  For many applications, a method for controlling the optical properties of a
solid-state film over a broad wavelength range is highly desirable and could
have significant commercial impact. One such application is smart glazing
technology where it is necessary to harvest near-infrared solar radiation in
the winter and reflect it in the summer--an impossibility for materials with
fixed thermal and optical properties. Here, we experimentally demonstrate a
smart window which uses a thin-film coating containing GeTe, a bi-stable,
chalcogenide-based phase-change material which can modulate near-infrared
absorption while maintaining neutral-colouration and constant transmission of
light at visible wavelengths. We additionally demonstrate controlled
down-conversion of absorbed near-infrared energy to far-infrared radiation
which can be used to heat a building's interior and show that these thin-films
also serve as low-emissivity coatings, reducing heat transfer between a
building and its external environment throughout the year. Finally, we
demonstrate fast, sub-millisecond switching using transparent electrical
heaters integrated on glass substrates. These combined properties result in a
smart window that is efficient, affordable, and aesthetically pleasing--three
aspects which are crucial for successful adoption of green technology.
