  Ultrasound-modulated optical tomography enables sharp 3D optical imaging of
tissues and other turbid media, but the light modulation signals are hard to
sensitively measure. A common solution, involving photorefractive crystals,
enables the measurement of a relatively slow and low-spatial-resolution signal
tracking the envelope of the ultrasound wave. We reexamine the photorefractive
detection principle both intuitively and quantitatively, and from this analysis
we predict that the photodetector should additionally see a fast response at
twice the ultrasound frequency, and correspondingly high spatial frequency. The
fast and slow response usually have similar amplitudes and reveal complementary
information, thus allowing ultrasound-modulated optical tomography to create
dramatically sharper tomographic images under the same measurement conditions,
integration time, and experimental complexity.
