The main objective of this research was to construct an optical pump system that would allow the study of Er:GaN materials under 980 nm resonant excitation to be carried out. The results obtained from the optically pumped studies could then be utilized to guide crystal growth and laser design.

Experimental setup for optically pumped studies under 980 nm laser excitation.

High energy and high power solid-state lasers have enabled a variety of applications which have had, and will continue to have, profound and far-reaching impacts on emerging technologies. The optical gain medium is the heart of a high energy laser (HEL) system.

Compared to the presently dominant gain material for HEL, which is Nd doped yttrium aluminum garnet (Nd:YAG), the most outstanding property of GaN as a host material for HEL application is its outstanding thermal properties. The thermal conductivity of GaN is very high (κ = 230 W/m·K) and is more than one order of magnitude higher than YAG (κ = 14 W/m·K), while its thermal expansion coefficient (α ≈ 4 × 10-6 °C-1) is about 2 times smaller than that of YAG (κ ≈ 8 × 10-6 °C-1). These together make Er doped GaN (Er:GaN) an excellent gain material with a potential to outperform Nd:YAG lasers by a factor of about 60 – 120.

Another important advantage of GaN for HEL application is its significantly higher fracture toughness figure compared to YAG. Moreover, the 1.54 μm emission resulting from the intra-4f transition from the first excited manifold (4|13/2) to the ground state (4|15/2) in Er 3+ ions is a relatively eye-safe wavelength, in that the upper limit of eye-safe laser exposure at 1.5 μm is more than 4 orders of magnitude higher than that of the wavelength range below or close to 1 μm.

To realize the full potential of Er:GaN as a gain medium for HEL, however, Er:GaN bulk crystals in large wafer sizes are required to enable the fabrication of gain media in disk, rod or slab geometry to provide high energy and high power operation. Furthermore, a resonant excitation (e.g. 980 nm) is more desirable than a non-resonant excitation as resonant excitation involves direct transition between the ground state to a higher-lying inner 4f manifold in Er3+ ions without invoking a non-radiative energy transfer, hence generating a much smaller amount of heat than a non-resonant excitation. Additionally, 980 nm pump appears to be a preferred pump wavelength in terms of providing the best trade-off between the optical absorption length, minimizing the quantum defect and managing the constraints in hydride vapor phase epitaxy (HVPE) growth for obtaining Er:GaN crystals with a reasonable thickness. Furthermore, the absorption cross section of Yb3+ at 980 nm is about an order of magnitude larger than that of Er3+. Under 980 nm pump, in an Er and Yb co-doped GaN, from Yb 3+ the energy can be transferred resonantly to the 4|11/2 state of Er3+. Therefore, under 980 nm pump, Yb and Er co-doping can enhance the effective excitation cross section by at least one order of magnitude.

This work was done by Jingyu Lin and Hongxing Jiang of Texas Technical University for the Army Research Laboratory. ARL-0203


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Erbium Doped GaN Lasers by Optical Pumping

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This article first appeared in the September, 2017 issue of Aerospace & Defense Technology Magazine.

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