The ultrafast nonlinear response of air molecules and its effect on femtosecond laser plasma filaments in atmosphere
Milchberg, Howard M
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The nonlinear propagation of an intense ultrafast laser pulse in atmosphere or other gas media leads to filamentation, a phenomenon useful for applications such as remote sensing, spectral broadening and shaping of ultrashort laser pulses, terahertz generation, and guiding of electrical discharges. Axially extended optical filaments result from the dynamic balance between nonlinear self-focusing in the gas and refraction from the free electron distribution generated by laser ionization. In the air, self-focusing is caused by two nonlinear optical processes: (1) the nearly-instantaneous, electronic response owing to the distortion of electron orbitals, and (2) the delayed, orientational effect due to the torque applied by the laser field on the molecules with anisotropic polarizability. To study their roles in filamentary propagation as well as influences on plasma generation in atmosphere, these effects were experimentally examined by a sensitive, space- and time-resolved technique based on single-shot supercontinuum spectral interferometry (SSSI), which is capable of measuring ultrafast refractive index shift in the optical medium. A proof-of-principle experiment was first performed in optical glass and argon gas, showing good agreement between the laser pulse shape and the refractive index temporal evolution owing to pure instantaneous <italic>n</italic><sub>2</sub> effect. Then the delayed occurrence of the molecular alignment in the temporal vicinity of the femtosecond laser pulse, as well as the subsequent periodic “alignment revivals” due to the coherently excited rotational wavepacket were measured in various linear gas molecules, and the results agreed well with quantum perturbation theory. It was found that the magnitude of orientational response is much higher than the electronic response in N<sub>2</sub> and O<sub>2</sub>, which implies that the molecular alignment is the dominant nonlinear effect in atmospheric propagation when the pulse duration is longer than ∼40 fs, the rotational response timescale of air molecules. Realizing the possibility of manipulating plasma generation by aligning air molecules, the molecular orientational effect was further investigated by a technique developed to directly measure, for the first time, the radial and axial plasma density in a meter-long filament. The experiment was performed using both ∼40 fs and ∼120 fs laser pulse durations while keeping the peak power fixed under various focusing conditions, and the alignment-assisted filamenation with ∼2–3 times plasma density and much longer axial length was consistently observed with the longer pulse, which experienced larger refractive index shift and thus stronger self-focusing. Simulations reproduced the axial electron density measurements well for both long and short pulse durations, when using a peak magnitude of instantaneous response as <15% of the rotational response.