Spatiotemporal continuum generation in polariton waveguides

Abstract

We demonstrate the generation of a spatiotemporal optical continuum in a highly nonlinear exciton–polariton waveguide using extremely low excitation powers (2-ps, 100-W peak power pulses) and a submillimeter device suitable for integrated optics applications. We observe contributions from several mechanisms over a range of powers and demonstrate that the strong light–matter coupling significantly modifies the physics involved in all of them. The experimental data are well understood in combination with theoretical modeling. The results are applicable to a wide range of systems with linear coupling between nonlinear oscillators and particularly to emerging polariton devices that incorporate materials, such as gallium nitride and transition metal dichalcogenide monolayers that exhibit large light–matter coupling at room temperature. These open the door to low-power experimental studies of spatiotemporal nonlinear optics in submillimeter waveguide devices.

Nonlinear optics: polariton waveguides

Miniature exciton-polariton waveguides make it possible to induce a variety of nonlinear optical effects at very low powers. Paul Walker and co-workers from a UK-Russian team fabricated semiconductor waveguides containing 3 InGaAs quantum wells and a GaAs core and an AlGaAs cladding. Picosecond pulses from a Ti:Sapphire laser (2 ps, 100 W peak power) were coupled into the waveguide from above via a gold grating coupler, they then propagated a distance of 100–600 μm along the waveguide prior to being out-coupled into free space by a second grating coupler. The strong light–matter coupling between excitons and photons inside the waveguide structure induced spectral broadening nonlinear effects such as continuum generation, self-phase modulation, modulational instability and the generation of Cherenkov radiation and X waves. The amount of spectral broadening is determined by the strength of the light–matter coupling.