Abstract-Terahertz broadband anti-reflection moth-eye structures fabricated by femtosecond laser processing

Haruyuki Sakurai, Natsuki Nemoto, Kuniaki Konishi, Ryota Takaku, Yuki Sakurai, Nobuhiko Katayama, Tomotake Matsumura, Junji Yumoto, and Makoto Kuwata-Gonokami

 Overview of the fabrication process. Moth-eye structures are fabricated by linearly scanning the sample in a microscopic grid pattern against a focused laser.

https://www.osapublishing.org/osac/abstract.cfm?uri=osac-2-9-2764

Anti-reflection (AR) coatings aiming at the reduction of Fresnel reflection losses has come into demand in the terahertz (THz) region. Implementation of such a coating in practice is a difficult task, partially because the broad spectrum of the THz signal is difficult to control. Here, we propose and demonstrate a moth-eye AR structure capable of suppressing reflection losses in the range of 0.3 to 2.5 THz for high-resistivity silicon, resulting in a maximum transmission of 91%. The structure comprises of pyramid-like structures with a height of about 100 μm created on the material surface by femtosecond laser processing. We demonstrate experimentally and theoretically that such micromachining considerably increases transmittance of the silicon in the spectral range of 0.3–2.5 THz. We also demonstrate experimentally that such a structure allows one to improve performance of the THz source based on the LiNbO3 crystal.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Ultrashort Pulsed-Laser Fabrication of Silicon Moth-Eye Structures for Terahertz Anti-Reflection

Haruyuki Sakurai, Natsuki Nemoto, Kuniaki Konishi, Yuki Sakurai, Nobuhiko Katayama, Tomotake Matsumura, Junji Yumoto, and Makoto Kuwata-Gonokami

https://www.osapublishing.org/abstract.cfm?uri=cleo_qels-2018-JW2A.77&origin=search

We fabricate moth-eye anti-reflection structures for the terahertz region on high-resistivity silicon substrates by ultrashort pulsed-laser ablation. We demonstrate 1.4 times increased THz yield from Mg:LN by using the moth-eye structure as an output coupler.

© 2018 The Author(s)

Abstract-Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields

Katsumasa Yoshioka, Ikufumi Katayama, Yusuke Arashida, Atsuhiko Ban, Yoichi Kawada, Kuniaki Konishi, Hironori Takahashi,  Jun Takeda,

https://pubs.acs.org/doi/10.1021/acs.nanolett.8b02161

Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip–sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription—in situ tailoring of single-cycle THz near fields in a tunnel junction—will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology

Abstract-Ultrafast zero-bias photocurrent and terahertz emission in hybrid perovskites

Petr A. Obraztsov, Dmitry Lyashenko, Pavel A. Chizhov, Kuniaki Konishi, Natsuki Nemoto, Makoto Kuwata-Gonokami, Eric Welch, Alexander N. Obraztsov,  Alex Zakhidov,

https://www.nature.com/articles/s42005-018-0013-8

Methylammonium lead iodide is a benchmark hybrid organic perovskite material used for low-cost printed solar cells with a power conversion efficiency of over 20%. Nevertheless, the nature of light–matter interaction in hybrid perovskites and the exact physical mechanism underlying device operation are currently debated. Here, we report room temperature, ultrafast photocurrent generation, and free-space terahertz emission from unbiased hybrid perovskites induced by femtosecond light pulses. The polarization dependence of the observed photoresponse is consistent with the bulk photovoltaic effect caused by a combination of injection and shift currents. Observation of this type of photocurrents sheds light on the low recombination and long carrier diffusion lengths arising from the indirect bandgap in CH3NH3PbI3. Naturally ballistic shift and injection photocurrents may enable third-generation perovskite solar cells with efficiency exceeding the Shockley–Queisser limit. The demonstrated control over photocurrents with light polarization also opens new venues toward perovskite spintronics and tunable THz devices.

