Wavelength dependence of laser pulse filamentation in the close spectral vicinity of atomic resonances

Abstract

We investigate the propagation and nonlinear self-focusing of TW power laser pulses that create 10-m-scale, highly homogeneous plasma channels in rubidium vapor. Using computational solutions of the relevant propagation equations, we study the effects of the ionizing pulse central wavelength in relation to the resonance frequencies of atomic rubidium. Recent experiments show that pulse propagation and plasma channel creation is distinctly different for 780 nm laser pulses (resonant with the rubidium $D_2$ line) and 810 nm laser pulses. We study pulse propagation in a $\pm$30 nm range around the $D_2$ resonance and find that the results are distinctly different when tuning to sub-resonant wavelengths from those obtained for super-resonant wavelengths. For pulse wavelengths below the resonance the behavior is similar to the resonant case, characterized by strong self-focusing and a sharp plasma boundary. Pulse wavelengths above 780 nm on the other hand yield gradually weaker self-focusing and an increasingly diffuse plasma boundary. Our results suggest that the observed behavior can be attributed to an interplay between multiple factors: anomalous dispersion around atomic resonances, resonant transitions between excited states of Rb lying in the 740-780 nm range and wavelength-dependent multiphoton ionization rates.