Nonreciprocal Wavefront Engineering with Time-Modulated Gradient Metasurfaces

Abstract

We propose a paradigm to realize nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit-cell whose reflection coefficient oscillates at low frequency. We demonstrate theoretically and experimentally that such modulation permits tailoring the phase and amplitude of any desired nonlinear harmonic and determines the behavior of all other emerging fields. By appropriately adjusting the phase-delay applied to the modulation of each unit-cell, we realize time-modulated gradient metasurfaces that provide efficient conversion between two desired frequencies and enable nonreciprocity by (i) imposing drastically different phase-gradients during the up/down conversion processes; and (ii) exploiting the interplay between the generation of certain nonlinear surface and propagative waves. To demonstrate the performance and broad reach of the proposed platform, we design and analyze metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-insensitive nonreciprocal responses. Our findings open a new direction in the field of gradient metasurfaces, in which wavefront control and magnetic-free nonreciprocity are locally merged to manipulate the scattered fields.

A Dynamically Modulated All‐Dielectric Metasurface Doublet for Directional Harmonic Generation and Manipulation in Transmission

“…Dynamic modulation of a metasurface provides the required momentum for the transition of fundamental frequency to higher‐order frequency harmonics defined as f n = f 0 + nf m with the integer denoting the order of generated frequency harmonics . As such, unlike static and quasi‐static metasurfaces which can only change the spatial features of scattered light, dynamically modulated metasurfaces provide control over the spectral content of the scattered light as well.…”

“…It has been recently shown that the imprinted phase gradient on the wavefront of higher‐order frequency harmonics generated by a time‐modulated metasurface can be controlled by tuning the modulation phase delay of the elements . Specifically, by introducing a modulation phase delay of α, the n th frequency harmonic acquires a phase shift of nα while maintaining a constant amplitude .…”

“…A high‐speed continuous tuning mechanism for all‐dielectric metasurfaces not only allows for real‐time control over the optical response but also enables realization of time‐modulated metasurfaces by applying time‐varying external stimuli. Despite their linearity, time‐modulated metasurfaces exhibit frequency‐mixing property and lead to generation of higher‐order frequency harmonics which are consisted of fundamental frequency up‐ and down‐modulated by the frequency of biasing . Unlike frequency conversion in nonlinear metasurfaces which requires high optical intensities and operates for one excitation at a time, frequency conversion in time‐modulated metasurfaces can be achieved for simultaneous excitations with arbitrary intensities.…”

“…Time modulation also renders a 4D design space and provides new degrees of freedom in light manipulation which can be used to overcome several limitations associated with quasi‐static tunable metasurfaces. In particular, the light acquires a dispersionless phase shift upon undergoing frequency conversion in a time‐modulated metasurfaces which is linearly proportional to the harmonic order and modulation phase delay, while maintaining a constant amplitude . This creates a new paradigm for designing dynamic metasurfaces with 2π phase span and uniform amplitude.…”

Surface-wave-assisted nonreciprocity in spatio-temporally modulated metasurfaces

“…However, a fully tailored electromagnetic response requires metasurfaces that can continuously alter their scattering properties simultaneously in time and space. These functionalities can be achieved in spatio-temporally modulated metasurfaces 31 (STMMs), which have the potential to revolutionize fundamental and applied photonics, including nonreciprocity 32,33 , through on-demand control of frequency and momentum harmonic contents of scattered light. For example, an STMM can focus a collimated beam ( Fig.…”