At the University of California, chemists have taken snapshots of the world’s smallest laser in action. Peidong Yang, assistant professor of chemistry at the UC Berkeley, reported the creation of an ultraviolet nanowire nonolaser shorter than the width of a human hair an done-hundredth the width.
These ultraviolet nanolasers have generated great excitement in the opto-electronics community because of their potential applications in miniature optical computer circuitry and communicationsdevices, Yang said.Yang and his laboratory colleagues have joined with Richard Saykally, professor of chemistry, of UC Berkeley lab 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 wiht high spatial resolution and th eoptical signal simultaneously,” Yang said. “It allowed us to characterise the laser and dtermine beam characteristics.” What they found is that the nanowire acts first as a waveguide, channelling ht eultraviolet light back and forth in the cavity, secondarily also 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 vapour-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 th earrays 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 ultraviolent radiation from a high power laser, after which they emit ultraviolet laser light that is “photographed” by the sharpened tip of an optical fibre 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, characterising the ability of the nanowires to generate visible colours for use in optical devices.
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