Browsing by Author "Tonouchi, Masayoshi"

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    Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides
    (The Optical Society, 2016) Razanoelina, Manjakavahoaka; Bagsican, Filchito Renee; Kawayama, Iwao; Zhang, Xiang; Ma, Lulu; Murakami, Hironaru; Vajtai, Robert; Ajayan, Pulickel M.; Kono, Junichiro; Tonouchi, Masayoshi
    As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 1011 cm−2, which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.
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    Terahertz and Infrared Spectroscopy of Gated Large-Area Graphene
    (American Chemical Society, 2012) Ren, Lei; Zhang, Qi; Yao, Jun; Sun, Zhengzong; Kaneko, Ryosuke; Yan, Zheng; Nanot, Sébastien L.; Jin, Zhong; Kawayama, Iwao; Tonouchi, Masayoshi; Tour, James M.; Kono, Junichiro; Applied Physics Program
    We have fabricated a centimeter-size single-layer graphene device with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10–10 000 cm–1), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods were able to effectively tune the Fermi energy, EF, which in turn modified the Drude-like intraband absorption in the terahertz as well as the “2EF onset” for interband absorption in the mid-infrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.
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    Terahertz Dynamics of Quantum-Confined Electrons in Carbon Nanomaterials
    (Springer, 2012) Ren, Lei; Zhang, Qi; Nanot, Sébastien; Kawayama, Iwao; Tonouchi, Masayoshi; Kono, Junichiro
    Low-dimensional carbon nanostructures, such as single-wall carbon nanotubes (SWCNTs) and graphene, offer new opportunities for terahertz science and technology. Being zero-gap systems with a linear, photon-like energy dispersion, metallic SWCNTs and graphene exhibit a variety of extraordinary properties. Their DC and linear electrical properties have been extensively studied in the last decade, but their unusual finite-frequency, nonlinear, and/or non-equilibrium properties are largely unexplored, although they are predicted to be useful for new terahertz device applications. Terahertz dynamic conductivity measurements allow us to probe the dynamics of such photon-like electrons, or massless Dirac fermions. Here, we use terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy to investigate terahertz conductivities of one-dimensional and two-dimensional electrons, respectively, in films of highly aligned SWCNTs and gated largearea graphene. In SWCNTs, we observe extremely anisotropic terahertz conductivities, promising for terahertz polarizer applications. In graphene, we demonstrate that terahertz and infrared properties sensitively change with the Fermi energy, which can be controlled by electrical gating and thermal annealing.
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    The 2023 terahertz science and technology roadmap
    (IOP Publishing, 2023) Leitenstorfer, Alfred; Moskalenko, Andrey S.; Kampfrath, Tobias; Kono, Junichiro; Castro-Camus, Enrique; Peng, Kun; Qureshi, Naser; Turchinovich, Dmitry; Tanaka, Koichiro; Markelz, Andrea G.; Havenith, Martina; Hough, Cameron; Joyce, Hannah J.; Padilla, Willie J.; Zhou, Binbin; Kim, Ki-Yong; Zhang, Xi-Cheng; Jepsen, Peter Uhd; Dhillon, Sukhdeep; Vitiello, Miriam; Linfield, Edmund; Davies, A. Giles; Hoffmann, Matthias C.; Lewis, Roger; Tonouchi, Masayoshi; Klarskov, Pernille; Seifert, Tom S.; Gerasimenko, Yaroslav A.; Mihailovic, Dragan; Huber, Rupert; Boland, Jessica L.; Mitrofanov, Oleg; Dean, Paul; Ellison, Brian N.; Huggard, Peter G.; Rea, Simon P.; Walker, Christopher; Leisawitz, David T.; Gao, Jian Rong; Li, Chong; Chen, Qin; Valušis, Gintaras; Wallace, Vincent P.; Pickwell-MacPherson, Emma; Shang, Xiaobang; Hesler, Jeffrey; Ridler, Nick; Renaud, Cyril C.; Kallfass, Ingmar; Nagatsuma, Tadao; Zeitler, J. Axel; Arnone, Don; Johnston, Michael B.; Cunningham, John
    Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.
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    Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control
    (American Chemical Society, 2023) Wais, Michael; Bagsican, Filchito Renee G.; Komatsu, Natsumi; Gao, Weilu; Serita, Kazunori; Murakami, Hironaru; Held, Karsten; Kawayama, Iwao; Kono, Junichiro; Battiato, Marco; Tonouchi, Masayoshi
    The one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy but also other types of behaviors are suppressed by disorder. Here, we compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks using a combination of terahertz emission and photocurrent experiments and out-of-equilibrium numerical simulations. We find that the degree of alignment strongly influences the excited quasiparticles’ dynamics, rerouting the thermalization pathways. This is, in particular, evidenced in the high-energy, high-momentum electronic population (probed through the formation of low energy excitons via exciton impact ionization) and the transport regime evolving from diffusive to superdiffusive.