We present a method to calculate the spatio-temporal electric field distribution of ultrashort focused pulses propagating in a nonlinear medium with no absorption. The method combines the angular spectrum technique, for propagating light beams, with the irradiance-dependent refractive index of the medium. We modeled the propagation of ultrashort mildly focused pulses, with different durations and peak powers, inside a Ti:sapphire crystal characterized by its linear and nonlinear refractive indices. Our method is able to reproduce the selffocusing effect expected for nonlinear materials for incident beams with powers above the critical power. We analyze the dependence of the focal length and spot size within the nonlinear medium on the incident power and compare them with the focal length and spot size for the same focusing lens in free space. We also analyze the spot size and pulse duration as a function of the distance traveled in the nonlinear medium for several incident powers. Finally, we analyze the effect of the nonlinear medium refractive index on the spatio-temporal distributions of the focused pulse.
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