Femto-Newton Sensitivity Opto-Mechanical Force Measurement

A common tool for measuring such minute effects is a torsion balance. Depending on its design, it gives the possibility of performing absolute and/or differential small force measurements. Our system was designed to allow absolute measurements of light pressure. It consists of a gold-coated glass plate, 50 mm x 10 mm x 0.15 mm in size, acting as the torsional body. In addition, a 50 g overdamped counter mass stabilizes the absolute wire position and acts as a mechanical low-pass filter for seismic noise. The whole setup is kept in a high vacuum environment which is mounted on top of an active vibration isolation system. The sensitivity of the system was demonstrated by pointing a laser beam on one highly reflecting arm of the torsion balance. We measured the pendulum´s elongation while the intensity of the incident laser beam was modulated at 0.03 Hz with an amplitude of 100 muW p-p shows that the equivalent force of 670 fN is clearly resolvable.