Ultrafast Spin-to-Charge Conversion

• Ultrafast spin-to-charge conversion is a process that converts spin currents into charge signals in sub-picosecond timescales using spin–orbit coupling in advanced heterostructures.
• Experimental methods like THz emission spectroscopy and FMR spin-pumping reveal SCC’s potential for ultrafast devices with bandwidths reaching up to 20 THz.
• Innovations in material platforms and interface engineering, supported by multiscale simulations, drive improvements that enable next-generation THz emitters and high-speed memory applications.

Ultrafast spin-to-charge conversion (SCC) refers to the transduction of spin currents or spin accumulations into charge currents on sub-picosecond to picosecond timescales, typically triggered by femtosecond laser photoexcitation in condensed matter heterostructures. This phenomenon underpins a core functionality of ultrafast spintronics: enabling all-optical or all-electrical detection, manipulation, and emission of spin information at terahertz (THz) frequencies. The physical mechanisms for SCC, including the inverse spin Hall effect (ISHE), inverse Rashba–Edelstein effect (IREE), and related interfacial or bulk spin–orbit driven processes, have been explored in a broad range of material systems from transition metal bilayers to topological insulators, noncollinear antiferromagnets, and Weyl semimetals.

1. Physical Principles and Primary Mechanisms

The essential mechanism of ultrafast SCC is the conversion, within hundreds of femtoseconds, of a transient spin current—typically injected from a photoexcited ferromagnet—into a transverse charge current via spin–orbit coupling. Two dominant paradigms are realized in experiment:

• Bulk Inverse Spin Hall Effect (ISHE):

$$\mathbf{j}_{c}^{\rm ISHE} = \theta_{\rm SH} \frac{2e}{\hbar}(\mathbf{j}_s \times \hat{\sigma})$$

where $\theta_{\rm SH}$ is the spin Hall angle, $\mathbf{j}_s$ the spin current density, and $\hat\sigma$ the polarization direction (Kampfrath et al., 2012, Dang et al., 2020). The bulk ISHE enables conversion in heavy metals, some antiferromagnets, metallic oxides, TMDCs, and Weyl systems.

• Interfacial Inverse Rashba–Edelstein Effect (IREE):

$$\mathbf{j}_c^{\rm IREE} = \frac{e \alpha_R \tau}{\hbar} \mathbf{s}$$

where $\alpha_R$ is the Rashba parameter, $\tau$ the momentum scattering time, and $\mathbf{s}$ the interfacial spin density (Rongione et al., 2023, Han et al., 20 Jul 2025). The IREE dominates in 2D systems with strong interfacial spin–orbit coupling, such as topological insulator surface states.

Ultrafast SCC is fundamentally distinct from steady-state transport SCC: the conversion occurs at unprecedented speed (sub-ps) and leads directly to THz transient emission or detection (Wu et al., 2018, Wang et al., 2018).

2. Material Platforms and Heterostructures

The rapid development of ultrafast SCC draws extensively on advances in material synthesis and interface engineering. Table 1 summarizes key material classes and their performance metrics.

Material / Structure	Dominant SCC Mechanism	Spin Hall Angle / Efficiency	Bandwidth/Time Scale
Fe/Au, Fe/Ru bilayers	ISHE	±1e-3 (Fe)	Up to 20 THz / 20–300 fs (Kampfrath et al., 2012)
CoFeB/Pt, CoFeB/W	ISHE (Pt), ISHE/iSOT(W)	Up to 0.1	<100 fs to 3 THz (Wu et al., 2018)
Bi₂Se₃, SnBi₂Te₄/Co (TIs)	IREE	λ_IEE ≈ 0.05 nm	0.1–8 THz / τ_SCC ≈ 120 fs (Wang et al., 2018, Rongione et al., 2022, Han et al., 20 Jul 2025)
NbSe₂, NbP (TMDCs, WSMs)	ISHE	–0.2%…–1.1%	Bandwidth up to 8 THz (Nádvorník et al., 2022, Han et al., 20 Jul 2025)
Py/Mn₃Pt (noncollinear AFMs)	ISHE/MSHE	θ_SH ~0.05–0.1 (Mn₃X series)	<80–100 ps (GHz–THz regime) (Sinha et al., 28 Aug 2025)
Py/L1₂–Mn₃Ir	ISHE	θ_SH = 0.035	Flat to 2.2 THz (Mao et al., 2023)
RuO₂ (altermagnet)	Anisotropic ISHE/ISSE	θ_SH ≈ 2.4e-3	THz, anisotropic effects (Jechumtál et al., 15 Aug 2025)

