  Tunable narrowband spectral filters with high light throughput and wide
dynamic range have remarkable applications such as in optical communications,
optical spectroscopy and spectral imaging. However, a cost is usually
associated with the filter narrowing either in the dynamic range, in the
throughput or the manufacturability. Here we report on a resonating planar
multilayered structure that exhibits transparency window in reflection with a
controllable full width at half maximum (sub-Angstroms till tens of nm) and
tunability over wide spectral range (>500nm in the visible and near infrared).
The phenomenon is observed in TE and TM polarizations with much higher contrast
in TE. Fano type resonance originating from coupling between waveguide modes
and lossy surface electromagnetic waves supported by field distribution
calculations explains the phenomenon. The wide tuning range with high contrast
is mainly achieved using an absorptive layer with high imaginary to real part
ratio of the dielectric constant that enables excitation of lossy surface waves
known to exist over a wide spectral band in thin films. To avoid large losses,
it is found that the lossy layer should be ultrathin (6nm Cr layer for
example). The tuning is achieved by small angular scan of less than 2 degrees
or by modulating the refractive index or thickness of the submicron thick
waveguide layer from the visible till the near infrared range and in principle
it can be designed to operate in any spectral range. Such a thin variable index
or thickness layer can allow tuning at ultrahigh speed using conventional
electrooptic, magnetooptic, piezoelectric or thermooptic materials at
relatively low external fields.
