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General laws of the propagation of few-cycle optical pulses with orbital angular momentum in free space
Physical Review A ( IF 2.9 ) Pub Date : 2020-09-23 , DOI: 10.1103/physreva.102.033522
Miguel A. Porras , Raúl García-Álvarez

We conduct a theoretical study of the propagation of few-cycle ultrafast vortices (UFVs) carrying orbital angular momentum (OAM) in free space. Our analysis reveals much more complex temporal dynamics than that of few-cycle fundamental Gaussian-like beams, particularly when approaching the single-cycle regime and the magnitude of the topological charge l is high. The recently described lower bound |l| to the number of oscillations of UFVs with propagation-invariant temporal shape (isodiffracting UFVs) is found to hold on average also for UFVs of general type, with variations along the propagation direction above and below that bound, even vanishing locally. These variations are determined by the so-called Porras factor, or g0 factor, characterizing the dependence of the Rayleigh distance of the spectral constituents with frequency. With a given available bandwidth, UFVs must widen temporally with increasing magnitude of the topological charge and must widen or may shrink temporally during propagation as a result of the axially varying g0-dependent lower bound. Under very restrictive conditions in their generation, an UFV can be shrunk below the lower bound |l| at a focus into a kind of locally compressed state of OAM, but it broadens well above |l| and is distorted in a tiny fraction of the depth of focus because of the dispersions introduced by Gouy's phase and wave-front mismatch. These propagation phenomena have implications and should be taken into account in experiments and applications of UFVs, such as the generation of high harmonics and attosecond pulses with high OAM, or in OAM-based ultrafast communications systems, as well as in other areas of physics such as acoustics or electron waves.

中文翻译:

具有自由空间轨道角动量的几周期光脉冲传播的一般规律

我们对自由空间中携带轨道角动量(OAM)的几个周期超快涡(UFV)的传播进行了理论研究。我们的分析揭示了比几周期基本高斯光束更复杂的时间动力学,尤其是在接近单周期状态和拓扑电荷量时高。最近描述的下限||对于一般类型的UFV,具有传播不变的时间形状的UFV的振荡数量(等衍射UFV)的平均振荡频率平均也保持不变,沿着传播方向在该界限之上和之下的变化,甚至局部消失。这些变化由所谓的Porras因子确定,或者G0系数,表征频谱成分的瑞利距离与频率的关系。在给定可用带宽的情况下,UFV必须随拓扑电荷量的增加而随时间变宽,并且在传播过程中由于轴向变化而必须随时间变宽或可能缩小G0依赖的下限。在非常严格的条件下,UFV可以缩小到下限以下|| 着眼于OAM的一种局部压缩状态,但它的扩展范围远超过 ||由于Gouy的相位和波前失配引入了色散,因此在焦点深度的很小一部分上就会失真。这些传播现象具有影响,在UFV的实验和应用中应予以考虑,例如在高OAM的情况下产生高次谐波和阿秒脉冲,或在基于OAM的超快速通信系统以及物理的其他领域,例如作为声波或电子波。
更新日期:2020-09-23
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