Terahertz (THz) science and technology is now attracting increasing attention around the world, and materials, devices, and systems exploiting the THz waveband are set to play an important role in a very diverse range of applications. In this paper, we provide an overview of the current status and future prospects of the THz technology.

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  Recent progresses of the terahertz time-domain spectroscopy are reviewed in view of broadening the detection bandwidth that can be measured. Using an ultra-short pulsed laser with the pulse duration of 15 fs and photo-conducting antennas in the reflection geometry, we can realize useful broadband terahertz spectrometers that are capable of measuring real and imaginary parts of the dielectric constants in materials over a wide frequency range. The detection bandwidth expands up to 20 THz with the continuous phase information with only a small discontinuity around 8 THz. Even the higher frequencies up to 170 THz can be generated and detected using a 5-fs pulsed laser with on organic nonlinear crystal (DAST) and a photo-conducting antenna. These methods have a wide range of the applications and the several examples such as characterization of dielectric thin films and light-pump terahertz-probe spectroscopy are reviewed.

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  Photon counting method is indispensable in visible/near-infrared optical measurements for detecting extremely weak radiation. The method, however, has been inaccessible in terahertz region, where the photon energies are more than 100 times smaller and catching individual photons is difficult. Here we review recent development of single terahertz photon detectors and passive sensing systems to locally pick up spontaneous emission in an object. Applying a passive terahertz microscope with quantum-dot single photon detectors, we image extremely weak cyclotron radiation emitted from quantum Hall effect devices. Owing to the unprecedented sensitivity, a variety of new features of electron kinetics are unveiled. Besides semiconductor electric devices studied here, the experimental method will find application in diverse areas of molecular dynamics, microthermography, and cell calorimetry.

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  Carbon nanotubes have potential to make breakthrough achievement in nano-electronics, photonics, and ultrafast devices. We report real-time observations of coherent phonons in aligned single-walled carbon nanotubes through the time-resolved reflectivity measurements with sub-10 fs laser pulses. In the isotropic reflectivity measurements, we observe the radial breathing modes, G and even D modes, while in the anisotropic reflectivity measurements, only the G mode appears. A complex polarization dependence of the G band phonon amplitude in the isotropic reflectivity indicates the superposition of G band phonons with different symmetries.

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  Significant progress in recent development of synchrotron radiation (SR) sources has offered the opportunity for time-resolved X-ray diffraction studies of fast lattice dynamics in the atomic scale. The coherent atomic motion, as seen in coherent optical and acoustic phonons which are induced by femtosecond pulsed laser irradiation of semiconductor surface has directly been observed with the time-resolved X-ray diffraction technique. In this article, the time-resolved diffraction methods using an SR beam are summarized, and the demonstrations for acoustic pulse echo, coherent acoustic phonon, and optical phonon are reviewed as examples of the investigation on coherent lattice dynamics. The future prospective on the investigation of lattice dynamics using time-resolved X-ray diffraction experiments is also discussed.

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  Ultrashort light pulses absorbed in a medium generate both bulk and surface acoustic waves: we focus here on imaging the surface waves and on monitoring the bulk acoustic waves that propagate in the depth direction. This is done in the time domain by delayed probe light pulses. For the bulk acoustic waves, the generated frequency ranges up to 1 THz. Some studies concerning the ultrafast electron-phonon relaxation in metal films, high frequency generation in semiconductor quantum wells, and high frequency shear acoustic wave generation and detection are reviewed here. For the surface acoustic waves the generated frequency ranges up to 1 GHz, and time-resolved two-dimensional images of the propagation are obtained with micron lateral spatial resolution. This imaging method is applied to anisotropic crystals and phononic crystals to elucidate their acoustic properties.

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  A gas separator that utilizes the thermal transpiration phenomena in a rarefied gas is proposed. The separator is devised to work without intermission and the device itself does not contain moving parts. One of its minimal implementation, which consists of an array of micro-channels with temperature gradient along the channel, is investigated numerically on the basis of the Boltzmann equation for binary gas mixture. It is shown that the separator generates a difference of molar concentration between its two exits when a gas flow with finite Mach number is supplied to the device.

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  As organic solar thin films fabricated by an active layer of organic materials are economical, lightweight, and flexible, as well as facilitating processing, organic solar cells have attracted considerable attention within the past few decades as a clean energy source. With this in mind, there have been global investigations and studies of the power conversion efficiency (PCE) within organic solar cells. In organic thin-film solar cells, the effect of the performance is not only dependent on an adopted active material but also the molecular orientation on the electrode. Using the mixed solution of Poly (3-hexylthiophene) and PCBM, both dissolved by solvent, an organic thin film is fabricated using the paint method (The conceptual diagram of the paint method is shown in Fig. 1) The form of the thin film was evaluated, an organic thin-film solar cell using the paint method for the active layer was made, and its performance was evaluated and examined.
  Using the mixed solution of Poly(3-hexylthiophene) and PCBM, both dissolved by solvent, an organic thin film is fabricated using the paint method (The conceptual diagram of the paint method is shown in Fig. 1) The morphology of the thin film was evaluated using an AFM image, UV/vis spectra, and so forth. Based on these data, an organic thin-film solar cell that used the paint method for the active layer was fabricated, and the performance was evaluated and examined. For the organic thin film solar cell fabricated using the brush painting method, the open-circuit voltage (Voc) is 0.41 V, the short circuit current density (Jsc) is 2.07 mA/cm², and the fill factor is 0.34. The efficiency η of PCE becomes 0.29%.

