Opto-mechanical Control of Cantilever Dynamics

We use laser radiation pressure rather than a traditional mechanical actuator in order to drive the oscillation of a micron sized cantilever as used in atomic force microscopy. We saw that the laser illumination causes a deflection not only via radiation pressure but also via thermal expansion because of residual absorption. By comparing experimental data against a finite-element simulation we could understand the thermal dynamics. The dominating contribution comes from the thermal expansion of the base rather than the cantilever itself with a time constant of tens of microseconds.

Creating an optical spring by using the cantilever as an end mirror of a detuned optical resonator enables the in situ tuning of the resonance frequency and the damping of the opto-mechanical oscillator. Here the optical restoring force comes from the dependence of the optical power, and hence the radiation pressure force, in the resonator on the resonator detuning, and hence the position of the cantilever. The damping/anti-damping results from the optical power in the resonator being retarded with respect the the dynamic position of the cantilever on the time scale of the photon life time in the resonator.  

The additional photothermal effect on the cantilever position will lead to even more interesting dynamics, because here the thermal deflection is retarded with respect to the optical power on the thermal time scale. In combination with the retardation of the optical power with respect to the cantilever position we expect depending on the oscillation frequency a modification of the spring constant and the damping with great flexibility in the parameter range.