  Halide perovskite semiconductors have emerged as promising materials for the
development of solution-processed, scalable, high performance optoelectronic
devices such as light-emitting diodes (LEDs) as well as coherent single photon
emitters. Their integration to nanophotonic cavities for radiative enhancement
and strong nonlinearity is underexplored. In this work, we demonstrate
cavity-enhanced emission and saturable absorption using colloidal CsPbBr$_3$
perovskite quantum dots coupled to a high-Q cavity mode of a circular Bragg
grating structure designed to facilitate integration of solution-processed
materials . We achieve an order of magnitude increase in brightness and 8-fold
increase in the spontaneous emission rate for the cavity-coupled emitters. This
result indicates the possibility of achieving transform-limited photon
coherence for the halide perovskites at cryogenic temperatures. We also observe
saturable absorption of the emitters through intensity-dependent cavity quality
factor. These results pave the way towards achieving improved photon
indistinguishability and strong optical nonlinearities for cavity coupled
perovskite systems.
