Abstract
We present an ultrasensitive plasmonic sensing system by introducing a nanostructured X-shaped plasmonic sensor (XPS) and measuring its localized optical properties in phase interrogation. Our tailored XPS exhibits two major resonant modes of a low-order dipole and a high-order quadrupole, between which the quadrupole resonance allows an ultrahigh sensitivity, due to its higher quality factor. Furthermore, we design an in-house common-path phase-interrogation system, in contrast to conventional wavelength-interrogation methods, to achieve greater sensing capability. The experimental measurement shows that the sensing resolution of the XPS reaches 1.15 × 10 -6 RIU, not only two orders of magnitude greater than the result of the controlled extinction measurement (i.e., 9.90 × 10 -5 RIU), but also superior than current reported plasmonic sensors.