# Nonlinear Polariton Fluids in a Flatband Reveal Discrete Gap Solitons

## TL;DR

This paper experimentally investigates nonlinear polariton dynamics in a flat band Lieb lattice, revealing discrete gap solitons with unique properties and complex multistability, advancing understanding of flat band physics and potential photon phase control.

## Contribution

It demonstrates the formation of discrete gap solitons in a driven-dissipative flat band system, a phenomenon previously observed only in closed systems, highlighting new nonlinear behaviors.

## Key findings

- Observation of quantized-size gap solitons
- Detection of abrupt edges indicating frozen propagation
- Identification of multistability in nonlinear domains

## Abstract

Phase frustration in periodic lattices is responsible for the formation of dispersionless flat bands. The absence of any kinetic energy scale makes flat band physics critically sensitive to perturbations and interactions. We report here on the experimental investigation of the nonlinear dynamics of cavity polaritons in the gapped flat band of a one-dimensional Lieb lattice. We observe the formation of gap solitons with quantized size and very abrupt edges, signature of the frozen propagation of switching fronts. This type of gap solitons belongs to the class of truncated Bloch waves, and had only been observed in closed systems up to now. Here the driven-dissipative character of the system gives rise to a complex multistability of the nonlinear domains generated in the flat band. These results open up interesting perspective regarding more complex 2D lattices and the generation of correlated photon phases.