Tunable Subradiant Mode in Free-Standing Metallic Nanohole Arrays for High-Performance Plasmofluidic Sensing

Free-standing metallic nanohole arrays (NAs) exhibit extraordinary benefits in combining nanofluidics and plasmonics in a single platform for flow-through plasmofluidic sensing. However, the sensing performance of NAs is usually limited by the low quality factor of the transmission peak, resulting from strong radiative damping. Here, we demonstrate a concept for high-performance plasmofluidic sensing based on the subradiant mode in free-standing metallic NAs. We show that the subradiant mode exhibits a good spectral resolution only when the dielectric media at the upper and lower metal surfaces are symmetric; otherwise, it easily emerges into other features present in the same spectral region. Therefore, free-standing NAs are particularly suitable for highly sensitive microfluidic sensing based on the subradiant mode. In addition, the subradiant mode exhibits a highly tunable frequency and refractive index sensitivity (RIS), depending strongly on the film thickness and periodicity and weakly on the hole radius. Compared with the superradiant mode, the subradiant mode exhibits a much narrower spectral characteristic and more intensely enhanced electric fields at metal surfaces with larger field extension and longer lifetime. The high RIS and good quality factor together afford the subradiant mode a much better sensing performance than the superradiant one. This work is significant for developing high-performance lab-on-a-chip devices.