In situ spatiotemporal patterning of light using ultrasonic virtual tunable optical waveguides

In this thesis, I discuss a novel technique for confining and patterning light in situ using ultrasonic waves. In this method, ultrasound is launched into the medium non-invasively to form virtual optical waveguides that can confine, guide, and pattern light within the volume of the target medium. I demonstrate that these optical patterns can be rapidly reconfigured by virtue of changing the interference pattern of the ultrasonic waves launched into the medium. This method presents unprecedented advantages for in situ spatiotemporal optical beam patterning that can be combined with state-of-the-art optical methods to enhance their reach and flexibility. Conventional external optical elements have been used to shape and pattern light prior to launching it into the target medium. Lack of control on the propagation of light beams into the target medium after it is launched poses limitations for these conventional methods based on external optics. To address this issue, invasive optical elements such as endoscopes have also been used to deliver light to the depth of the target medium to protect photons from scattering and refraction due to the inhomogeneity of the medium. These invasive optical methods disturb the medium, which can be detrimental, for example, when light is being delivered to or collected from biological tissue. 

In this thesis, I focus on demonstrating the concept of in situ optical beam patterning by ultrasound using a theoretical and experimental approach. Using rigorous simulations and analyses, I show the feasibility of the concept and I design ultrasonic-optical systems to demonstrate this technique. I discuss the design and implementation of a characterization setup to experimentally demonstrate this technique in transparent and scattering media. I show through experiments on mouse tissue that these in situ patterns of light can also be formed in heterogeneous mouse brain tissue through the skull. Finally, I discuss the combination of this technique with spectral-domain optical coherence tomography to extend the depth of imaging without compromising the lateral resolution. Using a combined approach of theory, simulations, and experiments, I demonstrate, for the first time, the applicability of ultrasonically sculpted virtual elements for non-invasive light delivery and imaging through scattering tissue. This presented method in this thesis can be used in tandem with existing conventional optical methods such as optical coherence tomography (OCT) to extend the reach and improve the flexibility of light-based methods.