Spintronic THz Emitters

• Spintronic THz emitters are ultrafast devices that use spin-to-charge conversion in ferromagnet/heavy-metal multilayers to produce gapless THz emission spanning 1–30 THz.
• They leverage mechanisms like the inverse spin Hall effect and engineered stack architectures (e.g., W/CoFeB/Pt) with photonic enhancements to achieve efficient amplitude scaling and polarization control.
• Theoretical models based on superdiffusive spin transport and impedance matching guide optimization efforts to improve performance in spectroscopy, imaging, and ultrafast photonics.

Spintronic terahertz (THz) emitters are a class of ultrafast photonic devices that generate broadband, single-cycle THz radiation via spin-to-charge conversion in magnetic metal heterostructures. These sources exploit ultrafast spin current generation and the inverse spin Hall effect (ISHE) in ferromagnet/heavy-metal multilayers, offering high flexibility in spectral coverage, polarization control, and device integration. Spintronic THz emitters have enabled gapless 1–30 THz coverage, efficient amplitude scaling, polarization tunability, and have become a foundational platform for spectroscopy, imaging, and nonlinear THz optics (Seifert et al., 2015, Seifert et al., 2021, Wu et al., 2018, Torosyan et al., 2017).

1. Ultrafast Spintronic Emission Mechanism

Spintronic THz emitters operate by rapid energy transduction across several physical channels:

• Photo-induced spin current: Upon femtosecond laser irradiation (typically τ_p < 100 fs, λ=800 nm, fluence 0.01–1 mJ/cm²), hot electrons in a nanometer-thick ferromagnetic (FM) layer (e.g., CoFeB, Fe, Co) are excited and undergo ultrafast demagnetization. This establishes a non-equilibrium spin population, resulting in a superdiffusive spin current density $J_s(t)$ flowing perpendicular to the film, typically along the z-direction (Seifert et al., 2015, Seifert et al., 2021):

$J_s(t) = \int G(t-t') \cdot P_s(t')\,dt'$

where $P_s(t')$ is the instantaneous spin polarization and $G$ is a material-dependent response kernel.

• ISHE-based spin-to-charge conversion: When $J_s(t)$ reaches adjacent heavy-metal (NM) layers (Pt, W), strong spin–orbit coupling facilitates conversion into a transverse charge current $J_c$:

$J_c = \theta_{SH} (J_s \times M/|M|)$

with $\theta_{SH}$ the spin Hall angle (opposite sign for Pt and W), and $M$ the FM magnetization vector (Wu et al., 2018, Seifert et al., 2021, Foggetti et al., 21 Nov 2024).

• Electromagnetic emission: The transient in-plane current $J_c(t)$ exhibits sub-picosecond time derivatives, radiating a broadband THz field due to Maxwell's equations:

$E_{THz}(t) \propto \partial_t J_c(t)$

Emission is gapless, with experimentally demonstrated bandwidths spanning 1–30 THz (Seifert et al., 2015, Foggetti et al., 21 Nov 2024).

2. Device Architectures, Materials, and Photonic Enhancement

The canonical spintronic THz emitter consists of a nanolaminate stack, with common architectures including:

Stack	Typical Thickness (nm)	Purpose
W/CoFeB/Pt	W(2)/CoFeB(1.8)/Pt(2)	Maximized ISHE (large ±θ_SH), hot-electron injection (Seifert et al., 2015, Wu et al., 2018)
Fe/Pt	Fe(2)/Pt(3)	Epitaxial, thickness-optimized (Torosyan et al., 2017)
[Co/Pt]_n multi	Co(2)/Pt(2), n=1–4	Stacked interfaces for field enhancement (Hawecker et al., 2022)
W/FeCo/TbCo2/Pt	bias-free (exchange-biased)	Zero-field operation (Paries et al., 2023, Yang et al., 26 Aug 2024)

Further enhancements are obtained by dielectric/photonic engineering:

