Optical actuation of a macroscopic mechanical oscillator

An intensity-modulated HeNe-laser beam was utilized to optically actuate the mechanical resonance of a macroscopic torsional silicon oscillator (f 0 = 67 700 Hz, Q = 42 100 at p = 1 mbar and T = 300 K). Both radiation pressure and photothermal effects may cause optical actuation of a mechanical device. Both excitation effects were studied. In actuation through radiation pressure, the actuating laser beam was focused on the high-reflectivity-coated oscillator surface. In the case where the intensity-modulated laser beam was incident on the uncoated silicon surface the photother-mal effect was shown to be the dominating excitation factor. Oscillation amplitudes due to the actuation through radiation pressure and photothermal effects were x rad = 1.4 pm and x ph = 4.3 pm with the same optical power of 1.5 mW. The measured resonance frequency and quality value were not changed when purely mechanical and radiation pressure actuation mechanisms were compared. With photothermal ac-tuation the absorbed optical power heats the oscillator, introducing a slight decrease in the resonance frequency. Our experiments demonstrate that optical actuation combined with sensitive optical interferometric measurements can be utilized to perform dynamic vibration analysis of micromechanical components. Prospects of using micromechanical devices for observing extremely weak external forces are discussed. 1 Introduction Optical actuation of mechanical components has been widely studied during recent years [1–3]. Lasers and consequent radiation pressure have been used, e.g. in optical levitation of small particles [4] and in laser cooling and trapping of atoms and molecules [5, 6]. Moreover, radiation pressure has been utilized in cooling of macroscopic objects such as vibrational modes of movable mirrors of optical cavities [7–9]. In cold damping a viscous feedback force is used to freeze the mirror motion by applying an additional radiation pressure on the oscillating mirror [8, 9]. Another possibility is to produce squeezed states of the thermal noise by using parametric amplification in which one quadrature of the thermal noise is cooled at the expense of the other, i.e. the other quadrature is heated [10]. It has been discussed how sensitivities of interferometric measurements can be increased by reducing the quantum back action noise [11]. In the quantum locking technique, an active feedback control locks the moving mirror with respect to the position of another less noisy reference mirror and the radiation pressure noise in the interferometer can be reduced. The subject of using optical coupling to the mechanical oscillator for …