Broadband Nonreciprocity Research Articles

Broadband nonreciprocal gyromagnetic metasurface via magnetic Kerker-type dimers

Optical nonreciprocity, stemming from the deviation of the Lorentz reciprocity theorem, holds significant interest in the realm of optics and electromagnetics. Here, we propose and experimentally demonstrate broadband nonreciprocal transmission via a low-biased magnetic Kerker-type dimer metasurface. The designed magneto-optical metasurface comprises three layers of metal sandwiched between two gyromagnetic near-zero thickness slabs. The Kerker-type dimers broaden the isolation bandwidth utilizing multiple resonances where the double-stacked metallic disks act as Kerker-type dipoles, enhancing the transmissibility of the metasurface. The multipole decomposition reveals that the magnetic dipole contribution arising from magnetization is the primary cause of the metasurface's nonreciprocal response. Microwave measurement demonstrates that the bandwidth for an isolation ratio exceeding 10 dB is over 3 GHz. The broadband nonreciprocal performance remains relatively stable, exhibiting strong robustness against the bias disturbance. Our findings provide an alternative avenue for enhancing broadband nonreciprocity transmission under a low-biased magnetic field.

Nonreciprocal transmission of electromagnetic waves with nonlinear active plasmonic metasurfaces

Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as an ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two silver nanostripes with different dimensions placed on both sides of an ultrathin active dielectric spacing layer. The introduced gain leads to an exceptional point formation in the linear operation regime, where unidirectional perfect transmission due to loss compensation is achieved from the presented non-Hermitian parity-time symmetric nanoscale system. The demonstrated plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when its nonlinear response is considered, mainly due to its spatially asymmetric geometry combined with enhanced nonlinearity. Strong and broadband nonreciprocity with unity transmission contrast from opposite directions is realized under relatively low-input laser intensity values. Furthermore, asymmetric nonlinear third-harmonic generation with a pronounced contrast is also illustrated by the same nonreciprocal metasurface design. The findings of this work can lead to the realization of various self-induced nonreciprocal nanophotonic configurations, such as integrated ultrafast switches, ultrathin protective coatings, and asymmetric directional imaging devices.

Nonlocal active metamaterial with feedback control for tunable bandgap and broadband nonreciprocity

In this paper, a nonlocal active metamaterial with feedback control is proposed. For a metamaterial with a symmetric feedback signal, the tunable bandgap and negative effective stiffness are discussed. Symmetric displacement feedback control significantly broadens the bandgap, and combined with velocity feedback control can rapidly attenuate the vibration amplitude. In the metamaterial with an asymmetric feedback signal, the spatial symmetries of the control force are broken by the asymmetrical feedback signal. The non-Hermitian skin effects are achieved by asymmetrical feedback signal. The modes of localized are determined by topological properties with winding number, and the asymmetrical feedback control gain can change the topological properties with winding number. The imaginary parts of the wavenumber are also asymmetrically distributed in space. Theoretical analysis shows that nonlocal active metamaterials with asymmetric feedback signals can achieve nonreciprocal wave propagation and directional amplification across a broad frequency range. Numerical results verify that this metamaterial possesses the characteristics of nonreciprocal wave propagation and directional amplification. The nonlocal active metamaterials in this paper can not only achieve superior bandgap tuning, but also nonreciprocal wave propagation and directional amplification. Function switching between bandgap control and nonreciprocity can be realized by switching the feedback control signal.

Unit cell interactions of non-local active acoustic media

Active acoustic metamaterials have greatly expanded upon the unique and unusual ways that passive metamaterials manipulate sound. Previously, we have shown how spatially non-local forces in an acoustic system can create band gaps and break acoustic reciprocity, and how such effects can be realized in practice using a non-local active acoustic metamaterial (NAM). We demonstrated experimentally the remarkable ability of a single NAM unit cell to generate large, subwavelength, broadband nonreciprocity using an open loop feedforward control mechanism where a measured local pressure is transmitted by an electronic controller downstream to actuate an acoustic source. In NAM systems with multiple unit cells, this control scheme strongly couples each cell with every other cell, and thus requires special design considerations. Here, we discuss the advantages and challenges of developing NAM systems with multiple unit cells by first considering a system with two cells, detailing the impacts of their complex interaction on both performance and stability. We validate model predictions with experiments and highlight the exciting prospects offered by larger multi-cell NAM arrays.

Passive Nonreciprocity in a System of Asymmetrical Rotational Oscillators

In this paper we investigate an elastically linked, nonlinear, in-plane rotator system and experimentally study its nonreciprocal impulse response. The nonlinearity of the system arises from the angled elastic linkage in rotational motion. A chain of rotators coupled with such linkages reaches an acoustic vacuum when the pretension of the elastic links vanish, leading to large nonlinearity tunable via small pretension. Using an analytical model and experimental exploration, we observe a broadband nonreciprocity in a weakly pretensioned, asymmetric, three-rotator system. In addition, we use a nonlinear normal-mode (NNM) analysis, capturing the main qualitative dynamics of the response, to explain the observed nonreciprocity mechanism. The analysis shows that equal applied impulses, combined with energy-dependent frequency and mode shapes, result in robust nonreciprocity features, contrary to the reciprocal response present in the linear counterpart of this system.

