The large nonlinearities of exciton-polaritons have been widely exploited in order to control the formation of temporal solitons, initially in microcavities and more recently in waveguides. Within a GaN ridge waveguide forming a Fabry-Perot cavity, with a length of a few tens of microns, we have recently demonstrated a mode-locked polariton laser based on cavity solitons [H. Souissi et al., “Mode-locked waveguide polariton laser”, Optica 11, 962 (2024)], with a very low energy per pulse (5 fJ).
Such waveguide polariton lasers can be optically pumped on a length much shorter than the cavity length since the polariton absorption along the unpumped sections is much smaller than the polaritonic gain arising from the stimulated relaxation of excitons towards the polariton branch in the pumped section. During this polariton condensation process, the solitons then form spontaneously as the third order polariton nonlinearity compensates the group velocity dispersion. The emission spectrum of the pulsed polariton laser in a 40 µm-long cavity pumped over 6.5 µm is composed of Fabry-Perot modes a characteristic secant-hyperbolic envelope. The pump is resonant with the GaN excitons, which large binding energy and Rabi splitting allow for a stable operation in the strong coupling regime up to 300K. Interestingly, the cavity soliton becomes instable when the excitation spot is moved away from the cavity center, leading to the simultaneous propagation of two cavity solitons. Longer cavity even lead to the observation of more than 2 solitons. This soliton dynamics can be modeled and understood when solving the Gross-Pitaevskii equations for the coupled exciton-photon system within the waveguide cavity.