Visualization of focusing-refocusing cycles during filamentation in BaF₂

Abstract

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 (p_max) to be 3.26× 10¹³ W cm^-2 and 2.81× 10¹⁹ cm^-3, respectively. Use of these values enables us to deduce that the 6PA cross-section in BaF₂ is 0.33×10^-70 cm¹²W^-6s^-1.