Abstract In this paper, generation of optical vortices with time-varying orbital angular momentum (OAM) and topological charge is theoretically demonstrated based on time-modulated metasurfaces with a linearly azimuthal frequency gradient. The topological charge of such dynamic structured light beams is shown to continuously and periodically change with time evolution while possessing a linear dependence on time and azimuthal frequency offset. The temporal variation of OAM yields a self-torqued beam exhibiting a continuous angular acceleration of light. The phenomenon is attributed to the azimuthal phase gradient in space-time generated by virtue of the spatiotemporal coherent path in the interference between different frequencies. In order to numerically authenticate this newly introduced concept, a reflective dielectric metasurface is modelled consisting of silicon nanodisk heterostructures integrated with indium-tin-oxide and gate dielectric layers on top of a mirror-backed silicon slab which renders an electrically tunable guided mode resonance mirror in near-infrared regime. The metasurface is divided into several azimuthal sections wherein nanodisk heterostructures are interconnected via nanobars serving as biasing lines. Addressing azimuthal sections with radio-frequency biasing signals of different frequencies, the direct dynamic photonic transitions of leaky-guided modes are leveraged for realization of an azimuthal frequency gradient in the optical field. Generation of dynamic twisted light beams with time-varying OAM by the metasurface is verified via performing several numerical simulations. Moreover, the role of modulation waveform and frequency gradient on the temporal evolution and diversity of generated optical vortices is investigated which offer a robust electrical control over the number of dynamic beams and their degree of self-torque. Our results point toward a new class of structured light for time-division multiple access in optical and quantum communication systems as well as unprecedented optomechanical manipulation of objects.

中文翻译：

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由时间调制超表面实现的时变光学涡流

摘要 在本文中，基于具有线性方位角频率梯度的时间调制超表面，理论上证明了具有时变轨道角动量 (OAM) 和拓扑电荷的光学涡旋的产生。这种动态结构光束的拓扑电荷显示出随着时间演变连续和周期性地变化，同时具有对时间和方位角频率偏移的线性依赖性。OAM 的时间变化产生一个自扭转光束，呈现出光的连续角加速度。该现象归因于不同频率之间的干扰中的时空相干路径所产生的时空方位角相位梯度。为了对这个新引入的概念进行数字验证，反射介电超表面由硅纳米盘异质结构组成，该异质结构与氧化铟锡和栅极介电层集成在镜背硅板顶部，在近红外区域呈现电可调导模谐振镜。超表面被分成几个方位角部分，其中纳米盘异质结构通过用作偏置线的纳米棒互连。解决具有不同频率的射频偏置信号的方位角部分，利用泄漏引导模式的直接动态光子跃迁来实现光场中的方位角频率梯度。通过执行几个数值模拟验证了超表面产生的具有时变 OAM 的动态扭曲光束。而且，研究了调制波形和频率梯度对生成的光学涡流的时间演变和多样性的作用，这对动态光束的数量及其自扭矩程度提供了强大的电气控制。我们的结果指向一类新的结构光，用于光和量子通信系统中的时分多路访问以及前所未有的物体光机械操作。