Non-Thermal Drift Photocurrents

• Non-thermal drift photocurrents are bias-free electrical currents produced by nonlinear optical mechanisms such as the inverse Faraday effect, shift/injection, and photon-drag processes.
• They rely on structural inversion symmetry breaking and polarization selectivity, enabling ultrafast (femtosecond-scale) responses in metals, semiconductors, and topological materials.
• Experimental techniques like dual-modulation lock-in detection and ultrafast pump-THz spectroscopy help isolate these non-thermal currents from photothermal contributions for reliable nanocircuit applications.

Non-thermal drift photocurrents are ultrafast, bias-free electrical currents generated in solids under illumination, attributed to nonlinear, non-thermal mechanisms such as the inverse Faraday effect, shift/injection photogalvanic response, and photon-drag processes. They appear in systems that lack inversion symmetry, including metals, semiconductors, low-dimensional crystals, and topological materials, and are fundamentally distinct from photothermal or photoconductive currents by virtue of their symmetry dependence, polarization selectivity, and absence of a requirement for dissipative heat or external bias. These currents are central to the physics of all-optical nanocircuitry, bulk photovoltaic effects, ultrafast photodetection, and terahertz emission.

1. Physical Mechanisms and Theoretical Foundation

Non-thermal drift photocurrents are fundamentally tied to the nonlinear optical response of materials, appearing at second or higher order in the electromagnetic field. The principal mechanisms include:

(a) Inverse Faraday Effect (IFE)-Driven Currents in Metals

• An oscillating optical field $\mathbf{E}(\mathbf{r},t)$ induces a local magnetization via the IFE, given by

$$\mathbf{M}_{\rm IFE} \propto i\, \mathbf{E} \times \mathbf{E}^*,$$

which in a dispersive conductor with conductivity $\sigma(\omega)$ produces a ponderomotive drift current:

$$\mathbf{j}_{\rm drift} = \frac{e n}{\omega} \Re\{\sigma(\omega)\} \nabla \times \left(\mathbf{E} \times \mathbf{E}^*\right),$$

generating femtosecond-fast, polarization-locked photocurrents localized within the optical skin depth (Zapata et al., 1 Dec 2025, Mou et al., 1 Feb 2024).

(b) Shift and Injection Currents in Non-centrosymmetric Insulators/Semiconductors

• Second-order nonlinear optical response in non-centrosymmetric systems is described by the third-rank photogalvanic tensor $\sigma_{ijk}(\omega)$:

$$j_i(0) = \sigma_{ijk}(\omega) E_j(\omega) E_k^*(\omega).$$

The symmetric part gives the linear photogalvanic effect (LPGE, or shift current), and the antisymmetric part yields the circular photogalvanic effect (CPGE, or injection current) (Sirica et al., 2018, Hipolito et al., 2016, Ishizuka et al., 2017):

$$j_i^{CPGE} = i \gamma_{ij}(E \times E^*)_j,\quad j_i^{LPGE} = \chi_{ijk} E_j E_k^*.$$

The shift current is controlled by the Berry-phase structure of the band manifold, and is “ballistic,” uniform, and non-dissipative.

(c) Photon-Drag Effect

• In both bulk and low-dimensional materials, the photon-drag current is expressed as

$$J_i(\omega) = \sum_{j,k,l} \gamma_{ijkl} E_j(\omega) E_k^*(\omega) q_l,$$

describing momentum transfer from photon wavevector $\mathbf{q}$ to electrons. This effect includes both linear (LPDE) and circular (CPDE) components depending on the symmetry and polarization of the incident light (Hamara et al., 2023).

(d) Structured Light and Higher-Order Effects

• With structured beams (including orbital angular momentum states), additional non-thermal photocurrent channels arise from intensity gradients, polarization (Stokes parameter) gradients, and spatial phase gradients, further enriching the phenomenology (Gunyaga et al., 2023).

2. Symmetry Constraints and Materials Platforms

The appearance and form of non-thermal drift photocurrents are governed by the symmetry of the host material:

• Inversion symmetry breaking (e.g., polar axis in TaAs, gapped graphene, surface Rashba alloys) is required for both shift and injection currents (Sirica et al., 2018, Hipolito et al., 2016).
• Magnetic symmetry (e.g., antiferromagnetic order in Mn$_3$Sn, ferromagnetic Rashba surfaces) enables additional channels such as the magnetic photogalvanic effect (Hamara et al., 2023, Adamantopoulos et al., 2022, Freimuth et al., 2017).
• The presence of strong spin–orbit coupling (Rashba, topological insulator surfaces) leads to large in-plane photocurrent responses with pronounced polarization and frequency dependence (Adamantopoulos et al., 2022, Freimuth et al., 2017).
• Structured light can be employed to break or mold effective symmetry locally, generating vortical, chiral, or spatially modulated drift currents in engineered 2D electron gases (Gunyaga et al., 2023).

