Researchers at the University of California Santa Barbara and the University of Oxford (England) have fabricated gallium nitride (GaN)-based continuous-wave room-temperature microdisk lasers with a lasing threshold of only 300 W/cm2. Previous GaN-based microdisk-laser systems have only achieved pulsed operation at 10 K with lasing thresholds on the order of 6 MW/cm2.
Lasing performance has been limited in the past by variables in material growth and fabrication process control; the researchers refined previous processes to reproducibly fabricate GaN microdisks with smaller diameter, smooth sidewalls, good circularity, and uniformity across the sample, which enabled them to achieve high-quality (high-Q) modes and low lasing thresholds. Primary factors in the process improvement included the use of a silicon dioxide hard mask instead of photoresist, to decrease mask erosion during the dry-etch step and improve sidewall smoothness; use of resist reflow to further smooth sidewalls; and the use of electron-beam lithography, rather than optical lithography, which improved disk circularity (contributing to high Q and low lasing threshold) and enabled fabrication of much smaller disks than previously obtainable, leading to fewer optical modes overlapping the quantum-well spectrum. Contact Adele Tamboli at [email protected].