Transmission resonances in a symmetry-broken square coaxial aperture in a metal film

Abstract

We present a computational study of the transmission resonances in a symmetry-broken square coaxial aperture with an inserted particle offset (δ) from the centre. According to the distribution characteristics of oscillating charges on the exit surface, the TE11- and TE21-like mode resonances in the hole may be regarded as dipole and quadrupole plasmon resonances, respectively. It is found that symmetry breaking, due to the particle offset, makes the quadrupole plasmon resonance, which is dark in symmetric geometry, visible. Based on the charge oscillation induced light emission mechanism, we qualitatively explain the variation of the transmission peaks of dipole and quadrupole plasmon resonances with the geometrical parameter δ, and the refractive index of the dielectric filled in the hole. Using polarization-dependent surface charge plots, we illustrate two distinct mode configurations of quadrupole plasmon resonance. Our findings will be of value in tuning the electromagnetic wave in subwavelength optics and designing chemical and biomedical sensors.