Coexistence of classical and quantum plasmonics in large plasmonic structures with subnanometer gaps

Researchers from DTU Physics in collaboration with DTU CEN have monitored a controlled transition from a classical to a quantum plasmonic system.

Shima Kadkhodazadeh1, Jakob B. Wagner1, Harald Kneipp2, Katrin Kneipp2
Appl. Phys. Lett. 103, 083103 (2013); doi: 10.1063/1.4819163 

Large metal nanostructures with subnanometer interparticle separations (gaps) can provide extremely high local fields and are of particular interest in plasmon supported spectroscopy, as well as for basic understanding of plasmonics. In this experimental electron energy loss study, we monitor the transition of plasmonic dimers from a classical to a quantum system by decreasing gaps to dimensions when tunneling occurs and a conductive nanobridge evolves. EELS spectra A, B and C illustrate this transition and represent the non-contact-, tunnel -, and contact-regimes, respectively by showing bright (iii), dark modes (i) as well as charge transfer plasmon modes (ii) and (iv) related to the tunnel current and an evolving nanobridge, respectively. Our studies show that silver dimers with atomic scale gaps can exhibit a regime, in which charge transfer plasmon modes, as a hallmark of a quantum nature, and “classical” bright and dark dipolar plasmon modes can be seen simultaneously. This finding can have important implications for local fields and their exploitation in plasmon-supported photonics.

Affiliations:
1Technical University of Denmark, Center for Electron Nanoscopy, Kgs. Lyngby, Denmark 
2Technical University of Denmark, Department of Physics, Kgs. Lyngby, Denmark