arXiv subjects

Mohammad Shafiei

Publications and source records attributed to Mohammad Shafiei.

14 recordsLinked to original sources

arXiv2026

Optical activation of nonlinear Hall effect in topological insulators with warped Fermi surface

Topological insulators (TIs) with hexagonally warped Fermi surface are natural platforms for the nonlinear Hall effect, as warping breaks inversion symmetry while preserving time-reversal symmetry (TRS). Here we show that this inversion breaking alone is insufficient: although warping generates a strongly anisotropic Berry curvature, the preserved threefold rotational symmetry forces the equilibrium Berry curvature dipole (BCD) to vanish identically. We demonstrate that linearly polarized light removes this symmetry obstruction: in the off-resonant Floquet regime, it lowers the rotational symmetry while preserving TRS, thereby generating a finite BCD whose magnitude, orientation, and sign are continuously tunable by the light intensity and polarization. For realistic Bi$_2$Te$_3$ parameters, we show that the induced BCD reaches $\sim$0.03~nm, yielding microampere-scale nonlinear Hall currents under experimentally accessible conditions. Our results therefore establish Floquet symmetry engineering as a route to activating the symmetry-forbidden nonlinear transport on TI surfaces without breaking the TRS.

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arXiv2026

Real-space Floquet topology written by the orbital angular momentum of light

Floquet engineering usually treats light as a uniform control field that changes the topology of an entire driven material. Here we show that structured light carrying orbital angular momentum (OAM) enables a different regime, in which topology is written directly in real space. For ultrathin topological insulator films, circularly polarized Laguerre--Gaussian beams generate a radial Floquet mass whose sign changes define a topological annulus bounded by two concentric chiral ring modes. The transition is helicity selective: below a thickness-dependent critical frequency, left-circularly polarized light drives mass inversion, whereas right-circularly polarized light increases the gap and leaves the film trivial. Independently, the OAM quantum number shifts and reshapes the annulus without changing the frequency, intensity, or helicity. In the decoupled-surfaces limit, the same mechanism produces a purely Floquet-induced topological mass and a vortex-core zero mode. These results identify photon OAM as a control parameter for nonequilibrium topology and provide a route to programmable topological landscapes in quantum materials.

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arXiv2025

Linearly polarized light enables chiral edge transport in quasi-2D Dirac materials

Floquet engineering with high-frequency light offers dynamic control over topological phases in quantum materials. While in 3D Dirac systems circularly polarized light is known to induce topological phase transitions via gap opening, linearly polarized light (LPL) has generally been considered ineffective. Here we show that in quasi-2D Dirac materials the second-order momentum term arising from the intersurface coupling can induce a topological phase transition under LPL, leading to chiral edge channels. Considering an ultrathin Bi$_2$Se$_3$ film as a representative system, we show that this transition occurs at experimentally accessible light intensities. Our results thus promote quasi-2D materials as viable platforms for light-controlled topological phases, expanding the potential of Floquet topological engineering.

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arXiv2025

Dissipationless transport by design in ultrathin magnetic topological insulator films

Magnetic topological insulators (MTIs) are among the prominent platforms for the next generation of high-speed and low-power spintronic devices. However, unlike their non-magnetic counterparts, where the surface spin-momentum locking prevents electrons from being scattered by non-magnetic impurities and results in a dissipationless electronic flow, magnetic impurities in MTIs cause dissipation by exerting magnetic torque on the electron spin. Decreasing this resistance is desired to reduce energy consumption and optimize performance of MTIs in envisaged applications. Here we reveal how electronic backscattering can be suppressed in a MTI thin film by external magnetic and/or electronic stimuli, to yield an entirely dissipationless spin-polarized charge transport. Our findings thus present an effective route to preserve spin coherence and enhance spin-current functionality in magnetic topological materials, suggesting design strategies for magneto-electronic and spintronic devices with strongly reduced energy consumption.

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arXiv2025

Planar Hall effect in ultrathin topological insulator films

The planar Hall effect (PHE), previously observed in Weyl and Dirac semimetals due to the chiral anomaly, emerges with a different origin in topological insulators (TIs), where in-plane magnetic fields induce resistivity anisotropy. In strictly two-dimensional TIs, PHE is generally suppressed due to the inability of the out-of-plane Berry curvature to couple to the in-plane band velocity of the charge carriers. Here, we demonstrate that in ultrathin TI films, a quasi-two-dimensional system, intersurface tunneling coupling with in-plane magnetization induces electronic anisotropy, enabling a finite PHE. In addition, we reveal that strong in-plane magnetization can stabilize the thickness-dependent quantum anomalous Hall effect, typically associated with out-of-plane magnetization. These insights advance the understanding of magnetic topological phases, paving the way for next-generation spintronic devices and magnetic sensing technologies.

