Helicity-dependent terahertz emission spectroscopy of topological insulator Sb2Te3 thin films

Abstract

We report on helicity-dependent terahertz emissions that originate from the helicity-dependent photocurrents in topological insulator Sb 2 Te 3 thin films due to ultrafast optical excitation. The polarity of the emitted terahertz radiation is controlled by both the incident angle and the helicity of optical pulses. Using an unprecedented decomposition-recombination procedure in the time domain, the signals of the Dirac fermions are fully separated from bulk contributions. These results provide insights into the optical coupling of topological surface states and open up opportunities for applying helicity-dependent terahertz emission spectroscopy in spintronics.

Generation and manipulation of chiral terahertz waves in the three-dimensional topological insulator Bi2Te3

“…Therefore, based on the geometric structure of the incident laser that was modulated via the quarter-wave plate (Supplementary Note 6 and Fig. S8 in the Supplementary Material), we can write the signal in the following form: 25,26,34 Sðt; αÞ ¼ CðtÞ sinð2αÞ þ L 1 ðtÞ sinð4αÞ þ L 2 ðtÞ cosð4αÞ þ DðtÞ:…”

“…4(e) and 4(f)], they both presented the same spectral shapes, strongly indicating that both C x and L 1x shared the same physical origin and are consistent with previous observation. 25,26,[35][36][37][38] However, the D x , L 2yz , and D yz terms also presented similar shapes in the frequency domain, and very few works pay attention to the underlying mechanism about these three terms. Ours is the first observation that all five terms resemble each other in nature, and we elucidate this phenomenon in a macroscopic phenomenological PGE-induced photocurrent framework.…”

“…The charge current and spin current inside topological insulators are simultaneously excited when being driven by femtosecond laser pulses. [24][25][26][27][28] However, topological insulator-based terahertz emission mechanisms are far more complicated when compared with conventional terahertz emitters. It is clarified that nonlinear effects play a predominant role in the ultrafast terahertz emission process.…”

Helicity-dependent THz emission induced by ultrafast spin photocurrent in nodal-line semimetal candidate Mg3Bi2

“…Mg 3 Bi 2 thin films were grown on the transparent sapphire (Al 2 O 3 ) substrates by using molecular beam epitaxy. A Se capping layer is essential to protect the Mg 3 Bi 2 film from oxidation in the atmosphere and prevent the thin film from the evaporation of pulsed-laser illumination 19 . The details of sample preparation are discussed in the supplementary information.…”

“…Terahertz time-domain spectroscopy (TDS) is a crucial method to confirm the properties of topological states and the dynamics of charge carriers on a sub-picosecond timescale 7,19,[21][22][23][24][25] . In the process of exploring topological properties, it is significant to separate the surface signals from the bulk backgrounds [26][27][28] .…”