Time-varying optical vortices enabled by time-modulated metasurfaces

Abstract

<jats:title>Abstract</jats:title><jats:p>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.</jats:p>

Manipulation of Scattering Spectra with Topology of Light and Matter

“…[49][50][51][52][53][54][55] Nevertheless, it is noteworthy that a majority of the related studies in the realm of light-matter interactions with complex resonant meta-atoms were limited to conventional Gaussian light beams. However, in parallel, several approaches including spiral phase plates, [56,57] q-plates, [58][59][60] spatial light modulators (SLM), [61] and optical metasurfaces [33,62,63] have been developed to generate and manipulate more complex beams, which has led to the emergence of a new category of light beams known as structured light. [64][65][66] Structured lights, including beams with a spin and orbital angular momentum (SAM and OAM, respectively), radially and azimuthally polarized vector beams, and spatiotemporal optical vortices have been shown to enable a plethora of distinct lightmatter interactions and applications in optical communication, particle manipulation, quantum information processing, sensing, and microscopy.…”

Spatiotemporal Optical Vortices: Toward Tailoring Orbital Angular Momentum of Light in Full Space-Time

“…In 1992, Allen et al proposed that an optical vortex with a phase factor exp­( i θ) (θ is the azimuth angle) carries orbital angular momentum (OAM) with a value of ℏ per photon, where ℏ is the Dirac constant and can be any integer, known as the topological charge. Since the arbitrary value of topological charge provides a new degree of freedom for manipulating light and its interaction with matter, the research on optical vortices attracts considerable attention and continues to deepen, from conventional spiral phase plates to integrated metasurfaces and microring resonators, from particle manipulation to OAM-based communication and quantum information processing, beyond traditional two-dimensional transverse fields to multidimensional tailored beams, especially the time-space modulation of optical vortices. − …”

Generation of time-varying orbital angular momentum beams with space-time-coding digital metasurface

“…26,27 However, such a high-harmonic approach in generating time-varying OAM is not easily applicable to other frequency regimes, hindering further demonstrations. Alternatively, Sedeh et al 28 proposed an optical metasurface with a spiral-staircase profile of modulation frequencies to implement time-varying OAM. In fact, such an approach can be feasible and generalized through a space-time-coding digital metasurface with field-programmablegate-array (FPGA) technology, particularly in the microwave regime.…”