A Subwavelength Microwave Bandpass Filter Based on a Chiral Waveguide

Wave manipulation at a subwavelength scale has recently attracted significant interest, especially for low-frequency wave propagation, from acoustic to microwave devices. Recently, a few methods achieving subwavelength routing using locally resonant metamaterials have been proposed, foreseeing new technologies for ultra-compact passive components, e.g. in satellite communications. In this paper, we aim at exploiting such a technique for designing a miniature microwave filter, which guides modes at the interface between two locally-resonant bang-gap materials with opposite chirality, forming a so-called chiral subwavelength waveguide. It has been recently demonstrated that such a chiral waveguide has inherent robustness against imperfections in both the position and resonance frequencies of the local resonators, which can be leveraged for realizing robust subwavelength systems. Here, we use this property in order to realize a bandpass filter, by purposely adding resonant defects to get rid of parasitic modes in the out-of-band region without perturbating the main guided mode. We demonstrate the potentiality of integrating subwavelength resonators directly into the active region of the filter for enhancing its order and suppressing spurious bands, a functionality conventionally obtained only by cascading bulky stages. Finally, we combine this approach for removing the parasitic modes with dispersion engineering methods, demonstrating a bandpass filter with improved frequency selection, as well as a high level of rejection, more than 40dB, in the out-of-band region of the filter.