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InxGa1−xSe mixed crystals have been successfully grown from an indium flux by the traveling heater method at three growth temperatures. The thickness of the grown InxGa1−xSe mixed crystal perpendicular to (001) was more than 3 mm. The lattice constant, and optical and electrical properties of the InxGa1−xSe mixed crystals and undoped GaSe crystals were investigated and compared. The indium content of the InxGa1−xSe mixed crystals was observed to increase with decreasing growth temperature, while the lattice constant along the c-axis was observed to follow Vegard’s law. It was confirmed that a bandgap of In0.020Ga0.980Se is narrower than that of undoped GaSe according to the photoluminescence (PL) spectra. Compared with undoped GaSe crystal, the carrier concentration p was decreased by the incorporation of indium (In0.020Ga0.980Se, p = 6.4 × 1014 cm−3 at 257 K; In0.037Ga0.963Se, p = 2.6 × 1014 cm−3 at 257 K). In addition, it was suggested that the dominant carrier scattering mechanism of high-indium-content crystals at low temperature is ionized impurity scattering.

Terahertz (THz) waves are transparent with respect to non-polar substances, such as plastics. The refractive index at THz frequencies and the specific spectral features of plastic materials, such as polypropene (PP), polyethylene (PE), polyvinyl alcohol (PVC) and polyethylene terephthalate (PET) can be used to discriminate waste plastics into each material regardless of color. This is important since colored plastic cannot be identified by the present method of near-infrared (NIR) reflection spectroscopy due to absorption of NIR light by the coloring. In addition, the THz refractive index of acrylonitrile butadiene styrene (ABS) increases in proportion to the content of bromine (Br), which can be used to quantitatively evaluate the presence of halogen additives in waste plastic. We show that the non-contact THz identification of materials and additives in plastics can be an effective method for sorting plastic waste. Safe & efficient waste management is one of the most urgent social requirements for smart city innovation.

The development of theoretical models for crystals has led to the evolution of computational methods with which much more thorough investigations than previously possible can be done, including studies of the nonlinear optical properties. There has recently been a rise in interest in 2-dimensional materials; unfortunately, measurements of the nonlinear susceptibility of these materials in the wavelength range of the order of hundreds of nanometers by traditional methods are difficult. Studies of second-harmonic generation (SHG) from the transition-metal dichalcogenides (TMDCs), MoS2 and MoSe2, have been reported; however, SHG from other typical van der Waals crystals such as GaSe and other transition metal monochalcogenides (TMMCs) has rarely been studied under the same conditions. In this study, the 211 (i = 2, j = 1, k = 1) elements in the susceptibility matrices of GaSe, InSe, MoS2 and WS2 were calculated and compared. A tendency for the SHG intensity to weaken as the wavelength increases from 500 nm to 1000 nm was observed for GaSe and InSe, and, apart from some periodic fluctuations, no clear increase could be seen for these two materials in the SHG response curve in the near infrared. By comparison, MoS2 and WS2 have obvious peaks in both the visible and infrared bands. Calculations of the SHG response show peaks at around 500 nm (for GaSe), 570 (for InSe), 660 nm, 980 nm (for MoS2) and 580 nm, 920 nm (for WS2). Moreover, similarities between the SHG curves for GaSe and InSe and for MoS2 and WS2 were revealed, which may be due to the similarities found for these two groups of crystals.

Terahertz (THz) region of the electromagnetic spectrum with the corresponding frequencies between 0.1 THz and 10 THz is very attractive. The generation and detection of THz radiation is difficult; scaling and extrapolating known concepts of the microwave or infrared ranges is not possible. Recent advances however make THz radiation attractive for a much wider range of activities. With increased number of many research teams and their constant efforts, recently this gap between photonics and electronics is filled and has a potential to fulfill real-world applications. We have focused on the development of dual-wavelength solid-state lasers operated at 1 μm. The realization of a compact, cost-efficient, and frequency-tunable THz sources was presented in this chapter. The principle of THz wave generation is based on the difference-frequency generation in nonlinear optical (NLO) crystals on the basis of NLO method. A high-resolution THz spectroscopic system with an automatic frequency scanning control was constructed. The THz spectroscopy system has potential applications in biomedical engineering, pharmaceutical industry, and nondestructive analysis of materials.

