Erbium-doped GaN bulk crystals as a gain medium for eye-safe high energy lasers

High energy lasers (HELs) operating around 1.5 μm are considered eye-safe and are highly sought-after for use in defense, industrial processing, communications, medicine, spectroscopy, imaging and various other applications where the laser is expected to travel long distances in free space. Compared to YAG, GaN has a higher thermal conductivity, lower thermal expansion coefficient, and lower temperature coefficient of the refractive index (dn/dT) around 1.5 μm. Hence, HELs based on erbium doped GaN (Er:GaN) are expected to outperform those based on Er:YAG in terms of average power, power density, temperature stability and beam quality. Here, we provide a brief overview on the progress made in authors' laboratory for the realization of freestanding Er:GaN wafers of 2-inches in diameter with a thickness on the millimeter scale. These freestanding wafers were obtained via growth by hydride vapor phase epitaxy (HVPE) in conjunction with a laser-lift-off process. Optical absorption and resonantly excited photoluminescence emission spectra have enabled the construction of energy level diagrams pertaining to the excitation and emission mechanisms of Er:GaN eye-safe HELs operating around 1.5 μm. The results have shown that the common excitation and emission scheme of Er:YAG is not entirely applicable to achieve Er:GaN eye-safe lasers. In addition to higher thermal conductivity of Er:GaN over Er:YAG gain medium, Er:GaN eye-safe lasers appear to possess lower quantum defects than Er:YAG lasers.