  We investigate theoretically and experimentally a first-order dissipative
phase transition, with diffusive boundary conditions and the ability to tune
the spatial dimension of the system. The considered physical system is a planar
semiconductor microcavity in the strong light-matter coupling regime, where
polariton excitations are injected by a quasi-resonant optical driving field.
The spatial dimension of the system from 1D to 2D is tuned by designing the
intensity profile of the driving field. We investigate the emergence of
criticality by increasing the spatial size of the driven region. The system is
nonlinear due to polariton-polariton interactions and the boundary conditions
are diffusive because the polaritons can freely diffuse out of the driven
region. We show that no phase transition occurs using a 1D driving geometry,
while for a 2D geometry we do observe both in theory and experiments the
emergence of a first-order phase transition.
