A polariton laser based on GaN waveguide: comparative gain study

We present a ridge waveguide polariton laser very similar to standard ridge lasers but, operating on a fundamentally different lasing scheme. The structure consists in a GaN waveguide, operating under strong coupling up to 300K with a 80meV Rabi splitting, and grown by metal-organic vapor phase epitaxy (MOVPE) on c-plane sapphire. Polariton in waveguides have the possibility to propagate with large group velocities while maintaining a large excitonic fraction and, thus, strong nonlinearities.In our study the longitudinal confinement is ensured by vertical GaN/air Bragg reflectors whereas the lateral confinement is produced by a deep-etched 1-µm-wide ridge. We study the effect of the excitation length, i.e. the size of the exciton reservoir, versus the cavity length by exciting the waveguide with a variable line-shaped pump spot matching the ridge, at 355nm, resonant with the exciton reservoir. The power-dependent study at 70K of the 20µm-long cavity by a 10µm long-pump, shows Fabry-Perot modes (3.38-3.45eV) below threshold, with a decrease of the free spectral range near the exciton resonance as expected in the strong coupling regime [1]. At threshold, polariton modes with a slightly positive detuning (δ = 10meV and ∼ 55% excitonic fraction) undergo a nonlinear intensity increase as well as a spectral narrowing, demonstrating the operation of the polariton laser. Contrary to the Bernard-Durrafourg inversion condition, we observe a striking feature specific to polariton lasers: the laser effect is achieved even for an excitation length of just 10% of the cavity, with only a threefold increase of the overall threshold. From the Fabry-Perot transmission, we estimate losses of about 430cm −1 and 760cm −1 at 3.42eV and 3.44eV, respectively, and a large gain characteristic of polariton stimulated scattering [2].[1] Brimont et al., Phys. Rev. Appl. 14, 054060 (2020).[2] Solnyshkov et al., Appl. Phys. Lett. 105, 231102 (2014).