Ultrafast Spin-to-Charge Conversion

• Ultrafast spin-to-charge conversion is a process where rapidly varying spin currents are transformed into electrical charge currents via mechanisms such as ISHE and IREE.
• Time-resolved techniques like THz emission spectroscopy are used to probe sub-picosecond dynamics in engineered multilayer heterostructures.
• Optimized material systems, including heavy metals and topological insulators, demonstrate high conversion efficiency, promising advanced spintronic and memory applications.

Ultrafast spin-to-charge current conversion refers to a set of physical mechanisms whereby a rapidly varying spin current—often driven by optical, electrical, or magnetization dynamics on sub-picosecond timescales—is converted into an electrical charge current. This process underpins many emerging spintronic devices and is typically probed by time-resolved techniques such as terahertz (THz) emission spectroscopy. The conversion can proceed via several microscopic routes, including the inverse spin Hall effect (ISHE), the inverse Rashba–Edelstein effect (IREE), topological surface state physics, and symmetry-driven interfacial or bulk mechanisms in antiferromagnets and altermagnets. Recent work also highlights the rich anisotropy, material dependence, and ultrashort (femtosecond-scale) dynamics central to efficient, ultrafast spin-to-charge interconversion.

1. Physical Principles and Mechanisms

Spin-to-charge current conversion fundamentally relies on coupling mechanisms that transfer angular momentum between electron spin and orbital (charge) degrees of freedom. Prominent microscopic channels include:

• Inverse Spin Hall Effect (ISHE): When a spin current $\mathbf{j}_s$ is injected into a material with strong spin–orbit coupling (e.g., Pt, W, RuO₂), electrons with opposite spin orientations are deflected transversely in opposite directions, producing a charge current $\mathbf{j}_c$ described by

$$\mathbf{j}_c = \theta_{\mathrm{SH}} (\mathbf{j}_s \times \mathbf{\sigma}),$$

where $\theta_{\mathrm{SH}}$ is the spin Hall angle and $\mathbf{\sigma}$ the spin polarization direction (Kampfrath et al., 2012, Dang et al., 2020, Jechumtál et al., 15 Aug 2025).

• Inverse Rashba–Edelstein Effect (IREE): At interfaces with strong structural inversion asymmetry and significant Rashba spin–orbit coupling (e.g., metal/Bi₂O₃, graphene/Pt), a non-equilibrium spin accumulation $\mathbf{S}$ leads to a transverse charge current, with conversion efficiency parameterized by the Edelstein length $\lambda_{\mathrm{IREE}}$ (Karube et al., 2016, Anadón et al., 6 Jun 2024, Rongione et al., 2023).
• Surface States in Topological Insulators: Spin-momentum-locked Dirac surface states enable efficient and robust spin-to-charge conversion via IREE, often with sub-picosecond response and temperature insensitivity (Wang et al., 2018, Rongione et al., 2023).
• Altermagnetic and Antiferromagnetic Mechanisms: In altermagnets with symmetry-protected spin splitting and in antiferromagnetic materials with broken inversion symmetry, ultrafast spin dynamics (such as magnon excitation or Néel vector fluctuations) can directly generate charge currents via internal spin–orbit coupling fields, sometimes with characteristic anisotropy and high efficiency (Lai et al., 9 Jun 2025, Huang et al., 2023).

2. Experimental Realizations and Time Scales

Ultrafast spin-to-charge conversion is routinely generated and probed using femtosecond laser pulses interacting with engineered multilayer heterostructures:

