Coherent lattice dynamics in topological insulator Bi2 Te3 probed with time-resolved optical second-harmonic generation

The dynamics of phonon modes in thin flakes of topological insulator Bi2Te3 has been studied using transient optical second-harmonic spectroscopy. Using the surface sensitivity of second-harmonic generation, the excitation and dynamics of a family of optical phonon modes are observed with a response dependent on the excitation polarization. Three coherent phonon modes, inaccessible in previous linear time-resolved optical measurements, were detected in the nonlinear dynamic response. A symmetry-allowed surface optical phonon mode in Bi2Te3 has been identified using surface and symmetry selectivity of the second-order up-conversion process. Possible second-harmonic generation mechanisms in Bi2Te3 as well as the enhanced sensitivity of the nonlinear processes for the phonon mode detection in topological insulators are discussed.