Optimizing spin-based terahertz emission from magnetic heterostructures

Terahertz radiation pulses can be generated efficiently through femtosecond laser excitation of a ferromagnetic/nonmagnetic heterostructure, wherein an ultrafast laser-induced spin current results in an electromagnetic THz pulse due to spin-charge conversion. It is, however, still poorly understood how the THz emission amplitude and its bandwidth can be optimized. Here, we perform a systematic analysis of the THz emission from various magnetic heterostructures. The dynamics of the spin current is described by the semiclassical, superdiffusive spin-transport model and the energy dependence of the spin Hall effect of hot electrons is taken into account, leading to emission profiles for Co(2 nm)/Pt(4 nm) bilayer in good agreement with experiment. To identify the optimal {conditions} for THz emission, {we study} the properties of the emitted THz wave profile by systematically varying the layer thicknesses of metallic bilayers, their interfacial spin-current transmission properties, their materials’ dependence, and influence of the pump laser-pulse width, allowing us to give optimization guidelines. We find that thin nonmagnetic layer thicknesses of 5-6 nm provide the largest bandwidth in the case of Co/Pt and that the peak frequency of the THz emission depends only on the geometry of the emitter and not on the laser pulse width. The THz bandwidth {is conversely found to} depend on several factors such as exciting laser pulse width, layers’ thicknesses, and interface transmission-reflection properties, with the limitation that an increase in the bandwidth by tuning the interface properties comes with a trade-off in the energy efficiency of the emitter. Lastly, we propose a double pulse excitation protocol of a trilayer system that could provide broadband THz emission with a large bandwidth. {Our results contribute to establishing guidelines for optimizing spintronic THz generation.

💡 Research Summary

The paper presents a comprehensive theoretical and experimental study of spin‑tronic terahertz (THz) emitters based on ferromagnet/non‑magnet (FM/NM) heterostructures. When a femtosecond laser pulse excites the FM layer, hot spin‑polarized electrons are generated and diffuse super‑diffusively toward the NM layer. There, the inverse spin Hall effect (ISHE) converts the longitudinal spin current into a transverse charge current, which radiates a THz pulse. The authors employ a semiclassical super‑diffusive spin‑transport model that resolves spin‑dependent electron velocities and lifetimes as functions of energy and depth, and they incorporate the energy‑dependent spin Hall angle of the NM material (primarily Pt). This framework allows them to calculate the time‑dependent charge current and, via a frequency‑domain proportionality that accounts for the parabolic mirror used in detection, the emitted THz electric field.

First, the model is benchmarked against experimental data for a Co(2 nm)/Pt(4 nm) bilayer excited by a 23 fs, 1030 nm laser pulse. By including realistic interface transmission/reflection coefficients (derived from first‑principles data for similar 3d metals) and the detector response of a 20 µm ZnTe crystal, the simulated THz spectrum reproduces the measured peak frequency and bandwidth, confirming the validity of the approach.

Having validated the model, the authors systematically vary key parameters to identify optimal design rules:

1. Non‑magnetic layer thickness – The THz bandwidth is maximized for Pt thicknesses of 5–6 nm. At this thickness the charge current changes most abruptly, enhancing high‑frequency components. Thinner Pt reduces the overall current amplitude, while thicker Pt broadens the temporal spread and narrows the spectrum.

2. Ferromagnetic layer thickness – Varying Co thickness from 1 to 3 nm has a minor effect on the emitted spectrum; the dominant factors are the NM thickness and interface properties.

3. Laser pulse duration – Shorter pump pulses (≤30 fs) increase the high‑frequency content because the initial ballistic transport of hot electrons is less smeared, but they also reduce the total charge‑current amplitude, leading to a trade‑off between peak field strength and bandwidth. Importantly, the peak frequency (ν_max) is essentially independent of pulse width and is set by the geometric dimensions of the emitter.

4. Interface transmission/reflection – Increasing the spin‑current transmission at the FM/NM interface boosts the overall THz amplitude, whereas enhancing reflection (i.e., reducing transmission) sharpens the current pulse, thereby widening the bandwidth. However, the bandwidth gain comes at the cost of reduced efficiency, establishing a clear trade‑off.

5. External boundaries – The reflectivity of the capping layer and substrate also influences the temporal shape of the charge current and thus the spectral width, though to a lesser extent than the internal FM/NM interface.

6. Double‑pulse excitation of a trilayer – The authors propose a novel configuration: an FM/NM/FM trilayer where both FM layers are excited by two temporally separated laser pulses (delay ≈100 fs). The resulting spin currents interfere within the NM layer, producing a charge‑current waveform that is a superposition of two shifted pulses. Simulations show that this interference can broaden the THz spectrum beyond 3 THz while maintaining a comparable field amplitude to the single‑pulse bilayer, effectively overcoming the bandwidth‑efficiency trade‑off inherent to simple bilayers.

From these investigations the authors distill practical guidelines for designing high‑performance spin‑tronic THz emitters: select a NM thickness of ~5 nm for maximal bandwidth, tune interface transmission to balance amplitude versus spectral width, use sub‑30 fs pump pulses when high‑frequency content is desired, and consider double‑pulse excitation of trilayer stacks for broadband applications. The work not only validates the super‑diffusive transport model with energy‑dependent ISHE but also provides a roadmap for engineering spintronic THz sources that can meet the demanding requirements of spectroscopy, imaging, and next‑generation wireless technologies.