  We report on the realization of an open plane-concave Fabry-P\'erot resonator
with a mode volume below $\lambda^3$ at optical frequencies. We discuss some of
the less common features of this new microcavity regime and show that the
ultrasmall mode volume allows us to detect cavity resonance shifts induced by
single nanoparticles even at quality factors as low as $100$. Being based on
low-reflectivity micromirrors fabricated on a silicon cantilever, our
experimental arrangement provides broadband operation, tunability of the cavity
resonance, lateral scanning and promise for optomechanical studies.
