Conductivity Mismatch in Topological-Insulator-Based Spintronic Terahertz Emitters

We report TeraHertz (THz) emission from magnetic heterostructures comprised of Fermi-level engineered Bi₂Se₃ to assess how the position of the chemical potential affects the resultant emission properties. The emission spectra of bare topological insulator (TI) films reflect the 3m point group symmetry of Bi₂Se₃, while that from magnetic heterostructures follows the magnetic dipole symmetry of ultrafast demagnetization. Bulk-insulating Bi₂Se₃ heterostructures show a ~ 5-fold reduction in the THz emission amplitude as compared to their bulk-conducting counterparts. Such behavior contrast with a reciprocal measure of charge-to-spin conversion from spin-orbit torque, and is attributed to a conductivity mismatch across the magnetic interface brought on by the low carrier density and high sheet resistance of the bulk-insulating film. Tunnel junctions and Kramers-Weyl semimetals are proposed as an effective strategy to enhance the degree of spin polarization injected across the interface, providing a promising future direction for designing THz spintronic emitters using advanced, functional materials beyond elemental heavy metals.