Optical Motion Detection
Abstract
Detecting the motion of an object involves the measurement of its position or velocity. In this thesis, we consider optical detection of motion for two different cases: the monitoring of particle displacement in optical tweezers and the detection of surface displacements due to ultrasonic vibrations. In the first part of the thesis, we consider the resolution limit of particle sensing in optical tweezers. Optical tweezers are formed by tightly focusing light. The resulting optical gradient then gives rise to a restoring force which can be used to trap a particle. Displacement measurements can then be carried out by monitoring the scattered light from the trapped particle. Optimal displacement measurements can be performed by measuring the position of every photon, for example, by using an idealised detector array consisting of infinitesimally small pixels. In practice, quadrant photodiodes are typically used; however, these have previously been shown to be non-optimal for measurements of small beam displacements. Here we developed a model of an optical tweezers system and similarly show that quadrant photodiodes are non-optimal for particle displacement measurements in optical tweezers. We propose an alternative detection scheme based on spatial homodyne detection, where the local oscillator beam is spatially tailored in order to optimally detect the particle displacement. We show that spatial homodyne detection is optimal and outperforms quadrant photodiodes for all cases…
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