Phototermal self-excitation of nanomechanical resonators in liquids

We report the use of the photothermal actuation for the self-excitation of a selected vibration mode of a microcantilever in liquid. The gain of the positive feedback loop is adjusted in order to obtain a negative effective damping. In this regime, the amplitude noise is squeezed due to the nonlinear saturation of the system and the phase noise is largely reduced. The microcantilever vibration achieved a frequency stability of the order of 1 ppm for a bandwidth of 1 Hz. This is at least two orders of magnitude better than previous measurements in liquids. The obtained sensitivity is applied for detecting in real time the change of the fluid properties when glycerol is added to water at a concentration of 1% (m/m). (c) 2008 American Institute of Physics.