  A laser threshold is determined by the gain condition, which has been
progressively reduced by the use of heterostructures and of quantum
confinement. The polariton laser is the ultimate step of this evolution:
coherent emission is obtained from the spontaneous decay of an
exciton-polariton condensate, without the achievement of any gain condition.
ZnO, with its unique excitonic properties, is the best choice for a
blue/UV-emitting polariton laser device. We report on the fabrication of a new
family of fully hybrid microcavities that combine the best-quality ZnO material
available (bulk substrate) and two dielectric distributed Bragg reflectors,
demonstrating large quality factors (>2500) and Rabi splittings (~200 meV). Low
threshold polariton lasing is achieved between 4 and 300 K and for excitonic
fractions ranging between 12% and 96 %. A phase diagram highlighting the role
of LO phonon-assisted relaxation in this polar semiconductor is established,
and a remarkable switching between polariton modes is demonstrated.
