Solid-State, High-Energy Lasers Based on Rare-Earth Doped Gallium Nitride

U.S. Army Research Laboratory, Adelphi, Maryland

Tuesday, December 01 2009

This technique eliminates the bottleneck in the heat removal process.

Laser-based directed-energy weapons (DEW) are important components for future Army missile defense systems. The diode-pumped, rare-earth (RE)-doped, solid-state laser is a very promising path towards achieving a DEW-sufficient level of average power from a reasonably compact device. Even so, the extreme pump power densities, combined with the inevitable non-radiative losses in the pump-lase process, introduce severe thermal loading in the gain medium. Regardless of the sophistication of the heat removal technique and its efficiency, the gain medium itself is the bottleneck for non-distortive heat removal due to the low thermal conductivity of known gain media compared to that of heat-sinking materials. The bestknown laser hosts, e.g., yttrium aluminum garnet (YAG), possess thermal conductivities (10–11 W/(m-K)) that are ~1.5 orders of magnitude lower than those of known heat-sinking materials. In order to eliminate this technical hurdle, an innovative gain medium with a thermal conductivity on the same order as copper (~390 W/(m- K)) had to be engineered. A qualitatively new approach to highly scalable diode-pumped solid-state lasers was developed based on rare-earth neodymium (Nd3+) doping of gallium nitride (GaN), a high-thermal-conductivity material. The goal was to fully eliminate the bottleneck in the heat removal process associated with the low thermal conductivity of the gain medium compared to that of heat-sinking materials. It was demonstrated, for the first time, in situ neodymium (Nd) doping of gallium nitride GaN by plasma-assisted molecular beam epitaxy (PA-MBE). The Nd doping is controlled by the GaN growth conditions and the Nd effusion cell temperature. The Rutherford backscattering spectroscopy (RBS) and secondary ion mass spectrometry (SIMS) data indicated Nd doping as high as ~8 at. %, with no evidence of phase segregation identified by x-ray diffraction (XRD) for Nd up to ~1 at. %. The Nd incorporation reached a limit while maintaining crystal quality.

Strong room-temperature (RT) luminescence corresponded to the three characteristic Nd emission multiplets, with the Stark energy levels resolved by photoluminescence (PL) and photoluminescence excitation (PLE). Although the 4f electrons were well shielded from the host material, weak electron-phonon interactions were observed. Spectral correlation of the multiplets for above (325 nm) and below (836 nm) GaN bandgap excitation implied enhanced substitutional doping at the Ga site. The highest RT PL intensities corresponded to a doping level between 0.1 and 1 at. %.

The enhanced substitutional doping at the Ga site and low optical loss in waveguide structures suggests GaN:Nd with a high enough Nd concentration has significant potential for use in simple, area-scalable, RT, diode-pumped, solid-state, high-energy lasers (HELs).

This work was done by Michael Wraback and Mark Dubinskiy of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. ARL-0075