Broadband Nonreciprocity Realized by Locally Controlling the Magnon’s Radiation

Interference between coherent and dissipative coupling results in nonreciprocity in cavity magnonic devices, in which the isolation ratio can theoretically be infinite when the magnon-mode frequency exactly matches a zero-damping condition (ZDC). In order to obtain a nonreciprocal device possessing both a high isolation ratio and a broad operating bandwidth, we tune the side ZDCs by integrating a movable loop antenna, which can effectively control the radiation damping of the magnon mode. As a consequence, the effective operating bandwidth of the giant nonreciprocity can be broadened to a few hundred times the magnon linewidth. Our study offers a feasible method to design a broadband nonreciprocal device with a high isolation ratio by engineering photon-magnon coupling, which may be of benefit for future quantum and coherent information processing.

Broadband Nonreciprocity Enabled by Strong Coupling of Magnons and Microwave Photons

Non-reciprocity of signal transmission enhances capacity of communication channels and protects transmission quality against possible signal instabilities, thus becoming an important component ensuring coherent information processing. However, non-reciprocal transmission requires breaking time-reversal symmetry (TRS) which poses challenges of both practical and fundamental character hindering the progress. Here we report a new scheme for achieving broadband non-reciprocity using a specially engineered hybrid microwave cavity. The TRS breaking is realized via strong coherent coupling between a selected chiral mode in the microwave cavity and a single collective spin excitation (magnon) in a ferromagnetic yttrium iron garnet (YIG) sphere. The non-reciprocity in transmission is observed spanning nearly a 0.5 GHz frequency band, which outperforms by two orders of magnitude the previously achieved bandwidths. Our findings open new directions for robust coherent information processing in a broad range of systems in both classical and quantum regimes.

Broadband non-reciprocity with robust signal integrity in a triangle-shaped nonlinear 1D metamaterial

In this paper, we propose and numerically study a nonlinear, asymmetric, passive metamaterial that achieves giant non-reciprocity with (i) broadband frequency operation and (ii) robust signal integrity. Previous studies have shown that nonlinearity and geometric asymmetry are necessary to break reciprocity passively. Herein, we employ strongly nonlinear coupling, triangle-shaped asymmetric cell topology, and spatial periodicity to break reciprocity with minimal frequency distortion. To investigate the nonlinear band structure of this system, we propose a new representation, namely a wavenumber–frequency–amplitude band structure, where amplitude-dependent dispersion is quantitatively computed and analyzed. Additionally, we observe and document the new nonlinear phenomenon of time-delayed wave transmission, whereby wave propagation in one direction is initially impeded and resumes only after a duration delay. Based on numerical evidence, we construct a nonlinear reduced-order model (ROM) to further study this phenomenon and show that it is caused by energy accumulation, instability, and a transition between distinct branches of certain nonlinear normal modes of the ROM. The implications and possible practical applications of our findings are discussed.

Synchronized conductivity modulation to realize broadband lossless magnetic-free non-reciprocity

Recent research has explored the spatiotemporal modulation of permittivity to break Lorentz reciprocity in a manner compatible with integrated-circuit fabrication. However, permittivity modulation is inherently weak and accompanied by loss due to carrier injection, particularly at higher frequencies, resulting in large insertion loss, size, and/or narrow operation bandwidths. Here, we show that the presence of absorption in an integrated electronic circuit may be counter-intuitively used to our advantage to realize a new generation of magnet-free non-reciprocal components. We exploit the fact that conductivity in semiconductors provides a modulation index several orders of magnitude larger than permittivity. While directly associated with loss in static systems, we show that properly synchronized conductivity modulation enables loss-free, compact and extremely broadband non-reciprocity. We apply these concepts to obtain a wide range of responses, from isolation to gyration and circulation, and verify our findings by realizing a millimeter-wave (25 GHz) circulator fully integrated in complementary metal-oxide-semiconductor technology.

Linear broadband nonreciprocity in waveguides using distributed feed forward control

Nonreciprocal waveguides can exhibit different properties depending on the direction, amplitude, and frequency content of an incident wave. These systems are the focus of intense recent research activity in engineering and in acoustics in particular driven by exciting applications such as cloaking, subwavelength anechoic termination, and full-duplex communication. In biology, nonreciprocity has been identified in cochlear mechanics and is hypothesized to play a role in its filtering and nonlinear processing of sound. Although there has been a surge in the study of unidirectional energy propagation in acoustic media, most of the work has been limited to non-linear effects and/or narrowband non-reciprocity. However, nonlinear effects generate additional harmonics, complicating the approach, and narrowband non-reciprocity constrains the bandwidth that can be used. We have developed the theory for linear nonreciprocal broadband acoustic waveguides using a distributed feed forward control scheme that exhibits significant broadband, nonreciprocity while maintaining stability. We discuss the different systems for which the paradigm may be adopted along with the theoretical formalism to predict the stability and dispersion relations for this class of waveguides.