arXiv subjects

Tomohiro Fujimoto

Publications and source records attributed to Tomohiro Fujimoto.

8 recordsLinked to original sources

arXiv2025

Signature of inverse orbital Hall effect in silicon studied using time-resolved terahertz polarimetry

We investigated the anomalous Hall conductivity induced in silicon by circularly polarized light at room temperature using near-infrared (NIR) pump-terahertz (THz) probe spectroscopy. The time-resolved detection scheme eliminates the large nonlinear current generated by the field-induced circular photogalvanic effect, allowing exclusive observation of a long-lived anomalous Hall conductivity of photocarriers that depends on the helicity of NIR light. The magnitude of this conductivity is comparable to that of GaAs despite silicon's much weaker spin-orbit coupling, and its robustness against NIR photon energy rules out a spin-polarization-based origin, which occurs only in the vicinity of the bandgap. These results suggest the emergence of the inverse orbital Hall effect, paving the way for silicon-based orbitronics.

cond-mat.mes-hall↗

arXiv2025

Light-induced inverse spin Hall effect and field-induced circular photogalvanic effect in GaAs revealed by two-dimensional terahertz Fourier analysis

The electromotive force transverse to a bias field under irradiation of circularly polarized light, namely the photovoltaic Hall response or light-induced anomalous Hall effect, has attracted considerable attention to investigate the topologically nontrivial states in Floquet engineering and the inverse spin Hall effect of spin-polarized carriers in spintronics. However, taking into account inversion symmetry breaking by the bias field, the circularly polarized light can excite photocarriers with asymmetric momentum distribution, which generates injection current transverse to the bias field. Therefore, the field-induced circular photogalvanic effect (FI-CPGE) should also emerge in the very same experimental configuration for light-induced anomalous Hall effect but has been overlooked in literature. In this work, using terahertz pulses as a bias field for a semiconductor GaAs, we conduct two-dimensional Fourier analysis and demonstrate that FI-CPGE can play a major role in the photovoltaic Hall response. Counterintuitively, FI-CPGE is significantly enhanced when the photocarriers are excited near the bandgap with small density of states and low group velocity, which can be explained by a three-level resonant nonlinear interaction near the band degeneracy point. We also clarified that FI-CPGE would be further largely detected in the contact-type measurement using electrodes because of the absence of a filtering effect inherent to terahertz pulses. This work provides a comprehensive, generalized view of the photovoltaic Hall response in biased materials, paving a new avenue for detecting topological monopoles in momentum space hidden in equilibrium using third-order nonlinear responses.

cond-mat.mtrl-sci↗

arXiv2025

Calculations in Unified theory of the photovoltaic Hall effect by field- and light-induced Berry curvatures

Photovoltaic Hall effect is an interesting platform of Berry curvature engineering by external fields. Floquet engineering aims at generation of light-induced Berry curvature associated with topological phase transition in solids, which may manifest itself as a light-induced anomalous Hall effect. However, recent studies have pointed out an important role of the bias electric field, which adds a field-induced circular photogalvanic effect to the photovoltaic Hall effect. Except for numerical studies, the two mechanisms have been described by different theoretical frameworks, hindering a coherent understanding. Here, we develop a unified theory of the photovoltaic Hall effect capable of describing both mechanisms on an equal footing. We reveal that the bias electric field alters the interband transition dipole moment, transition energy, and intraband velocity, all contributing to the field-induced circular photogalvanic effect in nonmagnetic materials. The first process can be expressed as a manifestation of the electric field-induced Berry curvature. Shift vector plays an essential role in determining the transition energy shift. We also clearly distinguish the anomalous Hall effect by light-dressed states within the density matrix calculation using the length gauge. Our theory unifies a number of nonlinear optical processes in a physically transparent way and reveals their geometric aspect.

cond-mat.mtrl-sci↗

arXiv2025

Unified theory of the photovoltaic Hall effect by field- and light-induced Berry curvatures