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  TiO₂ films were deposited by DC magnetron sputtering under various discharge gas pressures and annealed at various temperatures. The structure of the films was investigated with focus on porosity, which was evaluated quantitatively by density and effective surface area. Furthermore, the H₂ sensing property of the films was investigated at various operating temperatures in ambient air. As the pressure during deposition increased, pores developed between columnar grains. In the most porous film deposited at 12 Pa, the sidewalls of the columnar grains were mostly exposed. A dense film deposited under a low pressure showed sensitivity only at an operating temperature above 200°C. As the porosity increased, the sensing temperature decreased. A porous film annealed at 500°C showed a rather high sensitivity even at a low temperature of 100°C.

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  The most common use of the SPM, especially AFM, is the topography imaging. Therefore, the establishment of accurate imaging of surface microstructures is strongly demanded. The most significant distortion in AFM topography imaging is induced by the tip shape whenever the sample surface contains features with aspect ratios comparable to the tip apex size. The acquired AFM height image is a convolution or dilation between the tip shape and the sample topography. To restore the original profile, a numerical erosion procedure using a precise tip shape function is required. We already proposed a novel procedure for the restoration of dilated AFM images using standard nano-spheres. In this study, we compare the standard nanosphere method with a conventional blind reconstruction method in which any tip characterizers are required.

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  There are two advantageous points for fluorine-doped tin-oxide (FTO) films if used as transparent conducting electrodes; the one is high average transmittance in the wave length region between 400 nm and 1500 nm which is favorable for thin film solar cells and the second is to improve anti-heat or anti-humidity properties of Al-doped zinc oxide (AZO) films when employed as a laminated structure of FTO and AZO layers. Therefore, approximately 500 nm-thick multi-layer transparent conducting films (FTO film (100 nm-thick) as protection layer/AZO film (400 nm-thick)) have been prepared. FTO films and AZO films were deposited at substrate temperature of 420~500°C and 280°C, respectively. Sheet resistance of 6.7 Ω/□ was obtained for the structure of FTO layer (fabricated at oxygen partial pressure of 0.5 Pa) and AZO layer (fabricated at oxygen partial pressure of 0 Pa).

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  Chromium silicon carbide (Cr-Si-C) composite thin film has been deposited on the stainless steel substrate by radio frequency (RF) sputtering and the possibility of the force sensor is discussed. The characteristic of the Cr-Si-C composite thin film force sensor is also described. Small temperature coefficient of resistance (TCR) dispersion has been obtained by a Cr-Si-C composite thin film force sensor, and the TCR was between -20 and 10 ppm/°C at the aspect ratio of 510 and the film thickness of 15 nm. The force sensor with hysteresis-free and good linearity has been prepared on the stainless steel substrate by using the Cr-Si-C composite film. The gauge factor of force sensor has increased according to the reduction of film thickness and the increase of sensor pattern aspect ratio. The gauge factor reached 14.2 at the aspect ratio of 510 and the film thickness of 15 nm, which is 7 times larger than that of Ni-Cr strain gauge.

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  Surface roughness and soft x-ray reflectance of soft x-ray Mo/Si multilayer mirrors ion-milled for physical optical reflection wavefront correction with 0.1 nm accuracy have been measured. The multilayer mirror was milled by Ar ion beam accelerated at 500 V using area-selected mask template with 10×10 mm² opening. Milled surface at peripheral was rougher than that at center of milling area. Measured maximum roughness of 0.47 nm RMS would cause only under 0.01% reduction of reflectance by theoretical estimation. 44% peak reflectance of the ion-milled multilayer at λ=14.3 nm is practical enough for composing soft x-ray objectives such as Schwarzschild objective. These results prove that ion milling method is effective and gentle milling for precise figure error correction of soft x-ray multilayer mirrors.

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  BaTiO₃ (BTO)/Pt freestanding films were fabricated by depositing BTO on Pt foils using a RF magnetron sputtering method. From the results, we found that the flatness of the Pt foils was affected by that of substrates. Flat and preferentially (111)-oriented Pt foils were obtained by using the MgO substrates. Coercive field and remnant polarization of the BTO/Pt freestanding film were 13 kV/cm and 1.4 μC/cm², respectively. This was for the first time the report on the deposition of BTO films on the Pt foils, which is called BTO/Pt freestanding film.

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  The performance of a single chamber fuel cell (SCFC) using a novel boehmite proton conductor as the inorganic electrolyte film is studied on operating temperatures from room temperature (RT) to 120°C in the dry gas mixtures of hydrogen and oxygen. The boehmite electrolyte film generated by the soaked treatment of the alumina evaporated film using the autoclave apparatus. The open circuit voltage (OCV) of SCFC using the Pt only electrodes obtained 300-350 mV at operating temperatures between RT and 70°C in the dry gas mixtures, and however the OCV decreased at operating temperatures until 120°C. On the other hand, the OCV of SCFC using the Pt/CB electrodes was achieved 800 mV at RT operation that this value corresponded to a development index of polymer electrolyte fuel cell.

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