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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 Cd$_3$As$_2$ by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 $\mu$m), 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., Comment: 20 pages

We investigate ultrafast dynamics of the anomalous Hall effect (AHE) in the topological antiferromagnet Mn3Sn with sub-100 fs time resolution. Optical pulse excitations largely elevate the electron temperature up to 700 K, and terahertz probe pulses clearly resolves ultrafast suppression of the AHE before demagnetization. The result is well reproduced by microscopic calculation of the intrinsic Berry-curvature mechanism while the extrinsic contribution is clearly excluded. Our work opens a new avenue for the study of nonequilibrium AHE to identify the microscopic origin by drastic control of the electron temperature by light.

We demonstrate a jitter correction method for asynchronous optical sampling (ASOPS) terahertz (THz) time-domain spectroscopy using two free-running oscillators. This method simultaneously records the THz waveform and a harmonic of the laser repetition rate difference, to monitor the jitter information for software jitter correction. By suppressing the residual jitter below 0.1 ps, the accumulation of the THz waveform is achieved without losing the measurement bandwidth. Our measurement of water vapor successfully resolves the absorption linewidths below 1 GHz, demonstrating a robust ASOPS with a flexible, simple, and compact setup without any feedback control or additional continuous-wave THz source., 18 pages, 5 figures

We investigated ultrafast broadband (12-45 THz) response functions of a photoexcited Dirac semimetal Cd3As2. Under periodic driving at 30 THz, we observed coherent nonlinear response dominated by stimulated Rayleigh scattering between the photon-dressed states.

We develop Ti:sapphire- and Yb-laser pumped mid-infrared sources with stable carrier-envelope phases using LiGaS2 (LGS), KTiOAsO4 (KTA) and GaSe crystals. These light sources are used to explore the physics of high harmonic generation in solids.

Background and ObjectiveTo elucidate the relationship between melanoma cell adhesion molecule (MCAM)-expressing lymphocytes and pathogenesis of CNS inflammatory demyelinating diseases (IDDs).MethodsPatients with multiple sclerosis (MS) (n = 72) and neuromyelitis optica spectrum disorder (NMOSD, n = 29) were included. We analyzed the frequency and absolute numbers of MCAM+ lymphocytes (memory helper T [mTh] cells, naive helper T cells, CD8+ T cells, and B cells) in the peripheral blood (PB) and the CSF of patients with MS and NMOSD, treated with/without disease-modifying drugs (DMDs) or steroids, using flow cytometry.ResultsThe frequency of MCAM+ cells was higher in the mTh cell subset than that in other lymphocyte subsets. A significant increase in the frequency and the absolute number of MCAM+ mTh cells was observed in the PB of patients with NMOSD, whereas no increase was observed in the PB of patients with MS. The frequency of CSF MCAM+ mTh cells was higher in relapsing patients with MS and NMOSD than that in the control group. Although there was no difference in the frequencies of MCAM+ lymphocytes among the DMD-treated groups, fingolimod decreased the absolute number of MCAM+ lymphocytes.DiscussionMCAM+ mTh cells were elevated in the CSF of relapsing patients with MS and in both the PB and CSF of patients with NMOSD. These results indicate that MCAM contributes to the pathogenesis of MS and NMOSD through different mechanisms. MCAM could be a therapeutic target of CNS IDDs, and further study is needed to elucidate the underlying mechanism of MCAM in CNS IDD pathogenesis.

High-order harmonic generation (HHG) is currently utilized for developing compact table-top radiation sources to provide highly coherent extreme ultraviolet (XUV) and soft X-ray pulses; however, the low repetition rate of fundamental lasers, which is typically in the multi-kHz range, restricts the area of application for such HHG-based radiation sources. Here, we demonstrate a novel method for realizing a MHz-repetition-rate coherent XUV light source by utilizing intracavity HHG in a mode-locked oscillator with an Yb:YAG thin disk laser medium and a 100-m-long ring cavity. We have successfully implemented HHG by introducing two different rare gases into two separate foci and picking up each HH beam. Owing to the two different HH beams generated from one cavity, this XUV light source will open a new route to performing a time-resolved measurement with an XUV-pump and XUV-probe scheme at a MHz-repetition rate with a femtosecond resolution., Extreme ultraviolet: generating coherent pulses inside a laser A compact table-top laser system developed by researchers in Japan can generate extreme ultraviolet radiation (XUV) for ultrafast spectroscopy. Coherent XUV pulses are used to accurately measure data on electronic transitions in atoms and molecules, however they can usually only be generated via synchrotron radiation from large particle accelerators. A possible route to generating XUV on a smaller lab scale is to direct high-order harmonic pulses from a femtosecond laser onto a gas. Katsumi Midorikawa at the RIKEN Center for Advanced Photonics in Wako and co-workers have developed a novel method where the gas targets are actually placed inside the laser cavity. By using two different gases, they can simultaneously generate an XUV pump and a probe, opening a route to very accurate timing of atomic measurements, perhaps resolving events as short as a few femtoseconds.

