Thermal noise reduction of mechanical oscillators by actively controlled external dissipative forces

We show that the thermal fluctuations of very soft mechanical oscillators, such as the cantilever in an atomic force microscope (AFM), can be reduced without changing the stiffness of the spring or having to lower the environ ment temperature. We derive a theoretical relationship between the thermal fluctuations of an oscillator and an actively controlled external dissipati ve force. This relationship is verified by experiments with an AFM cantilev er where the external active force is coupled through a magnetic field. Wit h simple instrumentation, we have reduced the thermal noise amplitude of th e cantilever by a factor of 3.4, achieving an apparent temperature of 25 K with the environment at 295 K. This active noise reduction approach can sig nificantly improve the accuracy of static position or static force measurem ents in a number of practical applications. (C) 2000 Published by Elsevier Science B.V. All rights reserved.