Optimizing spin-based terahertz emission from magnetic heterostructures

TL;DR

This work uses a semiclassical superdiffusive spin-transport model, incorporating energy-dependent ISHE and REE, to analyze THz emission from FM/NM spintronic bilayers and trilayers. It validates the model against Co$(2\,\text{nm})$/Pt$(4\,\text{nm})$ experiments and then conducts a systematic parametric study of pulse width, layer thicknesses, and interface/boundary properties, establishing explicit design rules. Key findings include optimum thicknesses ($L_{\text{FM}}\approx 2-3$~nm, $L_{\text{NM}}\approx 5-6$~nm) for large bandwidth, the importance of asymmetric interface transmission to trap spin current in the NM layer, and the potential of a double-pulse trilayer to widen the THz spectrum. These insights offer practical guidelines for tailoring high-bandwidth, high-amplitude spintronic THz emitters for applications in imaging, telecommunications, and spectroscopy, and propose experimental paths to further validate the governing mechanisms.

Abstract

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.

Figures (12)

• Figure 1: Pictorial representation of a spin-based THz emitter. The fs laser pulse excites the ferromagnetic (FM) layer, reducing the magnetization and generating a longitudinal spin current, which is converted to a charge current in the nonmagnetic (NM) layer via the inverse spin Hall effect. The resulting time-varying transverse charge current is the source for the THz pulse emission.
• Figure 2: Comparison of our modeling (red) and experiment (blue) for the THz amplitude spectrum of a Co(2 nm)/Pt(4 nm) emitter. The response function of the ZnTe detector is taken into account and is responsible for the dip at 5 THz. The THz emission amplitude $E$ is normalized to its maximum value. The black-dashed line shows the theoretical emission accounting for a 15$^{\circ}$-tilt of the parabolic detection mirror with respect to the propagation direction of the THz radiation.
• Figure 3: Results of THz emission calculations. (a) THz emission spectrum of a Co(2)/Pt(4) bilayer computed for different pump durations. (b) Computed THz bandwidth as a function of the femtosecond pump pulse duration.
• Figure 4: (a) Schematic representation of the variation of layer thicknesses of the spintronic THz emitter. The thicknesses ${\text{L}}_{\text{FM}}$ of the FM and ${\text{L}}_{\text{NM}}$ of the NM layer are varied independently to study their influence on the THz properties of the system. (b) Simulated THz bandwidth and (c) peak frequency of a Co($\mathrm{L_{FM}}$)/Pt($\mathrm{L_{NM}}$) emitter as a function of the ferromagnetic and nonmagnetic layer thicknesses, $\mathrm {L_{FM}}$ and $\mathrm {L_{NM}}$, respectively. The grid in the peak frequency plot has been added for better visualization.
• Figure 5: (a) Schematic illustration of the interface transmission properties of the spintronic emitter. (b) Simulated signal bandwidth, (c) peak frequency, and (d) total integrated THz power as a function of the interface reflection coefficients for left- and right-moving electrons, $\mathrm {R_{Left}}$ and $\mathrm {R_{Right}}$, respectively. The signals are calculated for a Co(2)/Pt(4) bilayer.