Strategies and Progress towards Compact Deep UV Laser Sources (invited)

Abstract

The increasing interest in compact deep UV lasers is driven by applications such as native fluorescence or Raman spectroscopy for detection and identification of chemical species and bio-particles. Several possible strategies will be discussed to realize such lasers using the AlGaInN material system. The first is an AlGaN-based laser diode (LD) with direct current injection. PARC has pioneered the development of nitride UV lasers containing AlGaN quantum wells and LDs on bulk single-crystal AlN substrates. More recently, we have demonstrated optically pumped UV lasers with AlGaN MQWs on bulk AlN at wavelengths down to 237 nm and with record-low pump power thresholds, for example, 41 kW/cm2 at ? = 266 nm. Fundamental work has included experimental observation and computational understanding of polarization switching of the laser emission at wavelengths near 250 nm. To overcome the limitations of thermally activated p-type doping, nano-structured AlGaN-based composition- and doping-modulated superlattice materials were designed, develop, tested and integrated into sub-300-nm LD heterostructures and fully processed UV laser test devices. Significant progress has been made in all aspects of LDs, with maximum current density exceeding 40 kA/cm2 and improved carrier injection. Yet, challenges remain to the realization of deep UV LDs. These include optical absorption losses as well as further heterostructure optimization and device design. Alternative strategies have been proposed that are also based on AlGaInN semiconductors. One is the use of a high-energy electron beam to pump the UV laser heterostructures. Conceptual designs will be presented and the advantages of the approach discussed.