  Excitons in lead bromide perovskites exhibit high binding energy and high
oscillator strength, allowing for a strong light-matter coupling regime in the
perovskite-based cavities localizing photons at the nanoscale. This opens up
the way for the realization of exciton-polariton Bose-Einstein condensation and
polariton lasing at room temperature -- the inversion-free low-threshold
stimulated emission. However, polariton lasing in perovskite planar photon
cavities without Bragg mirrors has not yet been observed and proved
experimentally. In this work, we employ perovskite metasurface, fabricated with
nanoimprint lithography, supporting so-called exceptional points to demonstrate
the room-temperature polariton lasing. The exceptional points in
exciton-polariton dispersion of the metasurface appear upon optically pumping
in the nonlinear regime in the spectral vicinity of a symmetry-protected bound
state in the continuum providing high mode confinement with the enhanced local
density of states beneficial for polariton condensation. The observed lasing
emission possesses high directivity with a divergence angle of around 1$^\circ$
over one axis. The employed nanoimprinting approach for solution-processable
large-scale polariton lasers is compatible with various planar photonic
platforms suitable for on-chip integration.
