Confounding Effect of Spot-Size on the Wavelength-Dependence of Tissue Ablation Metrics

Tunable free-electron lasers have been used in several previous studies to investigate the mid-IR wavelength-dependence of tissue ablation. These studies gave conflicting results on an important question: do the ablation metrics depend on targeting the laser energy to a water or protein vibration? Here, we investigate the effects of two parameters that varied widely in previous studies -- fluence and focused spot-size. We measured ablation threshold, etch depth and collateral damage in porcine corneas for a set of five matched wavelengths -- same absorption coefficients, but different primary chromophores. Although the ablation thresholds are similar, the slope of etch depth versus fluence (ablation efficiency) differs by up to a factor of five. These differences are most strongly dependent on the focused spot diameter, not wavelength. When spot sizes are matched, protein-targeting wavelengths still leave less collateral damage, but they remove tissue less efficiently. The confounding roles of fluence and spot size have strong implications for the interpretation of previous wavelength-dependent results.