A little-known wide-bandgap material could open the way to ultraviolet-emitting semiconductor lasers. Researcher Zikang Tang and colleagues at Hong Kong University of Science and Technology (Kowloon, Hong Kong), Tokyo Institute of Technology (Midori-ku, Yokohama, Japan), and RIKEN (Sendai, Japan) recently reported room-temperature lasing at 387 nm from a thin film of self-organized zinc oxide (ZnO) crystals that self-form a Fabry-Perot cavity.1 The researchers measured a peak gain in the microcrystalline film an order of magnitude higher than has been measured previously in bulk ZnO.
The enormous market potential of short-wavelength diode lasers has produced significant research into materials such as indium gallium nitride (InGaN) and zinc selenide (ZnSe), but wide-bandgap ZnO has attracted remarkably little attention, perhaps because the gain of bulk materials is relatively low. Hence, although lasing in ZnO is not new, efficient operation is. And although researchers have not yet developed electrically pumped lasers from the material, that is a clear development path.
Ultraviolet (UV) lasing of bulk ZnO at cryogenic temperatures has been reported previously. In 1996, this same group of researchers reported room-temperature lasing in ZnO microstructures, as well as considerable theoretical research concerning the likely mechanism—a radiative recombination process of exciton-exciton collisions.2 In 1997, D. M. Bagnall and other researchers at Tohoku University (Sendai, Japan) reported room-temperature lasing in a conventional cleaved cavity made from an epitaxial film of ZnO, but the threshold intensity of 240 kW/cm2 was high.3
The new work is based on an entirely different—and unintentional—cavity design and exhibits a lower threshold intensity of 40 kW/cm2. The researchers suggest that two different mechanisms are at work.
Material specifics
To make the thin film, the group used laser molecular-beam epitaxy at a deposition temperature of 500°C and an oxygen pressure of 1 µTorr. A pure ZnO target was ablated in an ultrahigh-vacuum chamber by a krypton fluoride (KrF) excimer laser, and the material formed microcrystals on a sapphire substrate. The crystals self-assembled into closely packed hexagons, each about 50 nm in diameter.
To the researchers` surprise, when they pumped the thin film with a stripe of 355-nm light, the structure emitted "regularly spaced sharp lines in the lasing spectrum [that] look like cavity modes." But, they say, "because we did not intentionally form a lasing cavity, it was not clear where the required feedback came from."
After further investigation, they realized that the end mirrors were formed by facets of the crystals located at either end of the stripe of pump light. Within the pumped area, the high density of excitons created by photoexcitation leads to a decrease in the refractive index, so the refractive index between these microcrystals and those outside the pump region is different enough to cause light to reflect off the facets of crystals on the edge of the pump zone. To test this theory, the group measured the output power from the film while rotating it—the intensity of the laser emission rose to a maximum every 60°, which is consistent with the theory.
The reflectivity of the edge facets, however, is calculated to be a fairly small 1.5%. The strong contrast between Fabry-Perot oscillations and other photoluminescence would not make sense for such a small reflectivity unless the gain in the material was quite high. The measured peak gain reaches 320 cm-1 when the laser is pumped at 3.0 µJ/cm2. This is an order of magnitude higher gain than has been measured in bulk ZnO crystals that were pumped with much higher intensities. High gain is a promising characteristic for real-world uses.
Lasing mechanisms
The researchers discovered that two different mechanisms are responsible for UV lasing when the material is pumped with 355-nm, 15-ps pulses from a frequency-tripled Nd:YAG laser. Near threshold, a sharp peak at 387 nm appears. However, as the pump intensity increases to nearly three times the threshold, another group of lines appears, pulling energy away from the first line and red-shifting from 391 to 394 nm (see figure). The group concluded that the first line is due to radiative recombination from exciton-exciton collision, while the second set of lines is due to electron-hole plasma radiative recombination.