Enhancing the detection of laser-excited strain waves via transparent nanolayers

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.