Chemists at the University of California, Berkeley, have taken snapshots of the world's smallest laser in action. Peidong Yang, assistant professor of chemistry at UC Berkeley, reported in June the creation of an ultraviolet nanowire nanolaser shorter than the width of a human hair and one-hundredth the width.
These ultraviolet nanolasers have generated great excitement in the optoelectronics community because of their potential applications in miniature optical computer circuitry and communications devices, Yang said.
Now, as reported in the cover article of the Nov. 22 issue of The Journal of Physical Chemistry B, Yang and his laboratory colleagues have joined forces with the UC Berkeley lab of Richard Saykally, professor of chemistry, to image single zinc oxide nanowires in the act of producing ultraviolet laser radiation.
The snapshots were taken with a relatively new type of microscope called a near-field optical scanning microscope.
"Near-field optical microscopy is a very powerful technique, providing a snapshot with high spatial resolution and the optical signal simultaneously," Yang said. "It allowed us to characterize the laser and determine beam characteristics."
What they found is that the nanowire acts first as a waveguide, channeling the ultraviolet light back and forth in the cavity, and secondarily as a laser. Once emitted from the end of the laser, however, the light quickly diverges or spreads, in contrast to the highly collimated beam of a typical laser, which spreads little over distances as great as hundreds of miles.
This is not necessarily a disadvantage, Yang said. If a strong, collimated beam is needed, the light from many nanowires could be channeled into an optical waveguide. If a small footprint and tiny power are required, an individual laser could be used.
The nanolasers are fabricated by a complicated high temperature process, called vapor-liquid-solid epitaxy, that grows vertical arrays of nanowires on gold-coated sapphire.
In order to image single nanolasers, the wires were broken out of the arrays and spread onto a glass slide for imaging with a near-field optical microscope. Single nanowires are "pumped" with extremely short pulses - less than one-trillionth of a second - of ultraviolet radiation from a high power laser, after which they emit ultraviolet laser light that is "photographed" by the sharpened tip of an optical fiber in the microscope.
By measuring the patterns of the emitted laser light, the chemists were able to determine the mechanism of the laser action and the optical properties of the nanolaser. In subsequent experiments, radiation at both one-half and one-third the pump laser wavelength were imaged in the same way by the Nonlinear Chemical Imaging Nanomicroscope, characterizing the ability of the nanowires to generate visible colors for use in optical devices.
"In the array form, the nanowires couple together and produce a beam that has much less spread in the frequency or color of the light," Yang said.
The research was supported by the Camille and Henry Dreyfus Foundation, the 3M Corporation, the National Science Foundation, the U.S. Department of Energy and UC Berkeley. Coauthors of the paper are graduate students Justin C. Johnson, Haoquan Yan, Richard D. Schaller and Louis H. Haber.
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