3. Experimental Realization and Detection Protocols

Key developments in experimental technique for non-thermal drift photocurrents include:

• All-optical polarization switching: Reversal of photocurrent direction by changing the handedness or axis of the incident field, allowing for all-optical control with subwavelength precision (Zapata et al., 1 Dec 2025, Mou et al., 1 Feb 2024).
• Dual-modulation lock-in detection: Superimposing intensity and polarization modulation (with optical choppers and photo-elastic modulators) to isolate polarization-dependent drift currents from photothermal backgrounds (Zapata et al., 1 Dec 2025).
• Ultrafast pump-THz emission spectroscopy: Direct measurement of femtosecond-current transients and their radiated field to distinguish ultrafast, non-thermal drift processes from slower thermal signals (Sirica et al., 2018, Hamara et al., 2023, Mou et al., 1 Feb 2024).
• First-principles modeling and Wannier interpolation: DFT+Wannier approaches interface ab initio electronic structures and time-dependent nonequilibrium Green's function calculations to benchmark measured current amplitudes and their spectral/temperature dependence (Adamantopoulos et al., 2022).
• Scanning photocurrent microscopy: Mapping spatial variations of localized drift current generation at nanoscale for comparison with theoretical skin-depth predictions or domain wall gradients (Zapata et al., 1 Dec 2025, Gunyaga et al., 2023).

4. Quantitative and Qualitative Characteristics

Salient features of non-thermal drift photocurrents across material classes:

• Time scales: Ultrafast, with current rise/decay $\lesssim 100\,$fs for injection (CPGE) and up to a few ps for shift (LPGE) processes (Sirica et al., 2018, Zapata et al., 1 Dec 2025).
• Polarization dependence: Direction, amplitude, and sign of the drift current are programmable via light polarization (circular or linear) and, in tailored geometries, optical phase or beam position (Zapata et al., 1 Dec 2025, Gunyaga et al., 2023).
• Localization: In metals, photocurrent density is confined within the optical skin depth ($\sim 20$–$30$ nm), while in insulators and 2D systems the current is bulk and position-independent over the illuminated region (Zapata et al., 1 Dec 2025, Ishizuka et al., 2017).
• Amplitude scaling: Currents in Rashba interfaces and Co/Pt bilayers reach amplitudes of order $1$–$10\,{\rm A/m}$ under fs-pulse excitation, while atomically thin crystals show $1$–$10\,{\rm nA\,cm/W}$ depending on bandgap, doping, and photon energy (Adamantopoulos et al., 2022, Hipolito et al., 2016, Freimuth et al., 2017).
• Spectral tuning: Current direction and amplitude can be changed by gating (e.g., in biased bilayer graphene), frequency tuning, or via resonant enhancement in systems with excitonic structure (e.g., hBN) (Hipolito et al., 2016).

5. Separation from Photothermal Contributions

A central distinction of non-thermal drift photocurrents is their lack of reliance on heat-driven processes:

• Photothermal currents: Scale strictly with optical intensity gradients ($\nabla T$), insensitive to polarization modulation for fixed power, broadened over $\sim$ps–ns timescales (Zapata et al., 1 Dec 2025, Gunyaga et al., 2023).
• Drift (non-thermal) currents: Track polarization and phase structure, reverse with handedness, persist in uniform temperature, and are isolated by polarization-modulation lock-in protocols or symmetry-based channel decomposition in emission spectroscopy (Zapata et al., 1 Dec 2025, Sirica et al., 2018, Hamara et al., 2023).
• Collaborative “extraction” mechanisms in metals: Unavoidable macroscopic thermal gradients serve as driving fields to sweep locally generated, polarization-selective drift currents out to electrodes for remote detection, enabling collection of nanoscale signals (Zapata et al., 1 Dec 2025).

6. Broader Implications and Applications

Non-thermal drift photocurrents underpin several advances in optoelectronic nanodevice technology and physical understanding:

• Reconfigurable, ultrafast photonic circuitry: All-optical directional current steering with subwavelength spatial resolution, as realized in plasmonic nanocircuits and metasurfaces (Zapata et al., 1 Dec 2025, Mou et al., 1 Feb 2024).
• THz generation and photodetection: Local, strong, ultrafast drift currents are efficient sources of coherent THz radiation; polarization-selective injection and shift currents in topological semimetals and Rashba surfaces are exploited for high-speed photodetectors (Sirica et al., 2018, Hamara et al., 2023, Mou et al., 1 Feb 2024).
• Nonlinear and topological photonics: The robust connection of shift and injection currents to Berry curvature and quantum geometry enables optical probing and manipulation of topological band features, as well as the design of materials with targeted optoelectronic properties (Hipolito et al., 2016, Ishizuka et al., 2017).
• Structured-light optoelectronics: Use of vortex beams, spatially patterned polarizations, and ultrafast modulation schemes extends control over current topology, spatial distribution, and magnetic near-fields at the nanoscale (Gunyaga et al., 2023).

7. Outlook and Future Directions

Current research highlights several promising avenues:

• Inverse/femtosecond-optimized nanostructures: Inverse design of plasmonic antennas and metamaterials allows enhancement and real-time control over drift current density and magnetic-field pulses on nanometer scales (Mou et al., 1 Feb 2024).
• Engineering of Berry curvature and quantum geometry: By materials choice, biasing, and atomic design (e.g., in BBG or Weyl semimetals), one can preassign photocurrent magnitude, direction, and temporal response for target applications (Sirica et al., 2018, Hipolito et al., 2016).
• Integration with quantum and neuromorphic circuits: The non-dissipative, reversible, and ultrafast nature of these currents is anticipated to impact architectures for quantum photonics and analogue information processing.
• Advances in detection and characterization: Multi-channel lock-in protocols, on-chip THz detection, and high-fidelity spatial mapping will enable further disambiguation and exploitation of distinct non-thermal mechanisms (Zapata et al., 1 Dec 2025, Hamara et al., 2023).

Non-thermal drift photocurrents, as established by both theoretical formalism and diversified experimental platforms, represent a unifying and technologically pivotal phenomenon for ultrafast, polarization-controlled optoelectronics across metals, semiconductors, and low-dimensional quantum materials.