An apparatus may include a substrate, a light source system and a receiver system.
The light source system may be configured to emit light through an area of the substrate, from a first side of the substrate towards a target object in contact with a second and opposite side of the substrate. The receiver system may include one or more receivers residing in, on or proximate the substrate. The receiver system may be configured to detect surface acoustic waves propagating in the substrate corresponding to a photoacoustic response of the target object to light emitted by the light source system. In some examples, the apparatus may include a control system. The control system may be configured to receive surface acoustic wave signals from the receiver system corresponding to detected surface acoustic waves and to detect at least one structure within the target object based on the surface acoustic wave signals.