Abstract-Ultrafast polarization control of zero-bias photocurrent and terahertz emission in hybrid organic perovskites

Petr A. Obraztsov, Dmitry Lyashenko, Pavel A. Chizhov, Kuniaki Konishi, Natsuki Nemoto, Makoto Kuwata-Gonokami, Eric Welch, Alexander N. Obraztsov, Alex Zakhidov

(Submitted on 26 Oct 2017)

https://arxiv.org/abs/1710.09728

Methylammonium lead iodide (MAPI) is a benchmark hybrid organic perovskite material, which is used for the low-cost, printed solar cells with over 20 percent power conversion efficiency. Yet, the nature of light-matter interaction in MAPI as well as the exact physical mechanism behind device operation is currently debated. Here we report room temperature, ultrafast photocurrent and freespace terahertz (THz) emission generation from unbiased MAPI induced by 150 fs light pulses. Polarization dependence of the observed photoresponse is consistent with the Bulk Photovoltaic Effect (BPVE) caused by a combination of injection and shift currents. We believe that this observation of can shed light on low recombination, and long carrier diffusion lengths due to indirect bandgap. Moreover, ballistic by nature shift and injection BPVE photocurrents may enable third generation perovskite solar cells with efficiency that exceed the Shockley_Queisser limit. Our observations also open new venues for perovskite spintronics and tunable THz sources.

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.

Reconfigurable micro spirals control light

http://www.nanowerk.com/nanotechnology-news/newsid=42351.php

(Nanowerk News) A University of Tokyo research group has developed an active polarization filter which is able to switch the circular polarization state of light between right and left in the terahertz frequency region at wavelengths of several hundred micrometers ("Enantiomeric Switching of Chiral Metamaterial for Terahertz Polarization Modulation Employing Vertically Deformable MEMS Spirals").

A schematic illustration of polarization modulation by the spiral metamaterial. Passing light through a metamaterial composed of an array of reconfigurable metal spirals changes its polarization state. By changing the direction of the conic helices, the circular polarization of the light transmitted through the material can be switched from left to right and vice versa. (Image: Tetsuo Kan)

Spectroscopy using polarized light, in which the direction of oscillation of the light’s electric field is aligned, enables us to obtain detailed information about materials. Moreover, dynamic switching of polarization enables even more precise measurement. However, the lack of a simple device that can actively switch the polarization state of light in the terahertz region has prevented the development of technologies employing polarized terahertz light.

The research group of University of Tokyo Graduate School of Information Science and Technology of Professor Isao Shimoyama, Graduate School of Science Professor Makoto Gonokami (currently University President), and the Graduate School of Information Science and Technology Assistant Professor Tetsuo Kan, and Graduate School of Science Assistant Professor Kuniaki Konishi developed a terahertz metamaterial device to dynamically switch the polarization state of terahertz waves.

The metamaterial is composed of an array of reconfigurable metal spirals, which are 150-?m in diameter, using micro electro mechanical systems (MEMS) technology. Applying force perpendicular to the plane of the spirals causes the spirals to deform, creating three dimensional conic helices.

Depending on the direction of the force applied, the conic helices become left-handed or right-handed respectively. This handedness alternation brings about the switching of the polarization state of the transmitted light. Mechanically changing the handedness of the micro spirals has been difficult to achieve in a small, three-dimensional structure, making this the first time it has been achieved.

“The use of MEMS techniques made it possible to realize this structure,” says Professor Shimoyama. He continues, “Since the spectra of a material with respect to circularly polarized light contains clues to its three-dimensional molecular structure, this device may, for example, enable the development of technologies for easily obtaining information about large structural arrangements in materials with this new approach that is would otherwise be difficult to obtain from analysis of X-ray diffraction patterns. In the future, it may be possible to realize compact analytical instruments that can perform on-site analyses of dangerous drugs or other materials.”

Read more: Reconfigurable micro spirals control light 

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

• 
• 
• 
• 

• 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