Intense Signal Modulation of Nonlinear Optical Scattering and Multiphoton Fluorescence by Ultrasound Irradiation

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.