Highly nonlinear optical materials with strong effective photon-photon interactions are required for ultrafast and quantum optical signal processing circuitry. Here we report strong Kerr-like nonlinearities by employing efficient optical transitions of charged excitons (trions) observed in semiconducting transition metal dichalcogenides (TMDCs). By hybridising trions in monolayer MoSe 2 at low electron densities with a microcavity mode, we realise trionpolaritons exhibiting significant energy shifts at small photon fluxes due to phase space filling. We find the ratio of trion-to neutral exciton-polariton interaction strength is in the range from 10 to 100 in TMDC materials and that trion-polariton nonlinearity is comparable to that in other polariton systems. The results are in good agreement with a theory accounting for the composite nature of excitons and trions and deviation of their statistics from that of ideal bosons and fermions. Our findings open a way to scalable quantum optics applications with TMDCs.

We experimentally demonstrate the formation of a macroscopically occupied state (condensation) of excitonpolaritons in a tunable 3D open-access microcavity system under resonant excitation. We observe a high conversion efficiency of polaritons from the pumped state to the ground mode, leading to population of the condensate 1.5 times greater than that of the driven state for a total number of polaritons of ∼110. Comparative numerical simulations show that coherent polariton-polariton scattering mechanisms alone cannot explain the observed experimental results and that other relaxation channels play an important role.

Ultrafast nonlinear photonics enables a host of applications in advanced on-chip spectroscopy and information processing. These rely on a strong intensity dependent (nonlinear) refractive index capable of modulating optical pulses on sub-picosecond timescales and on length scales suitable for integrated photonics. Currently there is no platform that can provide this for the UV spectral range where broadband spectra generated by nonlinear modulation can pave the way to new on-chip ultrafast (bio-) chemical spectroscopy devices. We demonstrate the giant nonlinearity of UV hybrid light-matter states (exciton-polaritons) up to room temperature in an AlInGaN waveguide. We experimentally measure ultrafast nonlinear spectral broadening of UV pulses in a compact 100 μm long device and deduce a nonlinearity 1000 times that in common UV nonlinear materials and comparable to non-UV polariton devices. Our demonstration promises to underpin a new generation of integrated UV nonlinear light sources for advanced spectroscopy and measurement.