Microphotonic Mirrors:
Dielectric Omnidirectional Reflector Developed by Multidisciplinary Team
A team of MIT researchers from MIT?s Materials Science
and Engineering Department, Plasma Science Fusion Center and
Physics Department report in Science that they have come up
with a new kind of mirror that combines the best characteristics
of two existing kinds of mirrors-metallic and dielectric.
The familiar metallic mirror is omnidirectional, which
means it reflects light from every angle. It also absorbs a
significant portion of the incident light. Dielectric materials,
unlike metals, do not conduct electricity and can reflect light
more efficiently. Light travels in dielectric materials at speeds
that are lower than in air. When light traveling in a particular
direction through one type of dielectric material encounters
another type, part of the light is reflected while the other part is
transmitted at a different angle. Dielectric mirrors, are simply
made of multiple layers of transparent dielectric materials. Such
mirrors can be made to be extremely low loss compared to their
metallic counterparts, and are used to reflect a prescribed range
of frequencies coming from within a limited set of angles.
First reported in the November 27, 1998 edition of Science
magazine, the new kind of mirror developed at MIT can reflect
light from all angles and polarizations just like the metallic mirrors
and be as low loss as dielectric mirrors. In addition, it can be
tuned to reflect certain wavelength ranges and transmit the rest of
the spectrum. A device such as this, operating in visible light,
would appear to be one color ? red, for instance ? while also
being transparent. In the materials currently used by the research-ers,
the ?perfect mirror? looks like a dark gray film. It is designed
to reflect a portion of the infrared part of the spectrum.
Principle researcher and doctoral candidate Yoel Fink, of the
Fusion Center, and Jurgen Michel and Prof. Edwin L. Thomas of
the Department of Materials Science and Engineering. The team
began with theoretical modeling of the reflector from which they
developed the optimal materials choices and processing tech-niques
to create the film.
Polystyrene and tellurium were chosen because tellurium has a
high index of refraction and exhibits low power loss, and polysty-rene
is easily processed and is low cost.. Deposited at thicknesses
of 1.65 çm and 0.8 çm, respectively, these materials produce a
multilayer film which is omnidirectional and polarization indepen-dent
although it does not have a complete photonic band gap.
?Potential uses depend on the geometry of the system for
example coating an enclosure will result in a optical cavity a
hollow tube will produce a low loss broad band waveguide,
while a planar film could be used as an efficient heat barrier or
collector for thermoelectric devices,? the authors wrote of the
device, which has three patents pending. |
