Ultrasonically Steerable Graded-Index Optical Waveguides for Deep Tissue Light Delivery: Theory and Applications

Graded-index (GRIN) fibers have been used as implantable optical waveguides to guide light and relay images through the depth of the tissue. We have recently shown that non-invasive ultrasound can generate refractive index gradients within the tissue that form virtual GRIN lenses for imaging and photostimulation deep into the tissue. Here we present the theory behind this idea by analyzing the coupled acoustic-photonic system that models the interaction of light with the ultrasonically modulated medium. We will discuss how changing the parameters of ultrasound will change the confinement and guiding of light within the modulated medium. We will also demonstrate that using a custom-designed cylindrical ultrasonic array, the pressure interference can be controlled to sculpt complex patterns of light in the medium, such as dipole and quadrupole shapes, suitable for multisite imaging. Finally, we will discuss experimental results corroborating the theoretical predictions to generate single and multisite in situ virtual lenses that can be used for fluorescent imaging of mouse brain tissue that expresses green fluorescent protein (GFP).