Mechanical mode dependence of bolometric backaction in an atomic force microscopy microlever

Two backaction (BA) processes generated by an optical cavity-based detection device can deeply transform the dynamical behavior of an atomic force microscopy microlever: the photothermal force or the radiation pressure. Whereas noise damping or amplifying depends on the optical cavity response for radiation pressure BA, we present experimental results carried out under vacuum and at room temperature on the photothermal BA process which appears to be more complex. We show for the first time that it can simultaneously act on two vibration modes in opposite directions: Noise on one mode is amplified, whereas it is damped on another mode. Basic modeling of photothermal BA shows that the dynamical effect on the mechanical mode is laser spot position-dependent with respect to mode shape. This analysis accounts for opposite behaviors of different modes as observed.