Multilayer optical filter for energy-efficient smart-window applications.

In this study, a spectrally engineered broadband optical short-wave-pass (OSWP) multilayer filter is proposed for passive smart-window applications. The optical response of the structure is theoretically investigated using the transfer matrix method (TMM) to evaluate its transmission characteristics over the visible and near-infrared (NIR) spectral regions. The proposed design consists of a cascaded dielectric multilayer architecture combining SiOF, Si3N4, and BaSnO3 to achieve high visible-light transmittance together with broadband NIR rejection. The simulated spectra exhibit a well-defined photonic band gap with sharp spectral selectivity, resulting in efficient suppression of infrared wavelengths while preserving daylight transmission. The calculated solar heat gain coefficient is approximately 0.37 under the adopted lossless-dielectric approximation, indicating favorable theoretical solar-control performance for energy-efficient glazing applications. An angular analysis further demonstrates the intrinsic tunability of the photonic band gap, where increasing the incidence angle produces a systematic blue-shift in the spectral response. These results highlight the potential of the proposed multilayer platform as a passive optical coating improving solar control and potentially reducing solar heat gain. At normal incidence, the nominal cascaded structure provides a principal NIR rejection band extending from approximately 782.55 to 1279.55 nm, corresponding to a bandwidth of about 497 nm under the adopted T<10% criterion.