High-quality interfaces, careful doping/compositional control, and in-situ layer-by-layer growth are essential to maximize SCC efficiency and bandwidth, particularly in TIs and WSMs, where surface and bulk contributions can compete (Han et al., 20 Jul 2025).

3. Experimental Methodologies and Detection

Ultrafast SCC is predominantly studied with the following experimental frameworks:

• Time-Domain Terahertz Emission Spectroscopy (THz-TDS): An ultrafast optical pump pulse excites a ferromagnet or spin-source layer, launching a femtosecond spin current into the adjacent conversion layer. The SCC process transduces this to an in-plane ultrafast charge current, radiating a broadband THz pulse. Electro-optic sampling in ZnTe or GaP crystals measures the emitted $E_{\rm THz}(t)$, directly probing SCC and enabling extraction of conversion coefficients via Fourier analysis (Kampfrath et al., 2012, Dang et al., 2020, Wang et al., 2018, Rongione et al., 2022, Han et al., 20 Jul 2025).
• Ferromagnetic Resonance Spin-Pumping (FMR-SP) with ISHE Detection: Continuous-wave microwave excitation induces spin pumping in the FM, and the SCC-induced voltage (dc or time-resolved) is measured across the conversion layer (Dang et al., 2020, Sinha et al., 28 Aug 2025).
• All-Electric and Nonlinear Schemes: In semiconductor nanostructures with strong Rashba or Dresselhaus spin–orbit coupling, ultrafast spin-to-charge conversion can be achieved all-electrically, with quantum point contacts acting as nonlinear filters for spin accumulation (Pawłowski et al., 2018, Marcellina et al., 2019).

Common analysis strategies distinguish SCC-induced signals from optical rectification, shift currents, or other nonlinearities by leveraging the symmetry of the field (odd/even in magnetization), and controlling the angle of magnetization and light incidence.

4. Quantitative Performance and Limiting Factors

Critical figures of merit for ultrafast SCC include the spin Hall angle ($\theta_{\rm SH}$), interfacial conversion length (e.g., $\theta_{\rm SH}$0), spin diffusion length ($\theta_{\rm SH}$1), time resolution/bandwidth, and device-level charge current density $\theta_{\rm SH}$2.

• Spin Hall angle and SCC efficiency: Pt and related heavy metals exhibit $\theta_{\rm SH}$3; antiferromagnets such as L1₂–Mn₃Ir reach $\theta_{\rm SH}$4; TMDCs like NbSe₂ range from –0.2% to –1.1% (Nádvorník et al., 2022, Mao et al., 2023, Dang et al., 2020).
• Temporal response: SCC processes typically complete within 0.1–1 ps, enabling THz bandwidths up to 20 THz (Kampfrath et al., 2012, Wang et al., 2018).
• Efficiency limiting factors: High SCC efficiency requires (1) large $\theta_{\rm SH}$5 or $\theta_{\rm SH}$6, (2) long $\theta_{\rm SH}$7, (3) high spin-mixing conductance $\theta_{\rm SH}$8 at the interface, and (4) clean, atomically sharp interfaces. Increased resistivity, reduced spin-diffusion lengths, or lowered $\theta_{\rm SH}$9 (due to defects or oxidation) suppress amplitude and bandwidth (Dang et al., 2020, Han et al., 20 Jul 2025).

5. Ultrafast SCC in Advanced Materials: Topological and Quantum Systems

Topological insulators (e.g., Bi₂Se₃, Bi₁₋ₓSbₓ, SnBi₂Te₄), Weyl semimetals (e.g., NbP), and metallic TMDCs (e.g., NbSe₂) support ultrafast SCC dominated by surface or bulk effects as determined by Fermi level tuning and interface quality.