Indium selenide (InSe), which is one of the most promising layered III-chalcogenide compounds, is an attractive material for applications in infrared detection, solar energy conversion and high mobility transfer devices etc. In this work, InSe crystals were grown from the liquid phase using the temperature difference method under controlled vapor pressure (TDM-CVP) at a growth temperature of 582 °C, which is lower than that of the melt used in the Bridgman-Stockbarger technique. X-ray diffraction (XRD) and Raman spectroscopy results indicate that the grown crystal was γ-InSe with R3m space group symmetry. Photoluminescence measurements were carried out to determine the optical properties of the grown crystal, from which it was confirmed that the sample had a direct bandgap of 1.32 eV, an indirect bandgap of 1.28 eV and an exciton binding energy of 20 meV.

The interlayer van der Waals bonding force in crystalline InSe was directly measured using a mechanical test equipment. The bulk γ-InSe crystal was grown by the temperature difference method under controlled vapor pressure, a unique liquid phase solution crystal growth method with a low and fixed growth temperature. The measured bonding force in the crystal was 20.8 N/cm2, which is greater than that in 2D crystalline GaSe. We also made theoretical discussion of the van der Waals forces in InSe, based on the fluctuations in the electron cloud distributions around the atoms.The interlayer van der Waals bonding force in crystalline InSe was directly measured using a mechanical test equipment. The bulk γ-InSe crystal was grown by the temperature difference method under controlled vapor pressure, a unique liquid phase solution crystal growth method with a low and fixed growth temperature. The measured bonding force in the crystal was 20.8 N/cm2, which is greater than that in 2D crystalline GaSe. We also made theoretical discussion of the van der Waals forces in InSe, based on the fluctuations in the electron cloud distributions around the atoms.

Concrete is the most widely used for construction materials in the world. Water content of concrete is an important parameter in terms durability of concrete structures. Terahertz (THz) waves, for which concrete is a porous and absorbable material, have been studied in order to establish a new non-contact inspection technology for maintenance of concrete structures. In this study, THz transmittance and reflectance of concrete in drying process were measured with a 60 GHz GUNN diode and absorption coefficient is analyzed for concrete with various water contents. It is shown that quantitative detection below 10% is possible for the water content at surface area of concrete.

The structures of fiber, including the distance between fibers and the arrangement direction of fibers, were quantitatively investigated using coherent terahertz (THz) wave. Composite fibers with periodically arranged 57% silk and 43% nylon are measured, then the specific absorption spectra due to intermolecular vibration and interference effect are observed in THz frequency region. It is shown that, not only the period and direction of woven but also the type of material could be specified by the THz spectroscopy. This study enlightens a novel method for fiber nondestructive inspection beyond microscopy imaging, which has potential to be applied in quality control, infrastructure disaster prevention.

Thin films of crystalline molybdenum disulfide (MoS2) are grown by friction-induced synthesis at the interface between a n+-Si(1 0 0) substrate and a synthetic quartz ball. The thin films are formed as a result of the tribochemical reaction of molybdenum dithiocarbamates (MoDTC) in synthetic oil under sliding friction conditions. Raman mapping shows the width of the grown MoS2 film to be about 7 µm, and the difference between the Raman frequencies of the E12g and A1g modes indicate that there are more than 6 monolayers in the MoS2 film. A TEM image indicates that a layer-by-layer structure is formed at the top surface of the stacked films. These results show that it is possible to fabricate a 2D layered MoS2 film on a non-metal surface using this friction induced crystal growth method.

In order to improve conversion efficiency of THz wave generation, Germanium (Ge)-doped gallium selenide (GaSe) single crystals have been grown by Temperature Difference Method under Controlled Vapor Pressure (TDM-CVP). In this article, electrical property and lattice constant of Ge doped GaSe (GaSe:Ge) crystal are compared with that of not-intentionally doped GaSe crystal. Compared with non-doped GaSe crystal, carrier concentration p was decreased by Ge-doping (not-intentionally doped GaSe p = 3.2 × 10 15 cm −3 at 255 K, GaSe:Ge p = 4.9 × 10 14 cm −3 at 255 K). In addition, it has been confirmed that lattice constant of GaSe crystal increased with doping Ge concentration increased.

