  Recent technological advancements have allowed to implement in solid-state
cavity-based devices phenomena of quantum nature such as vacuum Rabi splitting,
controllable single photon emission and quantum entanglement. For a
sufficiently strong coupling between a quantum emitter and a cavity, large
quality factors ($Q$) along with small modal volume ($V_{eff}$) are essential.
Here we show that by applying a 5nm Al coating to the sidewalls of a
submicrometer-sized Fabry-P\'{e}rot microcavity, the cavity $Q$ can be
temperature-tuned from few hundreds at room temperatures to 2$\times$10$^5$
below 30~K. This is achieved by, first, a complete shielding of the sidewall
loss with ideally reflecting lateral metallic mirrors and, secondly, a dramatic
decrease of the cavity's axial loss for small-sized devices due to the largely
off-axis wavevector within the multilayered structure. Our findings offer a
novel temperature-tunable platform to study quantum electrodynamical phenomena
of emitter-cavity coupling. We demonstrate that a Rabi splitting of 2g=24~GHz
(0.142~nm) can be readily achieved at 40~K in a 0.8$\mu$m-sized device, which
has an $V_{eff}\approx0.0845~\mu$m$^3$, comparable to best 2D photonic crystal
(PhC) nanocavities.

Optics