• THz Emission Spectroscopy: A femtosecond optical excitation produces a transient spin current (e.g., via ultrafast demagnetization or superdiffusive transport) which, upon conversion to a charge current, emits an electromagnetic pulse in the THz range. The detection of this THz burst serves as a direct probe of spin-charge conversion efficiency and dynamics (Kampfrath et al., 2012, Dang et al., 2020, Nádvorník et al., 2022).
• Devices with Engineered Interfaces: The magnitude and anisotropy of spin-to-charge conversion are controlled by materials selection (e.g., 2D layers like graphene or NbSe₂, heavy metals like Pt, or topological insulators), interface quality, and inversion symmetry breaking. For instance, double-Rashba interfaces in Fe/graphene/Pt stacks afford thirty-fourfold enhancement and pronounced angle dependence in conversion (Anadón et al., 6 Jun 2024).
• Single-Electron and Mesoscopic Devices: In lateral semiconductor heterostructures or nanowires, energy-filtering (e.g., using quantum point contacts with gate tuning) and time-dependent Rashba interactions (e.g., by voltage pulsing gates) allow rapid, high-fidelity all-electrical spin-to-charge conversion, with potential for quantum information readout (Stano et al., 2010, Pawłowski et al., 2018, Marcellina et al., 2019).
• Ultimate Speeds and Limits: Ultrafast processes are not only robust (with efficiency in excess of 50% in optimized systems), but are also fundamentally limited by materials-specific response times—e.g., 0.12 ps for topological insulator surface states (Wang et al., 2018), ±100 fs in superdiffusive ferromagnet/semiconductor injection (Battiato et al., 2016), and even faster temporal resolutions mandated by the bandgap in 2D valleytronic materials (Gill et al., 4 Nov 2024).

3. Prototypical Systems and Symmetry Aspects

The efficiency and physical characteristics of ultrafast spin-to-charge conversion are closely linked to the electronic structure, interface hybridization, and symmetry:

Materials System	Dominant Mechanism	Key Parameter(s)
Pt, W, RuO₂	ISHE (sometimes anisotropic)	θ_SH, τ_sf, resistivity, T*
Bi₂Se₃, Bi₁₋ₓSbₓ TI films	IREE (surface, robust)	λ_IEE, SML, conversion speed
Fe/Gr/Pt, Py/Cu/Bi₂O₃, TMDC stacks	Double IREE / Rashba SOC	λ, α_R, interface asymmetry
Mn₂Au, altermagnetic RuO₂, KV₂O₂Se	NSOT reciprocity, ISSE, ISHE	θ', θ_SH, Fermi surf. geometry

Notably, altermagnetic materials such as KV₂O₂Se or RuO₂ present extreme anisotropy and near-maximum conversion efficiency due to flat Fermi surfaces and unconventional spin channel separation; under doping, theoretical upper bounds nearing 100% conversion may be achieved (Lai et al., 9 Jun 2025). In contrast, in RuO₂, the ISSE contribution is significantly smaller than the anisotropic ISHE at room temperature ($\theta' < 2\times 10^{-4}$), highlighting the pre-eminent role of conventional spin–orbit coupling (Jechumtál et al., 15 Aug 2025).

4. Theoretical Modelling and Quantitative Frameworks

Modelling ultrafast conversion necessitates consideration of:

• Spin and Charge Dynamics: Time-dependent nonequilibrium Green's function (TDNEGF) approaches capture charge and spin current pumping in response to time-dependent magnetization profiles, such as ultrafast demagnetization dynamics, which may generate both types of current even absent explicit spin–orbit coupling (Varela-Manjarres et al., 31 Mar 2024).
• Conversion Metrics: Conversion is often quantified via the spin Hall angle (θ_SH for ISHE), Edelstein length (λ_IEE for IREE), or generic conversion efficiency (CSE), distinguished by their physical origin and symmetry properties. For anisotropic Fermi surfaces in altermagnets, the CSE may reach theoretical maxima as flat spin-split Fermi surfaces are engineered (Lai et al., 9 Jun 2025).
• Role of Relaxation and Equilibration: THz emission spectra and current pulse shapes are determined not only by primary conversion efficiency but also by subsequent charge equilibration or backflow, modelled via local RC time constants. As shown in ultrafast THz emitter studies, these delay lines filter the frequency response and can complicate extraction of the intrinsic spin dynamics from the measured charge current profile (Schmidt et al., 2022).

