Time-modulated Microwave Devices

Time-varying circuits have recently gained significant attention to develop magnetic-free circulators. In these devices, the magnetic bias is replaced by a spatiotemporal modulation that breaks the Lorentz reciprocity principle and enables strong non-reciprocal responses. Inspired by these recent developments, we have exploited the concept of time-modulated resonators to put forward a wide variety of novel non-reciprocal devices and antennas operating at RF and microwaves frequencies exhibiting unprecedented electromagnetic responses.

For instance, we have recently introduced 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 demonstrated theoretically and experimentally at microwaves 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) preventing the generation of certain propagative harmonics due to their total internal reflection. To demonstrate the performance and broad reach of the proposed platform, we have designed and analyzed metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-independent 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.

In addition, we have introduced the concept of non-reciprocal phased-array antennas operating at the same frequency. 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. We have demonstrated that these structures 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, with important implications in communication, sensing, and radar systems, as well as in thermal management and energy harvesting. 

In a related thrust, we have introduced the concept of nonreciprocal bandpass filters based on time-modulated resonators that exhibit very low-loss forward transmission and high reverse isolation. with low-loss forward transmission. We have developed a theoretical formalism to quickly analyze and design these novel exciting devices. Additionally, we have extended this concept to a wide variety of microwave components, such as diplexers and power dividers.