Ultra-broadband detection of coherent infrared pulses by sum-frequency generation spectroscopy in reflection geometry.

We have newly developed, to the best of our knowledge, a detection method for broadband infrared pulses based on sum-frequency generation spectroscopy in reflection geometry, which can avoid a restriction of the detection bandwidth originating from the phase mismatch that is inevitable for the upconversion in transmission geometry. Using a GaAs crystal, we successfully demonstrated the ultra-broadband detection of the infrared pulses generated from a two-color laser-induced air plasma filament in a region from 300 to 3300 cm-1. With the advantage of ultra-short infrared pulses, the present detection method holds promise for application to time-resolved, ultra-broadband vibrational spectroscopy.

Time-domain characterization of electric field vector in multi-terahertz pulses using polarization-modulated electro-optic sampling.

We demonstrated characterizing the electric field waveform of multi-terahertz pulses (10 - 50 THz) as vector quantities in the time domain by applying the polarization modulated electro-optic sampling (POMEOS) method. The problem of an ultrabroadband gate pulse was solved by modifying the fitting function in POMEOS, and its validity was confirmed through numerical simulations. High accuracy and precision of approximately 1 mrad with 3 s accumulation were demonstrated. Our method can be applied not only to multi-terahertz polarization measurements for linear response but also to the evaluation of the driving field of intense pulses for nonlinear response or material control.

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 µrads 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.

Anomalous Hall Transport by Optically Injected Isospin Degree of Freedom in Dirac Semimetal Thin Film.

Chirality of massless fermions emerging in condensed matter is a key to understand their characteristic behavior as well as to exploit their functionality. However, the 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, Cd3As2. 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 a potential application of Dirac semimetals in future information technology.

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.

Stimulated Rayleigh Scattering Enhanced by a Longitudinal Plasma Mode in a Periodically Driven Dirac Semimetal Cd_{3}As_{2}.

Using broadband (12-45 THz) multi-terahertz spectroscopy, we show that stimulated Rayleigh scattering dominates the transient optical conductivity of cadmium arsenide, a Dirac semimetal, under an optical driving field at 30 THz. The characteristic dispersive line shape with net optical gain is accounted for by optical transitions between light-induced Floquet subbands, strikingly enhanced by the longitudinal plasma mode. Stimulated Rayleigh scattering with an unprecedentedly large refractive index change may pave the way for slow light generation in conductive solids at room temperature.

Tracking Ultrafast Change of Multiterahertz Broadband Response Functions in a Photoexcited Dirac Semimetal Cd3As2 Thin Film.

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd3As2 by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 µm), which covers the crossover between the inter- and intraband response. We resolve dynamics of the photoexcited nonthermal electrons, which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.

Light-Driven Electron-Hole Bardeen-Cooper-Schrieffer-Like State in Bulk GaAs.

We investigate the photon-dressed state of excitons in bulk GaAs by optical pump-probe spectroscopy. We reveal that the high-energy branch of the dressed states continuously evolves into a singular enhancement at the absorption edge in the high-density region where the exciton picture is no longer valid. Comparing the experimental result with a simulation based on semiconductor Bloch equations, we show that the dressed state in such a high-density region is better viewed as a Bardeen-Cooper-Schrieffer-like state, which has been theoretically anticipated to exist over decades. Having seen that the dressed state can be regarded as a macroscopic coherent state driven by an external light field, we also discuss the decoherence from the dressed state to an incoherent state after the photoexcitation in view of the Coulomb enhancement in the transient absorption.

Infrared Activation of the Higgs Mode by Supercurrent Injection in Superconducting NbN.

The Higgs mode in superconductors, i.e., the collective amplitude mode of the order parameter, does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. Contrary to this common understanding, here, we demonstrate that if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at ω=2Δ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.