Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators

Abstract

Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

“…Therefore, it cannot arise from the initial optical transition. In fact, previous works on Bi 2 Se 3 assigned the slow J yz component to a carrier drift in the surface field, consistent with the strong dependence of J yz on the doping level of Bi 2 Se 3 (refs 9, 10, 11). This notion is further supported by additional observations made in our experiment: first, the initial electron flow is directed toward the sample surface, along the direction of the space–charge field of our effectively n-doped sample.…”

“…Since the slow component of J yz is a drift current of photoexcited carriers in the space–charge field91011, it is also localized in a depth of ∼15 nm. In addition, its amplitude changes during sample aging must arise from gradual modifications of E SC (Fig.…”

“…Part of this considerable potential was demonstrated by recent works, which reported the exciting possibility of launching TI surface currents by simply illuminating the sample with light678910111213. The direction of the photocurrent could be controlled through the polarization state of the incident light beam.…”

“…Scenarios based on asymmetric depopulation of the Dirac cone6 transitions into other higher-lying cones13, and asymmetric scattering of electrons8 have been proposed. To directly resolve the generation of TI surface photocurrents, we need to boost the time resolution of the experiment from so far as ∼250 fs and longer910111213 to the scale of elementary scattering events, which can be shorter than 10 fs.…”

Possible flat band bending of the Bi1.5Sb0.5Te1.7Se1.3 crystal cleaved in an ambient air probed by terahertz emission spectroscopy

“…1(a). Among several THz generation mechanisms, we consider the optical rectification and the laser-induced surge current as a primary source for BSTS16. Particularly in the latter case, the emitted THz wave can provide important information about the charge dynamics as well as about the surface band bending.…”