Coherent Dirac plasmons in topological insulators

We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in Sb2Te3 topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the A(1g)(2) mode, which is attributed to quantum interference between continuumlike coherent Dirac plasmons and phonons. By analyzing the time-dependent asymmetric line shape using the two-temperature model (TTM), it was determined that a Fano-like resonance persisted up to approximate to 1 ps after photo excitation with a relaxation profile dominated by Gaussian decay at <= 200 fs. The asymmetry parameter could be well described by the TTM for >= 200 fs, therefore suggesting the coherence time of the Dirac plasmon is approximate to 200 fs.