Photorefractive second-harmonic detection for ultrasound-modulated optical tomography

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.