Tagging linear fluorescence signals by ultrasound has been proposed to achieve improved spatial resolution at known imaging depths. An alternative way to enhance imaging resolution is the use of nonlinear optical methods such as second-harmonic imaging microscopy and two-photon excitation fluorescence. Extending further along these lines, we have studied the fundamental aspects of the effect of ultrasound irradiation on two nonlinear optical effects. In contrast to the focused transducers that are traditionally used in the study of ultrasound -mediated fluorescence that cause a small fluorescence modulation, we have made use of a simple, flat disc-shaped transducer to generate standing waves in the sample cell. Upon ultrasound irradiation, a prompt increase of the hyper-Rayleigh scattering signal with 35% was observed. The signal enhancement shows a clear correlation with the standing wave pattern, as characterized by laser beam deflection scanning measurements. The viability of different possible mechanisms underlying the effect is discussed.

The investigation and detection of near-surface structures (cavities, caves, tunnels, mineshafts, buried objects, archeological ruins, water reservoir, etc.) is important to mitigate geo-and environmental hazards. We use a method inspired by seismic interferometry to estimate the location of a cavity in a scaled ultrasonic experiment, representative for geophysical field problems. We use only one source at the surface and retrieve ghost scattered waves by evaluating the correlation of scattered waves at different receiver locations. As an exploitation of the ghost arrival information, the ghost travel times are determined and combined to estimate the location of a cavity with good accuracy.