Selective Probing of Photoinduced Charge and Spin Dynamics in the Bulk and Surface of a Topological Insulator

D. Hsieh, F. Mahmood, J. W. McIver, D. R. Gardner, Y. S. Lee, and N. Gedik

Phys. Rev. Lett. 107, 077401 – Published 10 August 2011

Abstract

Topological insulators possess completely different spin-orbit coupled bulk and surface electronic spectra that are each predicted to exhibit exotic responses to light. Here we report time-resolved fundamental and second harmonic optical pump-probe measurements on the topological insulator ${Bi}_{2} {Se}_{3}$ to independently measure its photoinduced charge and spin dynamics with bulk and surface selectivity. Our results show that a transient net spin density can be optically induced in both the bulk and surface, which may drive spin transport in topological insulators. By utilizing a novel rotational anisotropy analysis we are able to separately resolve the spin depolarization, intraband cooling, and interband recombination processes following photoexcitation, which reveal that spin and charge degrees of freedom relax on very different time scales owing to strong spin-orbit coupling.

Received 30 March 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.077401

Authors & Affiliations

D. Hsieh¹, F. Mahmood¹, J. W. McIver^1,2, D. R. Gardner¹, Y. S. Lee¹, and N. Gedik¹

¹Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
²Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 107, Iss. 7 — 12 August 2011

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Schematic of the experimental geometry. $M$ denotes a mirror plane of the ${Bi}_{2} {Se}_{3}$ crystal. When the polarizing beam splitter (PBS) is oriented as drawn, the $s$ - and $p$ -polarized output probe photons are measured using detectors 1 and 2, respectively. The PBS is rotated by 45° to perform a balanced detection measurement of Kerr rotation. (b) The change in reflectivity of $s$ and $p$ fundamental probe photons following a $p$ pump pulse. The difference of the two traces is shown magnified by a factor of 2. The inset shows the $s$ -out trace measured out to long delay times. (c) Typical pump-probe signal measured from detectors 1 and 2 with the PBS rotated by 45° following excitation by obliquely incident left ( $σ^{+}$ ) and (d) right ( $σ^{-}$ ) circularly polarized pump pulses. (e) $σ^{+}$ and $σ^{-}$ pump-induced Kerr rotation $θ_{K}$ obtained via the signal difference of detectors 1 and 2 from (c) and (d).Reuse & Permissions
Figure 2
(a) $p$ pump-induced change in SHG intensity as a function of sample angle $ϕ$ measured with $p$ -in $s$ -out and (b) $p$ -in $p$ -out probe photons. Data are normalized to their minimum. Data are taken in the range $0 ° < ϕ < 120 °$ and then threefold symmetrized. (c) and (d) show constant $ϕ$ cuts through the data in (a) and (b), respectively. A normalized fundamental pump-probe trace taken with the same pump fluence is overlayed in (c).Reuse & Permissions
Figure 3
(a) Static SHG intensity as a function of $ϕ$ measured using $p$ -in and $p$ -out or $s$ -out probe photons. Time-resolved SHG Kerr rotation measurements are taken with $p$ pump and $ϕ = 0 °$ so that $θ_{K} = 0 °$ when there is no pump. (b) Typical SHG pump-probe signal measured from detectors 1 and 2 with a 45° oriented analyzer following excitation by obliquely incident $σ^{+}$ and (c) $σ^{-}$ pump pulses. (d) $σ^{+}$ and $σ^{-}$ pump-induced Kerr rotation measured at oblique pump incidence and (e) normal pump incidence. We note that there is some statistical fluctuation in the temporal widths of the Kerr rotation peaks due to noise in the raw data traces (b) and (c).Reuse & Permissions
Figure 4
Schematic of the transient surface charge and spin response to photoexcitation and their characteristic time scales $τ$ . Parabolic curves represent the valence and conduction bands of ${Bi}_{2} {Se}_{3}$ near the surface and the straight lines traversing the gap represent the Dirac surface states. Empty circles represent holes and filled circles represent electrons. (a) The circular pump pulse initially creates a spin-polarized excited carrier population. (b) The spin polarization is then rapidly depolarized because of spin-orbit coupling within the pump-probe overlap time. (c) Intraband carrier cooling occurs on a 1 ps time scale followed by (d) a much slower interband electron-hole recombination.Reuse & Permissions

Physical Review Letters