Tohru Suemoto

Synthesis of an Electromagnetic Wave Absorber for High-Speed Wireless Communication

Millimeter waves (30-300 GHz) are starting to be used in next generation high-speed wireless communications. To avoid electromagnetic interference in this wireless communication, finding a suitable electromagnetic wave absorber in the millimeter wave range is an urgent matter. In this work, we prepared a high-performance millimeter wave absorber composed of a series of aluminum-substituted epsilon-iron oxide, epsilon-Al(x)Fe(2-x)O(3), nanomagnets (0 < or = x < or = 0.40) with a particle size between 25 and 50 nm. The materials in this series have an orthorhombic crystal structure in the Pna2(1) space group, which has four nonequivalent Fe sites and Al ion that predominantly occupies the tetrahedral [FeO(4)] site. The field-cooled magnetization curves showed that the T(C) values were 448, 480, and 500 K for x = 0.40, 0.21, and 0, respectively. The magnetization versus external magnetic field showed that the coercive field H(c) values at 300 K were 10.2, 14.9, and 22.5 kOe for x = 0.40, 0.21, and 0, respectively. The millimeter wave absorption properties were measured at room temperature by terahertz time domain spectroscopy. The frequencies of the absorption peaks for x = 0.40, 0.30, 0.21, 0.09, 0.06, and 0 were observed at 112, 125, 145, 162, 172, and 182 GHz, respectively. These absorptions are due to the natural resonance achieved by the large magnetic anisotropies in this series. Such frequencies are the highest ones for magnetic materials. Because aluminum is the third most abundant atom, aluminum-substituted epsilon-iron oxide is very economical, and thus these materials are advantageous for industrial applications.

Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg, Zn)O multiple quantum wells

We report on temperature dependence of excitonic photoluminescence (PL) from ZnO/(Mg, Zn)O multiple quantum wells (MQWs). Two kinds of MQWs having different barrier heights grown by laser molecular-beam epitaxy showed significantly different temperature dependences of PL spectra; in ZnO/Mg0.27Zn0.73O MQWs, the PL peak energy at 50–200 K was a monotonically increasing function of temperature, which was opposite to that ascribed by band gap shrinkage. Moreover, spectra taken at 95–200 K encompassed two peaks, both of which originated from recombination of localized excitons. The temperature-induced shift (redshift-blueshift-peak duplication-redshift) at 5–300 K is caused by a change in the exciton dynamics with increasing temperature due to inhomogeneity and the exciton localization effect. On the other hand, the corresponding dependence in ZnO/Mg0.12Zn0.88O MQWs (lower barrier height) was similar to that in bulk II–VI semiconductors.

High collection efficiency in fluorescence microscopy with a solid immersion lens

High efficiency of light collection was demonstrated by applying solid immersion lenses (SILs) with refractive indices n=1.845 and 1.687 to fluorescence microscopy of 0.11-μm-radius dye-doped polystyrene sphere beads. This was analyzed with theories on the radiation of dipoles near the surface of the dielectric medium. The estimated collection efficiency with the NA=0.8 objective and the n=1.845 SIL was about 60%–70%, which well exceeds the diffraction-limit value of 50% for 2π-solid angle collection in conventional methods.

Hard magnetic ferrite with a gigantic coercivity and high frequency millimetre wave rotation

Magnetic ferrites such as Fe3O4 and Fe2O3 are extensively used in a range of applications because they are inexpensive and chemically stable. Here we show that rhodium-substituted ε-Fe2O3, ε-RhxFe2−xO3 nanomagnets prepared by a nanoscale chemical synthesis using mesoporous silica as a template, exhibit a huge coercive field (Hc) of 27 kOe at room temperature. Furthermore, a crystallographically oriented sample recorded an Hc value of 31 kOe, which is the largest value among metal-oxide-based magnets and is comparable to those of rare-earth magnets. In addition, ε-RhxFe2−xO3 shows high frequency millimetre wave absorption up to 209 GHz. ε-Rh0.14Fe1.86O3 exhibits a rotation of the polarization plane of the propagated millimetre wave at 220 GHz, which is one of the promising carrier frequencies (the window of air) for millimetre wave wireless communications.

