  The non-linear and tunable optical constants of phase-change materials
associated with their phase-switching have been utilized in reconfigurable
optical devices. For example, one possible application of phase-change thin
films 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 phase-change
material, 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
Brewster angle. Such vanishing reflectance values (points of darkness) are
associated with phase singularities with abrupt changes. Moreover, we show that
additional phase singularities can be realized by switching the active Sb2Se3
layer between amorphous and crystalline structures, extending the functionality
of our design. The realized phase singularities are susceptible to small
optical constant changes and can be utilized for ultra-sensing applications. As
a proof of concept, we demonstrate our design's CO2 gas sensing capabilities,
showing a linearly dependent optical response with gas flow rate. We believe
our lithography-free design with multiple phase singularities, from the
coexisting generalized Brewster effect and from the additional structural
switching, is highly promising for tunable optical sensing applications.
