Coherent lattice dynamics in topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ probed with time-resolved optical second-harmonic generation

The dynamics of phonon modes in thin flakes of topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ 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 ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ has been identified using surface and symmetry selectivity of the second-order up-conversion process. Possible second-harmonic generation mechanisms in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ as well as the enhanced sensitivity of the nonlinear processes for the phonon mode detection in topological insulators are discussed.

Figure 1

Time-resolved SHG signal for different polarizations of the control beam. S-polarized SHG was measured with the S-polarized fundamental light. Inset shows the SHG kinetics on large time scale.

Figure 2

Fourier spectra of the time-resolved SHG intensity from ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ flakes in $S_{\omega }\text{−}{S}_{2\omega }$ polarization configuration, measured for (a) P- and (b) S-polarized pump. Dashed lines indicate the phonon modes observed in the measurements. Solid lines indicate the phonon modes observed in [16, 17]. Dashed area represents the $A_1$ surface mode. (c) Dependence of the relative contribution of two phonon modes $I_{2.85THz}/I_{1.82THz}$ on the polarization of the pump. Zero angle corresponds to the S-polarized pump.

Figure 3

Fourier spectra of the time-resolved SHG intensity from ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ flakes in $P_{\omega }\text{−}{S}_{2\omega }$ polarization configuration, measured for (a) P- and (b) S-polarized pump. Dashed lines indicate the phonon modes observed in the measurements. Solid lines indicate the phonon modes observed in [16, 17]. Dashed area represents the $A_1$ surface mode.