3 The Stanford microcantilever
The behaviour of gravity at small distances can also be explored using micron-sized devices, such as this microcantilever at Stanford. The test mass measures 50 x 50 x 30 μm and rests on a cantilever, while the "drive" mass consists of gold bars (1 mm x 100 μm x 100 μm) embedded in silicon. The vertical separation of the test and drive masses is only 25 μm.
A piezoelectric actuator moves the drive mass from side to side, while a fibre-based interferometer measures the resulting motion of the test mass, which has a resonant frequency of about 300 Hz. The device has been designed such that any short-range interactions will produce an excess of motion at this resonant frequency, while the drive frequency is well below resonance. The conducting shield (yellow) suppresses electrostatic interactions that could mimic a deviation from the inverse-square law. The first results using this device provide the most sensitive limits for any new forces at distances below 10 μm.