Modeling of optoacoustic signal generation for high resolution near-surface imaging with experimental verification

Optoacoustic systems making use of optical detection probes are potentially advantageous over contact transducers for noncontact, noninvasive high-resolution near surface imaging applications. In this work, an interferometer is used for high-frequency optoacoustic microscopy. The limitations of this system in terms of both sensitivity and resolution are discussed. A theoretical model has been developed for two-dimensional excitation source geometries, which can be used to predict the optoacoustic signal from a target material with an arbitrary through-thickness optical absorption distribution. The model incorporates the temporal and spatial profile of the excitation laser pulse, and is used to predict the actual out-of-plane displacement at the target surface. An adaptive, photorefractive crystal-based interferometry system has been used to measure the optically induced displacement on the surface of target materials, and the results show reasonable quantitative agreement with theory. The detection system has a 200 MHz bandwidth allowing for high-resolution imaging, and the use of optical probes for both generation and detection allows for the probes to be easily co-aligned on the sample surface. Preliminary experimental results are presented demonstrating the feasibility of using all-optical optoacoustic microscopy for near surface imaging of small-scale spatial variations in optical absorption.