Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi 2 Se 3 revealed a strong dependence of the carrier relaxation time on the film thickness. For thicker films the relaxation dynamics are similar to those of bulk Bi 2 Se 3 , where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.Topological insulators (TIs) are novel electronic materials that have an insulator-type band gap in the bulk (for Bi 2 Se 3 E g ~ 0.3 eV) but have protected gapless conducting phase on their surface due to the combination of spin-orbit interactions and time-reversal symmetry. 1,2 The most effective experimental methods currently used to monitor metallic two-dimensional (2D) Dirac surface states (SS) of TIs are angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (TrARPES). 1-7 These techniques are equally sensitive to SS and the bulk atoms residing in the close proximity to the surface as a consequence of the extremely small penetration depth (a few nm) of incident energetic photons used for photoemission, combined with the limited escape depth of the electrons (also a few nm). Finite-size effects have also been studied for thin Bi 2 Se 3 films of only a few nm thick and a crossover of the three-dimensional (3D) TI Bi 2 Se 3 to the 2D limit (gapped SS) has been observed when the thickness is below six quintuple layers (~ 6 nm). 8 Reaching a similar sensitivity to SS using traditional optical pump-probe techniques (like transient reflectivity (TR)/transmission), which use less energetic photons in the visible/infrared range, seems problematic since the absorption length of the laser light normally used for these measurements (a few tens of nm) significantly exceeds the range where the effect of SS can actually be monitored. As a result, for bulk single crystals of Bi 2 Se 3 the transient optical response is dominated by the bulk contribution. To overcome the problem one can use SS/surface sensitive methods. An example of this approach has recently been demonstrated by illuminating Bi 2 Se 3 with circularly polarized near-infrared light. 9 The resulting photocurrent which reverses its direction with a reversal of the helicity of the light unambiguously proves the SS origin of the optical response. Another surface sensitive technique exploits the centrosymmetric nature of TI's, which governs exclusively the surface-related response which results in an optical second harmonic generation (SHG) process. 10,11 In this Letter we report on a new way to distinguish between the contributions from the TI (Bi 2 Se 3 ) bulk 3D states and the 2D gapless SS, which is based on differences in the carrier relaxation rates for the insulating and metallic phases. We demonstrate that the car...

Transient reflectivity (TR) from thin films (6 -40 nm thick) of the topological insulator Bi 2 Se 3 reveal ultrafast carrier dynamics, which suggest the existence of both radiative and non-radiative recombination between electrons residing in the upper cone of initially unoccupied high energy Dirac surface states (SS) and holes residing in the lower cone of occupied low energy Dirac SS. The modeling of measured TR traces allowed us to conclude that recombination is induced by the depletion of bulk electrons in films below ~20 nm thick due to the charge captured on the surface defects. We predict that such recombination processes can be observed using time-resolved photoluminescence techniques.Thin films of topological insulators (TIs) are threedimensional (3D) materials that are insulating in the bulk (bandgap of Bi 2 Se 3 , for example, E g ~ 0.3 eV), but conductive at the surfaces due to two-dimensional (2D) Dirac surface states (SS) caused by the combination of strong spin-orbit interaction and time-reversal symmetry.

Transient reflectivity traces measured for nanometer-sized films (6 -40 nm) of the topological insulator Bi 2 Se 3 revealed GHz-range oscillations driven within the relaxation of hot carriers photoexcited with ultrashort (~100 fs) laser pulses of 1.51 eV photon energy. These oscillations have been suggested to result from acoustic phonon dynamics, including coherent longitudinal acoustic phonons in the form of standing acoustic waves. An increase of oscillation frequency from ~35 to ~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the interplay between two different regimes employing traveling-acoustic-waves for films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR) modes for films thinner than 40 nm. The amplitude of oscillations decays rapidly for films below 15 nm thick when the indirect intersurface coupling in Bi 2 Se 3 films switches the FBAWR regime to that of the Lamb wave excitation. The frequency range of coherent longitudinal acoustic phonons is in good agreement with elastic properties of Bi 2 Se 3 .

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.

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 tunnelling between the opposite-surface Dirac states.