"Electrical tuning of a quantum plasmonic resonance"

Xiaoge Liu: Ju-Hyung Kang, Hongtao Yuan, Junghyun Park, Soo Jin Kim, Yi Cui, Harold Y. Hwang & Mark L. Brongersma; Nat Nano, 06/12/17.

Additional Authors: Ju-Hyung Kang, Hongtao Yuan, Junghyun Park, Soo Jin Kim, Yi Cui, Harold Y. Hwang & Mark L. Brongersma

Abstract:

Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light–matter interaction1,2. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λF of the electrons3,4,5,6. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles7,8,9,10,11,12, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements3, as well as imperfect control over size, shape, faceting, surface reconstructions13, contamination14, charging effects15 and surface roughness16 in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal17,18,19,20,21. An ionic liquid (IL)22,23 is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.