时空波包
Space-time wave packets
摘要：
“时空”（ST）波包构成了一大类脉冲光场，这些脉冲光场在线性介质中刚性传输而没有衍射或色散，因此在没有光学非线性或波导结构的情况下是传播不变的。这样的波包表现出独特的特性，如在自由空间中的可控群速度和奇异的折射现象。这些行为的根源是支撑ST波包的一个基本特征：它们的频谱在空间和时间自由度上是不可分离的。实际上，时空结构被赋予不可微的角色散，其中每个空间频率与单个规定波长相关联。此外，从这个特定的时空结构的偏差产生新的行为，离开传播不变性以精确的方式，如加速度与任意轴向分布的群速度，可调的色散曲线，和塔尔博特效应的时空。虽然ST波包的基本概念自20世纪80年代以来就已为人所知，但直到最近才出现了快速的实验发展。这些进步是通过时空傅立叶合成的创新而实现的，从而在束流光学和超快光学的交叉点为结构光开辟了新的前沿。
英文摘要：
"Space-time" (ST) wave packets constitute a broad class of pulsed optical fields that are rigidly transported in linear media without diffraction or dispersion, and are therefore propagation-invariant in absence of optical nonlinearities or waveguiding structures. Such wave packets exhibit unique characteristics, such as controllable group velocities in free space and exotic refractive phenomena. At the root of these behaviors is a fundamental feature underpinning ST wave packets: their spectra are not separable with respect to the spatial and temporal degrees of freedom. Indeed, the spatio-temporal structure is endowed with non-differentiable angular dispersion, in which each spatial frequency is associated with a single prescribed wavelength. Furthermore, deviation from this particular spatio-temporal structure yields novel behaviors that depart from propagation invariance in a precise manner, such as acceleration with an arbitrary axial distribution of the group velocity, tunable dispersion profiles, and Talbot effects in space-time. Although the basic concept of ST wave packets has been known since the 1980's, only very recently has rapid experimental development emerged. These advances are made possible by innovations in spatio-temporal Fourier synthesis, thereby opening a new frontier for structured light at the intersection of beam optics and ultrafast optics.
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