Our group works on improving the conversion efficiencies of terahertz (THz) wave generation using GaSe crystals. The operating principle is based on difference frequency generation (DFG) which has the advantages such as high output power, a single tunable frequency, and room temperature operation. In this study, GaSe crystals were grown by the temperature difference method under controlled vapor pressure (TDM-CVP). It is a liquid phase growth method with temperature 300 °C lower than that of the Bridgman method. Using this method, the point defects concentration is decreased and the polytype can be controlled. The transition metal Ti was used to dope the GaSe in order to suppress free carrier absorption in the low frequency THz region. As a result, a deep acceptor level of 38 meV was confirmed as being formed in GaSe with 1.4 at% Ti doping. Compared with undoped GaSe, a decrease in carrier concentration (~10 14 cm −3 ) at room temperature was also confirmed. THz wave transmittance measurements reveal the tendency for the absorption coefficient to increase as the amount of dopant is increased. It is expected that there is an optimum amount of dopant.

The real capacity of graphene and the lithium-storage process in graphite are two currently perplexing problems in the field of lithium ion batteries. Here we demonstrate a three-dimensional bilayer graphene foam with few defects and a predominant Bernal stacking configuration, and systematically investigate its lithium-storage capacity, process, kinetics, and resistances. We clarify that lithium atoms can be stored only in the graphene interlayer and propose the first ever planar lithium-intercalation model for graphenic carbons. Corroborated by theoretical calculations, various physiochemical characterizations of the staged lithium bilayer graphene products further reveal the regular lithium-intercalation phenomena and thus fully illustrate this elementary lithium storage pattern of two-dimension. These findings not only make the commercial graphite the first electrode with clear lithium-storage process, but also guide the development of graphene materials in lithium ion batteries., The mechanism of lithium storage in graphenic carbon remains a fundamental question to be addressed. Here the authors employ suitable bilayer graphene foam to investigate various physiochemical phenomena of lithium intercalation and propose a storage model.

We have presented a numerical analysis to describe the behavior of a second harmonic generation (SHG) in THz regime by taking into account for both linear and nonlinear optical susceptibility. We employed a nonlinear finite-difference-time-domain (nonlinear FDTD) method to simulate SHG output characteristics in THz photonic crystal waveguide based on semi insulating gallium phosphide crystal. Unique phase matching conditions originated from photonic band dispersions with low group velocity are appeared, resulting in SHG output characteristics. This numerical study provides spectral information of SHG output in THz PC waveguide. THz PC waveguides is one of the active nonlinear optical devices in THz regime, and nonlinear FDTD method is a powerful tool to design photonic nonlinear THz devices.

In this study, the Ga–Te binary system was reassessed by means of the CALPHAD method using a modified lattice stability parameter for Te as well as experimental data for this binary system. The two-sublattice ionic solution model was applied for the liquid phase, and the intermediate phases were described by the sublattice model. A set of self-consistent thermodynamic parameters was optimized for all the phases in the Ga–Te binary system, which reproduced the phase diagram and the thermodynamic properties well. Using the reevaluated Ga–Te system, previously assessed Ga–Se system, and modified Se–Te system, a critical evaluation of the Ga–Se–Te ternary system was performed. The calculated vertical sections, isothermal sections, and liquidus projection agreed reasonably well with the experimental data. Immiscibility in the liquid phase was observed, and the origin of this behavior is discussed from a thermodynamic perspective.

We have demonstrated enhancement of the spin-charge current interconversion by oxidation of rhenium (Re). Although amorphous rhenium oxide (ReOX) is electrically conductive, the spin–orbit torque efficiency measured by spin-torque ferromagnetic resonance is 0.074, and 37 times larger than the pure Re case whose amplitude is 0.002. Furthermore, we have also found that the values of the damping-like and field-like torque efficiencies are 0.030 and −0.052, respectively. These values are comparable with those for conventional spin–orbit materials such as Pt or Ta, suggesting that ReOX is a candidate material for spintronics devices. THz-time domain spectroscopy was performed to confirm the inverse spin-charge current conversion process in ReOX.

Terahertz (THz) waves at 9.7, 10.1 and 10.6 THz were generated via difference frequency generation in high-quality InxG1-xaSe mixed crystals with a relatively high indium compositions (x = 0.040, 0.048, 0.074) grown from an indium flux. The phase-matching angle for THz wave generation was measured for each indium content. As a result, it is confirmed that the incident angle of the excitation light satisfying the phase-matching condition is shifted to a higher angle with an increase in the indium content.

