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Processing improves GaN microdisk performance

11 Jan 2007

Low threshold, room temperature, continuous wave lasing from a GaN microdisk laser is now possible say researchers.

Researchers in the US and the UK have reported an eye-catching leap in performance from blue-emitting GaN microdisk lasers. Hailing from the University of California at Santa Barbara (UCSB) and the University of Oxford, the team says it has observed the first room temperature continuous wave lasing from a GaN microdisk and also the lowest threshold. (Nature Photonics 1 61)

"The threshold of about 300 W cm-2 is the lowest reported for GaN-based microdisks by several orders of magnitude," say the authors. "Our threshold is also many of orders of magnitude lower than current optically-pumped GaN-based vertical-cavity, surface-emitting lasers."

According to researcher Adele Tamboli from UCSB, several key factors have had a huge effect on device performance. "The first is the size of the microdisk," Tamboli told optics.org. "Previous reports have focused on larger microdisks about 4 microns in diameter and ours are significantly smaller."

"Sidewall roughness and the circularity of the disk are important to performance," she continued. "We have improved these factors considerably by using electron beam lithography coupled with resist reflow."

Unlike standard in-plane lasers where the cavity is formed by two parallel mirrors, the resonant cavity in a microdisk laser is circular. This means that light travels round the periphery of the microdisk and is reflected at the air-semiconductor interface creating standing waves in the material known as whispering gallery modes (WGMs). Because the WGMs are concentrated very close to the surface, especially in small disks, sidewall smoothness is crucial to producing low-loss modes.

The team fabricated a range of microdisks from 1.2 to 8 microns in diameter. Optical pumping of a 1.2 micron microdisk with a 3 mW HeCd laser emitting at 325 nm resulted a lasing mode at 428 nm at room temperature. The threshold for this disk was 300 W cm-2 and the researchers say this should be considered as an upper limit.

"It is difficult to measure the total output power because the disks emit radially in all directions," added Tamboli. "The input power for these lasers to reach threshold is very low. The fabrication is very simple compared to other designs for blue lasers. It takes about 10 to 12 hours for one chip and we can generally pack several hundred microdisks per chip."

The team is now working on an electrically-pumped microdisk. "The emission wavelength is determined by the material growth and the cavity size so we have control over that. In terms of improving performance, we think that some minor improvements can be made but the structure is already close to optimal for optically-pumped devices."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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