Ultraviolet nanosecond excimer lasers and near infrared (NIR) femtosecond lasers are two forms of laser technology that have been adopted in ophthalmic practice for the purpose of altering corneal optics and cutting corneal flaps. Ultraviolet excimer lasers use single-photon absorption to ablate corneal tissue, changing the cornea's thickness and curvature and thus its refractive state.1,2
This technology takes advantage of the fact that the cornea naturally absorbs ultraviolet light to photoablate the tissue. Femtosecond lasers use NIR light that is naturally transmitted through the cornea with little one-photon absorption. To affect the cornea, femtosecond laser light must be focused inside the tissue. The resultant increase in laser intensity at the focal point causes localized, nonlinear, multiphoton absorption, allowing for a range of modifications within the tissue; because the absorption is nonlinear, the surrounding tissue is left virtually unaffected.3–5
NIR femtosecond lasers are now used clinically for corneal flap cutting.6–12
Such lasers use low-repetition-rate (kilohertz to several megahertz range) pulses that induce photodisruption and optical breakdown of the corneal tissue, accompanied by high-density microplasma and bubbles. The layer of damaged tissue created by the laser can then be used to separate a tissue flap from the rest of the cornea.
In a very different set of applications, NIR femtosecond lasers have also gained popularity as powerful tools for micromachining patterns into different, mostly nonbiological materials.13–17
When the femtosecond pulses are tightly focused into transparent bulk materials, the resultant nonlinear absorption and highly localized energy deposition alter the material's properties, allowing for the fabrication of different components and devices such as gratings, waveguides, and photonic crystals.18–24
The changes seen in these materials are usually the result of laser-induced two-photon polymerization,18–24
and recent research has shown that this polymerization process can be enhanced by doping bulk materials with photoinitiators or chromophores that have large two-photon absorption (TPA) cross sections.18–20,25
In 2008, our group showed for the first time, that it is possible to use a low-pulse-energy femtosecond laser to induce low-scattering-loss RI changes in lightly fixed, postmortem corneal and lens tissues.26
These modifications were attained with a 27-fs pulsed laser in the NIR (800 nm) with pulse energies titrated to fall below the optical breakdown threshold of the tissue (<0.5 nJ). This process, termed intratissue refractive index shaping (IRIS), causes long-standing refractive index changes that range between 0.005 and 0.01 in the cornea and 0.015 and 0.021 in the lens.26
However, these changes are achievable only with a slow scanning speed of 0.7 μm/s, which makes realistic clinical application of this method unlikely. Recently, Ding et al.18
reported that both coumarin-1 and fluorescein can be used to enhance the scanning speed and magnitude of RI changes attainable during micromachining in hydrogels, which like the cornea, possess a relatively high water content. Fluorescein is of particular interest, because it is already commonly used in ophthalmic practice for the identification of corneal abrasions and epithelial defects.27
It has also been shown to be safe when injected systemically to visualize retinal vasculature leaks and other abnormalities.27
Thus, in the present study, our goal was to test two hypotheses: (1) that IRIS can be performed in living, unfixed corneas with greater effectiveness than in our previous report of this phenomenon in fixed postmortem corneas, and (2) that the effects of IRIS can be significantly enhanced if the TPA capability of the living tissue is increased.