Vibrational Manipulation of Microspeheres Using Optically Excited In-Plane Motion of NEMS Oscillators

Optical excitation plays an important role in the actuation of higher flexural and torsional modes of nanoelectromechanical oscillators. We show that optical fields are extremely efficient for excitation, direct control and measurement of in-plane motion of cantilever-type nanomechanical oscillators. As a model system, 200 nm and 250 nm thick single crystal silicon cantilevers with dissimilar lengths and widths ranging from 6 to 12 μm and 500 nm to 1 μm, respectively, were fabricated using surface micromachining and dynamically analyzed using optical excitation and interferrometric detection. Higher order modes of slender cantilevers, calculated using linear Euler-Bernoulli beam model, differed significantly from the measured values. Three dimensional finite element analysis incorporating shear, rotational inertia, cross-sectional deplanation and nonideal boundary conditions due to the structural undercut, are shown to adequately describe the dynamics of the nanomechanical structures. The quality factor of a particular in-plane harmonic was consistently higher than the transverse mode. The increased dissipation of the out of plane mode was attributed to material and acoustic loss mechanisms. The in-plane mode was used to demonstrate vibrational detachment of sub-micron polystyrene spheres on the oscillator surface. In contrast, the out of plane motion, even in the strong non-linear impact regime, was insufficient for the removal of bound polystyrene spheres. Our results suggest that optical excitation of in-plane mechanical modes provide a unique mechanism for controlled removal of particles bound on the surface of nanomechanical oscillators [1].