Optical properties of GaN/Er:GaN/GaN core-cladding planar waveguides

Erbium doped GaN (Er:GaN) is a promising candidate as a gain medium for high energy lasers (HELs) operating at the “retina-safe” spectral region around 1.5 µm due to outstanding thermal, mechanical and optical properties of GaN host. Compared to YAG, GaN has a much higher thermal conductivity of $\kappa \quad \approx $ 253 W/m\textbullet K and a smaller thermal expansion coefficient of $\alpha \quad \approx $ 3.53 x 10$^{\mathrm{-6}}$ °C$^{\mathrm{-1}}$ and the potential to significantly outperform YAG based HELs. We report here the successful fabrication and optical characterization of GaN/Er:GaN/GaN core-cladding planar waveguides (PWGs). Optical confinement in the core layer has been investigated. The measured optical loss coefficients of Er:GaN PWGs at 1.54 µm have been measured and are respectively 1.0 cm$^{\mathrm{-1}}$ for the transverse electric (TE) and 1.2 cm$^{\mathrm{-1}}$ for the transverse magnetic (TM) modes. Based on the observed transition lines, a detailed energy levels diagram in Er:GaN has been constructed. Approaches to further reduce the optical loss and optimal configuration for resonantly pump GaN/Er:GaN/GaN PWGs for achieving amplification near 1.5 µm have been identified.~