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

Abstract

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 freecarrier 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/cm3 (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 ̃GPa range. The pressure depended on the irradiance as P ̃ I0.59. In the case of loose focusing (NA = 0.035) filamentation of fs-pulses occurred at sub-critical plasma density with P ̃ 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. © 2009 SPIE.