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Silicon films on metal create perfectly
absorbing optical color filters
Using two of the earth’s most abundant
and inexpensive materials—silicon and
absorption optical color filters have been
developed by researchers at the University of Alabama in Huntsville.
1 Spectral-selective perfect absorption is caused by
the critical coupling of incident light to
the second order resonance mode of the
asymmetric Fabry–Perot cavity formed
by single-layer silicon films on the metal
surface. The perfect absorption wavelength falls in the visible spectral range
and changes with the silicon film thickness in the range from 20 to 150 nm.
Silicon is chosen not only because it is
Critical coupling to
inexpensive and widely used in the opto-
electronics industry, but because it has
a low optical extinction
with other high-
in the visible and near-
infrared (NIR) range.
the 2nd order mode
Perfect absorption can
only be achieved when
the critical coupling condition is met; that is, when
the optical power coupled from the air to
the silicon film is equal to the optical loss
per round trip in the nanooptical cavity.
For these devices, the second-order
resonance mode in the
films causes perfect
absorption (> 99%)
at the peak absorp-
tion wavelengths of
552, 605, 657, and
700 nm for silicon films
with thickness values of
110, 120, 130, and 140 nm,
respectively (see figure).
Thermal annealing the
devices blue-shifts the
and correspondingly changes their colors.
The reflectance spectra are measured at
different angles of incidence up to 70°.
As the angle of incidence is increased, the
reflectance slightly increases, the peak
absorption wavelengths become slightly
shorter, but the peak reflection wave-
lengths do not shift.
Strong light absorption in nanome-ter-scale germanium thin films on metal
surfaces was reported earlier by a group
at Harvard University.
2 However, the
absorption in the nanoscale germanium films was not complete because the
group investigated the enhanced optical
absorption of the first order optical cavity
resonance mode. The researchers at
the University of Alabama in Huntsville
used the second-order optical cavity resonance mode to achieve perfect light
absorption in single-layer silicon films at
designated optical wavelengths using a
technique called “selective mode critical
coupling.” For achieving perfect light
absorption with critical coupling to the
second-order cavity mode, the silicon
film thickness is slightly increased, but
still remains a small fraction of the peak
absorption wavelength. —Gail Overton
1. S. S. Mirshafieyan and J. Guo, Opt. Express
22, 25, 31545 (2014).
2. M. A. Kats et al., Nat. Mat. 12, 1, 20 (2012).
Single-layer silicon films
with different thicknesses
deposited on an aluminum