Measurement of electromagnetic radiation force using a capacitance-bridge interferometer

We present a mechanical cantilever-based tabletop interferometer to measure the radiation force exerted by light. Using a high-power (~ 1W) pulsed laser beam, we excite mechanical oscillations in a thin metallic cantilever. The cantilever forms a parallel-plate capacitor with a printed circuit board trace. Using a capacitance-bridge geometry, we measure small capacitance changes of the order of femto-farads, induced by the radiation forces of a few nano-newtons. This experiment uses equipment commonly found in an undergraduate teaching laboratory for physics and electronics while providing insight into electromagnetic wave theory, circuit design for low-noise measurements, and Fourier analysis.