The growth rate of crystalline GaSe from solution was increased by using indium as a solvent. The solubility and concentration gradient of Se were measured using differential scanning calorimetry (DSC). The Se solubility and the temperature coefficient of the solubility were respectively 15 times and 2.2 times greater in crystals grown from an In flux with GaSe at saturation compared with the case of Se dissolved in a Ga flux. In this study, we succeeded in growing InxGa1−xSe ingots from an In flux without the need for a seed crystal, and in increasing the growth rate of GaSe from solution. In addition, we used difference frequency generation to generate THz waves (with a frequency of 9.7 THz) in an InxGa1−xSe mixed crystal and investigated the relationship between the output energy of the THz radiation and the interaction length of the excitation light. The conversion efficiency of THz wave (9.7 THz) from the InxGa1−xSe mixed crystal with thickness of 860 μm was 26 times greater than that of GaSe crystal with thickness 100 μm grown from Ga solvent.

We demonstrate the generation of THz waves (frequency 9.7 THz) using difference frequency generation in an InxGa1-xSe mixed crystal grown from In flux. The amount of indium and the lattice constant of the crystal were evaluated using electron micro probe analysis and X-ray diffraction, respectively. We believe that the Ga sites were substituted by In atoms in the InxGa1-xSe crystal because the In content, estimated according to the Vegard’s law, was similar to that measured by EPMA. The maximum power of the generated THz wave was 39 pJ and the conversion efficiency was 1.7×10−5 J−1. This conversion efficiency was 28 times larger than that reported for undoped GaSe crystal.

Terahertz(THz) wave has both of characteristics as represented by transparency of radio wave to non-polarized substance and good reflectivity to metal of light wave. Our group has created a database of THz permeability characteristics for industrial materials, and successfully constructed non-destructive THz diagnosis of building blocks, insulated copper cable, hot-dip 2alvanized steel sheet and banknotes.

Graphene material is promising for harmonizing supercapacitor-like power density and battery-level energy density into one electrochemical energy storage (EES) system. Cost-effective, controlled, and massive production of graphene material and its successful application in practical EES are two significant challenges yet to be resolved. By developing a simple nickel nitrate-based hard-template preparation, this study demonstrates the smart design of nanocrystalline C-Ni hybrid monoliths with three-dimensionally ordered macroporous (3DOM) frameworks for high-efficiency EES. The abundant Li-storage sites and mixed high electronic and ionic conductivities grant such freestanding C-Ni composite electrode material large reversible capacity and high-rate capability through thousands of cycles even at a great thickness and using no extra current collector. This 3DOM strategy will facilitate real EES applications of graphene materials, and the yielded nanocrystalline-graphene material is promising to replace graphite anodes in current commercial LIBs with limited performances.

International audience; Chocolate is widely enjoyed in many sweets and foods all over the world. In cocoa butter, fatty acids combine together and give cocoa butter 6 different types of crystals: γ, α, β’(III), β’(IV), β(V) and β(VI). In the industry, the β(V) form that is required, is metastable and gives chocolate its marketable properties. The β(VI) form is also stable but not desirable, because of fat bloom. The objective of this study is to characterize these polytypes by terahertz(THz) spectroscopy. THz spectroscopy is expected to be a new tool in food industry, as THz energy corresponds to collective molecular macro-vibrations. Two chocolates from different factories were inspected with optical microscope, X-ray diffraction (XRD) and THz spectroscopy. Optical microscope showed different surface aspects, XRD showed the same species in the two brands and THz spectroscopy showed different features. These differences may be due to fatty acids combination of chocolate.

Magnetoconductance (MC) at low temperature was measured to investigate spin-related transport affected by spin-orbit interaction (SOI) in III-VI compound $n$-type GaSe thin films. Results reveal that MC shows weak antilocalization (WAL). Its temperature and gate voltage dependences reveal that the dominant spin relaxation is governed by the D'yakonov-Perel' mechanism associated with the Rashba SOI. The estimated Rashba SOI strength in GaSe is much stronger than that of III-V compound GaAs quantum wells, although the energy gap and spin split-off band in GaSe closely resemble those in GaAs. The angle dependence of WAL amplitude in the in-plane magnetic field direction is almost isotropic. This isotropy indicates that the strength of the Dresselhaus SOI is negligible compared with the Rashba SOI strength. The SOI effect in $n$-GaSe thin films differs greatly from those of III-V compound semiconductors and transition-metal dichalcogenides.

