Nature (2019)

Polaritons are particles made up of light and electrons, and can be used to enhance the coupling between photons via the electronic interactions. Patrick Knüppel and collaborators have shown that the interaction between polaritons in a two-dimensional electron gas embedded in an optical cavity can be substantially increased if the electronic part of the polaritons is in a fractional quantum Hall state. This leads to a strong nonlinear optical response.

The authors observed an increase in the reflected third harmonic of the incident light when the electrons were at filling fractions 2/3 and 2/5. This suggests that nonlinear spectroscopy can be used to investigate strongly correlated materials in addition to traditional linear spectroscopy and transport techniques. Also, the ‘polariton blockade’ regime — where the interactions shift the resonance condition by more than the linewidth so that the presence of a polariton blocks the injection of a second one — might now be within reach.