Nonreciprocal active metamaterials

We show experimentally that active electromagnetic metamaterials are suitable for the implementation of a large class of nonreciprocal media characterized by magnetoelectric constitutive parameters that cannot be achieved in passive materials. We design, fabricate, and characterize an isolator made of such an active metamaterial slab and show that it is essentially transparent to microwaves propagating in one direction and opaque in the opposite direction.

Figure 1

Nonreciprocal active medium. (a) Isolator placed inside the microstrip waveguide fills the entire section of the waveguide. The white lines mark the volume occupied by the effective medium. (b) Inverted photolithographic mask used to fabricate the unit cell. Red segments mark the position of the lumped elements inside the cell. (c) Photo of a unit cell. The cell is a rectangle of 5 cm by 5.6 cm. The substrate is FR4. (d) Medium composed of five unit cells. The distance between adjacent cells is 6 cm.

Figure 2

Measurements of metamaterial-based isolator. The transmission coefficients, $S_{21}$ and $S_{12}$, are measured with the metamaterial slab unpowered (black, dotted line) and powered (lines). For one direction of propagation the powered device is transparent (blue), for the other it is opaque (red).

Figure 3

Isolator $S$ parameters obtained in simulation [(a) and (c)] and measurements [(b) and (d)]. The amplitude in dB is shown in the top panels, while the phase is shown in the bottom panels.

Figure 4

Three-slab configuration that models the active metamaterial placed inside the parallel plate waveguide. The flow graph highlights the transmitted and reflected waves at each air-slab interface and is used to find the $S$ parameters as a function of local transmission ($T$) and reflection ($R$) coefficients.

Figure 5

Effective material parameters. (a) Wave vector for both directions of propagation, (b) effective magnetoelectric coupling coefficients $\xi$ and $\zeta$, (c) effective relative permittivity ($\epsilon$), and (d) effective relative permeability ($\mu$).