Magnet-free electromagnetic nonreciprocity in two-dimensional materials

HIGHLIGHTS

What: To this purpose, the authors provide a general conductivity model to describe gyrotropic metasurfaces that exhibit nonreciprocity through different physical mechanisms enabled by 2D materials, including optical pumping, drifting electrons, ferromagnetic monolayers, mechanical strain, and spatiotemporal modulation. Even though this approach has been demonstrated from microwaves43 to optical frequencies41 and leads to high isolation levels, it usually operates with high intensity signals and requires excitation from only one port at a time. This approach is intensity-independent and provides great design flexibility, as it can be implemented with varactors or switches at microwave and millimeter wave frequencies,48,51,54,58 modulating the properties of optical materials,50 or through mechanical modes.53 A large number of nonreciprocal devices has been demonstrated based on spatiotemporal modulation, ranging from RF circulators and isolators48,51,58,65,68,69 to filters, antennas,73,74 and metasurfaces . This approach has been proposed to achieve Faraday rotation at THz frequencies by transferring graphene onto a substrate with sub-lattice holes .
Who: TUTORIAL | JULY and colleagues from the (UNIVERSITY) have published the research work: (TITLE), in the Journal: (JOURNAL) of 02/09/2024
How: This feature is essential for enhancing photonic energy conversion and thermal management technologies with applications such as enhancing efficiency of solar energy harvesting thermo-photovoltaics and radiative cooling . This approach confirms that plasmonic resonators ribbons and nanostructures can be applied to enhance nonreciprocal responses in optically pumped systems following a similar strategy as in magnetically biased platforms.226-231 A third approach consists of locally enhance the in-plane field of the incoming circularly polarized pump for instance taking advantage of surface lattice resonances . This approach is general and can be applied to implement a wide variety of nonreciprocal mechanisms for instance the asymmetric bandgaps (leading to large isolation over a narrow bandwidth) and interband photonic transitions (leading to larger bandwidth) discussed in Sec III C. The authors presented a comprehensive conductivity model capable of characterizing anisotropic metasurfaces that captures nonreciprocal behavior through various physical mechanisms facilitated by 2D materials.
Future: Moving beyond the exotic properties offered by 2D materials provide the ideal playground to break and manipulate electromagnetic nonreciprocity and thus the authors envision that new approaches and configurations will be further pursued and higher performance levels will be reached in the near future.

SUMMARY

In this Tutorial, the authors overview recent developments to break and manipulate electromagnetic nonreciprocity in two-dimensional (2D) materials without relying on magnetic_fields. Historically, breaking reciprocity has mostly relied on electromagnetic waves interacting with a magneto-optical system biased with a static magnetic_field B0.5-11 As benchmark, the authors consider first applying an external magnetic_field-see Fig 1(a). September 2024 09:29:06 intuitively, nonreciprocity in these systems can be understood by considering that time-reversal affects all quantities involved in wave propagation except the external bias provided by the magnetic_field B0. This effect originates from the change of system topology in the presence of the magnetic_field. If the 2D material is subjected to an external biasing, such as a magnetic_field or circularly polarized light, its electromagnetic J. Appl. Equation highlights the role of the magnetic_field as the driving force behind the Hall conductivity term, which quantifies the system`s degree of nonreciprocity. As described in Sec II D, when a 2D non-magnetic material is layered onto a 2D ferromagnetic layer, the interaction between them creates an out-of-plane effective magnetic_field. At equilibrium, the induced effective magnetic_field reaches approximately 100 T, and graphene is hole-doped with a spin-dependent chemical potential of μ 0:3 eV. In this Tutorial, the authors surveyed recent advancements aimed at disrupting and controlling electromagnetic nonreciprocity using twodimensional materials, all without the need for magnetic_fields. Even more exciting is the potential combination of optical pumps with strain engineering: this platform should be able to access the large pseudo-magnetic_fields induced by the strain in the K and K0 valleys of graphene using circularly polarized light, breaking the valley symmetry and leading to significantly boosted nonreciprocal responses. @@

LAY DEFINITIONS

Magnetic field: A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetized materials. A charge that is moving in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field
Transverse magnetic: A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular to the radiation`s propagation direction. Transverse modes occur in radio waves and microwaves confined to a waveguide, and also in light waves in an optical fiber and in a laser`s optical resonator
Electric field: An electric field is the physical field that surrounds each electric charge and exerts force on all other charges in the field, either attracting or repelling them. Electric fields originate from electric charges, or from time-varying magnetic fields
Circularly polarized: In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary manner. Circular polarization is a limiting case of the more general condition of elliptical polarization
First brillouin zone: In mathematics and solid state physics, the first Brillouin zone is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigneru2013Seitz cells in the real lattice, the reciprocal lattice is broken up into Brillouin zones
Surface plasmon: Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface. SPs have lower energy than bulk plasmons which quantise the longitudinal electron oscillations about positive ion cores within the bulk of an electron gas
Hall conductivity: The quantum Hall effect (or integer quantum Hall effect) is a quantized version of the Hall effect, observed in two-dimensional electron systems subjected to low temperatures and strong magnetic fields, in which the Hall resistance R_xy exhibits steps that take on the quantized values at certain level R_xy = V_Hall /I_channel = h / (eu00b2u03bd), where V_Hall is the Hall voltage, I_channel is the channel current, e is the elementary charge and h is Planck`s constant. The divisor u03bd can take on either integer or fractional values

Licence: cc-by

Site reference: https://pubs.aip.org/aip/jap/article-pdf/doi/10.1063/5.0207377/20086680/041101_1_5.0207377.pdf

DOI reference: https://www.doi.org/10.1063/5.0207377

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