Résumé
The physics of gain in interband semiconductor lasers is mostly discussed in terms of population inversion for electrons and holes and the associated Bernard-Durrafourg condition. Due to the reciprocity between the processes of absorption and stimulated emission, one consequence is that lasing action can only be reached in standard ridge lasers if the pumped section of a laser is longer than the unpumped (absorptive) section. Here we present how a polariton laser, in a ridge waveguide laser geometry very similar to that of standard ridge lasers, operates on a fundamentally different lasing scheme. Alike multi-section lasers, this laser can be optically pumped over an adjustable length, and we show that lasing action is observed for a pumped length equal to only 15% of the cavity. Based on this striking feature, the comparison between polariton gain and population inversion will be didactically discussed.Historically, polariton lasers have mostly been studied in the geometry of vertical cavities, very similar to Vertical Cavity Surface Emitting Lasers. The polariton waveguides have been explored more recently, opening new potentialities to the field of polaritonics. They exhibit low waveguide losses, leading to attenuation lengths as long as hundreds of microns. Their intrinsic self-focusing nonlinearity lead to the formation of bright temporal solitons in GaAs waveguides at T=4K [1], as well as the generation of a super-continuum. Wide bandgap materials offer the possibility of a strong coupling regime stable up to room temperature. In a bulk ZnO waveguide, with cavities formed between natural cracks, the first waveguide polariton laser was recently realized [2].The studied system in the current study is a ridge polariton laser based on a GaN/(Al,Ga)N on sapphire etched structure. As shown on Fig. 1.a, GaN/air reflectors provide the longitudinal confinement and deep-etched 1µm-wide ridges provide the lateral confinement. A line-shaped pump spot (in yellow) matching the ridge shape allows us to adjust the length of the pumped section, i.e. the size of the exciton reservoir feeding the stimulated relaxation process towards the lasing polariton mode. Below threshold, the emission exhibits the Fabry-Perot modes of the longitudinal cavity (3.37-3.44eV, Fig. 1.b), with a decrease of the free spectral range near the exciton resonance as expected in the strong coupling regime [3]. At threshold, polariton modes with a slightly positive detuning (3.45eV) undergo a nonlinear intensity increase as well as a spectral narrowing, demonstrating the operation of the polariton laser. Very short cavity lengths (5 to 60µm) are enough to provide sufficient gain. Since the polariton attenuation in the unpumped section is much smaller than the gain under the pump spot [4], lasing is reached even for a pump length of 15% of the cavity length (Fig. 1.c), with only a twofold increase of the overall threshold for a 20µm-long cavity. This is a striking demonstration of the specificity of polariton lasers. Polariton lasing was maintained up to T=200K.[1] Walker et al., Appl. Phys. Lett. 102, 012109 (2013) and Nat. Comm. 6, 8317 (2015)[2] Jamadi et al. Light: Science & Applications 7, 82 (2018) [3] Brimont et al., Phys. Rev. Appl. 14, 054060 (2020)[4] Solnyshkov et al., Appl. Phys. Lett. 105, 231102 (2014)