Giant Photon-Drag Currents in Centrosymmetric Moiré Bilayers Arise with Wavevectors a Few Times Larger Than Free-Space Photons

Nonlinear optical effects, which convert light into electrical current, typically require materials lacking symmetry, but a new theory challenges this long-held belief. Zhuocheng Lu, Zhuang Qian from Westlake University, and Zhichao Guo from Zhejiang University, along with colleagues, demonstrate that substantial photon-drag currents, electrical currents generated by light, can arise even in centrosymmetric materials like twisted bilayer graphene. The team developed a geometric framework that explains how these currents emerge from the material’s structure, revealing that the arrangement of atoms, rather than a lack of symmetry, drives the effect. This breakthrough establishes a new understanding of nonlinear optical phenomena and suggests moiré bilayers represent a powerful platform for creating highly tunable and efficient optoelectronic devices, potentially revolutionising areas like light detection and energy harvesting.

Symmetry and Nonlinear Photoconductivity in Graphene

This work provides a detailed theoretical foundation for understanding nonlinear photon-drag photoconductivity in twisted bilayer graphene, comprehensively outlining methods, results, and supporting data. The research analyzes how photoconductivity transforms under different symmetry conditions, revealing how the electronic properties of twisted bilayer graphene evolve with twist angle and symmetry breaking through detailed band structure calculations. Deriving expressions for the nonlinear photoconductivity tensor using the Kubo formula and Green’s function formalism provides a robust theoretical framework, connecting the model to experimental measurements and establishing a basis for calculating the second-order photoconductivity tensor. The thoroughness, clarity, mathematical rigor, and physical insight presented significantly advance understanding of twisted bilayer graphene, providing a valuable resource for researchers and a means to verify and refine existing theoretical models.

Geometric Loops Explain Photon-Drag Currents in Graphene

Scientists have developed a new theoretical framework to understand nonlinear photon-drag currents, representing them as originating from geometric loops within materials and directly relating the current to the dipole moment of the loop and differences in band velocity. Applying this theory to twisted bilayer graphene achieves accuracy comparable to complex first-principles calculations. Solving the quantum Liouville equation determines how optical electric fields influence the material’s electronic state, expanding the density matrix to account for field strength and iteratively solving for complex interactions. This work establishes a geometric framework for understanding nonlinear photon-drag phenomena and highlights twisted bilayer graphene as a promising material for creating large, tunable optoelectronic devices, providing a foundation for designing and optimizing materials with enhanced light-matter interactions.

Large Photon-Drag Currents in Twisted Graphene

Scientists have developed a unified theory to explain nonlinear photon-drag currents in twisted bilayer graphene, establishing a geometric-loop framework that elegantly connects injection and shift currents. Calculations, performed using an exact continuum model with ab initio accuracy, reveal remarkably large photon-drag responses, comparable to photogalvanic effects observed in typical non-centrosymmetric two-dimensional materials. Experiments demonstrate that even a wavevector only a few times larger than that of free-space photons generates sizable photon-drag currents in centrosymmetric twisted bilayer graphene, with magnitude tunable by changes in twist angle, photon wavevector, and light polarization. The photon-drag injection current is directly related to the dipole moment of a geometric loop, while the shift current arises from the same loop weighted by a difference in band velocities.

Analysis of twisted bilayer graphene aligned with hexagonal boron nitride, where inversion symmetry is broken, distinguishes between wavevector-independent and photon-drag contributions. The results confirm the potential of moiré bilayers, like twisted bilayer graphene, as promising platforms for creating large, highly tunable optoelectronic responses, opening new avenues for advanced optical devices and materials. The developed geometric-loop formalism provides a powerful tool for understanding and optimizing photon-drag phenomena in a wide range of quantum materials.

Twist Angle Controls Photon-Drag Currents

This research establishes a unified theoretical framework to understand nonlinear photon-drag currents, successfully linking the injection and shift current contributions through a geometric-loop model. Applying this approach to twisted bilayer graphene reveals that even relatively small momentum photons can induce substantial currents within this centrosymmetric material, achieving magnitudes comparable to those observed in typical non-centrosymmetric two-dimensional materials. The team demonstrated that these currents are readily tunable by adjusting the twist angle, photon wavevector, and light polarization. The study identifies key symmetry constraints governing the nonlinear photon-drag conductivity tensor and details how the photon-drag currents evolve with varying twist angles.

Researchers found that the peak response increases significantly with photon wavevector before reaching saturation, and that the presence of hexagonal boron nitride can enhance the shift current, particularly at low photon momentum. Harnessing light’s momentum offers a pathway to generate large nonlinear photocurrents in moiré systems like twisted bilayer graphene, paving the way for novel photon-drag engineered optoelectronic devices. Future work could explore the impact of many-body interactions and investigate the potential for realizing these photon-drag currents in actual devices, potentially leading to new functionalities in optoelectronics.