We report strong terahertz ($\ensuremath{\sim}{10}^{12}\text{ }\text{ }\mathrm{Hz}$) high harmonic generation at room temperature in thin films of ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$, a three-dimensional Dirac semimetal. Third harmonics are detectable with a tabletop light source and can be as strong as $100\text{ }\text{ }\mathrm{V/}\mathrm{cm}$ by applying a fundamental field of $6.5\text{ }\text{ }\mathrm{kV}/\mathrm{cm}$ inside the film, demonstrating an unprecedented efficiency for terahertz frequency conversion. Our time-resolved terahertz spectroscopy and calculations also clarify the microscopic mechanism of the nonlinearity originating in the coherent acceleration of Dirac electrons in momentum space. Our results provide clear insights for nonlinear currents of Dirac electrons driven by the terahertz field under the influence of scattering, paving the way toward novel devices for high-speed electronics and photonics based on topological semimetals.

High harmonic generation in solids attracted great attentions as a new scheme for frequency conversion. We report observation of an extremely efficient terahertz (THz) third- and fifth-harmonic generation in thin films of Cd3As2, a three-dimensional Dirac semimetal with massless electron dispersion, as it is observable with tabletop THz source at room temperature. Our THz pump-THz probe study with subcycle time resolution elucidates that the intraband current of coherently accelerated Dirac electrons is the main source of the THz harmonics as expected theoretically. The results pave the way toward novel devices for ultrafast THz electronics and photonics based on topological semimetals.

Environment DNA (eDNA) is a nonlethal sampling method that has potential for evaluating fisheries resources. To evaluate whether eDNA information is useful for fisheries, we collected and analysed eDNA samples from 14 sites in Tokyo Bay from April 2018 to March 2020. Using MiFish universal primers, fish mitochondrial 12S rRNAs were amplified from the eDNA by polymerase chain reaction (PCR) and then sequencing. A total of 535 eDNA samples yielded 44.5 million reads, with 197 of teleosts and 16 of elasmobranchs identified. Of those, 42 identified species were commercially important, with the most abundant being Engraulis japonicus (Japanese anchovy), followed by Konosirus punctatus (dotted gizzard shad) and Lateolabrax japonicusi (Japanese seabass). Mitochondrial cytochrome b copy numbers of K. punctatus and L. japonicus showed a seasonal trend, although catch per unit effort for L. japonicus was not correlated with copy number. Although eDNA is a powerful tool that can detect the presence of fish species from a water sample, caution should be used when interpreting fish biomass based on eDNA dynamics until the method has been better studied.

We report strong terahertz (∼10^{12} Hz) high harmonic generation at room temperature in thin films of Cd_{3}As_{2}, a three-dimensional Dirac semimetal. Third harmonics are detectable with a tabletop light source and can be as strong as 100 V/cm by applying a fundamental field of 6.5 kV/cm inside the film, demonstrating an unprecedented efficiency for terahertz frequency conversion. Our time-resolved terahertz spectroscopy and calculations also clarify the microscopic mechanism of the nonlinearity originating in the coherent acceleration of Dirac electrons in momentum space. Our results provide clear insights for nonlinear currents of Dirac electrons driven by the terahertz field under the influence of scattering, paving the way toward novel devices for high-speed electronics and photonics based on topological semimetals.

This paper reports an actuation method of a MEMS spiral metamaterial designed for THz (terahertz) electromagnetic frequency. Two-dimensional micro spirals with a diameter of $150\ \mu\mathrm{m}$ transform into three-dimensional micro spirals by the center of each micro spiral being pulled to the out-of-plane direction individually (Fig. 1). At the center, each micro spiral, arrayed on a silicon substrate, have an SU-8 bonding to another Parylene coated silicon substrate, and the micro spirals were stretched by separating the two substrates. It was confirmed that micro spirals were stably stretched to $536\ \mu\mathrm{m}$ at the maximum. The transmission characteristics obtained with THz-TDS (time domain spectroscopy) presented a large increase of polarization effect by the increase of micro spiral height.

