  Hybrid light-matter states, polaritons, are one of the central concepts in
modern quantum optics and condensed matter physics. Polaritons emerge as a
result of strong interaction between an optical mode and a material resonance,
which is frequently realized in molecular, van der Waals, or solid-state
platforms (1-7). However, this route requires accurate (nano)fabrication and
often lacks simple means for tunability, which could be disadvantageous in some
applications. Here, we use a different approach to realize polaritonic states
by employing a stable equilibrium between two parallel gold nanoflakes in an
aqueous solution (8). Such plates form a self-assembled Fabry-Perot microcavity
with the fundamental optical mode in the visible spectral range. The
equilibrium distance between the plates is determined by a balance between
attractive Casimir and repulsive electrostatic forces (9-11) and can be
controlled by concentration of ligand molecules in the solution, temperature,
and light pressure, which allows active and facile tuning of the cavity
resonance by external stimuli. Using this Casimir approach, we demonstrate
self-assembled polaritons by placing an excitonic medium in the microcavity
region, as well as observe their laser-induced modulations in and out of the
strong coupling regime. These Casimir microcavities can be used as sensitive
and tunable polaritonic platforms for a variety of applications, including
opto-mechanics (12), nanomachinery (13), and cavity-induced effects, like
polaritonic chemistry (14).
