Helicity-Dependent Photocurrent (HDPC)

• HDPC is a phenomenon where circularly polarized light generates a photocurrent by coupling light’s angular momentum to carriers via symmetry breaking.
• Experimental techniques like modulated polarization and angle-resolved measurements isolate contributions from CPGE, CPDE, and interface effects.
• Advances in HDPC underpin the development of opto-spintronic devices, chiral sensors, and photodetectors by leveraging spin-momentum locking and Berry curvature.

Helicity-dependent photocurrent (HDPC) refers to a class of photocurrent responses in which the magnitude and/or direction of the generated current depends sensitively on the helicity (handedness) of circularly polarized light irradiating the material. HDPC is rooted in the interplay between spin-orbit coupling, material symmetry, and polarization selection rules. It is observed in a broad variety of systems, including graphene, topological insulators, semimetals, polar van der Waals heterostructures, semiconductors under bias, and hybrid spintronic devices. The underlying mechanisms include the circular photogalvanic effect (CPGE), photon drag effects, interface or edge-induced symmetry breaking, and spin-dependent carrier extraction, among others. HDPC is of central importance for opto-spintronics, chiral sensing, Berry curvature engineering, and the detection or control of symmetry and topology in quantum materials.

1. Fundamental Mechanisms and Theoretical Framework

The generation of HDPC arises predominantly through symmetry-breaking processes that allow light’s angular momentum to be coupled to carriers, resulting in a net photocurrent that reverses sign upon switching the light’s helicity. The principal microscopic mechanisms include:

• Circular Photogalvanic Effect (CPGE): In systems where inversion symmetry is broken (surface states of topological insulators, strained graphene on substrates, polar heterostructures), the CPGE dominates. The CPGE is typically described by

$$j_{i} \propto \gamma_{ij} (i\,\mathbf{e} \times \mathbf{e}^*)_j I$$

where $\gamma_{ij}$ is a material-specific tensor, $\mathbf{e}$ is the polarization unit vector, and $I$ is the intensity.

• Circular Photon Drag Effect (CPDE): Relevant for oblique incidence, CPDE involves the transfer of both linear and angular momentum from photons to carriers:

$$j_{\lambda} = \tilde{T}_{\lambda \mu \xi} q_{\mu} P_{\text{circ}} \hat{e}_{\xi} I$$

with $q_{\mu}$ the in-plane photon wavevector component, $P_{\text{circ}}$ the degree of circular polarization, and $\tilde{T}$ a tensor reflecting symmetry and microscopic relaxation processes.

• Edge/Interface and Surface Effects: At sample edges (graphene) or interfaces in bilayers and heterostructures (e.g., Fe/AlOx/GaAs, metal/Bi), local symmetry breaking supports helicity-dependent rectification mechanisms even when the bulk is symmetry-forbidden.
• Spin-Selective Tunneling and Magnetization Effects: In hybrid spintronic architectures, CP light creates spin-polarized carriers, which are selectively extracted through ferromagnetic electrodes, with the sign and magnitude of the photocurrent tracking the electrode’s magnetization hysteresis.
• Field-Driven Berry Curvature and Third-Order Effects: With application of a static electric field or strong spin-orbit coupling (e.g., Rashba-Edelstein effect), Berry curvature and shift vector contributions can render the CPGE highly tunable and nonlinear, producing unusually large and electrically switchable HDPC.

HDPC can generally be decomposed phenomenologically in polarization modulation experiments as:

$$J = C\sin(2\varphi) + L_1\sin(4\varphi) + L_2\cos(4\varphi) + D$$

where $C$ reflects the helicity-dependent component, $L_1$, $L_2$ linear-polarization-dependent backgrounds, and $D$ a polarization-independent term. Experimentally, the $C$ term is isolated by the characteristic $\sin(2\varphi)$ dependence (quarter-wave plate angle $\varphi$).