At present, one of the main inspection methods of electric wires is visual inspection. The development of a novel non-destructive inspection technology is required because of various problems, such as water invasion by the removal of insulators. Since terahertz (THz) waves have high transparency to nonpolar substances such as coatings of conductive wire, electric conductive wires are extremely suitable for THz non-destructive inspection. In this research, in order to investigate the quantitative possibility of detecting the defects on aluminum electric wire, THz wave reflection imaging measurement was performed for artificially disconnected wires. It is shown that quantitative detection is possible for the disconnect status of the aluminum electric wire by using THz waves.

GaSe crystals were grown by liquid phase solution growth, using the temperature difference method under controlled vapor pressure. Temperature-dependent Hall effect measurements were used to show the different thermal activation energies of the acceptor levels of the commercially available Bridgman-grown GaSe crystals and our liquid phase solution-grown crystals. Unintentionally doped GaSe crystals showed p-type conduction; also, the thermal activation energies of the acceptor levels of the Bridgman-grown crystals were 30 meV + E v and 50 meV + E v while that of liquid phase-grown crystals was 15 meV + E v (where E v is the energy of the valence band). We conclude that low temperature growth and stoichiometry control can effectively inhibit native point defect formation.

Terahertz (THz)-wave generation has been conducted based on difference frequency mixing (DFM) process with phonon-polariton excitation of e-GaSe single crystals implemented with liquid-phase solution growth using the temperature difference method under controlled vapour pressure for the first time. The type-eoo phase matching condition for the DFM process at around 10 THz is satisfied by changing the incident angle into the crystal. The maximum conversion efficiency in the present DFG process is about 10-6 J-1 using a 0.1-mm-thick GaSe single crystal with the only e- phase polytype, which can be greater than that of the commercially available Bridgman grown GaSe crystal including both e- and γ-phase polytypes.

A novel non-destructive inspection method using terahertz waves for the detection of broken wires in copper cables shielded by insulating polymer opaque in the visible and near infrared light region is presented. Terahertz reflection imaging, using a 0.14 THz IMPATT oscillator and a Schottky barrier detector, was applied to insulated copper cables that had been artificially damaged. The internal insulated wires embedded in the opaque polyethylene can be clearly visualized using THz radiation. It was shown that a gap in a wire can be detected by a decrease in the intensity of the reflected signal where the size of the gap is greater than the wavelength of the oscillator.

We proposed a surface-emitted THz-wave generation on the basis of difference frequency mixing in a GaP planar waveguide. By utilizing modal birefringence of fundamental TE and TM modes at telecom wavelengths in the GaP membrane waveguide, phase matching condition for the surface-emitted difference frequency mixing (SE-DFM) can be achieved. THz output power is enhanced near the phonon polariton resonance frequency owing to the strong coupling between the transverse optical (TO) phonon of GaP and THz radiation. The SE-DFM scheme can generate THz waves beyond the Reststrahlen band located between 11 and 12 THz, resulting in widely tunable THz wave generation. Our proposed broadband THz sources can be applicable for optically isotropic nonlinear optical materials such as GaAs and InP as well as GaP.

GaSe crystal has been expected as one of the promising nonlinear optical crystals for highly efficient terahertz (THz) wave generation. However there are several reasons why it is difficult to grow the bulk crystals with high quality. To overcome some difficulties, the temperature difference method under controlled vapor pressure (TDM-CVP) is applied for crystal growth. According to this method, stoichiometric composition can be controlled by the application of Se vapor during crystal growth. Crystal growth is carried out at a constant growth temperature without any mechanical disturbance or vibration. It is also noticed that lower temperature growth enables the reduction of point defect concentration in equilibrium. In this article, surface morphology is observed by an optical microscope using the Nomarski interference method. To identify polytypes of grown crystals, backscattered Raman spectra were measured. X-ray diffraction confirmed the polytypes and single crystalline phase. Infrared ( λ =1 μm) and terahertz wave (1–3 THz) transmittance measurements were performed to calculate the absorption coefficient in these wavelength regions. From these results, it is shown that the grown crystals have shown e-type single phase and the absorption coefficients of grown crystals have been improved according to the increase of applied Se vapor pressure during crystal growth.

