Visualization of focusing-refocusing cycles during filamentation in BaF2

Filamentation occurs within a 1.5 cm-long crystal of BaF2 during the propagation of intense, ultrashort (40 fs) pulses of 800 nm light; a systematic study as a function of incident power enables us to extract quantitative information on laser intensity within the condensed medium, the electron density and the six-photon absorption cross section. At low incident power, a single filament is formed within the crystal; two or more filaments are observed along the direction transverse to laser propagation at higher incident powers. Further, due to fluorescence from six-photon absorption (6PA), we are able to map the intensity variation in the focusing-refocusing cycles along the direction of laser propagation. At still higher incident powers, we observe splitting of multiple filaments. By measuring the radius (L (min) ) of single filament inside BaF2, we obtain estimates of peak intensities (I (max) ) and electron densities (rho (max) ) to be 3.26x10(13) W cm(-2) and 2.81x10(19) cm(-3), respectively. Use of these values enables us to deduce that the 6PA cross-section in BaF2 is 0.33x10(-70) cm(12) W-6 s(-1).