5. Advanced Functionalities and Applications

Ultrafast spin-to-charge conversion directly informs the design of future opto-spintronics and spin logic devices:

• On-Chip Broadband THz Emitters: Owing to their ultrafast response, heterostructures such as Fe/Au, BiSb/Co, and hybrid TMDC systems act as efficient, spectrally tunable THz sources for communication and spectroscopy (Kampfrath et al., 2012, Nádvorník et al., 2022, Rongione et al., 2023).
• Antiferromagnetic Spintronics: Reciprocal Néel spin–orbit torque effects in Mn₂Au and related compounds allow direct optical generation of magnonic charge currents and self-emission of THz pulses at room temperature, potentially circumventing ferromagnetic control elements (Huang et al., 2023).
• Scalable, Energy-Efficient Memory: Efficient, robust, and directionally controlled conversion in 2D/3D hybrid quantum systems (as in Fe/Gr/Pt interfaces) and high CSE altermagnets enables low-power operation, high-density integration, and neuromorphic logic or unconventional computation (Anadón et al., 6 Jun 2024, Lai et al., 9 Jun 2025).
• All-Optical Control: Structured light (e.g., optical vortex beams) and Floquet-engineered synthetic interactions in multiferroics provide all-optical pathways for dynamic and selective generation of spin and valley currents, with applications in ultrafast logic and valleytronics (Sato et al., 2016, Wätzel et al., 2018, Gill et al., 4 Nov 2024).

6. Current Challenges and Future Directions

Outstanding issues in this field include:

• Decoupling various spin-to-charge conversion pathways, especially in materials exhibiting both strong conventional SOC (yielding ISHE) and exotic symmetry-induced effects (such as ISSE in altermagnets or surface state IREE in topological insulators) (Jechumtál et al., 15 Aug 2025, Rongione et al., 2023).
• Controlling interfacial properties—hybridization, atomic ordering, Rashba field asymmetry—for enhanced and anisotropic conversion, as shown for graphene-based systems (Anadón et al., 6 Jun 2024).
• Extension of all-optical or field-free methods for generation, manipulation, and readout of spin currents in both bulk and nanostructured devices, with timing resolution down to tens of femtoseconds (Wang et al., 2018, Gill et al., 4 Nov 2024, Battiato et al., 2016).
• Exploration of high-CSE altermagnets for robust, ultrafast, and lossless spin information processing at room temperature, including materials optimization via Fermi surface engineering (Lai et al., 9 Jun 2025).

7. Summary Table: Key Mechanisms and Material Systems

Conversion Type	Material System	Efficiency / Feature	Reference
ISHE	Pt, W, RuO₂, Au-based alloys	θ_SH ≈ 0.1–2.4×10⁻³	(Kampfrath et al., 2012, Jechumtál et al., 15 Aug 2025)
IREE	Bi₂Se₃, BiSb, Fe/Gr/Pt, Cu/Bi₂O₃	λ_IEE ~ 0.05–0.6 nm; 34× gain	(Wang et al., 2018, Anadón et al., 6 Jun 2024, Karube et al., 2016)
Reciprocal NSOT	Mn₂Au (antiferromagnet)	Room-temp. direct THz emitter	(Huang et al., 2023)
Altermagnetic effects	KV₂O₂Se, RuO₂	CSE up to ~98%, θ' < 2×10⁻⁴	(Lai et al., 9 Jun 2025, Jechumtál et al., 15 Aug 2025)
Surface state SCC	Topological insulators (Bi₂Se₃, BiSb)	Ultrafast <0.12 ps, robust	(Wang et al., 2018, Rongione et al., 2023)

Ultrafast spin-to-charge current conversion thus encompasses a family of highly dynamic, symmetry-dependent, and interface-driven processes uniquely suited for the realization of future spintronic, optospintronic, and quantum information technologies operating at sub-picosecond timescales.