Space-Time Modulated Metasurfaces with Spatial Discretization: Free-Space N-path Systems

TL;DR

This paper investigates spatially-discrete traveling-wave modulated metasurfaces, introducing a modified Floquet analysis to understand their electromagnetic behavior, revealing regimes with frequency translation, angular deflection, and subharmonic mixing, validated through experiments.

Contribution

It introduces a new analysis method for discretized space-time metasurfaces and demonstrates their capabilities across different regimes with experimental validation.

Findings

Discretized modulation enables subharmonic mixing.

Metasurface can achieve simultaneous frequency translation and deflection.

Different regimes depend on the spatial modulation period.

Abstract

This work theoretically and experimentally studies metasurfaces with spatially-discrete, traveling-wave modulation (SD-TWM). A representative metasurface is considered consisting of columns of time-modulated subwavelength unit cells, referred to as stixels. SD-TWM is achieved by enforcing a time delay between temporal waveforms applied to adjacent columns. In contrast to the continuous traveling-wave modulation commonly assumed in studies of space-time metasurfaces, here the modulation is spatially discretized. In order to account for the discretized spatial modulation, a modified Floquet analysis is introduced based on a new boundary condition that has been derived for SD-TWM structures. The modified Floquet analysis separates the scattered field into its macroscopic and microscopic variations. The reported theoretical and experimental results reveal that the electromagnetic behavior of an SD-TWM metasurface can be categorized into three regimes. For electrically-large spatial modulation periods, the microscopic field variation across each stixel can be neglected. In this regime, the space-time metasurface allows simultaneous frequency translation and angular deflection. When the spatial modulation period on the metasurface is electrically small, the microscopic variation results in new metasurface capabilities such as subharmonic mixing. When the spatial modulation period of the metasurface is wavelength-scale, the metasurface allows both subharmonic mixing and angular deflection to be achieved simultaneously. To verify our analysis, a dual-polarized, spatio-temporally modulated metasurface, is developed and measured at X-band frequencies.