Coupling Phase Switching with Generalized Brewster Effect for Tunable Optical Sensor Designs

The nonlinear and tunable optical constants of phase change materials (PCM) associated with their phase switching have been utilized in reconfigurable optical devices. One possible application is for tunable perfect absorption designs, where p- polarized light reflectance vanishes at a specific incidence angle known as the Brewster angle. This work demonstrates a generalized Brewster effect (s- and p- polarized light absorption) for a multilayered heterostructure design based on the strong interference effect. The proposed design comprises a low-loss PCM, Sb₂Se₃, coated on a gold substrate. We experimentally and theoretically show the coexistence of vanishing reflectance values for both s- and p- polarized lights in the visible and near-IR wavelength range at a single angle. Such vanishing reflectance values are associated with phase singularities with abrupt changes. Moreover, it is shown that additional phase singularities can be realized by switching the active Sb₂Se₃ layer between phases, extending the functionality. The realized phase singularities are susceptible to small optical constant changes and can be utilized for ultrasensing applications. As a proof of concept, the design's CO₂ gas sensing capabilities are demonstrated, showing a linearly dependent optical response with gas flow rate. It is believed that this lithography-free design is highly promising for tunable optical sensing applications.