Guided polaritons in a GaN planar waveguide

Description

The slab waveguide geometry is attracting more and more interest in the field of polaritonics. Waveguide polaritons are obtained when an excitonic transition and a slab guided mode are brought in the strong coupling regime, in strong analogy with cavity polaritons in Fabry-Perot resonators. Waveguide polaritons have been first demonstrated in GaAs waveguide embedding InGaAs quantum wells, with a Rabi splitting of 6 meV [Walker et al., Nat. Comm. 6, 8317 (2015)]. There, at 10K, the polariton fluid of light presents strong optical nonlinearities, solitons are generated at very low power densities. Wide bandgap materials offer the possibility to combine stronc coupling, non-linearities and polariton lasing at 300K, as recently demonstrated for the first time in a ZnO slab waveguide [Jamadi et al., Arxiv:1708.00501]. In nitride materials, a first demonstration of strong coupling was reached in the waveguide geometry with InGaN multiple quantum wells, with a Rabi splitting of 63 meV [Ciers et al., Phys. Rev. Applied 7, 034019 (2015)].In this work we demonstrate the strong coupling in a bulk GaN waveguide grown on Silicon. The epitaxy of thick (500 nm) AlN buffer at high temperature (1000°C) followed by an AlGaN optical cladding layer allows for a good GaN quality. A thin SiO2 top layer is processed into gratings for the extraction of the guided polaritons. The dispersions of the polariton eigenmodes are measured through the diffraction at the gratings and exhibit a characteristic anti-crossing between the guided photons and the excitonic transition. A Rabi splitting Ω=150 ±30 meV for the TE0 polariton mode is deduced from the modeling of the dispersions.Further, the propagation of the guided polaritons is probed at T=10K under non-resonant excitation: the polaritons are generated at a focused excitation spot, and the attenuation of each polariton branch is measured through spatial filtering combined to far-field imaging. At negative detuning (-1.5Rabi) in the photonic regime, the propagation length is of the order of 200±80 µm (i.e. a attenuation of 50 cm-1). It decreases to 12±1µm (i.e. 840 cm-1) at zero detuning. Such waveguide polaritons have the potentiality for introducing non-linear polariton fluids into the realm of integrated photonics operating at room temperature.

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