Highly compact filters enabled by robust quarter-wavelength effective localized surface plasmonic resonators

Abstract This paper presents a design strategy for highly compact filters based on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>λ</mml:mi> <mml:mi>g</mml:mi> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:math> effective localized surface plasmons (ELSPs). Leveraging the robustness of ELSPs to their cross-sectional shapes, the proposed design facilitates easier integration into printed circuit boards (PCBs). However, challenges such as dielectric material stacking and excessive lateral dimensions remain. By employing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext> </mml:mtext> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>λ</mml:mi> <mml:mi>g</mml:mi> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:math> ELSPs, the longitudinal size of the ELSPs-based filters is significantly reduced. Furthermore, embedding the ELSPs directly into the PCB effectively solves the stability problems associated with dielectric material and PCB stacking in conventional dielectric resonator filters, while minimizing the overall filter size. To demonstrate this approach, third-order, fourth-order, and fifth-order ELSPs-based filters were fabricated on a single-layer PCB, achieving center frequencies of 3.6 GHz with fractional bandwidths of 9.3%, 5.1%, and 7.7%, respectively. Experimental results show excellent agreement with simulations. This work provides an ultra-compact filtering solution for next-generation microwave and radio-frequency devices, particularly for applications in 5G and satellite communications.