• Fabry–Pérot resonance: Due to total metal thickness $d \ll \lambda_{pump}$ and $\lambda_{THz}$, the stack acts as a broadband microcavity, providing constructive interference for both pump and THz field (Seifert et al., 2015).
• 1D photonic crystals / Bragg mirrors: Multi-period SiO₂/Si₃N₄ or HfO₂/SiO₂ distributed Bragg reflectors can trap pump photons, boosting absorptance in the FM to ≈95% and leading to >2× enhancement of emitted THz amplitude (Koleják et al., 8 Feb 2024, Feng et al., 2018, Yang et al., 26 Aug 2024).
• Plasmonic nanoparticle decoration: Drop-cast Au@SiO₂ core–shell nanoparticles yield local near-field enhancement, increasing emission up to ≈2.5× in pulse energy (Cecconi et al., 2 Dec 2025).
• Multilayer stacking: Periodic stacking ($n=2$–3) of FM/NM or NM/FM/NM with interfacial engineering (e.g., AuW-capped spin sinks) further enhances both spin current generation and suppression of spin backflow (Hawecker et al., 2022).

3. Polarization and Vectorial Control

Full vector control of the emitted THz field has been achieved through several approaches:

• Magnetic-field tuning: By engineering the static magnetic field or using nonuniform/twisted distributions, spintronic emitters allow independent tuning of chirality, azimuthal angle, and ellipticity of the THz polarization, enabling dynamic switching of polarization states (Wu et al., 2018).
• Remanent magnetization patterns: Lithographically patterned structures (micropatterned "chopped disks", metasurfaces) allow programmable on–off switching and arbitrary rotation of the polarization axis (Wu et al., 2022, Liu et al., 2021).
• Exchange-biased programmable emitters: Laser-assisted field-cooling of exchange-biased FM/AFM/HM trilayers enables programmable generation of structured beams—such as spatially separated circular polarizations, azimuthal, radial, and full Poincaré fields—by spatially varying the local magnetization direction $M(\mathbf{r})$ (Wang et al., 2023).
• Metasurface and cavity engineering: Patterned metasurfaces and integrated waveguides within the metal stack produce devices with broadband chiral and vectorial THz control, tunable via the angle and magnitude of $M$ (Liu et al., 2021).

4. Theoretical Models and Device Optimization

Quantitative modeling of spintronic THz emitters requires the simultaneous solution of spin generation, diffusion, and electromagnetics:

• Superdiffusive spin transport (Battiato et al., (Seifert et al., 2015, Foggetti et al., 21 Nov 2024, Yang et al., 2022)): Hot-electron superdiffusion is described by energy- and position-dependent transport equations, giving:

$$j_s(z, \omega) = j_s(d_{FM}, \omega) \frac{\sinh[(z - d_{FM})/2\lambda_{rel}]}{\sinh[d_{NM}/2\lambda_{rel}]}$$

• ISHE scaling and impedance matching: The emitted field amplitude depends on the conversion factor $\theta_{SH}$, velocity randomization length $\lambda_{rel}$, and photonic parameters (impedance $Z(\omega)$, dielectric environment). Optimum emission occurs for $d_{NM} \sim 2\lambda_{sf}$ (Pt: $\lambda_{sf} \sim$ 1.3–2 nm) (Seifert et al., 2015, Torosyan et al., 2017, Hawecker et al., 2022).
• Secondary spin currents: For thicker NM layers, secondary spin currents (from nonmagnetic-to-FM energy transfer) become significant and can even dominate, necessitating model terms proportional to the energy-diffusion length and pumped NM absorption (Agarwal et al., 2022).
• Two-temperature and thermal models: Device performance at high repetition rates is limited by electron–phonon equilibration and in-plane heat diffusion; for 6 nm stacks, a cooling time of ∼500 ps sets a GHz-scale thermal limit for damage-free operation at high pump fluence (Selz et al., 19 May 2025, Vaitsi et al., 25 Apr 2024).

