Abstract
Modulation of metasurfaces in time gives rise to several exotic space–time scattering phenomena by breaking the reciprocity constraint and generation of higher-order frequency harmonics. We introduce a new design paradigm for time-modulated metasurfaces, enabling tunable engineering of the generated frequency harmonics and their emerging wavefronts by electrically controlling the phase delay in modulation. It is demonstrated that the light acquires a dispersionless phase shift regardless of incident angle and polarization, upon undergoing frequency conversion in a time-modulated metasurface which is linearly proportional to the modulation phase delay and the order of generated frequency harmonic. The conversion efficiency to the frequency harmonics is independent of modulation phase delay and only depends on the modulation depth and resonant characteristics of the metasurface, with the highest efficiency occurring in the vicinity of resonance, and decreasing away from the resonant regime. The modulation-induced phase shift allows for creating tunable spatially varying phase discontinuities with 2π span in the wavefronts of generated frequency harmonics for a wide range of frequencies and incident angles. Specifically, we apply this approach to a time-modulated metasurface in the Teraherz regime consisted of graphene-wrapped silicon microwires. For this purpose, we use an accurate and efficient semi-analytical framework based on multipole scattering. We demonstrate the utility of the design rule for tunable beam steering and focusing of generated frequency harmonics giving rise to several intriguing effects such as spatial decomposition of harmonics, anomalous bending with full coverage of angles and dual-polarity lensing. Furthermore, we investigate the angular and spectral performance of the time-modulated metasurface in manipulation of generated frequency harmonics to verify its constant phase response versus incident wavelength and angle. The nonreciprocal response of the metasurface in wavefront engineering is also studied by establishing nonreciprocal links with large isolations via modulation-induced phase shift. The proposed design approach enables a new class of high-efficiency tunable metasurfaces with wide angular and frequency bandwidth, wavefront engineering capabilities, nonreciprocal response and multi-functionality.
Highlights
Optical metasurfaces have had a significant impact on development of novel optical devices owing to their capability in the engineering of electromagnetic wavefronts of light and their ultrathin structures which can reduce the footprint of optical platforms by replacing the bulky components
A half-wave plate element is rotated around its axis which leads to a geometric phase shift equal to twice as the rotation angle for a circularly polarized incident wave based on the Parancharatnam-Berry (PB) design rule [10,11,12]
We offer a new route for overcoming these limitations by introducing a dispersionless non-resonant phase shift of light upon undergoing temporal frequency transitions in time-modulated metasurfaces
Summary
Optical metasurfaces have had a significant impact on development of novel optical devices owing to their capability in the engineering of electromagnetic wavefronts of light and their ultrathin structures which can reduce the footprint of optical platforms by replacing the bulky components. We establish modulation-induced phase shift of light as the basis of a general design paradigm for time-modulated metasurfaces while highlighting its great promise for realization of broadband and wide-angle metasurfaces capable of manipulating the higher-order frequency harmonics and shaping their wavefronts with electrical tunability. The applicability of the design rule is subsequently studied in-depth and is verified beyond the dipolar regime by considering a time-modulated metasurface in THz regime based on graphene-wrapped microwires For this purpose we use a rigorous multipole scattering technique recently developed by authors which allows for accurate and efficient characterization of time-modulated metasurfaces with a large difference between time scales of optical and modulation frequencies [47]. The modulated-induced phase shift is leveraged to establish nonreciprocal links with large isolation
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