We show that the transient response of acoustically modulated optical flux in a turbid medium irradiated by a pulsed point source of light is delayed in time relative to the light-alone flux obtained in the ab sence of acoustic modulation. The time delay is shown to result from an initial phase of flux reversal, as determined by the time point of the input pulse onset with reference to the ultrasound cycle. Both the time delay and amplitude of modulation are shown to be dependent on the effective attenuation coefficient of the medium. Application of a periodic train of excitation pulses spaced at equal intervals at, or in multiples of, the ultrasound period enables a time-locked detection of the modulated light, without the deleterious effects caused by speckle artifacts.
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