Laser Micro- and Nanostructuring for Refractive Eye Surgery

Every year, more than a million refractive eye operations using femtosecond (fs) laser procedures are performed, and yet the cutting process in corneal tissue remains an area for development. In this chapter, we first review the state of the art of infrared (IR) fs laser dissection in laser in situ keratomileusis (LASIK) and small incision lenticule extraction (SMILE) and formulate the challenges for an improvement of precision and reduction of side effects. Since overcoming these challenges requires better knowledge of the cutting mechanisms, the plasma-mediated corneal dissection process is analyzed by high-speed photography of laser-induced bubble dynamics with up to 50 Mio frames/s, histological analysis of the cuts, and gas chromatography of the dissection products. Based on these results, we show that cutting efficiency and precision are improved through focus shaping by means of a helical phase plate, which converts the linear polarized Gaussian fs laser beam into a Laguerre-Gaussian vortex beam. The focus of the vortex beam has a ring shape with the same length in axial direction as the focus of a Gaussian beam but larger diameter. This greatly facilitates cleavage along the corneal lamellae, enabling cutting with low plasma energy density, higher precision, and fewer mechanical side effects. A shortening of the laser plasma length at constant focusing angle by use of UV-A laser pulses instead of IR pulses further improves precision. To compare the performance of UV and IR Gaussian and vortex beams, the incident and absorbed laser energy needed for easy removal of flaps created in porcine corneas are determined at various pulse durations and the smoothness of cuts is evaluated by scanning electron microscopy. Overall, vortex beams perform better than Gaussian beams for all wavelengths and can be easily implemented in clinical systems. Finally, we discuss a novel concept for refractive correction based on the introduction of refractive index changes in the corneal stroma by localized low-density plasma formation. Experimental findings that UV wavelengths work better for this purpose than IR wavelengths are explained through an analysis of the wavelength dependence of free electron density and energy spectrum that are obtained by numerical simulations.