CTS-SGER: Thermo-Mechanical Interactions of Ultrashort Laser Pulses with Subsurface Targets of Tissue Models

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ABSTRACT: Thermo-mechanical interactions of ultrashort laser pulses with subsurface targets of tissue modelsThe precise ablation of biological tissues using lasers with pulse durations longer than ~ 1 ns, while avoiding collateral thermal damage of the surrounding tissue is often quite a challenge and sometimes physically impossible. This is because the optical energy is coupled to the biological tissue through its wavelength-dependent absorption coefficient and transformed almost entirely into heat, which diffuses beyond the optically confined volume at a rate dictated by the tissue thermal diffusion time. In contrast, when laser pulses are much shorter than the characteristic diffusion time of tissue, non-linear laser-tissue interactions arise, such as the generation of plasma, shockwaves and cavitation bubbles. These physical phenomena consume a large portion of the optical energy and transform it into kinetic energy, allowing a more precise ablation while reducing and even eliminating collateral thermal damage to the surrounding tissue. Furthermore, since the coupling of the optical energy to the biological tissue does no longer depend on laser wavelength, it is possible to eliminate unwanted superficial heating and focus the interaction volume onto deep subsurface targets.The primary intellectual merit of the work lies in the proposed used of low energy (mJ) high power (GW) ultra short laser pulses (USLP) to induce a more precise mechanically-driven ablation of subsurface tissue structures with minimal collateral thermal damage. This is the first such use of USLP for tissue treatment. The objectives are to quantify the magnitude of the thermal (temperatures, heat fluxes) and mechanical (stress, strain) effects on subsurface tissue model targets induced by laser pulse durations ranging from the milli- to the femto-second time scale, with repetition rates between the Hz to the MHz frequency domain. Specifically, we intend to correlate the photoablation and extent of damage with the magnitude, pulse duration and repetition rates of a wide variety of lasers, particularly those of USLP. Furthermore, a thermo-mechanical mathematical model that accurately describes the experimental evidence will be developed. The proposed work will set the foundation for an innovative alternative to current laser therapy of vascular lesions and perhaps the only solution for the ablation of small vasculature and other tissue chromophores, such as benign and malignant pigmented lesions. With respect to the broader impacts of the work, the short-term impacts include the generation of experimental methods and better understanding of the interactions of USLP pulses with various tissue targets. The long-term impacts include the development of new technology where precise thermo-mechanical damage of subsurface structures is achieved while collateral thermal damage is minimized.