Browsing by Author "Katsutani, Fumiya"

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    Cross-Polarized Excitons and Plasmons Aligned Carbon Nanotubes
    (2018-07-17) Katsutani, Fumiya; Kono, Junichiro
    Single-wall carbon nanotubes (SWCNTs) exhibit strong absorption resonances for parallel -polarized light, arising from one-dimensional excitons with enormous binding energies, promising for a variety of applications in optoelectronics. However, how SWCNTs respond to light polarized perpendicular to the nanotube axis has not been explored experimentally due to the unavailability of suitable samples. In this study, we have observed new types of excitonic and plasmonic resonances for perpendicular-polarized light in aligned SWCNT films. Specifically, first, we observed an interband absorption resonance for perpendicular polarization through so-called cross-polarized excitons in a single-chirality aligned SWCNT film. This direct observation by absorption spectroscopy allowed us to determine the oscillator strength quantitatively. Second, we observed intersubband plasmons for perpendicular polarization when and only when gated and aligned SWCNT films. This observation allowed us to provide insight into the collective dynamic response of interacting electrons in one dimension.
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    Intersubband plasmons in the quantum limit in gated and aligned carbon nanotubes
    (Springer Nature, 2018) Yanagi, Kazuhiro; Okada, Ryotaro; Ichinose, Yota; Yomogida, Yohei; Katsutani, Fumiya; Gao, Weilu; Kono, Junichiro
    Confined electrons collectively oscillate in response to light, resulting in a plasmon resonance whose frequency is determined by the electron density and the size and shape of the confinement structure. Plasmons in metallic particles typically occur in the classical regime where the characteristic quantum level spacing is negligibly small compared to the plasma frequency. In doped semiconductor quantum wells, quantum plasmon excitations can be observed, where the quantization energy exceeds the plasma frequency. Such intersubband plasmons occur in the mid- and far-infrared ranges and exhibit a variety of dynamic many-body effects. Here, we report the observation of intersubband plasmons in carbon nanotubes, where both the quantization and plasma frequencies are larger than those of typical quantum wells by three orders of magnitude. As a result, we observed a pronounced absorption peak in the near-infrared. Specifically, we observed the near-infrared plasmon peak in gated films of aligned single-wall carbon nanotubes only for probe light polarized perpendicular to the nanotube axis and only when carriers are present either in the conduction or valence band. Both the intensity and frequency of the peak were found to increase with the carrier density, consistent with the plasmonic nature of the resonance. Our observation of gate-controlled quantum plasmons in aligned carbon nanotubes will not only pave the way for the development of carbon-based near-infrared optoelectronic devices but also allow us to study the collective dynamic response of interacting electrons in one dimension.
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    Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films
    (AIP Publishing LLC, 2018) Fukuhara, Kengo; Ichinose, Yota; Nishidome, Hiroyuki; Yomogida, Yohei; Katsutani, Fumiya; Komatsu, Natsumi; Gao, Weilu; Kono, Junichiro; Yanagi, Kazuhiro
    How the morphology of a macroscopic assembly of nanoobjects affects its properties is a long-standing question in nanomaterials science and engineering. Here, we examine how the thermoelectric properties of a flexible thin film of carbon nanotubes depend on macroscopic nanotube alignment. Specifically, we have investigated the anisotropy of the Seebeck coefficient of aligned and gated single-wall carbon nanotube thin films. We varied the Fermi level in a wide range, covering both theﾠp-type andﾠn-type regimes, using electrolyte gating. While we found the electrical conductivity along the nanotube alignment direction to be several times larger than that in the perpendicular direction, the Seebeck coefficient was found to be fully isotropic, irrespective of the Fermi level position. We provide an explanation for this striking difference in anisotropy between the conductivity and the Seebeck coefficient using Mott's theory of hopping conduction. Our experimental evidence for an isotropic Seebeck coefficient in an anisotropic nanotube assembly suggests a route toward controlling the thermoelectric performance of carbon nanotube thin films through morphology control.
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    Spectroscopy of Two-Dimensional Excitons in High Magnetic Fields to 30 Tesla
    (2020-08-13) Katsutani, Fumiya; Kono, Junichiro
    Excitons are hydrogen-like objects in solids, consisting of an electron and a hole bound together through their mutual Coulomb attraction. They play major roles in determining the optoelectronic properties of semiconductors. Magneto-optical spectroscopy provides valuable information on excitons through observation of diamagnetic shifts and Zeeman splittings, but high magnetic fields are required to observe these effects in two-dimensional semiconductors due to enhanced binding energies compared to bulk systems. We have developed a unique magnetospectroscopy setup to investigate materials in fields up to 30 T. The developed setup allows us to change the sample temperature (between 12 K and 300 K), the field direction (negative or positive), the sample configuration (Faraday or Voigt configurations), and the light source (a pulsed laser or a continuous-wave source). We first demonstrated diamagnetic shifts, Landau quantization, and superfluorescence in an InGaAs quantum well sample. Furthermore, we studied two-dimensional hybrid organic-inorganic halide perovskites, which attract much recent attention due to their promising properties for optoelectronic devices such as solar cells and light-emitting devices. We investigated their magnetooptical properties, observing unique temperature-dependent diamagnetic shifts.