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All-passive nonreciprocal metastructure - PubMed

  • ️Thu Jan 01 2015

All-passive nonreciprocal metastructure

Ahmed M Mahmoud et al. Nat Commun. 2015.

Abstract

One-way propagation of light, analogous to the directional flow of electrons in the presence of electric potential difference, has been an important goal in the wave-matter interaction. Breaking time-reversal symmetry in photonic flows is faced with challenges different from those for electron flows. In recent years several approaches and methods have been offered towards achieving this goal. Here we investigate another systematic approach to design all-passive relatively high-throughput metastructures that exhibit nonreciprocal properties and achieve wave-flow isolation. Moreover, we build on those findings and propose a paradigm for a quasi-two-dimensional metastructure that mimics the nonreciprocal property of Faraday rotation without using any magnetic or electric biasing. We envision that the proposed approaches may serve as a building block for all-passive time-reversal symmetry breaking with potential applications for future nonreciprocal systems and devices.

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Figures

Figure 1
Figure 1. Schematics of all-passive nonreciprocal metastructures.

(a) The difference between the ‘reciprocal' polarization rotation due to chirality and the ‘nonreciprocal' Faraday rotation phenomenon is shown, both mapped conceptually on the Poincaré sphere. (b) Depending on the illumination direction of the incident wave, one of the two constituent designs (shown as green and yellow), acting as a ‘wave diode', allows the wave to go through, interacting with the chiral element (not shown) in the unit. Owing to this interaction, the plane of polarization of the wave rotates clockwise by nearly 90 ° as it goes through this wave diode. The chiral elements in the diodes for waves going into the (+z) direction (‘yellow' design) are mirror image of the chiral elements in the diodes for waves going into the (−z) direction (‘green' design). The all-passive metastructure formed as the checkerboard pattern of such alternating designs may function as a nonreciprocal metasurface mimicking Faraday rotation, while no biasing electric or magnetic field is used.

Figure 2
Figure 2. Concept of the proposed electromagnetic wave ‘diode'.

(a) The relative electric field amplitude distribution within a bilayered asymmetric one-dimensional (1D) slab infinitely extent in the transverse directions (normalized to incident field amplitude E0). While the paired slab is reciprocal, the amplitude of the total electric field distribution in the partial standing waves within the structure is dependent on the side from which it is excited. The red and blue curves show the field amplitude distribution normalized to incident field amplitude E0 for the forward (+z)- and backward (−z)-propagating waves, respectively. The dashed lines show two of the locations of MLFR, where the ratio between the local field amplitude for the forward and backward illuminations is maximum. These are the planes where nonlinearly loaded resonator should be inserted for the most efficient nonlinear response. (b) MLFR (green curve) and transmission coefficient (yellow curve) versus normalized thickness t1. The dashed line shows the operating point for which the normalized field distribution is shown in a. (c) Bilayered asymmetric slab consisting of two layers with dielectric permittivities ɛ1 and ɛ2 and thicknesses t1 and t2 are inserted in the squared-cross-section metallic waveguide. The inset shows the varactor-loaded thin resonator placed at a distance l from one of the slab ends where one of the MLFR locations is. The resonator is made of two concentric rings loaded by four varactors distributed symmetrically along its perimeter. The bilayered slab and the ring resonator are placed within a rectangular waveguide.

Figure 3
Figure 3. Response of the nonlinearly loaded resonator ring and transmission coefficient of electromagnetic wave diode in the waveguide.

(a) Schematics of the wave diode. (b) The resonance behaviour of the ring (that is, its excited current in arbitrary units (a.u.) versus normalized frequency (with respect to operating frequency), when the ring is located in the waveguide filled with material with ɛ2=2ɛ0 for two different values of varactor capacitance. These capacitance values, Cf=0.157 pF and Cb=0.147 pF, are what the varactor experiences when the loaded ring in the bilayered structure and is illuminated with 5 dBm incident power in the forward (+z) and backward (−z) directions, respectively. In this case, we get almost the same excitation, which yields the same transmission characteristics for both. (c) Similar to b, except the varactor capacitance values Cf=0.1 pF and Cb=0.05 pF, which are for the case of 30-dBm incident power. Here we note significant difference in the resonance of the ring for different capacitance values, yielding two different transmission coefficients at the operating frequency. (d) Transmission coefficient versus input power level. The red and blue curves show the cases of the forward and backward illuminations, respectively.

Figure 4
Figure 4. Schematic of all-passive quasi-2D nonreciprocal metastructures.

(a) Geometry of the device with four units of the checkerboard including two pairs of mirror-imaged chiral elements shown in Fig. 1. (b) Cross-section of the electric field intensity distribution within the metastructure with the incident wave propagating in the +z direction, (c) Simulation results for the output field (shown as compared with the incident field) when the incident wave propagates in the +z direction (d,e) Similar to b,c but when the incident wave propagates in the −z direction.

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