5. Performance Benchmarks and Applications

Spintronic THz emitters exhibit the following experimentally validated figures of merit:

Metric	Typical Value / Range	Contexts/Comments
Bandwidth	1–30 THz (gapless)	Limited by pump-pulse, substrate phonon bands (Seifert et al., 2015, Seifert et al., 2021)
Peak amplitude (tight focus)	250 V/cm (few mm²), >1 MV/cm (scalable)	With photonic crystal, large area, or rotating emitters (Yang et al., 26 Aug 2024, Seifert et al., 2015, Vaitsi et al., 25 Apr 2024)
Conversion efficiency	Up to ~25% of ZnTe under identical fluence	With optimized stacking; further enhancements via photonic/ plasmonic integration (Wu et al., 2018, Cecconi et al., 2 Dec 2025, Koleják et al., 8 Feb 2024)
Polarization control	Chirality, azimuth, ellipticity, vector beams	Magnetic-vector engineering, microstructuring, exchange-bias patterning (Wu et al., 2018, Wang et al., 2023)
Dynamic range (TDS)	>80 dB	Cavity-enhanced, optimized substrate (Koleják et al., 8 Feb 2024)
Operation rate (MHz)	Up to 2 MHz (rotating, high-power)	μs-level stability at high rep rates (Vaitsi et al., 25 Apr 2024)

Applications include broadband time-domain THz spectroscopy, polarization-resolved ellipsometry, nonlinear pump–probe studies, THz imaging, ultrafast spin dynamics, structured beam (vectorial) generation, and programmable on-chip THz active devices.

6. Routes to Optimization and Advanced Concepts

Current research has identified diverse pathways for further improving spintronic THz emitter performance:

• Material engineering: Use of alloys (e.g., Pt$_{75}$Au$_{25}$ with $\theta_{SH}\sim0.2$–$0.25$), topological metals, Heusler semimetals for higher $\theta_{SH}$ and spin polarization (Shi et al., 29 Sep 2025, Hawecker et al., 2022).
• Stack optimization: Multiperiod stacking, dual-sign trilayers (e.g., W/FM/Pt), NM capping with spin sinks (e.g., AuW) to suppress backflow and improve efficiency (Hawecker et al., 2022, Feng et al., 2018).
• Photonic integration: 1D/2D photonic crystals, Bragg mirrors, or cavity–defect structures matched to the pump, providing >2× field enhancement and >5× increase in THz energy (Koleják et al., 8 Feb 2024, Feng et al., 2018, Yang et al., 26 Aug 2024).
• Plasmonic/metasurface hybridization: Core–shell nanoparticles, stripe metasurfaces, and programmable magnetic microtexturing for local field control, polarization tunability, and near-field engineering (Cecconi et al., 2 Dec 2025, Liu et al., 2021, Wang et al., 2023).
• Fiber-tip and on-chip integration: Fully-fiberized emitters for robust alignment-free applications, near-field THz imaging with ∼30 μm resolution (Paries et al., 2023).
• Thermal management: Rotating large-area emitters enable high average pump power (>18 W), >10 kV/cm peak field at MHz rates, outperforming nonlinear crystal sources in thermal robustness and bandwidth (Vaitsi et al., 25 Apr 2024, Selz et al., 19 May 2025).

7. Outlook and Research Directions

Spintronic THz emitters constitute a rapidly evolving technological platform with several outstanding challenges and opportunities (Seifert et al., 2021, Seifert et al., 2015):

• Increasing efficiency: Pushing optical-to-THz conversion toward $10^{-4}$ via further material, photonic, and interface engineering (Koleják et al., 8 Feb 2024).
• Advanced polarization/beam control: Implementation of real-time, fully programmable vector and structured THz fields for applications in communication, microscopy, and quantum information (Wang et al., 2023).
• On-chip integration: Wafer-scale emitters, THz-functional photonic circuits, and reconfigurable metasurfaces.
• Ultrafast dynamical studies: Using emitted polarization and structured pulses to probe elementary excitations (rotational, vibrational, spin) in matter (Wu et al., 2018).
• Thermal and mechanical robustness: Raising damage thresholds and lifetime under continuous operation by substrate and geometry optimization (Vaitsi et al., 25 Apr 2024, Selz et al., 19 May 2025).

These devices presently offer unique advantages in terms of area scaling, emission bandwidth, polarization programmability, and device integration, positioning spintronic THz emitters as leading candidates for next-generation ultrafast photonic systems (Seifert et al., 2015, Seifert et al., 2021, Yang et al., 26 Aug 2024).