CREOL, The College of Optics & Photonics, University of Central Florida, Orlando, FL, 32816, USA.
Department of Physics and Astronomy, Purdue University, West Lafayette, IN, 47907, USA.
Nat Commun. 2019 Feb 25;10(1):929. doi: 10.1038/s41467-019-08735-8.
Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Alternatively, the group velocity can be modified in free space by spatially structuring the beam profile, but the realizable deviation from the speed of light in vacuum is small. Here we demonstrate precise and versatile control over the group velocity of a propagation-invariant optical wave packet in free space through sculpting its spatio-temporal spectrum. By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backwards towards it.
控制光脉冲的群速度通常需要穿过具有精心设计的色散的材料或结构。或者，可以通过空间构造光束轮廓在自由空间中修改群速度，但是可以实现的与真空中光速的偏差很小。在这里，我们通过对自由空间中传播不变的光波包的时空频谱进行整形，展示了对其群速度的精确和通用控制。通过联合调制空间和时间自由度，可以在真空中的光速以上或以下的自由空间中明确观察到任意群速度，无论是在远离光源的正向传播方向还是甚至向其传播的反向传播方向。
