Wednesday, February 8, 2012
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Postdoctoral researcher Mercedeh Khajavikhan at work in the optics laboratory. Image: Josh Knoff, UC San Diego Jacobs School of Engineering
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A
team of University of California, San Diego researchers has built the
smallest room-temperature nanolaser to date, as well as an even more
startling device: a highly efficient, “thresholdless” laser that funnels
all its photons into lasing, without any waste.
The
two new lasers require very low power to operate, an important
breakthrough since lasers usually require greater and greater “pump
power” to begin lasing as they shrink to nano sizes. The small size and
extremely low power of these nanolasers could make them very useful
components for future optical circuits packed on to tiny computer chips,
Mercedeh Khajavikhan and her UC San Diego Jacobs School of Engineering
colleagues report in the Feb. 9 issue of the journal Nature.
They
suggest that the thresholdless laser may also help researchers as they
develop new metamaterials, artificially structured materials that are
already being studied for applications from super-lenses that can be
used to see individual viruses or DNA molecules to “cloaking” devices
that bend light around an object to make it appear invisible.
All
lasers require a certain amount of “pump power” from an outside source
to begin emitting a coherent beam of light or “lasing,” explained
Yeshaiahu (Shaya) Fainman, a professor in the Department of Electrical
and Computer Engineering at UC San Diego and co-author of the new study.
A laser’s threshold is the point where this coherent output is greater
than any spontaneous emission produced.
The
smaller a laser is, the greater the pump power needed to reach the
point of lasing. To overcome this problem, the UC San Diego researchers
developed a design for the new lasers that uses quantum electrodynamic
effects in coaxial nanocavities to alleviate the threshold constraint.
Like a coaxial cable hooked up to a television (only at a much smaller
scale), the laser cavity consists of a metal rod enclosed by a ring of
metal-coated, quantum wells of semiconductor material. Khajavikhan and
the rest of the team built the thresholdless laser by modifying the
geometry of this cavity.
The
new design also allowed them to build the smallest room-temperature,
continuous wave laser to date. The new room-temperature nanoscale
coaxial laser is more than an order of magnitude smaller than their previous record smallest nanolaser published in Nature Photonics
less than two years ago. The whole device is almost half a micrometer
in diameter—by comparison, the period at the end of this sentence is
nearly 600 μm wide.
These
highly efficient lasers would be useful in augmenting future computing
chips with optical communications, where the lasers are used to
establish communication links between distant points on the chip. Only a
small amount of pump power would be required to reach lasing, reducing
the number of photons needed to transmit information, said Fainman.
The
nanolaser designs appear to be scalable—meaning that they could be
shrunk to even smaller sizes—an extremely important feature that makes
it possible to harvest laser light from even smaller nanoscale
structures, the researchers note. This feature eventually could make
them useful for creating and analyzing metamaterials with structures
smaller than the wavelength of light currently emitted by the lasers.
Fainman
said other applications for the new lasers could include tiny
biochemical sensors or high-resolution displays, but the researchers are
still working out the theory behind how these tiny lasers operate. They
would also like to find a way to pump the lasers electrically instead
of optically.
The
nanolasers are fabricated at the university’s NANO3 facility. The
research was funded by the Defense Advanced Research Projects Agency,
the National Science Foundation, the NSF Center for Integrated Access
Networks (CIAN), the Cymer Corporation and the U.S. Army Research
Office.
Thresholdless Nanoscale Coaxial Lasers
SOURCE
