Abstract-Room-temperature terahertz anomalous Hall effect in Weyl antiferromagnet Mn3Sn thin films

• Takuya Matsuda, 
• Natsuki Kanda, 
• Tomoya Higo, 
• N. P. Armitage, 
• Satoru Nakatsuji & 
• Ryusuke Matsunaga 

• 

   Crystal and magnetic structures of Mn3Sn and the THz longetudinal conductivity spectra.

https://www.nature.com/articles/s41467-020-14690-6

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 Reσxy(ω)∼20Ω−1cm−1$\mathrm{R}\mathrm{e}{\sigma }_{xy}\left(\omega \right)\sim 20{\mathrm{\Omega }}^{-1}{\mathrm{c}\mathrm{m}}^{-1}$ 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.

Abstract-Real-time broadband terahertz spectroscopic imaging by using a high-sensitivity terahertz camera

• Natsuki Kanda
• , Kuniaki Konishi
• , Natsuki Nemoto
• , Katsumi Midorikawa
•  & Makoto Kuwata-Gonokami

http://www.nature.com/articles/srep42540

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.

Abstract-Enantiomeric switching of chiral metamaterial for terahertz polarization modulation employing vertically deformable MEMS spirals

• Tetsuo Kan,
• Akihiro Isozaki,
• Natsuki Kanda,
• Natsuki Nemoto,
• Kuniaki Konishi,
• Hidetoshi Takahashi,
• Makoto Kuwata-Gonokami,
• Kiyoshi Matsumoto
• & Isao Shimoyama
• http://www.nature.com/ncomms/2015/151001/ncomms9422/full/ncomms9422.html
• 
  

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.

Abstract-Photon-drag-induced terahertz emission from graphene

Petr A. Obraztsov1,2,*, Natsuki Kanda3,4, Kuniaki Konishi5, Makoto Kuwata-Gonokami4,5,6, Sergey V. Garnov1, Alexander N. Obraztsov2,7, and Yuri P. Svirko2

• 1A. M. Prokhorov General Physics Institute, Moscow, Russia
• 2Department of Physics and Mathematics, University of Eastern Finland, Joensuu, Finland
• 3Laser Technology Laboratory, RIKEN, Saitama, Japan
• 4Photon Science Center, The University of Tokyo, Tokyo, Japan
• 5Institute for Photon Science and Technology, The University of Tokyo, Tokyo, Japan
• 6Department of Physics, The University of Tokyo, Tokyo, Japan
• 7Department of Physics, M. V. Lomonosov Moscow State University, Moscow, Russia

• *p.obraztsov@gmail.com
• https://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.241416

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.

DOI: http://dx.doi.org/10.1103/PhysRevB.90.241416

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• 
• 

• Published 29 December 2014
• Received 9 October 2014

©2014 American Physical Society

Abstract-Highly precise and accurate terahertz polarization measurements based on electro-optic sampling with polarization modulation of probe pulses

Natsuki Nemoto, Takuya Higuchi, Natsuki Kanda, Kuniaki Konishi, and Makoto Kuwata-Gonokami  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-15-17915

Optics Express, Vol. 22, Issue 15, pp. 17915-17929 (2014)
http://dx.doi.org/10.1364/OE.22.017915

We have developed an electro-optic (EO) sampling method with polarization modulation of probe pulses; this method allows us to measure the direction of a terahertz (THz) electric-field vector with a precision of 0.1 mrad in a data acquisition time of 660 ms using a 14.0-kHz repetition rate pulsed light source. Through combination with a THz time-domain spectroscopy technique, a time-dependent two-dimensional THz electric field was obtained. We used a photoelastic modulator for probe-polarization modulation and a (111)-oriented zincblende crystal as the EO crystal. Using the tilted pulse front excitation method with stable regeneratively amplified pulses, we prepared stable and intense THz pulses and performed pulse-by-pulse analog-to-digital conversion of the signals. These techniques significantly reduced statistical errors and enabled sub-mrad THz polarization measurements. We examined the performance of this method by measuring a wire-grid polarizer as a sample. The present method will open a new frontier of high-precision THz polarization sensitive measurements.

© 2014 Optical Society of America

Abstract-Generation of broadband terahertz vortex beams

Ryo Imai, Natsuki Kanda, Takuya Higuchi, Kuniaki Konishi, and Makoto Kuwata-Gonokami  »View Author Affiliations

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-39-13-3714

Optics Letters, Vol. 39, Issue 13, pp. 3714-3717 (2014)
http://dx.doi.org/10.1364/OL.39.003714

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.

© 2014 Optical Society of America

Abstract-Efficient coupling of propagating broadband terahertz radial beams to metal wires

Zhu Zheng, Natsuki Kanda, Kuniaki Konishi, and Makoto Kuwata-Gonokami »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-9-10642

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.
© 2013 OSA

Abstract-Dynamics of photo-induced terahertz optical activity in metal chiral gratings

http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-37-17-3510

Natsuki Kanda, Kuniaki Konishi, and Makoto Kuwata-Gonokami

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.

© 2012 Optical Society of America