Chemist creates nanoscale laser

A chemist at the University of California, Berkeley, has grown the world’s smallest laser – a nanowire nanolaser one thousand times thinner than a human hair.

Among the potential applications are chemical analysis on microchips, high-density information storage and photonics. The laser emits ultraviolet light, but can be tuned from blue to deep ultraviolet.

‘The ability to produce high-density arrays of nanowires opens up lots of possible applications that today’s gallium arsenide devices can’t do,’ said creator Peidong Yang, assistant professor of chemistry at UC Berkeley and a member of the Materials Science Division at the Lawrence Berkeley National Laboratory. ‘This process works, it is ultracheap, and it’s the first real application of nanowires.’

Gallium arsenide and gallium nitride lasers are today’s leading solid state lasers and are cheap enough to be used in laser pointers. Made of multilayer thin films, they are several micrometers in size. The nanolaser is about 100 times smaller.

Yang and his team grew the lasers, which are pure crystals of zinc oxide, using a standard technique called epitaxy, which is employed broadly in the semiconductor industry. In epitaxy, a device is immersed in a hot vapour that is deposited in a thin layer.

The scientists painted a gold catalyst onto a piece of sapphire and placed it in a hot gas of zinc oxide (ZnO). The gold, when heated, formed regularly spaced nanocrystals that stimulated the growth of extremely pure zinc oxide wires only 20 to 150 nanometers in diameter.

The solid wires, which are hexagonal in cross section, grew to about 10 microns in length before the growth process was stopped, typically after two to 10 minutes.

Under an electron microscope, the arrays of nanowire nanolasers look like bristles of a brush, each bristle an individual laser. Bunched together, the nanolasers are bright enough to be used in different applications.

The key to getting these solid state lasers to emit coherent UV light is a perfectly flat tip that acts as a mirror in the way that, from underwater, the water surface acts like a mirror.

The end attached to the semiconductor also is a mirror, so that light emitted by excited zinc oxide bounces back and forth between them, causing more molecules to emit and amplifying the light. The amplified photons produced by this stimulated emission eventually pass through the mirrored free end, producing a flash of UV light.

Though Yang now must use another optical laser to excite the zinc oxide molecules so that they will emit UV light he hopes eventually to ‘pump’ the zinc oxide with electrons.

Once configured to work with electron pumping, the nanolaser could be put to any number of uses, Yang said. ‘Lab-on-a-chip’ devices could contain small laser analysis kits — nanodetectors — capable of such things as Raman spectroscopy.

A short-wavelength ultraviolet laser also could increase the amount of data that can be stored on a high-density compact disk, just as the advent of blue-light gallium nitride lasers boosted data density.