As far as basic studies and practical applications are concerned, special benefits are offered by terahertz radiation, which falls between infrared and microwave radiation in the electromagnetic spectrum.
Schematic of the generation of chiral terahertz waveforms from a stripe-patterned spintronic-metasurface terahertz emitter under an external magnetic field. Image credit: Liu et al., doi.org/10.1117/1.AP.3.5.056002.
The potential to produce and manipulate broadband chiral terahertz waves is crucial for applications in medical diagnosis, terahertz sensing, and material imaging. Also, it can offer new chances for nonlinear terahertz spectroscopy, as well as coherent control of chiral molecules and magnetic materials, which could finally allow new drug development and quick data storage.
Generating and regulating circularly polarized coherent terahertz waves can be done in a range of ways. There are three types of methods presently in use. The first technique is direct generation from gas plasmas, by employing external fields or an integrated two-color laser scheme. The second is the unique frequency-conversion methods in nonlinear crystals and magnetic and novel topological materials.
The third is the implementation of passive optical components. Generally, these techniques suffer from low efficiency, poor flexibility, or narrow bandwidth. A high-performance, flexible, and economical solution for producing and manipulating broadband, circularly polarized, and coherent waves in the terahertz regime is sought after.
In recent times, novel spintronic terahertz emitters have gained attention as affordable, extremely reliable, efficient, and flexible options. Such emitters are made of magnetic multilayer heterostructures, which are just a few nanometers thick. On applying an external magnetic field and laser illumination, a longitudinal spin flow is excited in a ferromagnetic material layer.
Furthermore, as a result of powerful interaction that takes place between spins and orbitals in a nonmagnetic layer, the spin flow is transformed into a transverse charge current, which provides an increase to coherent terahertz-wave radiation. The fixed micro-nano fabrication of thin metallic films enables such spintronic emitters to be processed into metasurfaces, which uncovers the great potential for applications.
As published in the Advanced Photonics journal, scientists from Fudan University have recently suggested and designed a novel spintronic-metasurface terahertz emitter that enables the generation and manipulation of chiral terahertz waves in a highly flexible and efficient way.
The emitter is made up of alternating magnetic heterostructures fixed in multilayer stripes. The terahertz radiation is produced by exciting the emitter with laser pulses under an oriented external magnetic field. The transverse anisotropic confinement of the laser-induced charge currents enforced by the metasurface structure results in chiral terahertz-wave emission.
The scientists illustrated that an outstanding circular polarization (ε > 0.75) can be realized more than a broad terahertz bandwidth ranging from 1 to 5 THz. This exhibits generation efficiency similar to nonlinear crystals available on the market. Also, the design enables flexible manipulation of the terahertz polarization state and helicity with the magnetic field.
The hybrid terahertz emitters integrate the benefits of spintronic emitters (ultra-broadband, highly flexible, and efficient) with those of metasurfaces, for strong control over the polarization state of emitted terahertz waves on an ultra-compact platform.
This work opens a new pathway to metasurface-tailored spintronic emitters for efficient generation and control of terahertz waves. The combination of ultrabroadband, efficient spintronic emitters, and metasurfaces with predesigned functionality, could lead to many more types of emitting devices for different spatial and temporal terahertz waveforms.
Zhensheng Tao, State Key Laboratory of Surface Physics, Fudan University
Ultimately, Tao observed that this technological advance might result in arbitrary field design of a terahertz wave with subcycle accuracy in both time and space.
Journal Reference:
Liu, C., et al. (2021) Active spintronic-metasurface terahertz emitters with tunable chirality. Advanced Photonics. doi.org/10.1117/1.AP.3.5.056002.