Photoinduced metallic state in VO2 proved by the terahertz pump-probe spectroscopy

We investigated the ultrafast terahertz response to the photoexcitation for vanadium dioxide single crystals and thin films using the optical-pump terahertz-probe technique at room temperature. The optical excitation induced an ultrafast decrease of the transmittance of the terahertz radiation within 0.7ps. Since we expect only the free carrier response in the terahertz range, the decrease of the transmittance is unambiguously assigned to the appearance of the high electronic conductivity due to free carriers. The conductivity increases more than ten times in the picosecond time range after photoexcitation and it is concluded that the electronic states are metallic.

Ultrafast time domain demonstration of bulk magnetization precession at zero magnetic field ferromagnetic resonance induced by terahertz magnetic field

We report the first observation of sub-terahertz bulk-magnetization precession, using terahertz time-domain spectroscopy. The magnetization precession in gallium-substituted epsilon-iron oxide nano-ferromagnets under zero magnetic field is induced by the impulsive magnetic field of the THz wave through the gyromagnetic effect. Just at the resonance frequency, the linear to circular polarized wave conversion is realized. This is understood as the free induction decay signal radiated from a rotating magnetic dipole corresponding to the natural resonance. Furthermore, this demonstration reveals that the series of gallium-substituted epsilon-iron oxide nano-ferromagnets is very prospective for magneto-optic devices, which work at room temperature without external magnetic field, in next-generation wireless communication.

High-power THz wave generation in plasma induced by polarization adjusted two-color laser pulses

We introduce a simple and efficient method of enhancing the terahertz field in an air plasma produced by two-color laser pulses, by inserting a specially designed dual-wavelength wave plate between the non-linear optical crystal and the plasma. Adjusting the polarization of the two laser pulses yielded an electric field of 1.4 MV/cm, which was 1.7 times as intense as that obtained from the unmodified system. Additionally, taking a dispersion of the group velocities of the two-color laser pulses into account, we discussed the validity of the enhancement factor.

Time-Resolved Absorption Spectroscopy of Self-Trapped Excitons in Condensed Ne, Ar, and Kr

The triplet self-trapped excitons (STE) in liquid and solid rare gases are investigated by means of absorption and luminescence spectroscopy under pulsed electron beam excitation. Absorptions from 3 Σ u state of molecular-type STE are observed in Ne, Ar, and Kr, and an intense band in the near infrared region is ascribed to the \(^{3}\varSigma_{\text{u}}{\rightarrow}\varPi_{\text{g}}\) Rydberg type transition. In solid and liquid Ne, the 3s-3p Rydberg transitions from 3 P 2 state of atomic-type STE are found, in addition to the molecular-type STE. The radiative lifetime of 3 Σ u is also determined for Ar and Kr from decay of the vacuum UV luminescence, and is found to agree with that of the absorption from 3 Σ u ; the lifetimes are 1.41±0.05 µsec and 0.09±0.005 µsec for solid Ar and Kr, respectively.

Non-thermal hot electrons ultrafastly generating hot optical phonons in graphite

Investigation of the non-equilibrium dynamics after an impulsive impact provides insights into couplings among various excitations. A two-temperature model (TTM) is often a starting point to understand the coupled dynamics of electrons and lattice vibrations: the optical pulse primarily raises the electronic temperature Tel while leaving the lattice temperature Tl low; subsequently the hot electrons heat up the lattice until Tel = Tl is reached. This temporal hierarchy owes to the assumption that the electron-electron scattering rate is much larger than the electron-phonon scattering rate. We report herein that the TTM scheme is seriously invalidated in semimetal graphite. Time-resolved photoemission spectroscopy (TrPES) of graphite reveals that fingerprints of coupled optical phonons (COPs) occur from the initial moments where Tel is still not definable. Our study shows that ultrafast-and-efficient phonon generations occur beyond the TTM scheme, presumably associated to the long duration of the non-thermal electrons in graphite.

Formation Process of Self-Trapped Exciton Bubbles in Solid Neon

The optical absorption spectrum of the atomic-type self-trapped excitons in solid neon is studied by time-resolved spectroscopy under pulsed electron beam excitation. The time evolution of the spectrum indicates that a microscopic cavity is formed around the excited atom resulting in a bubble state of the self-trapped exciton. A bubble growth model based on capture of the random walking vacancies by the excited atom is developed, and the time development of the spectrum and its temperature dependence are reasonably interpreted in terms of this model.