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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.

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.

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.

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.

We have presented a numerical analysis to describe the behavior of a quasi-monochromatic pulsed terahertz (THz) wave generation in photonic crystal waveguide (PCW) reported in our group. The frequency-dependent characteristic of the THz-wave generation is investigated numerically by the finite-difference time-domain method. Two different types of phase matching condition are achieved in the basics of the guided mode or air band in the PCW. The calculated THz output spectrum agrees well with the previous experiment one. This letter provides an effective estimation of THz-wave output characteristics and could be used to design the efficient coherent THz-wave source.

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.

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)

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.

Conductance-voltage (G -V) and current-voltage (J -V) measurements were applied to reveal the tunneling mechanisms of GaAs ultra-shallow sidewall tunnel junctions, which were fabricated by molecular layer epitaxy. From the G -V results, peaks were detected at 2, 12 and 14 THz at 6 K. In the J -V measurements, the fine structure near the peak voltage and a step in the tunneling current in the negative resistance region can also be observed. The tunneling mechanism is discussed in terms of the band transition, deep level and phonon-assisted tunneling. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaSe is one of the most promising semiconductor crystals for wide frequency-tunable terahertz (THz) wave generation by photo mixing. The e-type monocrystalline GaSe crystals were successfully grown by the liquid-phase epitaxy at constant and low (530–590 °C) growth temperatures under the controlled different selenium (Se) vapor pressures ( P Se ∼0–7.75 Torr). From the coherent THz-wave spectroscopy, the absorption spectra have shown different resonant frequencies and absorption coefficients due to the stoichiometry-dependent point defects which depend on the applied P Se . It is shown that the resonance in GaSe under P Se ∼0 Torr shifts towards the lower THz frequencies compared with those under high P Se , maybe due to the degraded intermolecular interactions caused by the introduction of Se vacancy-related defects. The absorption coefficients (1–5 THz) decreased according to the increase of Se vapor pressure, thus the transparency of GaSe under higher P Se is improved by an amount of 25% compared with that of Bridgman-grown crystals. By using Bridgman-grown GaSe crystals, coherent-THz wave was generated by the difference frequency method (DFM) in a wide frequency range of 0.1–70 THz. Coherent-THz spectroscopy is a revolutionary method for the evaluation of molecular structures and defects in organic molecule also could be analyzed.

Low temperature (290 °C) area selective regrowth (ASR) by the intermittent injection of TEGa and AsH 3 in UHV was applied for the fabrication of ultra-shallow sidewall GaAs tunnel junctions. Tunnel junction characteristics are seriously dependent on the sidewall orientations and the surface treatment conditions just prior to the regrowth. The junction characteristics and those associations with defects were shown in view of the doping characteristics and the photocapacitance results. And the effects of quantization by the reduction of junction area were also shown.

Selective liquid-phase (LPE) epitaxial growth of InP was performed on patterned Si(1 0 0) substrates. The growth temperature was rapidly cooled, then the crystal growth proceeded at constant temperature. As a result, area-selective epitaxy (ASE) and slight epitaxial lateral overgrowth (ELO) layers were achieved in narrow openings and small nuclei were observed in the wide openings. X-ray diffraction results show that the lattice strain in the InP nuclei on the Si substrate was fully relaxed. Huber etching revealed the dislocation-related etch pits in ASE layers, and it is shown that the etch pit density (EPD) is dependent on the length of open seed area.

Electrical and structural evaluations have been performed on high-k TiOx dielectrics which had been de-posited on {001} oriented n-GaAs at 100 °C by electron beam evaporation of TiO2 in a mixed atmosphere of O2/Ar. A permittivity, es, as high as 100 has been achieved, and a breakdown field strength of 0.5 MV/cm has been obtained. Room temperature transistor action was realized utilising a p+n+ tunnel transistor with TiOx gate dielectric, both in enhancement and depletion mode operation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

