Resolution and definition in photothermal imaging

Abstract

The influences of pump and probe beam sizes, as well as chopping frequency, on resolution and definition in photothermal laser-beam-deflection imaging of subsurface structure are investigated experimentally and theoretically. It is shown that the resolution of nearby subsurface structures is improved by decreasing pump and probe beam sizes to dimensions less than, but not necessarily much less than, the characteristic dimension of the subsurface structures. It is also shown that the photothermal image width (full width at half maximum) may be different from the structure size, and that the width may vary with frequency, for certain structure geometry. A theoretical model of thermal-wave scattering gives results consistent with the present experimental data and with previous, apparently contradictory, results in the appropriate geometric limits.