Thickness tunable quantum interference between surface phonon and Dirac plasmon states in thin-films of the topological insulator Bi2Se3

TL;DR

This study demonstrates a significant enhancement in Raman responses of Bi2Se3 thin films due to optical coupling with Dirac surface states and nonlinear Dirac plasmon excitation, revealing quantum interference effects in surface phonon and plasmon states.

Contribution

It uncovers thickness-independent and dependent Raman enhancements linked to Dirac surface states and plasmons, and observes quantum interference effects in thin films.

Findings

>100-fold Raman enhancement at specific photon energies

Observation of Fano lineshape indicating quantum interference

Thickness-dependent nonlinear Dirac plasmon excitation

Abstract

We report on a >100-fold enhancement of Raman responses from Bi2Se3 thin films if laser photon energy switches from 2.33 eV (532 nm) to 1.58 eV (785 nm), which is due to direct optical coupling to Dirac surface states (SS) at the resonance energy of ~1.5 eV (a thickness-independent enhancement) and due to nonlinearly excited Dirac plasmon (a thickness-dependent enhancement). Owing to the direct optical coupling, we observed an in-plane phonon mode of hexagonally arranged Se-atoms associated with a continuous network of Dirac SS. This mode revealed a Fano lineshape for films <15 nm thick, resulting from quantum interference between surface phonon and Dirac plasmon states.