2. Material and Symmetry Dependence

Graphene and Dirac Systems

In monolayer graphene, bulk inversion symmetry forbids HDPC at normal incidence, but edge states or symmetry breaking by substrate or interface can restore sensitivity. Circular photon drag prevails under oblique incidence, while edge photogalvanic effects dominate at $\theta_0 = 0$ (Karch et al., 2010). In Dirac semimetals and Weyl semimetals, HDPC results from Berry curvature and unique dispersion, often showing “universal” scaling with fundamental constants, the light frequency, and the degree of circular polarization (Golub et al., 2018, Kawaguchi et al., 2020).

Topological Insulators

Surface states of topological insulators, with strong spin-momentum locking and reduced (typically $C_{3v}$ or $C_s$) symmetry, are prototypical platforms for helicity-dependent currents (Pan et al., 2017, Movafagh et al., 30 Jan 2025). The sign and amplitude of HDPC can serve as a fingerprint of broken inversion symmetry, surface vs. edge nature, or the presence of valley polarization. The thickness and external gating (which controls chemical potential and surface–surface coupling) are critical tuning parameters (Shafiei et al., 18 Nov 2024, Liu et al., 4 Sep 2025).

Polar van der Waals and Heterostructures

In polar van der Waals heterostructures or TMD monolayers placed on substrates, built-in perpendicular polarization enables both in-plane and out-of-plane field components to interfere, giving rise to distinct valley and helicity-dependent photocurrents. Here, the valley degree of freedom introduces additional selection rules, and magneto-induced or Berry curvature effects further modify HDPC (Lyanda-Geller et al., 2015).

Semiconductors with Bias or Hybrid Devices

In devices with strong Rashba splitting and externally applied bias, such as MAPbI$_3$ perovskites, the Rashba-Edelstein effect induces a non-equilibrium spin polarization, enabling a magnetic shift current that is highly nonlocal and orders of magnitude stronger than conventional injection currents (Zhu et al., 22 Mar 2024, Murotani et al., 9 May 2025). In GaAs/AlGaAs field-effect transistors, interference of plasma oscillations creates a fast, all-electrical helicity-resolved photoresponse (Drexler et al., 2012).

3. Experimental Methodologies and Diagnostics

Common HDPC measurements involve:

• Modulated Polarization Excitation: Use of a quarter-wave plate to continuously transform linear into circular polarization, measuring current as a function of $\varphi$ to extract $\sin(2\varphi)$ (circular) and $\sin(4\varphi)$/$\cos(4\varphi)$ (linear) components.
• Angle-Resolved Measurements: Varying the light’s incidence angle $\theta$ provides information on the underlying mechanism. CPDE and photon drag scale as $\sin(2\theta)$, while CPGE in topological insulator surfaces typically scales as $\sin(\theta)$.
• Bias and Gating: Application of a static electric field or back gate enhances and tunes HDPC magnitude and sometimes reverses its sign by controlling inversion symmetry or Rashba-Edelstein effects.
• Spatially-Resolved and Nonlocal Probing: Scanning photocurrent microscopy reveals nonlocal spin diffusion lengths in excess of tens of microns, indicating highly mobile spin or excitonic information spreading away from the photo-excitation region (Zhu et al., 22 Mar 2024).

4. Sensitivity to Symmetry, Topology, and Phase Transitions

HDPC serves as a sensitive probe for both symmetry and topological phase:

• Point-Group Symmetry Analysis: The angular dependence (odd vs. even in incident angle, e.g., $\sin\theta$ or $\cos\theta$) directly reflects the underlying point group (e.g., $C_{2v}$ vs. $C_s$), enabling discrimination between bulk- or surface-driven electronic states (Liu et al., 4 Sep 2025).
• Topological Phase Transitions: In α-Sn/CdTe(110) films, HDPC reveals a thickness-driven 2D–3D topological insulator transition, changing its angular parity as the dominant channel evolves from edge (2D) to surface (3D) states (Liu et al., 4 Sep 2025).
• Crystallographic Domains and Defects: Competing domain symmetries (e.g., twin boundaries in Bi$_2$Se$_3$) can suppress or mask intrinsic three-fold HDPC, underlining the necessity for high structural quality in nonlinear symmetry- and topology-sensitive photogalvanic studies (Connelly et al., 2023).

