Optical lever for broadband detection of fluid interface fluctuations

We exploit the optical lever principle to detect minute fluctuations of a liquid-air interface. Waves propagating on the interface deflect a specularly reflected laser beam, inducing angular deviations captured by a dual-element photodiode. We implement this principle in a compact set-up that includes a temperature-controlled fluid sample. This allows us to detect deflection angle fluctuations across five orders of magnitude in frequency, from individual low-frequency surface eigenmodes to the thermal distribution of high-frequency capillary waves. In addition to demonstrating the method's versatility and broad dynamical range, we highlight practical considerations in characterising liquid interface dynamics, bridging established optical methods with their application to fluid and soft-matter systems.