GaN-based Waveguide polariton lasers: from quasi-CW to mode-locked lasers

Polariton lasers rely on the stimulated relaxation of polaritons instead of the population inversion involved in conventional semiconductor lasers. Exciton-polaritons are hybrid quasi-particles arising from the strong coupling between excitons and photons. Polariton lasers have first been investigated in vertical microcavities, and more recently demonstrated in the ridge geometry, with a design very similar to most ridge lasers, so that the two lasing mechanisms can readily be compared. In such waveguides, wide bandgap semiconductors, together with organic semiconductors and perovskites, provide large exciton-photon coupling strength and room-temperature polariton stability [1]. They allowed the demonstration of waveguide polariton lasers, first in ZnO waveguides [2], and then in GaN waveguides [3,4].We here present two lasing regimes demonstrated in waveguide polariton lasers, which are compared to conventional semiconductor lasers, focusing on GaN-based devices. (i) The quasi-continuous (CW) operation of the laser is exemplified at T=70K by measuring the laser threshold when reducing the gain length within the cavity under optical pumping, down to 15% of the cavity length [3] which would prevent lasing in conventional cavities. (ii) The mode-locked pulsed operation of the same laser device is achieved as the detuning of polaritonic gain/stimulated relaxation is brought to negative values, which is obtained as the operating temperature is increased to T=150K. The onset of mode-locking is explained by the specific self-focusing condition realized in a polariton waveguide, which combine a strong positive group velocity dispersion and a strong repulsive polariton-polariton interaction – a situation which is inversed compared to most mode-locked lasers based on optical fibers. It should be noticed that mode-locking does not require to insert a saturable absorber within the cavity.[1] C. Brimont et al., Phys. Rev. Appl. 14, 054060 (2020)[2] O.Jamadi et al. Light: Science & Applications 7, 82 (2018)[3] H. Souissi et al., Phys. Rev. Appl. 18, 044029 (2022); arXiv:2201.04348 (2022)[4] A. Delphan et al., APL Photonics 8, 021302 (2023)