Photovoltaic Hall effect, i.e., generation of a photocurrent perpendicular to the bias electric field, is an interesting platform of Berry curvature engineering by external fields. Floquet engineering aims at generation of light-induced Berry curvature associated with topological phase transition in solids, which may manifest itself as a light-induced anomalous Hall effect. However, recent studies have pointed out a larger contribution by momentum asymmetry of photocarriers, termed a field-induced circular photogalvanic effect. Except for numerical studies, the two mechanisms have been described by different theoretical frameworks, hindering a coherent understanding. Here, we develop a unified theory of the photovoltaic Hall effect capable of describing both mechanisms on an equal footing. We reveal that the bias electric field alters the interband transition dipole moment and transition energy, both contributing to the field-induced circular photogalvanic effect in nonmagnetic materials. These effects are governed by an electric field-induced Berry curvature and the shift vector coupled to bias field, respectively. A resonant enhancement of the transverse photocurrent is found in GaAs owing to the topological character of the valence band. We also clearly distinguish the anomalous Hall effect by light-dressed states within the density matrix calculation using the length gauge. Our theory unifies a number of nonlinear optical processes in a physically transparent way and presents a geometric picture of the third-order nonlinear response under light and bias fields, shedding new light on Berry curvature engineering.

cond-mat.mtrl-sci↗

arXiv2025

Valley polarization dynamics of photoinjected carriers at the band edge in room-temperature silicon studied by terahertz polarimetry

Sixfold-degenerate valleys in Si have attracted considerable attention for valleytronics application. Using optical pump-terahertz (THz) probe spectroscopy, we study the dynamics of valley polarization in bulk Si(001) at room temperature. Linearly polarized pump pulses excite electrons and holes with asymmetric distributions in momentum space, leading to in-plane anisotropic conductivity. By varying the polarization directions of the pump light relative to the in-plane crystalline axes, the valley polarization of electrons and the momentum asymmetry of holes are separately probed through observing the polarization rotation of THz pulses. We demonstrate that the valley relaxation time of electrons near the conduction band minimum exceeds 1.5 ps at room temperature, in good agreement with theoretically calculated intervalley phonon scattering with f process. This work paves the way for Si-based room-temperature valleytronics.

cond-mat.mtrl-sci↗

arXiv2024

Observation of Terahertz Spin Hall Conductivity Spectrum in GaAs with Optical Spin Injection

We report the first observation of the spin Hall conductivity spectrum in GaAs at room temperature. Our terahertz polarimetry with a precision of several $μ$rads resolves the Faraday rotation of terahertz pulses arising from the inverse spin Hall effect of optically injected spin-polarized electrons. The obtained spin Hall conductivity spectrum exhibits an excellent quantitative agreement with theory, demonstrating a crossover in the dominant origin from impurity scattering in the DC regime to the intrinsic Berry-curvature mechanism in the terahertz regime. Our spectroscopic technique opens a new pathway to analyze anomalous transports related to spin, valley, or orbital degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous Hall transport by optically injected isospin degree of freedom in Dirac semimetal thin film

Chirality of massless fermions emergent in condensed matter is a key to understand their characteristic behavior as well as to exploit their functionality. However, chiral nature of massless fermions in Dirac semimetals has remained elusive, due to equivalent occupation of carriers with the opposite chirality in thermal equilibrium. Here, we show that the isospin degree of freedom, which labels the chirality of massless carriers from a crystallographic point of view, can be injected by circularly polarized light. Terahertz Faraday rotation spectroscopy successfully detects the anomalous Hall conductivity by a light-induced isospin polarization in a three-dimensional Dirac semimetal, Cd$_3$As$_2$. Spectral analysis of the Hall conductivity reveals a long scattering time and a long decay time, which are characteristic of the isospin. The long-lived, robust, and reversible character of the isospin promises potential application of Dirac semimetals in future information technology.

cond-mat.mes-hall↗

arXiv2023

Disentangling the Competing Mechanisms of Light-Induced Anomalous Hall Conductivity in Three-Dimensional Dirac Semimetal

We experimentally elucidate the origin of the anomalous Hall conductivity in a three-dimensional Dirac semimetal, Cd$_3$As$_2$, driven by circularly polarized light. Using time-resolved terahertz Faraday rotation spectroscopy, we determine the transient Hall conductivity spectrum with special attention to its sign. Our results clearly show the dominance of direct photocurrent generation assisted by the terahertz electric field. The contribution from the Floquet-Weyl nodes is found to be minor when the driving light is in resonance with interband transitions. We develop a generally applicable classification of microscopic mechanisms of light-induced anomalous Hall conductivity.

cond-mat.mtrl-sci↗