The effects of the delayed Kerr nonlinearity on the annular Gaussian filaments nearby the characteristic time of the molecular rotational respond are numerically investigated. The simulated results show that the delayed Kerr nonlinearity leads to the advancement of the filament onset distance when the pulse duration is fixed. Moreover, in the presence of the delayed Kerr nonlinearity, the length of the filament is obviously extended, and the peak plasma density appears the great oscillations that makes the filament become unstable and nonuniform. These results are mainly induced by the redistribution of the fluence and the modulation of the speed of the total energy loss in the presence of the delayed Kerr nonlinearity. Moreover, the delayed Kerr nonlinearity enhances the self-focusing of the trailing edge and leads further to the extension of the optical filament. This research is of great significance to deeply understand the long-distance transmission characteristics of the annular Gaussian filaments.
We have numerically investigated the supercontinuum generation by a femtosecond annular Gaussian beam in air. Compared with the spectra broadening of the Gaussian filament under the same initial condition, the smooth supercontinuum spectra of the annular Gaussian filament are wide enough to cover the whole visible frequency range in the first focusing cycle. The spectra broadening is analyzed from the frequency shift induced by the ionization and the self-phase modulation. For the blue side of the enhancement of the spectra broadening, the ionization-induced and intensity-induced frequency shifts play the dominant role in the onset distance of the filament and the first maximum value of the spectra broadening, respectively. Then, we have also discussed the influences of the initial pulse energy and the spatial chirp for wide and smooth supercontinuum spectra to cover the whole visible frequency range. Therefore, it is an efficient route to produce the supercontinuum spectra by the annular Gaussian beam.
Due to the inevitability of the nonlinear collision between the particles in the experiment of the ultracold atomic condensates, it is of great significance to study the nonlinear effect for producing the ultracold molecules from the atomic condensates. In this work, we investigate theoretically the nonlinear effect induced by the two-body collision for producing ultracold Cs2 molecules from ultracold atoms by the stimulated Raman adiabatic passage in a tunneling four-level structure. Under the two-photon resonance condition and the coherent population trapping state, the conversion efficiency is influenced significantly by the nonlinear parameter. Meanwhile, the optimized parameter ranges of the resonance tunneling strength at the unstable excited molecular states, the effective Rabi frequencies of the two-laser fields and the detuning of pump field for different nonlinear parameter are suggested to obtain the high conversion efficiency of ground Cs2 molecules. At last, the maximum conversion efficiency up to 88.56% can be realized in the optimized parameter ranges.
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