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The schematic view of the arrangement of CLC molecules in a cholesteric microdroplet with parallel anchoring of the LC molecules at the surface. The helical structure of the liquid crystal originates from the center of the droplet and gives rise to concentric shells of constant refractive index. This dielectric structure is optically equivalent to the well-known Bragg-onion optical microcavity.
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Versatile
electronic gadgets should employ a number of important criteria: small
in size, quick in operation, inexpensive to fabricate, and deliver high
precision output. A new microlaser, developed at the Jožef Stefan
Institute in Ljubljana, Slovenia embodies all these qualities. It is
small, tunable, cheap, and is essentially the world's first practical
three-dimensional laser.
As
described in Optics Express, an open-access journal published by the
Optical Society (OSA), Slovenian scientists Matjaž Humar and Igor
Muševič have developed a microdroplet 3-D laser system in which laser
light shines forth in all directions from dye molecules lodged within
spherical drops of helical molecules dispersed in a liquid solution.
This
is the first practical 3-D laser ever produced," says Muševič, who
expects that the microdroplet lasers, which can be made by the millions
in seconds, will be used in making arrays of coherent light emitters.
These will be handy for a variety of imaging purposes, for example
"internal-source holography." Here a 3-D laser would be embedded inside
the object which is to be imaged; light coming directly from the source
interferes with the light scattered by the surroundings. A
three-dimensional image of the object can then be reconstructed from the
interference pattern.
The
helical molecules are cholesteric liquid crystals, related to the
molecules that form the backbone of liquid crystal displays. The
cholesteric molecules don't mix well with the surrounding polymer
liquid. This incompatibility sets up a curious condition: the index of
refraction of the cholesteric liquid crystal varies periodically
outwards through the body of the 15-micron-sized droplet. It's as if the
droplet were an onion with the layers corresponding to materials with a
different index of refraction.
Most
lasers possess two important ingredients: an active medium in which
energy can be turned into light and amplified, and some resonant
enclosure in which the developing coherent light can build up to a
potent beam emerging as laser light. In the case of the microdroplet
laser, the active medium consists of all those fluorescent dye molecules
nestled in the liquid crystals. And the resonant enclosure consists not
of the usual longitudinal shaped mirrored cavity, but of the nested
sequence of "onion-layer" regions of changing index of refraction.
Two
more features make this laser design highly workable. First, the laser
components are self-assembled. Instead of an expensive fabrication
process, the parts of the laser assemble spontaneously through
chemistry. Second, the laser can be tuned: by changing the pitch size of
the helical molecules --the degree of their corkscrew thread-- the
wavelength of the light can be altered.
"Scientists
have been trying to make these lasers from solid state materials, but
you can imagine how difficult it is to make hundreds of alternating
shells of optical materials, which should be very uniform," said
Muševič. "The beauty of our approach is that such a 3-D onion droplet is
self-assembled in a fraction of a second."
To
tune the laser you don't even have to replace the droplets. Their
optical properties can be changed by modifying the temperature. Tuning
might even be accomplished by applying an extra electric field to the
drops.
Last
year, an early version of the 3-D laser resonator was reported. Now in
the journal Optics Express, the fully tunable version of the laser is
described.
The
paper "3D microlasers from self-assembled cholesteric liquid-crystal
microdroplets" by Matjaž Humar and Igor Muševič appears in the journal
Optics Express. Study abstract
The authors' lab website is located at http://www.softmatter.si.
SOURCE: Optical Society of America
