Perspectives and opportunities with multisubband plasmonics

Abstract

In highly doped semiconductor quantum wells (QWs), electrons populate various energy states from different subbands and, therefore, several optical intersubband transitions (ISBTs) can occur simultaneously. Coulomb coupling between these ISBTs gathers the strength of all the individual transitions and concentrates all the oscillator strength in a single collective excitation: the multisubband plasmon (MSP). MSPs are an excellent platform for the study of collective and exotic effects in semiconductors and for the demonstration of novel device concepts. Indeed, the high electronic densities involved in the collective excitation greatly enhance the coupling strength and enable the ultra-strong coupling regime between MSPs and either optical modes in a cavity or phonons in the semiconductor. In this Perspectives paper, after addressing the basic physics of MSPs and the state of the art, we outline the most promising paths for the research community in this topic from the point of view of basic physics, material platforms, and applications of MSPs.