30K-photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions. In the dislocation-free layers, the dominant deep level was located at Ec-1.30 eV, whereas in the dislocated area, dominant deep levels were detected at Ec-0.86 eV and 1.05 eV. A neutralized state was also detected at 0.66 eV+Ev. Excitation photocapacitance results have shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts of 0.28 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Area selective epitaxy (ASE) of InP was performed by liquid phase epitaxy on Si (100) substrates without the use of buffer layers. At the start of epitaxy, the growth temperature was rapidly lowered to obtain high supersaturation. Epitaxial growth was then carried out at constant temperature. While small InP nuclei were observed in the broader open areas, ASE layers and layers showing slight epitaxial lateral overgrowth were formed in the narrow openings. Etch pit density of the InP ASE layers on Si is dependent on the length of open seed region and X-ray diffraction results indicate that the lattice strain in the InP nuclei on the Si substrate was fully relaxed.

The terahertz (THz) region, which lies between the microwave and infrared regions, offers a wealth of untapped potential. We generated widely frequency-tunable coherent THz waves from GaP crystal pumped using continuous-wave (CW) semiconductor lasers and compared it with pulse pumping using Q-switched high-power lasers at 1–1.2 μm. THz wave generation was based on difference-frequency generation via the excitation of phonon-polaritons in GaP. CW THz waves were generated from GaP by enhancing the power density of the pumping light from semiconductor lasers. The power and phase-matched condition for THz wave generation are discussed with respect to the pumping method compared to pulse pumping. Semiconductor lasers have light power stability with a narrow linewidth. Therefore, CW THz waves can be used as a light source in high-resolution THz spectroscopy, as well as in multichannel communication.

We designed and simulated a GaP-based sheet cavity structure for room-temperature THz wave detection via a nonlinear optical (NLO) parametric process, using photonic conversion from THz frequencies to the optical domain. The sheet cavity structure consisted of a GaP rectangular waveguide for THz waves. Pump enhancement in the cavity and strong confinement of the THz wave in the waveguide resulted in a high power conversion efficiency of 1% (detection at 5 THz). The noise equivalent power for THz wave detection using an optical single-photon detector was estimated to be in the order of a few fW Hz(-1/2), which was higher than that obtained using room-temperature bolometers, field-effect transistors, and other NLO processes.

Photocapacitance measurements have been made on GaAs ultra-shallow sidewall p+i n+ junctions, fabricated by low-temperature area-selective re-growth molecular layer epitaxy. Emission spectra were obtained of the photocapacitance, affected by the deep levels at the re-growth interface region. From the photocapacitance results, it is shown that the defect levels and their density differed, depending on the device sidewall mesa orientations, with the magnitude of deep level density increasing in the order of: normal mesa < 45°-inclined configuration < reverse mesa orientation. When photocapacitance results of sidewall p+i n+ junctions were compared with current-voltage characteristics of sidewall p+n+ tunnel junctions, a good agreement was found. It is considered that the deep levels at the re-growth interface are a major factor controlling excess current in sidewall tunnel junctions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of current-voltage (I -V ) characteristics of quantum-confined GaAs ultra-shallow sidewall p+n+ tunnel junctions has been investigated. The sidewall tunnel junctions with junction depths ranging from 5 nm to 50 nm were achieved by the combination of intermittent injection of TEG/AsH3 in an ultra high vacuum and a wet etching process of the GaAs growth layer. From the I -V results, abrupt negative differential resistances (NDR) were observed, which relate to direct/indirect tunneling and sub-band formation. The change in the number instances of NDR and their voltage positions also depended on the junction depth. Mechanisms of tunneling in the present sidewall tunnel junction will be discussed from the point of the sub-band formation in conduction bands. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

One of the most important uses of THz spectrometry is to detect defects in molecular structure or in crystals efficiently. We applied GaP Raman THz (GRT) spectrometer to detect and evaluate defects in inorganic and organic materials. High THz-wave absorption due to high defect density of GaSe crystal lowered the efficiency of THz wave generation, when the crystal is used as nonlinear material for DFG (Difference Frequency Generation). Defects in organic molecules could be observed as changes in frequency, intensities of the absorption, and broadenings of the spectra.

