arXiv subjects

Zhanni Wu

Publications and source records attributed to Zhanni Wu.

3 recordsLinked to original sources

arXiv2021

Efficient Computation of Spatially-Discrete Traveling-Wave Modulated Structures

Traveling-wave modulation is a form of space-time modulation which has been shown to enable unique electromagnetic phenomena such as non-reciprocity, beam-steering, frequency conversion, and amplification. In practice, traveling-wave modulation is achieved by applying a staggered time-modulation signal to a spatially-discrete array of unit cells. Therefore, the capability to accurately simulate spatially-discrete traveling-wave modulated structures is critical to design. However, simulating these structures is challenging due to the complex space-time dependence of the constituent unit cells. In this paper, a field relation (referred to as the interpath relation) is derived for spatially-discrete traveling-wave modulated structures. The interpath relation reveals that the field within a single time-modulated unit cell (rather than an entire spatial period) is sufficient to determine the field solution throughout space. It will be shown that the interpath relation can be incorporated into existing periodic method of moments solvers simply by modifying the source basis functions. As a result, the computational domain is reduced from an entire spatial period to a single time-modulated unit cell, dramatically reducing the number of unknowns. In the context of traveling-wave modulation, this enables researchers to efficiently simulate both complex structures with patterned unit cells in addition to continuous structures with infinitesimal unit cells.

physics.app-ph↗

arXiv2020

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

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.

physics.app-ph↗

arXiv2019

Serrodyne frequency translation using time-modulated metasurfaces

Temporally modulated metamaterials have attracted significant attention recently due to their non-reciprocal and frequency converting properties. Here, a transparent, time-modulated metasurface that functions as a serrodyne frequency translator, is reported at X-band frequencies. With a simple biasing architecture, the metasurface provides electrically-tunable transmission phase that covers $360^\circ$. A sawtooth waveform is used to modulate the metasurface, allowing Doppler-like frequency translation. Two such metasurfaces can be cascaded together to achieve magnetless devices that perform either phase or amplitude non-reciprocity.

physics.class-ph↗