In this paper, the possibility of utilizing characteristic absorption spectra in the terahertz (THz) frequency region for the non-destructive evaluation of a corroded metal surface coated with an insulator is discussed. The infrared activity of corrosion products in response to THz waves was characterized. The THz absorption spectra of Al(OH)3 and ZnCl2∙4Zn(OH)2 powders, which are typical corrosion products in Zn-Al alloy, were measured with a frequency tunable terahertz light source based on difference frequency generation from a GaP crystal. Characteristic absorption peaks were observed in both spectra. Subsequently, THz reflection imaging was conducted on a metal surface partially covered with these corrosion products and a coating. By using the characteristic absorption peaks of the corrosion products, the spatial distribution of these products was clearly obtained. From the results, it was shown that THz waves can be applied to the non-destructive evaluation of corroded metal surfaces coated with insulators.

In order to investigate the infrared activities of corrosion products of copper in terahertz (THz) frequency band, THz absorption spectra of copper (I) oxide (Cu2O) and copper (II) oxide (CuO) powders are measured. In view of the practical application for non-destructive inspection of copper wire deterioration, THz reflectance spectra of copper conductive wires with and without oxidization were also measured. THz spectroscopy results have shown some clear specific absorption bands in 1-5THz region, and the THz reflection spectra have revealed clear reflection intensity difference at around 1.7THz band between corroded and not-corroded copper wires. It is also shown that the surrounding polyethylene (PE) of copper electric wire is very transparent for THz wave, thus non-destructive inspection of the corrosion status of copper wire surrounded by the invisible PE insulator can be possible by using THz wave.

An efficient continuous wave (CW) THz source working at nominal room temperature is described. Optically pumped room temperature THz emission is observed from various kinds of semiconductor bulk crystals. In order to investigate the emission mechanism, temperature dependences of terahertz emission intensity in various semiconductors are measured. Semiconductor samples used are InSb, InSb:Ge, InAs, GaSb, Ge, and Si. From these results, it is shown that the temperature dependences of emission characteristics are different between direct and indirect transition semiconductors, and that the high resistive Ge is suitable for room temperature THz emitter via intracenter transitions excited by IR pump lasers.

GaP terahertz (THz) two-dimensional (2D) photonic crystal (PC) waveguides with line defects were fabricated using inductively-coupled plasma reactive-ion etching (ICP-RIE) in Ar/Cl2 gas chemistries. THz-wave generation from the fabricated PC waveguides was demonstrated under collinear phase-matched difference-frequency generation (DFG), using Cr:Forsterite (Cr:F) lasers as the incident source. We compared the THz-wave output characteristics of the PC waveguides with that of GaP planar waveguides. The collinear phase-matching conditions in the DFG process were satisfied for 300- and 500-μm-wide PC waveguide structures at 0.7 and 0.6 THz, respectively. The additional output peaks that appeared near the edge of the photonic band gap, around 0.5 THz, were attributed to the guiding modes in the PC waveguide; no such peaks appeared in the non-patterned ridge waveguides. These experimental results suggest that the phonon-polariton confinement in THz-PC waveguides based on the GaP crystal could be used to enhance the nonlinear optical effect for THz-wave generation.

1.2μm – laser pumped wide frequency tunable coherent terahertz (THz) light source is demonstrated. The operation principle is based on difference frequency generation (DFG) with an excitation of phonon – polariton in Gallium Selenide (GaSe) crystal. The pump and signal lasers used are 1.2μm Cr:Forsterite lasers. The tuning range of the THz wave frequency covers from 0.3THz to 4.8THz (typeeoophase matching) and 8.3THz to 10.2THz (typeeoophase matching) under collinear phase matching conditions. It is shown that the maximum conversion efficiency is up to ~10-6, which is 3 orders in magnitude larger than that of Gallium Phosphide (GaP) crystal.

We constructed two types of terahertz (THz) spectrometers with automatic scanning control based on the difference frequency generation method by the excitation of the phonon-polariton mode in GaP. The pulsed THz wave spectroscopic systems were based on an optical parametric oscillator and Nd: YAG laser sources having a frequency resolution of 1.5 GHz, and on Cr:fosterite sources with a resolution of 20 GHz. Following these, we recently constructed a continuous wave (CW) THz wave spectroscopic system with laser diode excitation. One of the advantages of the CW THz wave spectrometer is its wide frequency tuning range with fine frequency resolution of < 8 MHz. In this study, we compare both types of spectrometers (pulsed versus CW) to show the characteristics of each system in terms of frequency resolution. The absorption spectra of a non-deformed white polyethylene crystal and ultra-high molecular weight polyethylene with/without deformation are measured by using the CW THz wave spectrometer and pulsed THz wave spectrometer. The effect of the high-resolution CW THz wave spectrometer is shown based on the THz spectroscopic results.

