Interface Dominated Spin-to-Charge Conversion in Terahertz Emission by Band Structure Engineering of Topological Surface States

The rapid advancement of future information technologies necessitates the development of high-efficiency and cost-effective solutions for terahertz emitters, which hold significant practical value in next-generation communication, terahertz sensing, and quantum computing applications. Distinguished from trivial materials, three-dimensional topological insulators exhibit spin-momentum locking in helical Dirac surface states, making them highly efficient spin-to-charge converters that have the potential to revolutionize electronics. However, the efficiency of utilizing topological insulators for spin terahertz emission has not yet matched that of spin manipulation in other spintronic devices. Here, we investigate the spin terahertz emission properties of high crystalline quality (Bi_1-xSb_x)₂Te₃/Fe heterostructures through band structure engineering. Notably, contrary to expectations, the strongest terahertz radiation is not achieved at the charge neutrality point. Through an analysis of influencing factors and a temperature-independent investigation, we identify interface transparency as the primary factor affecting emission efficiency. To optimize interfaces and enhance spin-to-charge conversion efficiency, a Rashba-mediated Dirac surface state is constructed by attaching a Bi layer. Furthermore, with doping concentrations of 0, 0.5, and 1, respectively, we observe enhancements in intensity by 35.1, 50.3, and 44.3%. These results provide a detailed assessment of interfacial and doping effects in topological-insulator-based terahertz emitters and contribute to the understanding of spin-to-charge dynamics in topological materials.