Cavity-enhanced continuous-wave microscopy with potentially unstable cavity length

Microscopy gives access to spatially resolved dynamics in different systems, from biological cells to cold atoms. A big challenge is maximizing the information per used probe particle to limit the damage to the probed system. We present a cavity-enhanced continuous-wave microscopy approach that provides enhanced signal-to-noise ratios at fixed damage compared to standard single-pass microscopy. Employing a self-imaging 4f cavity, we show contrast enhancement for controlled test samples as well as biological samples. For thick samples, the imaging cavity leads to a new form of dark-field microscopy, where the separation of scattered and unscattered light is based on optical path length. We theoretically show that enhanced signal, signal-to-noise, and signal-to-noise per damage are also retrieved when the cavity is not length-stabilized. Our results provide an approach to cavity-enhanced microscopy with non-length-stabilized cavities and might be used to enhance the performance of dispersive imaging of ultracold atoms.