Active hybrid spintronic THz emitters with tunable chirality

The ability to generate and manipulate polarization-shaped terahertz (THz) waves plays a crucial role in applications such as THz sensing, THz wireless communication, and materials imaging. However, there is great demand to develop an efficient and flexible THz emitter. Recently, THz emission from RuO₂(101)/metal heterostructure via electrically anisotropic conductors (EACs) has been demonstrated. Compared to the inverse spin Hall effect (ISHE) in ferromagnetic metal (FM)/nonmagnetic metal (NM) bilayers, the THz wave from EACs is sensitive to crystal orientation and independent of an external magnetic field. Here, we combine the EACs with ISHE to fabricate a hybrid THz source composed of Pt/RuO₂(101)/Al₂O₃(sub)/Ni/Pt multilayers. We use RuO₂(101)/Pt bilayers as the pinned layer and Ni/Pt bilayers as the free layer. By optimizing the substrate thickness and incident laser power, efficient generation of circularly polarized THz waves is achieved at the design frequency. In addition, THz polarization is tunable from linear to circular by varying the azimuth angle between the crystal orientation and magnetic field. Besides, the ellipticity of chiral THz waves can be modulated by changing the incident laser power. In this way, THz waveform can also be controlled. This work not only expands the applications of EACs and Spintronic THz emission but also meets diverse demands for THz sources.