Terahertz (THz) waves can be used to probe the magnetic properties of new materials. Efficient THz wave generation could advance applications in energy harvesting, ultrafast electronics, and THz spectroscopy.

Bright, coherent THz light sources can be achieved via high-frequency, high-density charge currents. It is possible to produce strong current density that is several orders of magnitude higher than what is typically found in electronic devices, by exciting nanometer-scale, metallic interfaces with femtosecond laser pulses.

To harness the power of high-density charge currents for generating THz waves, researchers at Fudan University, Shanghai Research Center for Quantum Sciences, and Beijing Normal University developed an approach that, unlike other techniques, is nonrelativistic and nonmagnetic. The researchers’ approach directly exploits laser-excited, high-density charge currents across nanoscale metallic interfaces. It takes advantage of the anisotropic electrical conductivity of some materials and eliminates the need to convert charge currents to spin-polarized currents.

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The researchers developed a nonrelativistic mechanism for THz pulse formation using an electrically anisotropic, conductor-based heterostructure. (a) Ellipsoid of the conductivity tensor of the anisotropic conductors RuO₂ and IrO₂. (b) Characterization of the generated pulse (c, d). Courtesy of Zhang et al., doi: 10.1117/1.AP.5.5.056006.

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To convert laser-excited charge currents into efficient, broadband THz radiation, the researchers used anisotropic, conductive heterostructures. They relied specifically on the electrical anisotropy of two conductive rutile oxides: antiferromagnetic ruthenium(IV) oxide (RuO₂) and nonmagnetic iridium(IV) oxide (IrO₂).

The single-crystal films of these oxides can deflect superdiffusive charge currents injected from an optically excited metal thin film, and redirect the currents from a longitudinal to a transverse direction. The researchers found that the direct conversion of laser-excited, high-density, longitudinal charge currents to transverse currents led to efficient THz wave generation without the need for external fields.

The researchers determined that, out of several different metals, platinum (Pt) was the most suitable for fabricating the thin films needed for their approach. They fabricated Pt/RuO₂(101) and Pt/IrO₂(101) thin-film heterostructures and measured the structures’ THz amplitudes. The Ir-based system produced threefold-stronger signals, comparable to those generated by commercial THz sources based on nonlinear optical crystals and photoconductive switches.

Existing methods for generating THz radiation, including the inverse spin-Hall effect (ISHE), the inverse Rashba-Edelstein effect, and the inverse spin-orbit-torque effect, convert longitudinally injected, spin-polarized currents from magnetic materials to transverse charge currents in order to produce THz waves. These relativistic methods rely on external magnetic fields and can experience low spin-polarization rates and relativistic spin-to-charge conversion efficiencies, characterized by spin-Hall angle.

In contrast to traditional approaches to generating THz waves, the nonrelativistic, nonmagnetic method capitalizes on the inherent properties of conductive materials, eliminating the need for spin-polarization. In addition, the nonrelativistic, nonmagnetic approach offers a high THz conversion efficiency, comparable to that of ISHE.

The researchers said that the use of readily available conductive materials with highly anisotropic electrical conductivity was key to enhancing conversion efficiency using their technique.

The new, nonrelativistic, nonmagnetic approach could provide more flexibility and scalability than existing techniques, which are faced with the challenge of increasing the spin-Hall angle of heavy-metal materials. The efficient generation of THz waves, achieved by harnessing the potential of high-density charge currents across metallic interfaces, could lead to advancements in solar cell technology, artificial photosynthesis, and high-efficiency optoelectronic devices.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.5.5.056006).

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