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arXiv2025

Light-induced dissipationless states in magnetic topological insulators with hexagonal warping

Magnetic impurities in topological insulators (TIs) induce backscattering via magnetic torque, unlike pristine TIs where spin-orbit locking promotes dissipationless surface states. Here we reveal that one can suppress that unwanted backscattering and dissipation in magnetic TIs using high-frequency linearly polarized light (LPL). By carefully considering the hexagonal warping of the Fermi surface of the TI, we demonstrate how the coupling between Dirac surface states and LPL can effectively reduce backscattering on magnetic dopants, enhance carrier mobility and suppress resistance, even entirely. These findings open up avenues for designing ultra low-power sensing and spintronic technology.

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arXiv2024

Towards Advanced Chiral Sensors: Enhanced Helicity-Dependent Photocurrent in Ultrathin Topological Insulator Films

Chirality, a fundamental property of asymmetric structures, plays a crucial role in pharmaceutical, biological and chemical systems, offering a powerful tool for screening organic compounds. While the conventional optical chirality detectors are often bulky and involuted, the topological insulators (TIs) offer a promising platform for developing compact yet sensitive devices - owing to their inherent chirality. However, the complex interplay of photoresponses in TIs can limit the ultimate accuracy of chirality detection. Therefore, we here analyze the underlying mechanisms governing the photoresponses in TIs and reveal strategies to enhance the helicity-dependent photocurrent (HDPC). By attentively analyzing the symmetries and behavior of competing photoresponses, we show that it is possible to effectively eliminate unwanted contributions and isolate the HDPC. Moreover, we reveal that HDPC is strongly amplified in ultrathin TI films, and can be further enhanced by optimizing the illumination parameters, sensor strain and/or back gating. Our findings thereby provide a roadmap for design and optimization of miniaturized, high-performance TI chirality detectors, with potential to revolutionize chiral analysis in biomedical and material sciences.

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arXiv2021

Learning Neural Transmittance for Efficient Rendering of Reflectance Fields

Recently neural volumetric representations such as neural reflectance fields have been widely applied to faithfully reproduce the appearance of real-world objects and scenes under novel viewpoints and lighting conditions. However, it remains challenging and time-consuming to render such representations under complex lighting such as environment maps, which requires individual ray marching towards each single light to calculate the transmittance at every sampled point. In this paper, we propose a novel method based on precomputed Neural Transmittance Functions to accelerate the rendering of neural reflectance fields. Our neural transmittance functions enable us to efficiently query the transmittance at an arbitrary point in space along an arbitrary ray without tedious ray marching, which effectively reduces the time-complexity of the rendering. We propose a novel formulation for the neural transmittance function, and train it jointly with the neural reflectance fields on images captured under collocated camera and light, while enforcing monotonicity. Results on real and synthetic scenes demonstrate almost two order of magnitude speedup for renderings under environment maps with minimal accuracy loss.

cs.CV↗

arXiv2021

Real-Time Global Illumination Decomposition of Videos

We propose the first approach for the decomposition of a monocular color video into direct and indirect illumination components in real time. We retrieve, in separate layers, the contribution made to the scene appearance by the scene reflectance, the light sources and the reflections from various coherent scene regions to one another. Existing techniques that invert global light transport require image capture under multiplexed controlled lighting, or only enable the decomposition of a single image at slow off-line frame rates. In contrast, our approach works for regular videos and produces temporally coherent decomposition layers at real-time frame rates. At the core of our approach are several sparsity priors that enable the estimation of the per-pixel direct and indirect illumination layers based on a small set of jointly estimated base reflectance colors. The resulting variational decomposition problem uses a new formulation based on sparse and dense sets of non-linear equations that we solve efficiently using a novel alternating data-parallel optimization strategy. We evaluate our approach qualitatively and quantitatively, and show improvements over the state of the art in this field, in both quality and runtime. In addition, we demonstrate various real-time appearance editing applications for videos with consistent illumination.

