Frequency domain laser ultrasonics: optical transduction of acoustic waves and nanomechanical devices

Abstract

The concept of optical excitation and detection of nanoscale mechanical motion has led to a variety of tools for non-destructive materials characterization and remote sensing. These techniques, commonly referred to as laser ultrasonics, offer the benefit of high-bandwidth, highly localized measurements, and also allow for the ability to investigate nanoscale devices. The impact of laser ultrasonic systems has been felt in industries ranging from semiconductor metrology to biological and chemical sensing. In this thesis, we develop a variety of techniques utilizing a frequency domain laser ultrasonic approach, where amplitude modulated continuous wave laser light is used instead of traditional pulsed laser sources, and we apply these systems in free-space, optical fiber based. and integrated on-chip configuration. In doing so , we demonstrate the ability to efficiently transduce various types of mechanical motion including surface and bulk acoustic waves, guided acoustic waves, and resonant motion from nanomechanical systems (EMS). First, we develop a superheterodyne free-space ultrasonic inspection system in an effort to characterize tiurface acoustic wave dispersion in thin-film material systems. We utilize a similar system to study negative refraction and focusing behavior of guided elastic waves in a thin metal plate, providing a novel approach for the study of negative index physics. Furthermore, we develop a nearfield optical technique using optical fibers to simultaneously t ransduce the motion of 70 NEMS resonators using a single channel. This multiplexed approach serves as a crucial step in moving NEMS technology out of the research laboratory. Finally, we go on to study opto-mechanical interactions between optical whispering gallery mode (WGM) resonators and integrated EMS devices on the same chip, using the enhanced interactions to tudy optical forces acting on the nanoscale mechanical devices. This integrated system provides a very efficient mechanical sensing platform as well as a robust test-bed for the study of new optical interactions including the presence of both attractive and repulsive optical forces. The overall goal of the work is to further the state-of-the art for optically transduced nano mechanical sensing as well as to advance the understanding of optomechanical interactions of nanoscale devices.