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

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

Topics:
Aluminum Conductivity Copper Directed Energy Weapons, Detection, and Targeting Lasers Materials properties Metals Missiles Optics Parts Photonics Pumps Research and development Sensors and actuators Spectroscopy Vehicles
This Brief includes a Technical Support Package (TSP).
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Solid-State, High-Energy Lasers Based on Rare-Earth Doped Gallium Nitride

(reference ARL-0075) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine (Vol. 3 No. 6).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Rare-earth Doped Gallium Nitride (GaN)—An Innovative Path Toward Area-scalable Solid-state High Energy Lasers Without Thermal Distortion" presents research conducted by Michael Wraback and Mark Dubinskiy at the U.S. Army Research Laboratory. The report, finalized in April 2009, covers the period from December 2007 to December 2008 and focuses on the development of high-energy laser systems using rare-earth doped GaN.

    The primary objective of the research is to address the challenges associated with thermal distortion in solid-state lasers, which can limit their performance and scalability. Traditional laser materials often suffer from thermal management issues, leading to reduced efficiency and beam quality. The authors propose that rare-earth doped GaN can provide a solution to these problems, enabling the creation of scalable laser systems that maintain high performance even at larger sizes.

    The report outlines the properties of GaN, including its wide bandgap and high thermal conductivity, which make it an attractive candidate for laser applications. By doping GaN with rare-earth elements, the researchers aim to enhance its optical properties and improve the efficiency of laser light generation. The document discusses various experimental approaches and results that demonstrate the potential of this material in producing high-energy laser outputs.

    Additionally, the report emphasizes the significance of developing area-scalable laser systems for military and industrial applications. Such systems could be used in a variety of fields, including defense, telecommunications, and medical technologies. The findings suggest that rare-earth doped GaN could lead to advancements in laser technology that are both innovative and practical.

    The document also includes disclaimers regarding the findings, stating that they do not represent an official position of the Department of the Army unless specified otherwise. It encourages the destruction of the report when it is no longer needed, rather than returning it to the originator.

    In summary, this research highlights the potential of rare-earth doped GaN as a transformative material for high-energy lasers, addressing thermal distortion issues and paving the way for scalable laser systems that could have wide-ranging applications.

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