Spintronic terahertz (THz) emitters, typically designed as magnetic-nonmagnetic thin film heterostructures, have garnered attention due to theoretically high near infrared-THz conversion efficiency. Such devices allow spin current generation upon optical pumping, meanwhile convert spin currents into charge currents through mechanisms such as inverse spin Hall effect and radiate THz pulses. 

Among various material options for spintronic THz emitters, antiferromagnets (AFMs) exhibit high-speed spin response compared to ferromagnets. Topological insulators (TIs), on the other hand, offer larger spin Hall angles than heavy metals. Due to the lack of proper AFM and TI materials to form a lattice-matched heterostructure, AFM-TI structure has never been realized for THz emission or other ultrafast property.

In a study published in Nature Communications, Prof. ZHAO Jimin's group from the Institute of Physics of the Chinese Academy of Sciences, collaborating with researchers from Beihang University and the University of Hamburg, investigated THz emission spectra of AFM-TI heterostructure MnSe/(Bi,Sb)₂Te₃ under different optical pumping conditions and temperatures using the ultrafast THz spectroscopy, and they confirmed that AFM-TI heterostructure is an ideal THz emitter with controllable polarization state.

The researchers used ultrafast THz spectroscopy to probe the THz pulses emitted from MnSe/(Bi,Sb)₂Te₃ heterostructure upon near-infrared light pumping, and observed effective THz radiation at room temperature without auxiliary fields. By analyzing the pump polarization- and sample azimuth-dependent data, they discovered two interrelated mechanisms: magnetic-dipole radiation and spin-to-charge conversion, which jointly participated in the THz emission process. 

On this basis, the researchers adjusted the pump light polarization angle, which effectively tuned the ratio of two THz components, thereby controlling the polarization state of the combined THz emission pulses. They obtained near-to-perfect circularly polarized THz emission with practical application value at specific angles. 

This study reveals the AFM-TI heterostructure to be a new class of polarization-controllable spintronic THz emitters, placing the last piece of jigsaw puzzle of magnetic-nonmagnetic heterostructure-based THz spintronic devices.