  Tunable light filters are critical components for many optical applications
in which light in-coupling, out-coupling or rejection is crucial, such as
lasing, sensing, photovoltaics and information and communication technology.
For this purpose, Bragg mirrors, band-pass filters with high reflectivity
represent good candidates. However, their optical characteristics are
determined at the stage of fabrication. Heavily doped semiconductor
nanocrystals (NCs) on the other hand deliver a high degree of optical
tunability through the active modulation of their carrier density ultimately
influencing their plasmonic absorption properties. Here, we propose the design
of a tunable light filter composed of a Bragg mirror and a layer of plasmonic
semiconductor NCs. We demonstrate that the filtering properties of the coupled
device can be tuned to cover a wide range of frequencies from the visible to
the near infrared (vis-NIR) spectral region when employing varying carrier
densities. As tunable component we implemented a dispersion of copper selenide
(Cu2-xSe) NCs and a film of indium tin oxide (ITO) NCs, which are known to show
optical tunablility by chemical or electrochemical treatments. We utilized the
Mie theory to describe the carrier dependent plasmonic properties of the
Cu2-xSe NC dispersion and the effective medium theory to describe the optical
characteristics of the ITO film. The transmission properties of the Bragg
mirror have been modelled with the transfer matrix method. We foresee ease of
experimental realization of the coupled device, where filtering modulation is
achieved upon chemical and electrochemical post-fabrication treatment of the
heavily doped semiconductor NC component, eventually resulting in tunable
transmission properties of the coupled device.
