The dynamic response in a translucent tissue-mimicking phantom resulting from an acoustic radiation force impulse (ARFI) was investigated with optically-based methods. Embedded microspheres (10-Â¿m diameter) were tracked axially and laterally with an optical microscope; 0.5-Â¿m displacement resolution was achieved in both dimensions with frame rates of up to 36 kHz. Tracking was achieved before, during, and after the ARFI excitation at depths of up to 4.8 mm from the phantom's nearest material boundary. Both on- and off-axis (i.e., shear wave) results are presented; these results are then corroborated with matched finite element method (FEM) modeling results. This optically-based tracking method provides high-frame-rate, 2D tracking data, and thus it could prove fruitful in the future investigation of complex ARFI-induced dynamics in experimental settings.