Measuring acoustic fields in an optical trap

Abstract

This thesis describes progress in the use of optical traps for measurement of acoustic fields, a first step toward acoustic micromanipulation in fluids. The optical trap used throughout this study is carefully characterized. Eccentricity of measured distributions of thermal fluctuations of trapped dielectric spheres is attributed to a large, directional 60 Hz noise source. Analyses of trap stiffness insensitive to this noise are discussed and their results plotted. Knowledge of the trap and the dynamics of trapped objects are used to make measurements of acoustic fields. Cavity modes are identified and their central frequencies are shown to shift by a part in a thousand over minutes. This change is attributed to the temperature dependence of the speed of sound in the medium. Fluctuations in estimators of cavity mode resonance peak height are shown and the need for acoustic source stability is discussed.