Аннотация:Stimulated by the recently observed strong delocalization of fs radiation in c: Si bulk, the numerical study of fs laser beam propagation and photoexcitation of silicon material has been performed. Two fundamental aspects-dissipation of laser energy due to two-photon absorption (TPA) and nonlinear beam transformation-have been elaborated to clarify the nature of this effect. It has been found that for incident pulse energy 100 mu J and tight beam focusing onto a spot of 6.5 mu m diameter, due to TPA only a small part of the laser pulse energy (<1%) could reach the focal plane inside the Si bulk. As a result, the laser-produced e-h plasma density did not exceed similar to 10(19) cm(-3) and the bulk material damage (permanent modification) threshold was never reached. Additionally, the e-h plasma defocusing has been shown to give a contribution to the light delocalization, significantly widening the pulse propagation path. The data obtained are quite consistent with the experimental results.