Force-detected, single-molecule spectroscopy and imaging using nanoscale mechanical resonators

Abstract

Novel methods for spectroscopic probing of single-molecules are described that sense the optically induced, molecular dipole through the force/torque it generates on a submicron mechanical probe. The probe comprises a mechanical resonator with a high-Q mode of oscillation at frequency vh, to which is attached a nanoparticle with dipole moment p. This dipole is either the optically induced dipole of a metal nanoparticle irradiated at plasmon resonance, or the static dipole moment of a ferroelectric nanocrystal. The electric force or torque between the probe dipole and molecular dipole drives the motion of the resonator at the resonance frequency. Three novel optical scattering mechanisms, which encode the mechanical motion into the phase, amplitude, or polarization of the light scattered by the resonator are investigated and quantified. A novel single-molecule sensor will also be described that comprises a mechanical torsional resonator with an attached ferroelectric nanoparticle. The observable quantity is the shift in the oscillation frequency of the mechanical resonator as a molecule becomes polarized by the rf near-field of the ferroelectric particle. The ferroelectric particle couples electrostatically to a nearby nanoscale capacitor which is used to electrically drive and detect the resonant mechanical motion. Due to this coupling, the electric and mechanical coordinates, which specify the state of this electromechanical device, are no longer the eigenmodes of the system. This gives rise to interesting dynamical effects that are best analyzed using the Lagrange formulation of mechanics. Finally, we discuss experimental progress toward fiber-optic interferometric detection of submicron mechanical resonators.