We developed a high power storage ring oscillator using a Yb:YAG thin-disk gain medium pumped with 969nm absorption line. The intra-cavity average and peak power of 1kW and 200MW were attained, respectively at 14MHz operation. © 2020 The Author(s)

We perform polarization-resolved spectroscopy to study terahertz anomalous Hall effect (AHE) in Weyl antiferromagnet MmSn thin film. Large AHE comparable to ferromagnets is observed at room temperature, paving a new avenue for high-speed spintronics application.

Antiferromagnetic spin motion at terahertz (THz) frequencies attracts growing interests for fast spintronics, however, their smaller responses to external field inhibit device application. Recently the noncollinear antiferromagnet Mn3Sn, a Weyl semimetal candidate, was reported to show large anomalous Hall effect (AHE) at room temperature comparable to ferromagnets. Dynamical aspect of such large responses is an important issue to be clarified for future THz data processing. Here the THz anomalous Hall conductivity in Mn3Sn thin films is investigated by polarization-resolved spectroscopy. Large anomalous Hall conductivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Re}}\;\sigma _{xy}\left( \omega \right) \sim 20\;{\mathrm{\Omega }}^{ - 1}{\mathrm{cm}}^{ - 1}$$\end{document}Reσxyω~20Ω−1cm−1 at THz frequencies is clearly observed as polarization rotation. A peculiar temperature dependence corresponding to the breaking/recovery of symmetry in the spin texture is also discussed. Observation of the THz AHE at room temperature demonstrates the ultrafast readout for the antiferromagnetic spintronics using Mn3Sn, and will also open new avenue for studying nonequilibrium dynamics in Weyl antiferromagnets., Antiferromagnets promise great potential for spintronic applications due to their spin excitations at terahertz frequencies, however, read out of the spin state is hindered by the small response to external fields. Here the authors demonstrate all optical readout of the spin state of antiferromagnetic Mn3Sn at THz frequency.

Terahertz (THz) cameras comprising an uncooled micro-bolometer array have been developed for simple THz imaging, and the improvement of their sensitivity is one of the important issues. We fabricated a new micro-bolometer array with a resonant cavity structure for a real-time THz camera, alongside a new method for evaluating the sensitivity across a wide range of the THz frequency region. The frequency dependence of the sensitivity of the THz camera is measured in the 0.5–2.0-THz frequency range taking the polarization dependence into account. It was found that the resonant cavity structure effectively increased the sensitivity of the THz camera, and, actually, the improvement by one order of magnitude was achieved in the frequency range below 1 THz. The THz camera with much enhanced sensitivity will expand the frontiers of real-time THz imaging such as molecular imaging and nondestructive inspection.

We report the first experimental observation of backward terahertz emission from the two-color laser induced plasma filament in air. The ratio of measured forward-to-backward terahertz radiation is ∼25/1. This result agrees with numerical simulations based on interferometric model assuming 0.3 mm long plasma source.

We study the angular and frequency-angulardistributions of the low-frequency part (0.3–3 THz) of the terahertz emission from two-color femtosecond plasma spark experimentally and in 3D + time numerical simulations. Our experiments and simulations show, that the weakening in the focusing conditions results in the squeezing of the THz angular distribution and the formation of the bright conical emission.

A terahertz pulse shaper based on optical rectification is proposed. The polarization of the terahertz pulses depends on the polarization selection rules for the rectification process in a GaP crystal. The terahertz pulse shaper can arbitrarily control the chirality, phase, pulse duration and frequency of circularly polarized few-cycle terahertz pulses.

Terahertz (THz) imaging has a strong potential for applications because many molecules have fingerprint spectra in this frequency region. Spectroscopic imaging in the THz region is a promising technique to fully exploit this characteristic. However, the performance of conventional techniques is restricted by the requirement of multidimensional scanning, which implies an image data acquisition time of several minutes. In this study, we propose and demonstrate a novel broadband THz spectroscopic imaging method that enables real-time image acquisition using a high-sensitivity THz camera. By exploiting the two-dimensionality of the detector, a broadband multi-channel spectrometer near 1 THz was constructed with a reflection type diffraction grating and a high-power THz source. To demonstrate the advantages of the developed technique, we performed molecule-specific imaging and high-speed acquisition of two-dimensional (2D) images. Two different sugar molecules (lactose and D-fructose) were identified with fingerprint spectra, and their distributions in one-dimensional space were obtained at a fast video rate (15 frames per second). Combined with the one-dimensional (1D) mechanical scanning of the sample, two-dimensional molecule-specific images can be obtained only in a few seconds. Our method can be applied in various important fields such as security and biomedicine.

