Résumé
Polaritonic waveguides are bringing new potentialities to the field of polaritonics. The low waveguide losses allow for long propagation distances, up to hundreds of microns. In GaAs waveguides at T=4K, the formation of bright temporal solitons was demonstrated [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].Here we present the demonstration of a waveguide polariton laser in a GaN etched ridge structure with processed Bragg reflectors (Fig. 1a). The GaN waveguide is grown on a sapphire substrate and a thick AlGaN cladding. The transverse optical confinement is provided by a deep-etched 1µm-wide ridge, while the longitudinal confinement is obtained thanks to GaN/air reflectors. This design provides a full control of the cavity geometry, a major progress with respect to the previous demonstration based on uncontrolled cracks. The waveguide is excited with a line-shaped pump spot (in yellow on Fig 1a) matching the ridge (4ns pulse at 7kHz, =355nm resonant with the excitonic reservoir). The polariton emission is recorded at the DBRs, which act simultaneously as mirrors and extractors for the guided modes. Below threshold, the emission exhibits the Fabry-Perot modes of the longitudinal cavity (3.37-3.44eV), with a decrease of the free spectral range near the exciton resonance as expected in the strong coupling regime. 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. Polariton lasing was maintained up to T=200K.Polariton lasing is observed for a wide range of cavity lengths (from 5µm to 60µm), showing the large gain that can be reached in this geometry. For long cavities, lasing occurs whatever the length of the line-excitation spot, i.e. of the exciton reservoir. It should be underlined that this is a very nice illustration of the specific gain mechanism of a polariton laser: the Bernard-Durrafourg inversion condition in a standard ridge laser forbids lasing action if the pumped length is smaller than half the cavity length, since the gain under the pump cannot be larger than the absorption in the absence of pump. In the current work, a pump length equal to 10% of the cavity length still allows lasing, with only a slight increase (threefold) of the overall threshold. These results will be compared to the model of gain in a polaritonic waveguide [3].[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] Solnyshkov et al., Appl. Phys. Lett. 105, 231102 (2014)