Ultrasonically Sculpted Virtual Optical Elements for In Situ Light Confinement and Collection

Traditional external optical elements are commonly used to shape light through the target medium before it is launched into the medium. In some cases, optical elements are implanted into the target medium to enhance light delivery or collection by overcoming geometrical limitations and preventing deviations of light from its intended trajectory (e.g., due to scattering, diffraction, or inhomogeneities). However, implanting a physical element into the target medium is not always desirable or possible since it may cause damage to the medium. 

A novel technique is introduced in this thesis, in which ultrasound is used to sculpt noninvasive virtual optical waveguides into the target medium, enabling in situ guiding, shaping, and collecting light. Ultrasonic waves are launched through the target medium, creating traveling or standing pressure patterns that modulate the local refractive index, thus forming virtual optical waveguides in the medium to confine and guide photons without implanting an invasive physical optical element. These virtual waveguides can shape light even after entering the target medium (i.e., in situ), providing continuous control over the trajectory of photons. Furthermore, the virtual waveguides can be reconfigured in near real-time by changing the ultrasound parameters, such as its pattern, frequency, amplitude, and phase, thus enabling active correction of light trajectory. 

The fundamental physics behind this novel technique, as well as a few application scenarios, are discussed in this thesis. First, it is shown that virtual optical waveguides can be sculpted in the target medium using ultrasound, providing a geometrical advantage over traditional external optics. In particular, when combined with external optical systems, these in situ virtual waveguides can help overcome the tradeoff between penetration depth, spatial resolution (i.e., spot size), and the input beam size of a traditional external optical lens by relaying an externally focused beam of light through the medium. Second, virtual optical waveguides are demonstrated to alleviate the effect of optical scattering by re-routing the trajectory of scattered photons towards the desired target and, consequently, enhancing the light throughput in turbid media. For example, it is shown that in an optically thick medium (5 transport mean free paths), a virtual optical waveguide sculpted by ultrasound enhances light throughput by 15% compared to a traditional external lens with comparable optical performance in non-scattering media. The effect of ultrasound parameters on the performance of these virtual optical waveguides is systematically studied using a custom-designed simulator. This study provides insights on designing the optimal parameters to maximize light throughput in transparent and scattering media. For example, simulations show that a properly designed virtual waveguide can enhance the light throughput by about 50%, compared to an ideal external lens, in a medium that mimics the optical properties of the human tissue. Finally, virtual optical waveguides can be sculpted in situ to collect photons from targets deep within the medium, enabling non-invasive relay imaging without inserting physical optical components, such as GRIN lenses. Experiments demonstrate that the in situ relay imaging can resolve small structures (e.g., 22 μm features) in transparent and turbid media. Proof-of-concept computational analysis is presented to improve the quality of the relayed experimental images affected by blur, which is the result of nonidealities of the virtual optical elements in practice.