A GaAs/GaAsSb/GaAs structure was grown on (100), (111)A, (111)B and (110) GaAs substrates by intermittent injection of AsH3, TEGa, and TMSb. The maximum Sb composition was GaAs0.79Sb0.21 on (100) GaAs. From the SIMS, PL and XRD measurements, the abruptness of the quantum well interface and lattice strain were discussed. The growth of GaSb dot was also carried out, and a 20 nm average diameter and a dot density of 7×1010 cm–2 were achieved on (111)A. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanically extensional–deformation of ultra-high molecular weight polyethylene (UHMWPE) was examined by using polarization terahertz (THz) spectroscopy as well as X-ray diffraction (XRD) analysis. Due to the B 1u lattice translational mode, a THz absorption band was observed around 2.2 THz. The band intensity of the non-deformed UHMWPE is isotropic. For deformed UHMWPE, the dichroism appeared in the 2.2 THz band. The absorption intensity was smaller in spectra in which the THz wave electric polarization direction was parallel to the deformation direction than that in which the polarization was perpendicular. The intensities in each polarization direction are dependent on the amount of elongation strain. Based on the influences drawn from the THz and XRD results, THz non-destructive diagnosis of the tensile strain in deformed UHMWPE is proposed based on the dichroism of B 1u band intensities observed in polarization THz spectroscopy.

Steel sheet with an insulator to prevent corrosion is used for various purposes including in building and car manufacture. Terahertz waves, for which insulators are highly permeable and metal surfaces are highly reflective, have been studied in order to establish a new inspection technology for these steel plates. In our previous research, spectroscopic measurements in the 1.0-4.0 THz range, generated by a GaP crystal, were carried out in order to collect information on the infrared activity of the metal corrosion products formed on Zn-Al hot-dip galvanized steel sheet. In the previous work, the infrared activity of Fe-based corrosion products was not examined. To examine these products, we conducted THz spectroscopy on goethite (α-FeOOH) in the range from 8.4 to 11.0 THz, generated by a GaSe crystal. The results of Attenuated Total Reflectance (ATR) FTIR spectral measurements and molecular vibration calculations were analyzed, on the basis of which the natural vibration modes of α-FeOOH in the THz frequency range were assigned.

We investigate the photoluminescence (PL) polarization of millimeter-scale GaSe films with 100 nm and 150 ?m thicknesses by excitation with linearly and circularly polarized light. Although free exciton emission predominates at 140 K, multiple peaks appear at 30 K owing to the recombination with impurity and/or localized states. Despite the circularly and linearly polarized excitations, the 100-nm-thick GaSe film constantly shows linear polarization in both band-edge and impurity/localized PLs. By rotating the sample geometry, the observed linear polarization is also rotated. This indicates that linear polarization is sensitive to the sample geometry and originates from the optical waveguide effect.

We investigated methods for improvement of continuous-wave (CW) terahertz (THz) output power by laser diode (LD) pumping in noncollinear phase-matched difference frequency generation (DFG). The effects of interaction length and beam spot size of input lasers (near-IR) in GaP crystals were studied. The THz wave power dependence on various sizes of GaP crystals was investigated and it was observed that an output power of 4 nW was obtained with a 20 mm long GaP crystal at 1.5 THz. Also, the THz wave absorption coefficient was dominant for longer GaP crystals at high frequencies (above 2.5 THz). The THz wave power dependence on beam spot size (1.2 mm-300 mum) of near-IR lasers at 1.62 THz was studied, and an improvement of THz wave power being seen with a 500 mum beam spot size, while the beam divergence effect was dominant for beam spot sizes below 500 mum.

A compact sub-terahertz (THz) imaging system with a 0.2 THz-band GaAs TUNNETT diode oscillator and its application for non-destructive and harmless inspections of timbers, concrete and ceramic tiles are shown. Sub-THz transmission characteristics of various kinds of woods were investigated. It is shown that wood and concrete show a high transmittance in this frequency range, and that the measured absorption coefficients correlate well with the densities of woods. Then, the invisible grains, knots and diffused water inside the timbers were investigated by sub-THz transmission imaging. It is also shown that the sub-THz wave is a very efficient tool for defect recognition in concrete. Invisible cracks, diffused water and the quality of adhesion of tiles were investigated. It is concluded that the sub-THz wave has shown a high sensitivity in detecting these defects in building blocks.