  Controlling the relaxation dynamics of excitons is key to improving the
efficiencies of semiconductor--based applications. Confined semiconductor
nanocrystals (NCs) offer additional handles to control the properties of
excitons, for example, by changing their size or shape, resulting in a mismatch
between excitonic gaps and phonon frequencies. This has led to the hypothesis
of a significant slowing--down of exciton relaxation in strongly confined NCs,
but in practice due to increasing exciton--phonon coupling and rapid
multiphonon relaxation channels, the exciton relaxation depends only weakly on
the size or shape. Here, we focus on elucidating the nonradiative relaxation of
excitons in NCs placed in an optical cavity. We find that multiphonon emission
of carrier governs the decay resulting in a polariton--induced phonon
bottleneck with relaxation timescales that are slower by orders of magnitude
compared to the cavity--free case, while the photon fraction plays a secondary
role.
