通过数值模拟分析了初始脉冲能量对环形高斯光束在大气中成丝的影响.研究表明,当增加初始脉冲能量时,环形高斯光束在大气中生成的等离子体通道的长度被延长;而当脉冲能量增大到一定值时,虽然等离子体通道的长度继续增加,但是等离子体通道的稳定性却有所下降.因此,对于环形高斯光束,合适的初始脉冲能量将有利于稳定的等离子体通道的远距离传输.

Due to the inevitability of nonlinear collisions between particles in ultracold atomic condensate experiments, it is of great significance to study the nonlinear effect for producing ultracold molecules from 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 the pump field for different nonlinear parameters are suggested to obtain the high conversion efficiency of Cs2 molecules at the ground state. Finally, a maximum conversion efficiency of up to 88.56% can be realized in the optimized parameter ranges.

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.

We have explored a novel approach to generate a long-distance two-color filament by three collinear ultrashort pulses in air. The characteristics and dynamics of the two-color filament are investigated numerically. It is attractive that the coherent superposition of two low-intensity annular beams (both 400 nm) makes the length of the two-color filament increase about one order of magnitude over the Gaussian filament alone. Because two annular beams supply coherently the energy to the filament core and the clamp intensity of 400 nm filament is lower than the 800 nm one, the low input energy (1.2 mJ) can maintain sufficiently a long-distance propagation of the two-color filament, which is great efficient and economical.

用数值模拟的方法研究了不同透镜聚焦和不同输入脉冲能量对环形高斯光束在大气中传输的影响.结果显示,在初始能量固定时,光丝的特性及其超连续波谱强烈依赖于凸透镜的焦距f和锥透镜引入的空间啁啾系数C.透镜聚焦能力减弱(f增大和C减小),成丝起点延后,光丝变长且不稳定,但更有利于波谱在短波方向上的展宽.由锥透镜聚焦所产生的超连续波谱会更加的光滑,在传播方向的作用区域也更长.频率变化最大值(Δωmax)的峰值更有利于光谱展宽,但与最大展宽位置并不完全对应.此外,固定透镜焦距f和空间啁啾系数C,适当的初始脉冲能量对获得宽而光滑的超连续谱也起着关键作用.

本文利用一个具有隧穿耦合效应的四能级结构模型来描述超冷铯原子制备超冷铯分子的过程,理论分析了受激拉曼绝热通道过程中的两激光脉冲参数对铯原子-分子转换效率的影响。结果表明,在原子-分子暗态条件下,通过双色光缔合利用双光子共振条件的最优化处理方法可以使转换效率提高到90%以上,并给出了两激光脉冲参数的最优化区间。另外,通过分析原子态和束缚分子基态上粒子数布居的动力学演化,给出了光场的最优化条件,只有适当的匹配两激光场的有效Rabi频率、延迟系数和脉冲宽度时,才能高效地实现超冷铯原子-分子的相干布居转换。

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.