Topological Space‐Time Photonic Transitions in Angular‐Momentum‐Biased Metasurfaces

Abstract

In this paper, topological space‐time photonic transition of light in angular‐momentum‐biased metasurfaces is established, which yields a superposition of orbital‐angular‐momentum (OAM)‐carrying beams at distinct frequency harmonics upon scattering whose topological charges and frequency shifts are correlated. A reflective dielectric metasurface is considered that consists of silicon nanodisk heterostructures integrated with indium‐tin‐oxide and gate dielectric layers placed on a back mirror forming a dual‐gated field effect modulator. The metasurface is divided into several azimuthal sections wherein the nanodisk heterostructures are interconnected via biasing lines. Addressing each azimuthal section with radio‐frequency biasing signals separately allows for flexible implementation of different spatiotemporal modulation profiles. Active tuning of OAM states with high mode‐purity is demonstrated, which yields minimal cross‐talk between OAM channels in a mode‐multiplexed communication system. Moreover, the role of spatiotemporal modulation profile on the spectral and spatial diversity of OAM states is explored. It is shown that angular‐momentum‐biased metasurfaces allow opportunities for hybridized mode‐division and wavelength‐division multiple access through multiplexing and multicasting across distinct OAM states and wavelengths. The nonreciprocity of topological space‐time photonic transitions across the temporal frequency domain and Hilbert space of OAM states is also investigated giving rise to distinct twisted light channels in up‐ and down‐links.

Manipulation of Scattering Spectra with Topology of Light and Matter

“…Complementary to recent experimental studies that addressed some aspects of the OAM beam interaction with spherical [98] and chiral nanostructures, [99] our study predicted the possibility of the excitation of various multipolar moments which are not accessible via unstructured light illumination and elucidated the role of the OAM in the scattering response of the alldielectric meta-atoms with arbitrary shapes and orientations, enabling new ways of manipulating their optical responses on demand and/or in time-modulated fashion. [100][101][102][103][104][105]…”

Optical Pulse Compression Assisted by High‐Q Time‐Modulated Transmissive Metasurfaces

“…With such a choice for the modulation frequency, a one‐to‐one mapping can be made between the steady‐state time‐domain transmission coefficient at each instant of time to the quasi‐static stationary transmission coefficient according to the bias at that time instant. [ 63 ] In general, the transmission coefficients of the first layer can be expressed in terms of its amplitude and phase as $$T_1(t)=|T_1(t)|\exp (i\theta (t))$$, which on account of operation in the Huygens' regime, we can safely disregard its amplitude contribution, that is $$|T_1(t)|=1$$. Owing to the periodic variations in the optical response of the first layer, its transmission coefficients can be expanded in the form of a discrete Fourier series as $$T_{1}(t)=\exp (i\theta (t))=\sum _n \mathcal {T}_n\exp (in\omega _mt)$$, wherein the coefficients of $$\mathcal {T}_n$$ are obtained based on the well‐known relation of $$\mathcal {T}_n=1/T_m \int _0^{T_m}\exp (i(\theta (t)-n\omega _mt))$$ and $$T_m=2\pi /\omega _m$$.…”

“…In other words, TMMs provide control over both spatial and spectral content of the scattered light, which can significantly extend the degree of light manipulation compared to their static and quasi‐static counterparts. This is manifested in a myriad of novel physical phenomena for different applications such as acquiring a nonreciprocal response, [ 44–53 ] pulse shaping and time reversal, [ 54–56 ] dynamic beam steering, [ 57–59 ] signal processing, [ 60 ] spatiotemporal light manipulation, [ 61–63 ] signal amplification, [ 64 ] extreme energy accumulation, [ 65 ] wideband impedance matching, [ 66 ] and wave camouflaging. [ 67,68 ] It should be remarked that the concept of time‐modulation has recently found its way into other realms of physics such as acoustics and thermal sciences, and enabled a wide spectrum of novel functionalities.…”

Optical Manipulation of Nanoparticles: A Selective Excitation Approach Using Highly Focused Orbital Angular Momentum Beams

“…[20][21][22] The ability of carrying orbital angular momentum (OAM) is one of the prosperous aspects of optical fields, which can offer a variety of applications in nanophotonics instruments. [23][24][25][26][27][28] In addition, beams carrying OAM can be characterized using an integer or semiinteger constant called the topological charge (m), defining the phase singularity order at the beam center, translating to a spiral phase profile as illustrated in Figure 1a. Figure 1 also shows the amplitude (up) and phase (bottom) profiles for the azimuthally (b) and radially (c) polarized beams with m ¼ þ1.…”