Frequency shifting of light via multiple ultrasound waves in scattering media.

We derive analytical expressions to describe how light is frequency-shifted when interacting with ultrasound within scattering media, due to the modulation of the refractive index induced by the ultrasound pressure waves. The model is validated through Monte Carlo simulations, works for high ultrasound pressures, and allows for many simultaneous ultrasound waves or frequency components, which is important due to the non-linear propagation effects in tissue. We also provide critical insights into how the ultrasound properties can be optimized for an enhanced efficiency of the light to be frequency-shifted, facilitating applications in ultrasound optical tomography and other photonic diagnostic techniques.