Hexagonal Prism Blue Diode Laser Using Whispering Gallery Mode (WGM) Resonances
23 Oct 2007 | Online Presentations | Contributor(s): Sangho Kim
Semiconductor lasers have many important applications, including communication technologies, optical storage, printing, and molecular detection. The range of applications could be broadened significantly if the lasers could be made smaller and with lower threshold currents. Today’s in-plane semiconductor diode lasers are large discrete devices (about 100 x 400 microns), a form factor that results in part from the performance requirements associated with the optical cavity. To reduce the cavity length, it is necessary to increase the reflectivity of the mirrors – for example, by cleaving the crystal and coating the facets with a dielectric multilayer - adding complexity to the fabrication process. Although high-performance vertical cavity lasers can be fabricated from III-V semiconductor heterostructures, the requisite high reflectivities of the mirrors necessitate the fabrication of multilayers with hundreds of periods and thickness control on the order of 1% or better.
An alternative design that permits a smaller cavity and active region with a simple fabrication process is the hexagonal prism laser diode. With the hexagonal cavity geometry, whispering gallery mode (WGM) resonances are established by total internal reflection (TIR), which alleviates the need for multilayer reflector coatings as required for small in-plane and vertical cavity lasers. In addition, the high reflection efficiency from TIR enables a lower current density threshold, which is of great utility from a device perspective. The WGM resonance path for two-dimensional hexagonal devices allows reduced device sizes (~2.6x) while maintaining resonance lengths equivalent to one-dimensional in-plane lasers.
Hexagonal prism lasers were fabricated by reactive ion etching of (In,Ga)N multi-quantum-well light-emitting diode heterostructures designed to emit in the blue portion of the spectrum, Unlike the optically-pumped microdisk lasers that have been reported recently, the 160 micron diameter hexagonal prism laser is electrically-pumped and demonstrates lasing at 480nm (+/-5nm) with a threshold voltage and current density of 9V and 7kA/cm2, respectively. The performance of these hexagonal prism diode lasers is expected to improve significantly with the further development of a selective growth process to fabricate self-organized hexagonal prism structures.