Propagation of Ultrashort, Long Wavelength Laser Pulses

Abstract

This chapter summarizes some recent work on the propagation of long wavelength, intense, ultrashort laser pulses through different media. The spectral extent and spectral profile of the supercontinuum (SC) generated in transparent solids like ${\text {BaF}}_{2}$, ${\text {CaF}}_{2}$ and fused silica in the normal and anomalous group velocity dispersion regime (AGVD) are explored. Propagation of ultrashort pulses results in formation of filaments and gives rise to SC generation which is distinctly different in the AGVD regime compared to the normal GVD regime. A signature of SC generation in the AGVD regime is the appearance of an isolated wing on the blue-side of the spectrum in fused silica and ${\text {CaF}}_{2}$. Fifteen-photon absorption induced fluorescence is utilized to visualize femtosecond filaments in ${\text {BaF}}_{2}$ crystals and its absorption cross-section is deduced to be $6.5 \times 10^{-190}$ ${\text {cm}}^{30}\,{\text {W}}^{-15}\,{\text {s}}^{-1}$. Water shows significant linear absorption in the anomalous dispersion regime. Results of a systematic study on SC generation in water, carried out in different GVD regimes at longer wavelengths, highlights the robustness of SC generation despite the presence of linear absorption in water. Long wavelength propagation facilitates generation of lower order harmonics (fifth and seventh) and permits the testing of the validity of higher-order Kerr effects in air. A recent advance in application of long wavelength filamentation in biological media is described, specifically relating to DNA damage induced at long wavelengths.