We demonstrated intra-cavity high harmonic generation (HHG) in a Yb:YAG thin disk mode-locked oscillator with a repetition rate of 3MHz. Two HHG ports are simultaneously operated in a single mode-locked oscillator.

We study angular and frequency-angular distributions of the terahertz (THz) emission of the low-frequency region (0.3-3 THz) from a two-color femtosecond plasma spark experimentally and in three-dimensional numerical simulations. We investigate the dependence of the angular shapes of the THz radiation on focusing conditions and pulse durations by using two laser facilities (pulse durations 35 and 150 fs) for different focusing geometries. Our experiments and simulations show that decrease in the numerical aperture from NA ≈0.2 to NA ≈0.02 results simultaneously in (I) squeezing of the THz angular distribution and (II) formation of the bright conical emission in the THz range. The moderate focusing NA ≈0.05, which forms the relatively narrow unimodal THz angular distribution, is identified as optimal in terms of angular divergence. Numerical simulations with carrier wave resolved show that bright THz ring structures appear at the frequencies ≥2 THz for longer focuses (NA ≈0.02), while for optimal focusing conditions NA ≈0.05 the conical emission develops at THz frequencies higher than 10 THz.

Active modulation of the polarization states of terahertz light is indispensable for polarization-sensitive spectroscopy, having important applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. In the terahertz region, the lack of a polarization modulator similar to a photoelastic modulator in the visible range hampers expansion of such spectroscopy. A terahertz chiral metamaterial has a huge optical activity unavailable in nature; nevertheless, its modulation is still challenging. Here we demonstrate a handedness-switchable chiral metamaterial for polarization modulation employing vertically deformable Micro Electro Mechanical Systems. Vertical deformation of a planar spiral by a pneumatic force creates a three-dimensional spiral. Enantiomeric switching is realized by selecting the deformation direction, where the polarity of the optical activity is altered while maintaining the spectral shape. A polarization rotation as high as 28° is experimentally observed, thus providing a practical and compact polarization modulator for the terahertz range., Spectroscopy in the terahertz range requires polarization modulation. Here, Kan et al. demonstrate a chirality switchable metamaterial for polarization modulation employing deformable spirals. A polarization rotation as high as 28° is achieved, providing a practical and compact polarization modulator for the terahertz range.

We report the investigation of a strong interband photon drag effect in multilayer graphene leading to efficient emission of terahertz radiation. The obtained terahertz photoresponse of graphene layers exhibits peculiarities fundamentally predicted for free carrier transport in two-dimensional electronic systems. Owing to significant light absorption in gapless graphene, where each absorbed photon produces an electron-hole pair with the highest possible kinetic energy, the photon drag mechanism provides a possibility to achieve efficient conversion of light into broadband terahertz radiation as well as new ways towards vectorial control of the generated terahertz radiation in graphene-based materials.

We propose and demonstrate a method for generating broadband terahertz (THz) vortex beams. We convert a THz radially polarized beam into a THz vortex beam via achromatic polarization optical elements for THz waves and characterize the topological charge of the generated vortex beam by measuring the spatial distribution of the phase of the THz wave at its focal plane. For example, a uniform topological charge of +1 is achieved over a wide frequency range. We also demonstrate that the sign of the topological charge can be easily controlled. By utilizing the orbital angular momentum of the beam, these results open new THz wave technologies for sensing, manipulation, and telecommunication.

We propose a tunable metamaterial actuated by pneumatic force. The unit of the proposed metamaterial has a pair of sprit-ring-resonators (SRR), and the gap between them is controllable around sub-micron-order in size. One SRR is formed on a thin cantilever, which can be bent due to differential pressure between the upper and lower surfaces of the cantilever, whereas the other is fixed on a thin membrane. We confirmed that controlling the gap ranging from sub-micron-order to a few-micron-order was suitable for tuning a resonant frequency of a terahertz metamaterial.

