  Abstract: An induced-transmission filter (ITF) uses an ultrathin layer of
metal positioned at an electric-field node within a dielectric thin-film
bandpass filter to select one transmission band while suppressing other
transmission bands that would have been present without the metal layer. Here,
we introduce a switchable mid-infrared ITF where the metal film can be
"switched on and off", enabling the modulation of the filter response from
single-band to multiband. The switching is enabled by a deeply subwavelength
film of vanadium dioxide (VO2), which undergoes a reversible insulator-to-metal
phase transition. We designed and experimentally demonstrated an ITF that can
switch between two states: one broad passband across the long-wave infrared
(LWIR, 8 - 12 um) and one narrow passband at ~8.8 um. Our work generalizes the
ITF -- previously a niche type of bandpass filter -- into a new class of
tunable devices. Furthermore, our unique fabrication process -- which begins
with thin-film VO2 on a suspended membrane -- enables the integration of VO2
into any thin-film assembly that is compatible with physical vapor deposition
(PVD) processes, and is thus a new platform for realizing tunable thin-film
filters.
