Point defect states emergence in a plasmonic crystal

TL;DR

This study uses electron energy-loss spectroscopy to observe and analyze localized defect states within the bandgap of a plasmonic crystal, revealing how defect states emerge with minimal structural complexity.

Contribution

It provides the first experimental observation of defect-related energy levels within the plasmonic bandgap using EELS and explores their emergence through systematic structural modifications.

Findings

Defect states are observed within the plasmonic bandgap.

Defect-like states form with as few as two shells in the crystal.

Experimental and theoretical analysis confirms defect state emergence.

Abstract

Plasmonic crystals are well known to have band structure including a bandgap, enabling the control of surface plasmon propagation and confinement. The band dispersion relation of bulk crystals has been generally measured by momentum-resolved spectroscopy using far field optical techniques while the defects introduced in the crystals have separately been investigated by near field imaging techniques so far. Particularly, defect related energy levels introduced in the plasmonic band gap have not been observed experimentally. In order to investigate such a localized mode, we performed electron energy-loss spectroscopy (EELS), on a point defect introduced in a plasmonic crystal made up of flat cylinders protruding out of a metal film and arranged on a triangular lattice. The energy level of the defect mode was observed to lie within the full band-gap energy range. This was confirmed by a momentum-resolved EELS measurement of the band gap performed on the same plasmonic crystal. Furthermore, we experimentally and theoretically investigated the emergence of the defect states by starting with a corral of flat cylinders protrusions and adding sequentially additional shells of those in order to eventually forming a plasmonic band-gap crystal encompassing a single point defect. It is demonstrated that a defect-like state already forms with a crystal made up of only two shells.