Nonreciprocity in millimeter wave devices using a magnetic grating metamaterial

The control and manipulation of many of light’s fundamental properties, such as reflectivity, has become a topic of increasing interest since the advent of engineered electromagnetic structures—now known as metamaterials. Many of these metamaterial structures are based on the properties of dielectric materials. Magnetic materials on the other hand, have long been known to interact with electromagnetic waves in unusual ways; in particular, their nonreciprocal properties have enabled rapid advances in millimeter wave technology. Here, we show how a structured magnetic grating can be employed to engineer electromagnetic response at frequencies upwards of hundreds of GHz. In particular, we investigate how nonreciprocal reflection can be induced and controlled in this spectral region through the composition of the magnetic grating. Moreover, we find that both surface and guided polaritons contribute to high-frequency nonreciprocity; the nature of these is also investigated. Control of electromagnetic radiation at high frequencies is a current challenge of communications technology, where our magnetic gradient might be employed in devices including signal processing filters and unidirectional isolators. KEYWORDS: millimitre wave, RF devices, grating metamaterial, ATR, nonreciprocal reflection 1. INTRODUCTION Magnetic materials have long been used for a variety of microwave applications including circulators and isolators for radar systems and mobile telephone relay stations [1,2]. Some of the main properties of magnets that enable such versatility are their negative effective permeability [3-5] and non-reciprocal response ……()[3,6] near their ferromagnetic resonance frequency. This means that their electromagnetic characteristics depend on the direction of motion of the wave crossing through them. For just as long, ferrites and garnets – in particular, yttrium iron garnet (YIG) – have been the first-choice materials for most of these applications .()()[7,8]. The interest in this class of materials stems from their typical operating frequency in the low GHz frequency range, their low damping. as well as their non-metallic behavior. With the advent of new information technologies, such as 5G and even 6G, high frequency bands, from the high GHz to the THz range, are now being considered in order to enable data rates in the order of hundreds of Gbps [9]. Thus, making it now necessary to develop new high-frequency operating devices [10] including on-chip THz isolators [11], amplifiers and oscillators [12]. This brings about three main drawbacks when incorporating magnetic materials into electronic devices: The aforementioned class of materials is no longer the obvious choice as they would require extremely large applied magnetic fields in order to obtain such high operating frequencies [6,13,14]; Antiferromagnetic materials, which can have sub-terahertz operating frequencies, typically require extremely low temperature [15]; and Many of the other candidates are metallic structures wherein the electron motion induced by the electromagnetic waves near the surface of the magnet prevents the interior of the sample from interacting with the wave [16].