5. Technological Applications and Device Concepts

HDPC underpins several emerging opto-spintronic and photonic technologies:

• All-Electrical THz Polarization Detection: FETs and HEMTs provide fast, electric field-only, helicity-resolved detection for THz ellipsometry and imaging (Drexler et al., 2012).
• Spin-Photodiodes and Chiral Detectors: The sign and magnitude of HDPC in Fe/AlOx/GaAs and perovskite structures enable robust circular-polarization detectors, chiral analysis, and even room-temperature operation in devices with perpendicular magnetic anisotropy (Ikeda et al., 2014, Roca et al., 2017, Cadiz et al., 2020, Shafiei et al., 18 Nov 2024).
• Optical Receivers by Helicity-Driven Switching: Magnetization switching via light helicity (HDS) in magnetic tunnel junctions allows ultra-low energy direct optical-to-digital conversion, bypassing photodiodes entirely (Azim et al., 2018).
• Spin Current Photo-Transistors: Asymmetry-engineered ultrathin TI films support pure spin currents tunable by optical polarization and surface potential, with “s”-wave (rotation invariant) and “d”-wave (rotation odd) symmetry components. Device concepts include spin current amplifiers and logic elements based on polarization selection (Movafagh et al., 30 Jan 2025).
• Berry Curvature Engineering: By applying both light and bias, third-order nonlinear effects such as the photovoltaic Hall effect (field-tuned HDPC) provide access to Berry curvature correction and energy-shift mechanisms, leading to resonant manipulation of the photocurrent in semiconductors like GaAs (Murotani et al., 9 May 2025).

6. Enhancement Strategies and Outlook

HDPC magnitude and selectivity can be enhanced through:

• Strain Tuning and Sensor Strain (Straintronics): Biaxial compressive strain in ultrathin TIs modifies hybridization gaps and Berry curvature, enhancing the CPGE component (Shafiei et al., 18 Nov 2024).
• Optimized Incidence and Frequency: Larger incidence angle and optimal photon energy (above the hybridization gap) boost the circular response, though limitations arise due to in-plane field geometry and absorption (Shafiei et al., 18 Nov 2024).
• Electric Field and Gating: External gating can break inversion symmetry, induce Rashba-type splitting, or control the balance between surface-bulk contributions, thereby switching or amplifying HDPC (Zhu et al., 22 Mar 2024, Shafiei et al., 18 Nov 2024).
• Interface Engineering: Increasing spin-orbit coupled interface quality (e.g., metal/Bi or magnetic insulators) or employing ultrathin layers with built-in symmetry breaking can dramatically enhance HDPC and enable nonreciprocal control (Hirose et al., 2020).
• Phase and Domain Control: Suppressing twinning or controlling crystalline domain orientation permits the isolation of intrinsic symmetry-driven HDPC contributions (Connelly et al., 2023).

7. Open Questions and Prospective Developments

Several avenues for future research and practical development include:

• Determining the role of secondary states (Rashba split bands, higher Dirac cones) and the contribution of bulk versus surface states.
• Quantifying the relaxation dynamics and interplay between nonlocal spin information, recombination, and pure spin current transport.
• Integration of HDPC-based detection and switching mechanisms into large-scale circuitry for chiral sensing, ultrafast logic, and information transduction.
• Exploiting thickness-, strain-, and symmetry-driven control for quantum device platforms and Berry phase engineering.
• Advancing theoretical frameworks to include many-body effects, quantum interference, and the interplay of geometric and topological factors under intense bias or photon fields.

Helicity-dependent photocurrent stands at the intersection of quantum geometry, symmetry, and device physics, providing both a powerful probe of fundamental properties and a basis for advanced opto-spintronics and chiral photonic device engineering.