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arXiv2019

Inverse Rendering for Complex Indoor Scenes: Shape, Spatially-Varying Lighting and SVBRDF from a Single Image

We propose a deep inverse rendering framework for indoor scenes. From a single RGB image of an arbitrary indoor scene, we create a complete scene reconstruction, estimating shape, spatially-varying lighting, and spatially-varying, non-Lambertian surface reflectance. To train this network, we augment the SUNCG indoor scene dataset with real-world materials and render them with a fast, high-quality, physically-based GPU renderer to create a large-scale, photorealistic indoor dataset. Our inverse rendering network incorporates physical insights -- including a spatially-varying spherical Gaussian lighting representation, a differentiable rendering layer to model scene appearance, a cascade structure to iteratively refine the predictions and a bilateral solver for refinement -- allowing us to jointly reason about shape, lighting, and reflectance. Experiments show that our framework outperforms previous methods for estimating individual scene components, which also enables various novel applications for augmented reality, such as photorealistic object insertion and material editing. Code and data will be made publicly available.

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arXiv2017

VNect: Real-time 3D Human Pose Estimation with a Single RGB Camera

We present the first real-time method to capture the full global 3D skeletal pose of a human in a stable, temporally consistent manner using a single RGB camera. Our method combines a new convolutional neural network (CNN) based pose regressor with kinematic skeleton fitting. Our novel fully-convolutional pose formulation regresses 2D and 3D joint positions jointly in real time and does not require tightly cropped input frames. A real-time kinematic skeleton fitting method uses the CNN output to yield temporally stable 3D global pose reconstructions on the basis of a coherent kinematic skeleton. This makes our approach the first monocular RGB method usable in real-time applications such as 3D character control---thus far, the only monocular methods for such applications employed specialized RGB-D cameras. Our method's accuracy is quantitatively on par with the best offline 3D monocular RGB pose estimation methods. Our results are qualitatively comparable to, and sometimes better than, results from monocular RGB-D approaches, such as the Kinect. However, we show that our approach is more broadly applicable than RGB-D solutions, i.e. it works for outdoor scenes, community videos, and low quality commodity RGB cameras.

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arXiv2016

EgoCap: Egocentric Marker-less Motion Capture with Two Fisheye Cameras (Extended Abstract)

cs.CV↗

arXiv2016

EgoCap: Egocentric Marker-less Motion Capture with Two Fisheye Cameras

Marker-based and marker-less optical skeletal motion-capture methods use an outside-in arrangement of cameras placed around a scene, with viewpoints converging on the center. They often create discomfort by possibly needed marker suits, and their recording volume is severely restricted and often constrained to indoor scenes with controlled backgrounds. Alternative suit-based systems use several inertial measurement units or an exoskeleton to capture motion. This makes capturing independent of a confined volume, but requires substantial, often constraining, and hard to set up body instrumentation. We therefore propose a new method for real-time, marker-less and egocentric motion capture which estimates the full-body skeleton pose from a lightweight stereo pair of fisheye cameras that are attached to a helmet or virtual reality headset. It combines the strength of a new generative pose estimation framework for fisheye views with a ConvNet-based body-part detector trained on a large new dataset. Our inside-in method captures full-body motion in general indoor and outdoor scenes, and also crowded scenes with many people in close vicinity. The captured user can freely move around, which enables reconstruction of larger-scale activities and is particularly useful in virtual reality to freely roam and interact, while seeing the fully motion-captured virtual body.

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arXiv2015

Single-Spin CCD

Spin-based electronics or spintronics relies on the ability to store, transport and manipulate electron spin polarization with great precision. In its ultimate limit, information is stored in the spin state of a single electron, at which point also quantum information processing becomes a possibility. Here we demonstrate the manipulation, transport and read-out of individual electron spins in a linear array of three semiconductor quantum dots. First, we demonstrate single-shot read-out of three spins with fidelities of 97% on average, using an approach analogous to the operation of a charge-coupled-device (CCD). Next, we perform site-selective control of the three spins thereby writing the content of each pixel of this "Single-Spin CCD". Finally, we show that shuttling an electron back and forth in the array hundreds of times, covering a cumulative distance of 80 $μ$m, has negligible influence on its spin projection. Extrapolating these results to the case of much larger arrays, points at a diverse range of potential applications, from quantum information to imaging and sensing.

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