Space-Time-Modulated Metasurfaces with Spatial Discretization: Free-Space <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>N</mml:mi></mml:math>-Path Systems
Zhanni Wu, Cody Scarborough, Anthony Grbic
Abstract
This work theoretically and experimentally studies metasurfaces with spatially discrete, traveling-wave modulation (SDTWM). A representative metasurface is considered consisting of columns of time-modulated subwavelength unit cells, referred to as stixels. SDTWM 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 SDTWM 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 a SDTWM 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 unique 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, spatiotemporally modulated metasurface, is developed and measured at $X$-band frequencies.