Ultrashort laser transport in turbid media

The advent of ultrashort laser pulses with durations of picoseconds and femtoseconds has opened up many emerging areas. A fundamental understanding of thermophysical phenomena associated with the ultrashort pulse laser transport is crucial for the advance of ultrashort pulse laser technology in engineering and biomedical applications. Laser transport in biological tissue is the basis of understanding the biophysical mechanisms of all types of laser-tissue interactions. In this dissertation three different transient radiative transfer models, i.e., Monte Carlo (MC) Model, Discrete Ordinates Method (DOM) and Radiation Element Method (REM), are developed for the simulation of short pulse laser radiation transport in three-dimensional configurations containing anisotropically scattering, absorbing and emitting media with inhomogeneity. Experimental validation is performed using a Nd:YAG laser with pulse duration of 60 ps. Comprehensive parametric studies for one-, two- and three-dimensional problems are conducted. It is found that the three models are accurate and efficient. The MC method is powerful in dealing with realistic physical conditions, strongly anisotropic scattering and Fresnel reflection. The transient REM is excellent in handling thick media and irregular shapes and requires the least CPU time. The transient DOM is simple in formulation and can be integrated by the use of numerous existing CFD algorithms. The development of three different models covers most of the applications. It is also found that the equivalent isotropic scattering model generally overestimates the transmittance at early time stage for forward scattering media, and it is not a good approximation for backward scattering media. Both experimental and numerical studies demonstrate that, when the optical thickness of the medium is large, the difference of temporal shapes of the transmittance between different azimuthal angles is slight. The temporal signals of reflectance and transmittance are very sensitive to the absorption coefficient, the inhomogeneity of the medium, and the spatial position of the detector. When the input pulse width is comparable to the system propagation time, the input pulse parameters must be included in modeling. Duhamel's theorem is powerful in the study of transient responses for various types of input laser pulses.