Chen Jian, Wan Chenhao, Chong Andy, Zhan Qiwen
Opt Express. 2020 Jun 8;28(12):18472-18478. doi: 10.1364/OE.394428.
We report the method of producing a spatio-temporal (ST) wave packet carrying pure transverse orbital angular momentum (OAM) with subwavelength spatial sizes. Due to the lack of temporal focusing, an ST wave packet focused by a high numerical aperture (NA) objective lens experiences a "spatio-temporal astigmatism" effect similar to the focusing action of a cylindrical lens on the transverse profile of optical field. Thus an ST vortex with a spiral phase in the ST domain focused through a high NA objective will be distorted and lose the ST characteristic spiral phase pattern. With the understanding of such an ST astigmatism, the ST wave packet can be pre-conditioned such that an ST vortex carrying OAM with subwavelength transverse sizes can be obtained after strong focusing. This is the first revelation that a tightly focused ST vortex beam with transverse OAM can be realized, paving the way for potential applications including microscopy, optical trapping, laser machining, nonlinear light-matter interactions, and so on. The ST astigmatism effect also offers insights for the focusing and propagation studies of other types of ST wave packets.
我们报告了一种产生具有亚波长空间尺寸且携带纯横向轨道角动量（OAM）的时空（ST）波包的方法。由于缺乏时间聚焦，由高数值孔径（NA）物镜聚焦的ST波包会经历一种“时空像散”效应，类似于柱面透镜对光场横向分布的聚焦作用。因此，通过高NA物镜聚焦的在ST域具有螺旋相位的ST涡旋会发生畸变，并失去ST特征螺旋相位图案。基于对这种ST像散的理解，可以对ST波包进行预处理，使得在强聚焦后能够获得具有亚波长横向尺寸且携带OAM的ST涡旋。这是首次揭示可以实现具有横向OAM的紧密聚焦ST涡旋光束，为包括显微镜术、光镊、激光加工、非线性光与物质相互作用等潜在应用铺平了道路。ST像散效应也为其他类型ST波包的聚焦和传播研究提供了见解。
