Laser Focus World - January 2015

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Pulsed CO2 Lasers

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January 2015 www.laserfocusworld.com Laser Focus World 24 THIN-FILM FILTERS

Silicon films on metal create perfectly
absorbing optical color filters

Using two of the earth’s most abundant and inexpensive materials—silicon and aluminum—wavelength-selective perfect 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
inexpensive and widely used in the opto-
electronics industry, but because it has
a low optical extinction
coefficient compared
with other high-
refractive-index semi-
conductor materials
in the visible and near-
infrared (NIR) range.

Critical coupling to
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
single-layer silicon
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
absorption wavelengths
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