  Cavity-polaritons in semiconductor photonic structures have emerged as a test
bed for exploring non-equilibrium dynamics of quantum fluids in an integrated
solid-state device setting. Several recent experiments demonstrated the
potential of these systems for revealing quantum many-body physics in
driven-dissipative systems. So far, all experiments have relied on fully
integrated devices with little to no flexibility for modification of device
properties. Here, we present a novel approach for realizing confined
cavity-polaritons that enables in-situ tuning of the cavity length and thereby
of the polariton energy and lifetime. Our setup is based on a versatile
semi-integrated low-temperature fiber-cavity platform, which allows us to
demonstrate the formation of confined polaritons (or polariton boxes) with
unprecedented quality factors. At high pump powers, we observe clear signatures
of polariton lasing. In the strong-confinement limit, the fiber-cavity system
could enable the observation of the polariton-blockade effect.