Epitaxial lateral overgrowth (ELO) was performed on {0 0 1}-oriented InP substrates by liquid phase epitaxy at constant growth temperature (450–650 °C). In this work, ‘line and space’ (L&S) and L-shaped patterns were formed on the substrates before epitaxy, and then the as-grown and etched surfaces were observed by Nomarski interference optical microscopy. Defect etching revealed dislocation etch pits on both the open seed areas and the coalescence regions of the ELO layer formed on the L&S patterns. However, when using the L-shaped pattern, etch pits were only observed on the open seed regions of the ELO layer. We show that the ELO layers that spread laterally inside the L-shaped pattern are dislocation-free. In addition, large area ELO layers were obtained when the L-shaped pattern was modified by rotating the direction in which it was facing. In the case of the L-shaped pattern, the optical properties of the ELO layer were improved compared with those on the open seed area.

Low-temperature (290 °C) area-selective re-growth (LT-ASR) by molecular layer epitaxy (MLE) was applied to the fabrication of ultra shallow (50 nm) sidewall GaAs tunnel junctions. As a result, these tunnel junctions have exhibited record peak current densities of up to 31,000 A/cm 2 at RT, with a peak-to-valley current ratio of 2.1. It was also shown that the tunnel junction characteristics are strongly dependent on the sidewall mesa orientation. In order to investigate the midgap levels at the re-growth interface region, photocapacitance (PC) measurements were made on area-selective re-grown sidewall p + i n + junctions.

Low-temperature (290°C) area-selective regrowth by molecular layer epitaxy (MLE) was applied for the fabrication of an ultra-shallow sidewall (50 nm) GaAs tunnel junction. Fabricated tunnel junctions have shown a record peak current density up to 35,000 A/cm2. It is shown that the tunnel junction characteristics are strongly dependent on the sidewall orientation and the AsH3 surface treatment conditions just prior to regrowth. The effects of AsH3 surface treatment are discussed in view of the control of surface stoichiometry.

Low-temperature (290 °C) area-selective regrowth (ASR) by the intermittent injection of triethylgallium (TEGa) and arsine (AsH 3 ) in an ultra-high vacuum (UHV) was applied for the fabrication of ultra-shallow sidewall GaAs tunnel junctions with the junction area in the order of 10 −8 cm 2 . Fabricated tunnel junctions have shown the record peak current density up to 35,000 A/cm 2 at 100 μm long strip structure. It is shown that the tunnel junction characteristics are seriously dependent on the sidewall orientation and the regrown interface quality, which was determined by the surface treatment conditions under AsH 3 just prior to regrowth. The junction characteristics and AsH 3 surface treatment effects are discussed in view of the orientation dependence of Be doping and control of surface stoichiometry.

Pump enhanced optical parametric oscillation under a cavity phase matching configuration is an effective way to obtain monochromatic THz waves with high pulse energy. Numerical simulations are conducted for THz wave generations using a GaP sheet cavity. By optimizing the optical pulse duration and cavity configuration, the estimated peak power of THz waves is 4 MW at 3 THz, which corresponds to the photon conversion efficiency of η≈0.81. Our proposed scheme can generate a THz wave with high pulse energy, which is suitable for the nonlinear optical effects in the THz frequency region.

We design a GaP/Si composite waveguide to achieve efficient terahertz (THz) wave generation under collinear phase-matched difference frequency mixing (DFM) between near-infrared light sources. This waveguide structure provides a strong mode confinement of both near-infrared sources and THz wave, resulting in an efficient mode overlapping. The numerical results show that the waveguide can produce guided THz wave (5.93 THz) with a power conversion efficiency of 6.6×10(-4) W(-1). This value is larger than previously obtained with the bulk GaP crystal: 0.5×10(-9) W(-1) [J. Lightwave Technol.27, 3057 (2009)]. Our proposed composite waveguide can be achieved by bridging the telecom wavelength and THz frequency region.

We constructed a two-directional continuous-wave (CW) terahertz (THz) wave generation system with laser diode pumping. The generation method is based on difference frequency generation (DFG) of two near-infrared (NIR) lasers by excitation of phonon polaritons in a GaP crystal. The two NIR lasers for DFG are amplified simultaneously using one ytterbium-doped fiber amplifier (FA). Efficient beam overlapping of the NIR lasers was achieved on the GaP crystal surface. This system consists of a simple optical design with a single FA. Narrow linewidth CW THz waves, which are generated in two directions, can be applied to high-resolution spectroscopy.

