Tunable Terahertz Wave via Synthesis of Spin and Charge Induced Radiations in Topological Insulator

Abstract

Elliptically or circularly polarized terahertz (THz) radiation plays increasingly important roles in both scientific research and practical applications, such as wireless communication, composition analysis and undamaged imaging1. Recently, the realization of highly efficient THz radiation in ferromagnetic multilayer via spin-charge conversion (SCC) brings new hope for the emerging spin THz emitter at room temperature2. However, achieving an easily-tunned THz emitter with polarization and ellipticity manipulable still be challenging. Topological insulator (TI), with strong spin-orbital coupling and spin-momentum-locked surface states, has been regarded as the most effective spin-charge convertor in spin THz emitter3. In addition, the broken inversion symmetry at surface provides topological insulator numerous fascinating nonlinear optical phenomena, the dominant one is linear photogalvanic effect (LPGE), which could yield a polarization-dependent terahertz radiation in topological insulator4. By contrast, the terahertz wave induced by SCC is polarization-independent. This difference in polarization-dependence of two terahertz components makes it possible to control them separately, inspiring a new approach in chirality-tunned terahertz emission by synthesizing the SCC-induced and LPGE-induced terahertz waves only by tailoring the polarization of incident pumping laser.Here we prepared a Bi2Te3 (10nm)/Fe (3nm) heterostructure by molecular beam epitaxy (MBE) and measure the terahertz response after illumination by a linear polarized femtosecond laser. The terahertz wave was probed by a terahertz time domain spectroscopy (THz-TDS). As shown in Figure 1a, the amplitude of the terahertz generated in Bi2Te3/Fe heterostructure is higher than single Bi2Te3 or Fe thin film, almost keeps in the same level with W/CoFeB/Pt trilayer5. The latter is usually regarded as a typical spin terahertz emitter and the result proves the high efficiency of Bi2Te3/Fe heterostructure in terahertz emission. After further careful examination, we divided the overall terahertz radiation into two components. The first is SCC-induced spin terahertz. Generated from the spin-charge conversion in the Bi2Te3/Fe interface, this component is only magnetization-dependent. Meanwhile, an ultrafast photocurrent can also originate in Bi2Te3 film due to LPGE in the inversion symmetry breaking surface, leading to a terahertz wave. Notably, the direction of the LPGE-induced terahertz is related with the polarization of incident laser, which provides a simple method to construct orthogonal terahertz components by adjusting the incident polarization. More importantly, a phase difference between these two terahertz components could be observe because of the delayed time between SCC and LPGE. After deliberately tunning the incident laser to promise both terahertz components in the same magnitude, an elliptical polarized terahertz wave appears in Bi2Te3/Fe heterostructure. Furthermore, a terahertz wave with arbitrarily tailored chirality and ellipticity could be got by tailoring the pumping laser polarization.Overall, we have demonstrated a highly efficient terahertz emission in Bi2Te3/Fe heterostructure. Compared with conventional spintronic terahertz emitter, Bi2Te3/Fe can realize terahertz wave with arbitrarily tailored chirality only by adjusting the incident polarization. The engineering of topological insulator and ferromagnet heterostructure may open up a great opportunity to study high-performance TI-based spintronic terahertz devices with easy controllability. **