Crystal nanowires cultivate lasers

17 Jun 2002

US scientists have coaxed ultraviolet lasers from zinc oxide nanowires.

Scientists at the US University of California, Berkeley, have demonstrated mirrorless, ultraviolet lasing in zinc oxide semiconductor nanowires at room temperature. By persuading whisker-sized crystals to grow on a sapphire substrate, the scientists believe that they have created an ideal miniaturized laser light source that could find use in optical computing and information storage.

Michael Huang and his colleagues grew zinc oxide nanowire crystals on a sapphire substrate via the crystal-deposition technique of epitaxy. Under carefully controlled vapour and condensation conditions the team created vertical arrays of nanowires with diameters ranging from 20 to 150 nm.

"By adjusting the growth time [of the crystals] between two and ten minutes," explained Huang, "we varied the crystal lengths from between two and ten microns."

The team also noticed that the tips of each nanowire crystal had formed well faceted hexagons. Realizing that these could replace the need for mirrors, which are typically used to induce lasing, they decided to try stimulating photon emission. The team pumped a focused laser beam at the nanowires and were excited to see that ramping up the power led to lasing at ultraviolet wavelengths.

"The lasing action in these nanowires without fabricated mirrors prompts us to think of these single-crystalline well faceted nanowires as natural resonance cavities," said Huang. "It is possible that a giant oscillator strength effect that can occur in high-quality nanowire crystals [leads to] excitonic stimulated emission in our nanowire arrays."

The team also believes that the natural cavity formation in nanowires could offer an easier alternative to forging cavities through etching or cleavage.