Temporal metamaterials with passive switching as impedance-matched absorbers

Abstract

Recent experiments on temporal reflection in transmission line metamaterials and theoretical treatments of dispersive time-varying media have unearthed the fundamental role of modulation mechanisms on the interface conditions, underpinning the introduction of passive photonic time crystals with stable momentum band gaps. Drawing from these concepts, it is shown that temporal metamaterials with simultaneous passive permittivity and permeability switching exhibit wideband absorption with impedance-matching, effectively behaving as one-dimensional perfectly matched layers. Under the effective medium theory, the loss mechanism is attributed to the emergent effective electric and magnetic conductivities, which are used to derive an approximate matching condition for asynchronous modulation and to engineer lossy material properties. The proposed approach and its performance beyond the Rozanov bound are verified with semi-analytical calculations as well as full-wave simulations, and the possibility of realizing a two-dimensional temporal perfectly matched layer is discussed.

Figures (7)

• Figure 1: Unit cell of a transmission line medium under reactance switching with series inductors and parallel capacitors. The second switch on C2 acts to discharge the residual charges in between modulation.
• Figure 2: Band diagram of piecewise time-periodic media: a) Band diagram of an active PTC with permittivity modulation; b) temporal decay and growth in the active PTC; c) band diagram of an passive PTC with permittivity modulation; d) temporal decay in the passive PTC; e) band diagram of an passive PTC with impedance-matched modulation; f) temporal decay in the passive PTC with impedance-matched modulation
• Figure 3: FDTD simulation of reflection from a grounded slab with impedance-matched permittivity and permeability switching: a) Spatiotemporal electric field profile; b) reflectance of Floquet harmonics.
• Figure 4: Absorption efficiency of a band limited signal with respect to the maximum permeability during modulation.
• Figure 5: Reflectance of a time-varying half-space with respect to the time delay $(T_d)$ between permittivity and permeability modulations and the modulation depth. Line and scatter plots show the Floquet expansion and FDTD results, respectively.