Visualization of focusing–refocusing cycles during filamentation in BaF2
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- Dharmadhikari, A.K., Dharmadhikari, J.A. & Mathur, D. Appl. Phys. B (2009) 94: 259. doi:10.1007/s00340-008-3317-7
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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 (Lmin ) of single filament inside BaF2, we obtain estimates of peak intensities (Imax ) and electron densities (ρmax ) 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 cm12 W−6 s−1.