The strong delocalization of the energy of femtosecond pulses in silicon appears to be an essential factor for preventing laser damage inside a crystal and seemingly excludes the possibility of direct laser writing in the bulk, at least in the one-and two-photon absorption (1 PA and 2 PA) wavelength regions. Previously, the prefocal depletion of the pulse energy and laser-induced free-carrier plasma defocusing of the light were considered to be the main causes of the unlocalized dissipation of light energy. Here, we consider whether the delocalization could be significantly reduced by using longer wavelengths, at which the role of 1 PA and 2 PA decreases and higher orders of nonlinearity come into play. We numerically simulate propagation of focused femtosecond pulses at a wavelength of 1.2-5.25 μm. Plasma defocusing was found to be the crucial delocalization mechanism that prevents the enhancement of material excitation, even in the five-photon absorption region.

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