Monitoring of microplasma formation and filamentation of tightly focused femtosecond laser pulses in dielectrics

Generation of free-carrier plasma and filamentation of the ultra-short laser pulse were investigated and modeled. Experimental results of filamentation are supported by numerical model which takes into account accumulation of refractive index modifications due to multi-pulse exposure. A contact acoustic monitoring technique was employed to perform spatially-resolved in situ detection of micro-plasma formation and filamentation of focused femtosecond laser pulses with critical and sub-critical powers in glass. The recorded acoustic signals reveal free-carrier generation mechanisms associated with the formation of plasma and filamentation of the propagating laser pulses. Optical opacity of the plasma region, which sets in at the irradiance of a few kJ/cm(3) (close to the dielectric breakdown threshold) using pulse focusing optics with numerical aperture NA = 0.75, reveals its critical character, and allows the estimation of acoustic pressure in the similar to GPa range. The pressure depended on the irradiance as P similar to I-0.59. In the case of loose focusing (NA = 0.035) filamentation of fs-pulses occurred at sub-critical plasma density with P similar to I. Detection and interpretation of these acoustic signatures thus enable real-time in situ monitoring of optical ionization, pulse filamentation in bulk dielectrics under the irradiation by femtosecond laser pulses.