Robust wave transport at subwavelength scale with chiral metamaterials

Rapid progress in all types of communication systems imposes each time more strict requirements on the communication devices, requiring having the overall device's size as small as possible, but also increasing the demands on the robustness of the transmission channels to the disorders with an aim of achieving most efficient signal transmission. The existing schemes for transferring signals, based on the conventional materials, are tied to the operation wavelength of the propagating signal and therefore fundamentally limited by it. Moreover, in such schemes the absence of any sort of protection renders them vulnerable to possible defects in the channel, forcing the use of additional elements (for instance filters, amplifiers, etc.) and increasing the overall size and cost of the devices. However, recent developments in the field of artificial media, known as metamaterials [1], showed a great potential for achieving more control over the wave propagation and providing viable solutions for an efficient signal transmission. Unfortunately, since these artificial media consist of resonant inclusions - meta-atoms, they are inherently susceptible to geometrical imperfections and disorder-induced backscattering, which significantly reduces their performance and limits their real applications. In recent years, it has been demonstrated that the topological concepts, that originally have been derived in solidstate physics, can be also applied to photonic crystals [2] and locally-resonant crystalline metamaterials [3], providing a certain degree of protection against disorders. However, such photonic topological designs are based on preserving the time-reversal symmetry and, therefore, rely on the lattice structure of the media and frequency dispersion of the crystal. Thus, such time-reversal invariant topological designs are vulnerable to any disruption of the lattice symmetry that can couple time-reversed modes, which is the case for the most of disorders (in the location or in resonance frequency of the resonant inclusions). In this work, we experimentally demonstrate in the microwave regime that by exploiting a chiral metamaterial [4], [5] a robust-to-disorder subwavelength waveguiding can be achieved. Moreover, we quantitatively demonstrate the superiority of the proposed waveguiding scheme in terms of robustness to both spatial or frequency disorders to the previously proposed subwavelength waveguiding schemes: frequency defect lines, symmetry-based topological edge modes and valley interface states. To this end, we compare their performance in the presence of disorders by performing ensemble averages on disorder realizations along the path of the guided wave. The obtained results clearly demonstrate the superior robustness of the chiral metamaterial waveguide against both spatial and frequency designs while other analyzed designs are robust for one type of disorder [6].