• TI/FM bilayers: Spin injection into spin–momentum locked surface states yields sub-ps IREE with $\mathbf{j}_s$0 in the 0.05–1 nm range; conversion is robust and nearly temperature-independent, with device-level efficiency comparable to, or exceeding, heavy-metal analogs (Wang et al., 2018, Rongione et al., 2022, Rongione et al., 2023).
• Weyl/TMDC systems: Berry curvature and intrinsic SOC facilitate bulk ISHE, with efficiency determined both by bulk properties and spin transparency at interfaces (Han et al., 20 Jul 2025, Nádvorník et al., 2022).
• Noncollinear antiferromagnets and altermagnets: Materials like Mn₃Pt, Mn₃Ir, and RuO₂ exhibit ISHE (and in some cases magnetic spin Hall effect or anisotropic SCC) with large $\mathbf{j}_s$1, fast dynamics, and prospects for ultradense integration due to negligible stray fields (Sinha et al., 28 Aug 2025, Mao et al., 2023, Jechumtál et al., 15 Aug 2025).

6. Device Implications, Applications, and Prospective Directions

Ultrafast SCC establishes the foundation for fully optically driven or ultrafast electrically read-out spintronic devices, including:

• THz spintronic emitters and detectors: Broadband (>8 THz) radiation for ultrafast spectroscopy and on-chip communications (Han et al., 20 Jul 2025, Wu et al., 2018).
• High-speed logic and memory: Picosecond SCC enables terahertz-bandwidth nonvolatile devices exploiting all-optical or voltage-driven switching (Wu et al., 2018, Sinha et al., 28 Aug 2025).
• Spin-photonic integration: Efficient SCC at buried interfaces or within atomically thin layers supports vision for CMOS-compatible, on-chip THz generation, and interconnects (Mao et al., 2023, Nádvorník et al., 2022).

Anticipated research frontiers include engineered multilayer architectures for tailored SCC spectra, exploitation of magnonic and phononic resonances for further bandwidth enhancement, and integration of TIs, WSMs, or NCAFM layers for quantum-coherent devices. Distinguishing mechanisms (ISHE vs. IREE) and optimizing relative contributions according to application (directionality, bandwidth, efficiency) remain key open challenges (Han et al., 20 Jul 2025).

7. Theoretical Modelling and Multiscale Simulations

Quantitative understanding of ultrafast SCC leverages multiscale frameworks:

• Spin-dependent Boltzmann and wave-diffusion models: Capture superdiffusive hot-electron transport and pulse shaping effects (Kampfrath et al., 2012).
• Tight-binding and nonequilibrium Green’s function methods: Simulate spin–charge pumping dynamics under femtosecond demagnetization and interface-specific phenomena, revealing both conventional and unconventional SCC pathways (e.g., direct charge pumping without SOC) (Varela-Manjarres et al., 2024).
• Ab initio interface transmission and spin-mixing conductance calculations: Predict $\mathbf{j}_s$2, interface transparency, and Fermi-surface-resolved IREE parameters, crucial for rational heterostructure design (Dang et al., 2020, Rongione et al., 2023).

Modeling approaches emphasize the role of Fermi surface topology, spin Berry curvature, interfacial spin-memory loss, and disorder, guiding the development of superior SCC materials and device geometries.

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In sum, ultrafast spin-to-charge conversion enables the direct transduction of spin information to charge signals at sub-picosecond timescales across a wide variety of materials and interfaces. Advances in optical, electrical, and material engineering, combined with predictive modeling, are catalyzing the realization of next-generation ultrafast spintronic and THz devices, with performance metrics now mainly limited by interface quality, underlying spin–orbit coupling strength, and device integration considerations (Kampfrath et al., 2012, Wang et al., 2018, Dang et al., 2020, Han et al., 20 Jul 2025, Rongione et al., 2023, Varela-Manjarres et al., 2024, Sinha et al., 28 Aug 2025).