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.

This paper addresses the analysis and design of non-reciprocal filters based on time modulated resonators. We analytically show that time modulating a resonator leads to a set of harmonic resonators composed of the unmodulated lumped elements plus a frequency invariant element that accounts for differences in the resonant frequencies. We then demonstrate that harmonic resonators of different order are coupled through nonreciprocal admittance inverters whereas harmonic resonators of the same order couple with the admittance inverter coming from the unmodulated filter network. This coupling topology provides useful insights to understand and quickly design nonreciprocal filters and permits their characterization using an asynchronously tuned coupled resonators network together with the coupling matrix formalism. Two designed filters, of orders three and four, are experimentally demonstrated using quarter wavelength resonators implemented in microstrip technology and terminated by a varactor on one side. The varactors are biased using coplanar waveguides integrated in the ground plane of the device. Measured results are found to be in good agreement with numerical results, validating the proposed theory.

A phased-array antenna is a device that generates radiation patterns whose shape and direction can be electronically controlled by tailoring the amplitude and phase of the signals that feed each element of the array. These devices provide identical responses in transmission and reception due to the constrains imposed by time-reversal symmetry. Here, we introduce the concept of nonreciprocal phased-array antennas and we demonstrate that they can exhibit drastically different radiation patterns when operated in transmission or in reception. The building block of the array consists of a time-modulated resonant antenna element that provides very efficient frequency conversion between only two frequencies: one associated to waves propagating in free-space and the other related to guided signals. Controlling the tunable nonreciprocal phase response of these elements with the phase of low-frequency modulation signals permits to independently tailor the transmission and reception radiation patterns of the entire array. Measured results at microwaves confirm isolation levels over 40 dB at desired directions in space with an overall loss below 4 dB. We believe that this concept can be extended across the electromagnetic spectrum provided adequate tuning elements are available, with important implications in communication, sensing, and radar systems, as well as in thermal management and energy harvesting. I. IntroductionPhased-array antennas consist of multiple antennas appropriately arranged in space to provide tailored and highly directive radiation patterns that can be electronically controlled without the need of mechanical rotation [1][2][3][4][5]. They are ubiquitous in modern technology from radiofrequencies (RF) to optical frequencies and find wide applications in military radar systems and tracking platforms, civilian automotive radars [6,7], light-detection-and-ranging (LIDAR) devices [8,9], satellite, wireless, and optical communications [10-13], radio astronomy [14,15], imaging [16], and remote and biological sensing [17][18][19][20], among many others. The first phased array antenna was demonstrated in 1909 by employing a three-element switchable configuration to enhance the transmission of radio waves in one direction [21]. Although there has been continuous progress in phased-array antennas during the last decades, their basic operation principle has remained essentially unchanged since its invention: the amplitude and phase excitation of each antenna element are individually tailored in such a way that the radiated waves interfere constructively in desired directions and destructively in undesired ones. The advantages of phased-array antennas over single radiating elements include significantly higher transmission gain, reception sensitivity, and power handling, as well as the ability to synthesize a large variety of radiation patterns. Additionally, ultra-rapid beam scanning and shaping can be realized by electrically manipulating the excitation of the antenna elements, usually through tunable feeding netwo...