Ultrafast laser microsurgery : strategies for improving endoscopic bulk tissue ablation and cellular microsurgery

Ultrafast lasers are uniquely suited for tissue ablation in situations where precision and preservation of the surrounding tissue architecture is crucial. However, their clinical adoption has been limited by challenges to flexible endoscopic delivery, and the slow cutting speeds and material removal rates these lasers have historically produced. Solving the challenges surrounding miniaturization, fiber-optic delivery and the limited cutting speeds can provide surgeons with a viable tool for precise tissue excision in biologically sensitive regions like the vocal fold and spine. This dissertation focuses on the addressing the engineering challenges surrounding ultrafast laser surgery. The first part of the dissertation describes the design and realization of a 5 mm ultrafast laser scalpel capable of reaching material removal rates in the order of 0.6 mm³/min/W, a first for a miniaturized ultrafast laser device. Micro-Joules energy delivery at the tissue is made possible by air-core Kagome optical fibers. Diffraction limited performance is achieved via objectives with Zinc Sulfide (ZnS), and Calcium Fluoride (CaF₂) lenses respectively. The probe’s performance is tested via metal and tissue ablation studies, characterizing high-speed ablation parameters and uniformity of ablation over the scan area. The onset of three-photon absorption in our ZnS lenses limits the energy deliverable for these systems. Through additional z-scan studies, we characterize the nonlinear behavior of ZnS crystals at 776 nm and 1552 nm wavelengths, with an eye towards future endoscopic imaging systems. As a second part of the dissertation, we explore how the near field plasmonic enhancement around gold nanoparticles can be exploited to improve ultrafast laser surgery rates, especially on a cellular level. We detail bench-top pump-probe studies exploring the threshold reduction brought about in ultrafast laser bubble formation in nanoparticle suspensions. The nanoparticle’s spatial position within the focal volume is extracted from the size and frequency of bubbles generated. This information is then used to establish a relationship between the fluence experienced and size of the photo-disruptive bubble formed. With experimental results indicating nearly 20-fold reduction in threshold fluences, the use of such systems can potentially reduce treatment times as large volumes of tissue can be rapidly irradiated at a given time