Nanomechanical Characterization with Near-field Optical Microscopy

A highly sensitive non-destructive material characterization tool is developed with the goal of measuring the high frequency motion of laser generated ultrasound with nanometer scale lateral spatial resolution. The spatial resolution is achieved through the incorporation of near-field scanning optical microscope (NSOM) techniques, which rely on the measurement of the back scattered near-field light intensity from a illuminated probe-tip placed in close proximity to the sample surface. The weak signal level of the NSOM is enhanced by coupling light to surface plasmon polaritons (SPPs) that are localized at the apex of the probe-tip, and a novel heterodyne demodulation technique is additionally developed for efficient suppression of the high background signal content. A series of near-field imaging experiments along with the theoretical confirmations are provided as a proof of concept to the deep sub-wavelength optical imaging capabilities of the NSOM and the plasmonic nanofocusing probe. The plasmonic near-field scanning optical microscope (p-NSOM) is subsequently used for local detection of the laser generated ultrasound and nanomechanical characterization of doubly clamped resonators. An optoacoustic transducer integrating constrained generation is fabricated, and acoustic waves excited by sub-surface absorption are measured using the plasmonic probe. The p-NSOM is also used for dynamic characterization of nanoelectromechanical systems (NEMS): the heterodyne demodulation approach is utilized in the steady measurement of harmonic vibrations of a NEMS resonator, and laser excitation is used to measure the transient response of the resonator due to a pulsed source in both time and space. These experimental results demonstrate that the p-NSOM is able to measure mechanical motion greater than 100 megahertz and provide a clear indication that the bandwidth of the system is not dependent on the mechanical response of the cantilever probe. This technique, which offers local sensitivity to optical and mechanical properties, has broad applications in nanometrology, nanomechanical characterization, and molecular imaging in biological structures.