We report the direct observation of the orientation of jet-cooled molecules induced by a THz pulse. The intense THz field ($\ensuremath{\sim}$30 kV/cm) creates a superposition between the rotational ground state ($J=0$) and the excited state ($J=1$) of HBr molecules. The rotational dynamics is probed by Coulomb explosion imaging of the fragment ions produced by a near-infrared femtosecond laser pulse. Comparing with the simulations based on a time-dependent Schr\"odinger equation, the rotational wave packets are completely reconstructed, which reveal that $\ensuremath{\sim}$$7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ of the ground-state population is transferred to the $J=1$ state at an initial temperature of 8 K, achieving a degree of orientation of $\ensuremath{\langle}cos\ensuremath{\theta}\ensuremath{\rangle}\ensuremath{\sim}0.017$.

Bare metal wires have recently been demonstrated as waveguides for transporting terahertz (THz) radiation, where the guiding mode is radially polarized surface Sommerfeld waves. In this study, we demonstrate high-efficiency coupling of a broadband radially polarized THz pulsed beam, which is generated with a polarization-controlled beam by a segmented half-wave-plate mode converter, to bare copper wires. A total coupling efficiency up to 16.8% is observed, and at 0.3 THz, the maximum coupling efficiency is 66.3%. The results of mode-overlap calculation and numerical simulation support the experimental data well.

Summary form only given. Terahertz (THz) imaging technology attracts a lot of attentions for security and biochemical applications because many biochemical molecules have characteristic absorption spectra in the THz frequency region. THz camera, which can take real-time THz images is one of the most important detection devices for the development of THz imaging technology. NEC has developed a THz camera called IRV-T0830 consisting of uncooled microbolometer THz focal plane arrays (THz-FPAs) [1]. Although the operation of real-time imaging system with this camera was demonstrated recently [2], improvement of performance is still required. In this study, we have improved structure of the THz-FPAs to increase the sensitivity in the THz frequency region, and evaluated frequency-dependent sensitivity of the improved THz camera systematically.IRV-T0830 detects THz waves by sensing resistance change of the microbolometers. The resistance change is originated from the temperature increase by the absorption of incident THz waves. In addition, reflection layer is formed at the bottom of air gap located under the microbolometer layer in order to increase the THz absorption efficiency. Optical cavity is formed between reflection layer and THz absorption layer and it was found that the thickness of air gap is closely related to sensitivity of the THz camera [1]. Here, we have developed a new THz camera which air-gap thickness is near to wave length of THz frequency region to increase the sensitivity. THz beam used in sensitivity-measurement experiment was generated by tilted-pulse-front excitation of lithium niobate (LN) crystal. Spectrum of generated THz beam was obtained by electro-optic (EO) sampling method. The spectrum extended to 2 THz. We have measured frequency dependence of sensitivity of THz camera by putting THz band-pass filters in front of THz camera, and measured incident polarization direction angle dependence by rotating THz camera. It was found that sensitivities of both the new THz camera and IRV-T0830 depend on polarization of incident THz beam. Therefore, we estimated frequency dependence of sensitivities by averaging count numbers of each THz camera throughout all polarization angles. Frequency dependence of sensitivity for each THz camera is shown in Fig.1. The sensitivity of the new THz camera is three or four times larger than IRV-T0830 in frequency range of 1-2 THz. THz beam-spot images obtained by two cameras are shown in Fig.2. A band-pass filter which has transmission peak at 1.0 THz and full width at half maximum of 0.24 THz was used, and the power of incident THz beam penetrating the band-pass filter is about 100 μW. The image obtained by the new THz camera is much clearer than IRV-T0830. These results indicate that the structural modification of FPA successfully improved the performance of the THz camera, which should become a critical tool for the development of THz imaging technology.

We propose an electrostatically actuated spiral structures as a metamaterial for circularly polarized light in Terahertz (THz) frequency. Actuation of the spiral structures changes their geometry, and produces a tunable chirality resulting in circular dichroism, which is the differential absorption between the left and right circularly polarized light. Results of the THz-spectroscopy confirmed that the ON/OFF of circular dichroism was achieved with the proposing structures.

We have generated 1-mJ pulses with a pulse duration of 520 fs at a repetition rate of 2.85 MHz in a mode-locked oscillator cavity. These pulses can be used for generating high-harmonics in the cavity.

