Optimized Virtual Optical Waveguides Enhance Light Throughput in Scattering Media

Ultrasonically sculpted gradient-index optical waveguides make it possible to non-invasively steer and confine light inside scattering media. This confinement capability has applications in tissue and brain imaging, where virtual optical waveguides can be used on their own or cascaded with physical optical elements. The level of light confinement strongly depends on ultrasound parameters such as modulation pattern, frequency, and amplitude, as well as the material parameters of the scattering medium such as the refractive index, scattering coefficient, and phase function. We provide a characterization of these dependencies for a radially symmetric virtual optical waveguide. To this end, we develop a physically-accurate simulator, and use it to quantify how different ultrasound and material parameters affect light confinement. We explain our observations through a qualitative analysis of the behavior of multiply scattered light. We use the results of this analysis to demonstrate that, by properly designing ultrasound parameters, we can achieve a fourfold improvement in light confinement compared to previous virtual optical waveguide designs. We additionally show that virtual optical waveguides can achieve up to 50% light throughput enhancement compared to an ideal external lens, in a medium that mimics the scattering properties of human bladder, and at an optical thickness of one transport mean free path. Lastly, we show experimental results that corroborate the simulation predictions. In particular, we demonstrate for the first time that virtual optical waveguides effectively recycle scattered light in turbid media, and can achieve a 15% light throughput enhancement at five transport mean free paths.