Maximising the electromagnetic efficiency of spintronic terahertz transmitters
Spintronic terahertz (THz) emitters are devices that use spintronics to generate radiation in the terahertz frequency range (0.1 to 30 THz). These devices represent a promising alternative to conventional terahertz sources thanks to their simplicity, efficiency and compatibility with integrated systems.
In these devices, created by stacking layers of nanometric thickness, a spin current created in a ferromagnetic layer by excitation by a femtosecond laser pulse is injected into a non-magnetic material with a strong spin-orbit coupling, such as platinum or tungsten. The spin current is converted into an oscillating charge current via the inverse spin Hall effect, generating electromagnetic radiation in the terahertz range, as shown schematically (Figure a).
These emitters have several advantages: they are compact, robust, inexpensive and operate at room temperature. What’s more, their emission spectrum is very broad, with no absorption lines due to the phonons present in other sources. Spintronic terahertz transmitters have applications in a variety of fields, including spectroscopy, imaging, ultra-fast wireless communications and fundamental studies of spin dynamics.
On the other hand,the efficiency of optical conversion to terahertz is quite low, which is the main drawback preventing wider adoption of spintronic THz technology. It is typically of the order of 10-5-10-6 relative to the total input energy, which is at least one to two orders of magnitude lower than traditional optically pumped THz sources. STEs invariably require femtosecond lasers operating at average power levels of several watts. Improving their conversion efficiency is a critical step towards achieving competitiveness with conventional THz sources. In this context, researchers from the IEMN’s PHOTONIQUE-THz and AIMAN-FILMS groups have been working in collaboration with the Technical University of Ostrava (VSB) on improving conversion efficiency using photonic structures.
Fabry-Pérot integrated photonic cavities are composed of a stack of materials of different optical indices, such as silicon oxide and silicon nitride. In order to determine the thicknesses and number of stacks required, we have developed an electromagnetic theoretical framework that deals rigorously with the pulsed THz excitation and emission processes in spintronic emitters. This approach has enabled us to design an optimised photonic cavity that efficiently traps almost all the spectral power of the pump laser in the spin pumping magnetic layer. We opted for 1D 1D SiO2/SiNx distributed dielectric layer pairs (Figure b), as the resulting photonic crystal structure is only a few microns thick, which has minimal impact on the phase of the THz wavelengths generated and therefore hinders THz emission very little. The device was built in the IEMN’s CMNF platform on a sapphire substrate. The emitter is a tri-layer W(2nm)/FeCoB (1.8nm)/Pt(2nm). Compared with the most powerful spintronic THz emitters, the cavity improves the emitted THz field by 250% (figure c) and, consequently, increases the emitted power by 8 dB. In parallel, with researchers from the physics laboratory at ENS Paris, we are also working on a terahertz cavity that also amplifies the emitted signal. Currently under study, the combination of these two principles should enable spintronic transmitters to get closer to the symbolic milliWatt barrier.
Articles :
Koleják, P., Lezier, G., Vala, D., Mathmann, B., Halagačka, L., Gelnárová, Z., Dusch, Y., Lampin, J.-F., Tiercelin, N., Postava, K. and Vanwolleghem, M. (2024), “Maximizing the Electromagnetic Efficiency of Spintronic Terahertz Emitters”. Adv. Photonics Res., 5: 2400064.
https://doi.org/10.1002/adpr.202400064
Mičica, A. Wright, P. Koleják, G. Lezier, K. Postava, J. Hawecker, A. De Vetter, J. Tignon, J. Mangeney, H. Jaffres, R. Lebrun, N. Tiercelin, M. Vanwolleghem, and S. Dhillon, “Spintronic terahertz emitters with integrated metallic terahertz cavities”. Nanophotonics.
https://doi.org/10.1515/nanoph-2023-0807
Contact :
Mathias Vanwolleghem
Nicolas Tiercelin