Self-adaptation Optical Effects in Photothermal Treatment of Skin Structures

Photothermal effects form basis for many uses of optical energy to treat various medical conditions. With an increase in the optical power density, thermally induced phase transitions and chemical transformations take place and the optical properties of the tissue are affected; as a result, local intensity may become non-linear with respect to the instantaneous incident power. These nonlinearities may profoundly affect resulting patterns of the light-tissue interaction; yet they are still poorly understood due to their complexity. In this work, we briefly review the state of the art of clinically relevant nonlinear light-tissue interactions. Then, we report on investigation of the role of opto-thermal nonlinearities in one particular important case: non-ablative fractional treatment of skin. We used the Er:glass laser emitting at the wavelength of 1540 nm, the wavelength mostly absorbed by water. We have shown that propagation of optical energy in the tissue as well as the resulting temperature and damage distributions are significantly influenced by the nonlinear effects. Moreover, the observed nonlinearities led to a self-adaptation phenomenon, manifesting itself in canalization of the optical propagation pathway. Such phenomena may be used to optimize photothermal procedures.