Abstract
Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (ΔR/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the ΔR/R to show their intrinsic ultrafast dynamics.
Highlights
MethodsOptical pump and ultra-broadband MIR probe spectroscopy[23] consists of three stages:
The dynamics www.nature.com/scientificreports in the n-type Bi2Te2Se is dominated by bulk carriers because the ΔR/R spectra show a blue-shift in the plasma edge due to FCA
For p-type Sb2TeSe2, the dynamics is dominated by the Dirac fermion from the red-shift of the plasma edge in the ΔR/R spectra
Summary
Optical pump and ultra-broadband MIR probe spectroscopy[23] consists of three stages:. (i) 800-nm optical pulses with a duration of 30 fs were generated, (ii) ultra-broadband MIR probe pulses were generated in nitrogen and (iii) chirped pulses were generated for detection. The fundamental pulses (800 nm) and the second harmonic pulses (400 nm, which were generated by a type I β-BaB2O4 crystal with a thickness of 0.1 mm) from a Ti:sapphire amplifier (790 nm, 30 fs, 0.85 mJ at 1 kHz, Femtopower compactPro, FEMTOLASERS) were focused into nitrogen gas to generate MIR pulses. The filamentation occurred via four-wave DFG when the pulse was focused using a concave mirror (r = 1 m). When the MIR pulses were reflected from the sample with an incident angle of 45°, they were converted to ~400-nm pulses for the detection using a chirped-pulse up conversion (CPU) in nitrogen gas. To prevent significant absorption from vapor, the system was placed in boxes whose interior was purged with nitrogen
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