Nonlinear Optical Observation of Coherent Acoustic Dirac Plasmons in Thin-Film Topological Insulators

TL;DR

This paper reports the first observation of coherent acoustic Dirac plasmon modes in thin-film topological insulators using transient second harmonic generation, revealing undamped GHz oscillations linked to coupled surface states.

Contribution

It demonstrates the experimental detection of Landau-undamped coherent acoustic Dirac plasmons in Bi2Se3 thin films via a novel optical method.

Findings

Observation of GHz-range oscillations in second harmonic generation signals.

Identification of these oscillations as guided acoustic Dirac plasmon modes.

Evidence of Landau undamping due to fermion tunneling between surface states.

Abstract

Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient second harmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunneling between the opposite-surface Dirac states.