability is based on the thermo-optic effect, i.e., dependence of the refractive index on temperature. Notably, the optical response of the BS to temperature can be significantly enhanced by varying the relative humidity of the environment. Thus, the magnitude of the spectral shift increases more than fourfold from 4.4 to 21.9 nm with a change in relative humidity from 25% to 55% in the temperature range between 15 and 60 °C. Thus, humidity-enhanced thermal tuning causes shifts of the transmission spectra by up to −1.66 nm K^–1. The simulations of the wavelength shift based on the measurement of the effective thermo-optic coefficient of the individual TiO₂ and SiO₂ layers at ambient conditions closely correspond to the experimental values. Owing to their high inherent porosities and ease of fabrication, nanoparticle-based BSs offer a great potential for the development of sensitive, label-free photonic crystal temperature and humidity sensors