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 Muševiè 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 Muševiè, 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 Muševiè. "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 Muševiè appears in the journal Optics Express. It can be accessed at: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-18-26-26995
The authors' lab website is located at http://www.softmatter.si.
About Optics Express
Optics Express reports on new developments in all fields of optical science and technology every two weeks. The journal provides rapid publication of original, peer-reviewed papers. It is published by the Optical Society and edited by C. Martijn de Sterke of the University of Sydney. Optics Express is an open-access journal and is available at no cost to readers online at http://www.OpticsInfoBase.org/OE.
Uniting more than 106,000 professionals from 134 countries, the Optical Society (OSA) brings together the global optics community through its programs and initiatives. Since 1916 OSA has worked to advance the common interests of the field, providing educational resources to the scientists, engineers and business leaders who work in the field by promoting the science of light and the advanced technologies made possible by optics and photonics. OSA publications, events, technical groups and programs foster optics knowledge and scientific collaboration among all those with an interest in optics and photonics. For more information, visit www.osa.org.
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