A Sub-λ³-Volume Cantilever-based Fabry-P\'erot Cavity

We report on the realization of an open plane-concave Fabry-P\'erot resonator with a mode volume below λ³ 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.