THERMAL LENSING IN BIOLOGIC MEDIUM

A nonlinear effect for tissue response is reported which results from macroscopic changes in the refractive index of tissue due to its localized heating during laser irradiation. The effect, termed thermal lensing, arises when a high-intensity beam of light interacts with an absorbing medium and highly localized heating occurs. Localized heating causes thermal expansion which leads to a decrease in tissue density, inducing macroscopic spatial gradients in the index of refraction. A description is given of a series of experiments designed and performed to establish the existence of thermal lensing in a biological medium and quantify the effect of exposure parameters and medium properties.