Abstract
Light-induced acoustic waves can be used as sensitive probes, providing a pathway toward microscopic imaging and metrology in optically inaccessible media. The ability to detect such waves depends on the interaction of an optical probe pulse with the acoustic waves in the topmost layers of the structure. Therefore, the interplay between optoacoustic coupling and material boundaries, combined with the properties of acoustic waves near free surfaces is of prime importance. Here we show an approach toward optimized optical detection of such laser-excited acoustic waves. We explore the physics underlying this detection, finding that the presence of a free surface actually reduces the optoacoustic interaction, and subsequently enhancing this interaction via adding transparent nanolayers on the free surface. Our work uncovers an important yet rarely explored aspect in optical detection of strain waves via free surfaces and may lead to strategies for signal enhancement in the imaging and characterization of subsurface structures using laser-excited strain waves.
Highlights
Ultrafast laser generation and detection of acoustic waves in solids have attracted increasing attention in recent decades
We show an approach toward optimized optical detection of such laser-excited acoustic waves
When the acoustic waves are measured via the photoelastic effect as variations in the reflected probe intensity, there is no contribution from the surface displacement, as it only changes the phase of the probe pulse [13]
Summary
Ultrafast laser generation and detection of acoustic waves in solids have attracted increasing attention in recent decades. (3) The absorption of a femtosecond laser pulse results in a swift heating of the solid surface, followed by a prompt surface expansion, leading to static surface stress and propagating stress σprop From this analysis, in ultrafast photoacoustics, the total stress is the sum of three contributions: σtotal = σth + σstat + σprop. When the acoustic waves are measured via the photoelastic effect as variations in the reflected probe intensity, there is no contribution from the surface displacement, as it only changes the phase of the probe pulse [13] It is the purpose of this paper to experimentally verify the role of a free surface in strain detection by optical means. In those applications [8,10], the free-surface effect may be explored to enhance the strain detection via the photoelastic effect
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