Delocalization of femtosecond radiation in silicon.

The processes, induced by local action of the IR femtosecond laser pulse (λ=1.2 μm, τ=250 fs) in the bulk of silicon monocrystal, are studied. Infrared femtosecond interferometry was for the first time applied for visualization of beam propagation inside opaque materials. Dependences of laser-induced variation of material polarizability on pulse energy were obtained and essential wave-packet spreading in space was revealed. This leads to huge delocalization of light-scattering outside the beam caustic exceeds 99% of pulse energy. This effect results in extremely high optical damage threshold of crystalline silicon bulk-irreversible changes in material structure and optical properties were not observed for pulse energy up to 90 μJ. The role of beam Kerr self-focusing and defocusing by an electron-hole plasma inside c:Si is discussed.