Abstract
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.
Highlights
A light beam has two different forms of momenta, namely linear and angular momenta
To the authors’ best knowledge, the hidden physics behind such kind of dynamic optical vortices had remained unexplored until 2019 by the pioneering work of Rego et al [31]. As it has been reported in this work, generating a time-varying orbital angular momentum (OAM) beam is possible by employing two time-delayed, collinear and linearly polarized infrared pulses possessing different topological charges of l1 and l2 but at the same wavelength, which are focused into an argon gas acting as a high harmonic generation (HHG) medium
The topological charge of these structure light beams varies continuously and periodically at the steady-state with a linear dependence on time and the modulation frequency offset between azimuthal sections of the metasurface, which yields a selftorqued light beam
Summary
A light beam has two different forms of momenta, namely linear and angular momenta. While the former is obtained from P=ħk , with ħ being the reduced Planck’s constant and k being the wave vector, the latter can be decomposed into two distinct contributions of spin angular momentum (SAM) and orbital angular momentum (OAM) [1]. Spatiotemporal optical vortices (STOV) have been recently introduced as a new class of OAM-carrying light beams whose topological charge has a space-time dependence [22, 23]. To the authors’ best knowledge, the hidden physics behind such kind of dynamic optical vortices had remained unexplored until 2019 by the pioneering work of Rego et al [31] As it has been reported in this work, generating a time-varying OAM beam is possible by employing two time-delayed, collinear and linearly polarized infrared pulses possessing different topological charges of l1 and l2 but at the same wavelength, which are focused into an argon gas acting as a high harmonic generation (HHG) medium.
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