Modeling Nonlinear Plasma Formation for Femtosecond Processing ofTransparent Materials and Biological Cells at High NA Focusing

Abstract

Ultrashort laser pulses recently found extensive application in micro- and nanostructuring, in refractive surgery of the eye, and in biophotonics. Due to the high laser intensity required to induce optical breakdown, nonlinear plasma formation is generally accompanied by a number of undesired nonlinear side-effects such as self-focusing, filamentation and plasma-defocusing, seriously limiting achievable precision and reproducibility. To reduce pulse energy, enhance precision, and limit nonlinear side effects, applications of ultrashort pulses have recently evolved towards tight focusing using high numerical aperture microscope objectives. However, from the theoretical and numerical point of view, generation of optical breakdown at high numerical aperture focusing was barely studied. To simulate the interaction of ultrashort laser pulses with transparent materials at high NA focusing, a comprehensive numerical model was introduced by the authors in [1], taking into account nonlinear propagation, plasma generation as well as the pulse's interaction with the generated plasma. The multiple rate equation (MRE) model [2] is used to simultaneously calculate the generation of free electrons. Nonparaxial and vectorial diffraction theory provides initial conditions. The theoretical model derived in [1] is applied to numerically study the generation of optical breakdown plasmas, concentrating on parameters usually found in experimental applications of cell surgery. Water is used as a model substance for biological soft tissue and cellular constituents. For focusing conditions of low to moderate numerical aperture (NA < 0.9) generation of optical breakdown is shown to be strongly influenced by plasma defocusing, resulting in spatially distorted breakdown plasmas of expanded size. For focusing conditions of high numerical aperture (NA ≥ 0.9) on the other hand generation of optical breakdown is found to be almost unaffected by distortive side-effects, perfectly suited for material manipulation of highest precision.