We investigated the dynamics of photo-induced optical activity of metal chiral gratings on an Si substrate for terahertz (THz) waves. We employed a new technique that enables optical-pump and THz-probe measurements via broadband THz spectroscopy at the microsecond time scale using a low-repetition-rate pump and a high-repetition-rate probe. We revealed that the THz optical activity decays as a result of the carrier diffusion effect because this optical activity is because of the presence of three-dimensional chiral structures of photo-carriers in the Si substrate.

We proposed and demonstrated active control of terahertz optical activity via chiral patterned photo-excitation in a semiconductor with a spatial light modulator. This technique provides a new route to terahertz polarization modulators.

We demonstrated a new method of Terahertz cylindrical vector beam generation using segmented nonlinear crystals with three-fold rotational symmetry. The polarization dependent in-plane images obtained with a Terahertz camera clearly showed axial-symmetric polarization distributions. The electro-optic sampling image clearly visualized the longitudinal electric field at the focal point.

Although it is well accepted that the ultrafast manipulation of spins or magnetization in solid promises potential applications in coherent terahertz (THz) radiation source, spintronics and quantum information processing, their performance is significantly limited by the weak coupling between radiation field and magnetic dipole oscillation. For such ‘weak’ magnetic system, we propose an effective and simple route based on the cavity-based phase modulation technique towards the efficient energy extraction, demonstrated via controlling the magnetic dipole THz radiation generated in the nonlinear Raman process from antiferromagnetic (AFM) NiO. An asymmetric coupled Fabry-Perot (FP) cavity is constituted by simply placing a metallic planar mirror in the vicinity of a NiO slab. The energy-extraction (THz radiation) can be effectively manipulated by changing the NiO-mirror distance to modulate the phase relation between the magnetic wave and the induced magnetization in NiO. The distinct radiation control can be observed and the experiments are well explained by numerically analyzing the radiation dynamics that highlights the role of phase modulation during the radiation process.

Application of coherent light–matter interactions has recently been extended to the ultrafast control of magnetization. An important but unrealized technique is the manipulation of magnetization vector motion to make it follow an arbitrarily designed multidimensional trajectory. Here we demonstrate a full manipulation of two-dimensional magnetic oscillations in antiferromagnetic NiO with a pair of polarization-twisted femtosecond laser pulses. We employ Raman-type nonlinear optical processes, wherein magnetic oscillations are impulsively induced with a controlled initial phase. Their azimuthal angle follows well-defined selection rules that have been determined by the symmetries of the materials. We emphasize that the temporal variation of the laser-pulse polarization angle enables us to control the phase and amplitude of the two degenerate modes, independently. These results lead to a new concept of the vectorial control of magnetization by light., Light–matter interactions can be used to manipulate magnetization in solids, but light-controlled magnetization vector motion has not been demonstrated. Here, two-dimensional magnetic oscillations in NiO are manipulated with optical pulses leading to vectorial control of magnetization by light.

Recently, ultrafast control of magnetization via instantaneous stimulated Raman-type nonlinear optical processes has gained considerable interest. Here, we discuss that the conservation of angular momentum rules the light induced magnetization [1]. In light-magnetization interactions in solids, the balance of angular momentum included contributions from the light, the magnetization, and the crystalline lattice. In this presentation, we high-light the role of the lattice, in that it provides angular momentum in discrete units in correspondence to its rotational symmetry, which is the rotational analogue of the Umklapp process [2].

We demonstrated strong enhancement of coherent magnetic dipole radiation by a factor of up to ~9 in antiferromagnetic NiO with the direct phase manipulation by cavity effect. The results are reproduced well by FDTD calculation.

We demonstrated arbitrarily polarized magnetization control in an antiferromagnet by stimulated Raman processes with time- and polarization- controlled double pulses. This technique has lead to a new concept of vectorial control of magnetization by light.

We propose Raman-induced collinear difference-frequency generation (DFG) as a method to manipulate dynamical magnetization. When a fundamental beam propagates along a threefold rotational axis, this coherent second-order optical process is permitted by angular momentum conservation through the rotational analogue of the umklapp process. As a demonstration, we experimentally obtained polarization properties of collinear magnetic DFG along a [111] axis of a single crystal of antiferromagnetic NiO with micromultidomain structure, which excellently agreed with the theoretical prediction.

We report pronounced light-induced change of the optical activity at terahertz frequency in metal chiral gratings on semiconductor substrates. This result opens new horizons in the active terahertz polarization control.