Intermittent injection of trisdimethylaminophosphorus (TDMAP) is applied for selective etching of the InP in ultrahigh vacuum by using molecular layer epitaxy equipment. The etching depth is controlled by the number of injection cycles of TDMAP in a self-limiting fashion. The etch rate per injection cycle increases with the injection time of TDMAP and is saturated. The etch rate is well described by a Langmuir-type equation. The etch rate is independent of the injection time at times longer than 0.3 s, and is also independent of the injection pressure of TDMAP. This is almost independent of the evacuation time. The activation energy for etching with TDMAP is determined to be 1.27 eV, which is half of that for the decomposition of phosphorus from InP substrate. Well-controlled digital etching is realized at substrate temperatures as low as 350°C.

We have fabricated GaP two-dimensional photonic crystals (PCs) for terahertz (THz) wave generation by a reactive ion etching in Ar/Cl2 gas chemistries. We performed 75-mm-deep etching of GaP, in which Al2O3 layer is applied as a hard mask with its selectivity as high as 125. We demonstrated the THz-wave generation from the fabricated GaP slab waveguide with the PC structure as a cladding layer under a collinear phase-matched difference frequency generation. In the frequency dependence of THz output power for the PC slab waveguide is seen at around 1.1 THz. From the in-plane transmission spectrum of THz-wave, we confirmed that the THz output characteristics had relation with the photonic structure for THz wave. [doi:10.2320/matertrans.MAW200728]

Terahertz (THz) generation was demonstrated from GaP rod-type waveguides via difference-frequency-mixing of near-infrared light using a collinear phase-matching condition. THz output peaks were observed, and appeared at frequencies corresponding to the fundamental and high-order waveguide modes. Interestingly, the position of the fundamental mode shifted to a higher frequency for a smaller waveguide cross-section, which is attributed to the waveguide confinement of the THz wave. The conversion efficiency was enhanced in the waveguide with a cross section of 200 mumtimes160 mum as compared to that in bulk GaP crystals

Continuous-wave (CW) single-frequency terahertz (THz) waves were generated using difference-frequency generation via excitation of phonon-polaritons in GaP. The two pump sources were an external cavity laser diode (LD) and an LD-pumped Nd : YAG laser. The power density of the latter beam was enhanced by using a ytterbium-doped fiber amplifier. The two incident beams were focused to near the wavelength of THz waves. This optical alignment enabled us to generate frequency-tunable CW THz waves in the 0.69-2.74 THz range. With a fixed angle between the pump beams, we obtained a frequency bandwidth as large as 600 GHz

Dynamic SIMS has been applied to investigate the influence of impurity profiles on the characteristics of ultra-shallow GaAs sidewall tunnel junctions. SIMS depth profile on test-element-group areas on the device chips have shown that the Be profiles pile-up, with concentrations of up to 1020 cm−3 at the tunnel junction interfaces. This result illustrates one of the dominant causes why very high peak current densities are achieved.

Improvement of the intensity and purity of emission from GaP green light emitting diodes (LEDs) has been achieved by pre-annealing of the substrate. With annealing the substrate under 20 kPa of phosphorus vapor for an hour just before the epitaxial growth, emission from deep level disappeared and saturation current density was decreased to 1×10 −16 A/cm 2 . The output power of LED depends on the n-layer thickness in the case of 5-min pre-annealing. In contrast, with 1-h pre-annealing the dependence was not observed. This fact means that some kinds of nonstoichiometric defects diffuse from the surface region of the substrate during the growth of epitaxial layers, and affect the emission characteristics.

We carried out terahertz (THz)-wave generation from the GaP planar waveguides under collinear phase-matched difference-frequency mixing of two near-infrared sources. TE- and TM-mode of THz-waves were generated simultaneously by adjusting the polarization direction of two incident infrared sources. The phase shift between TE- and TM-mode of THz-wave in the waveguide was dependent on the waveguide length and contributed to the generation of the elliptical polarized THz-wave. The ellipticity of generated THz-wave increased as waveguide length increased. We indicated the possibility of control of rotational direction of elliptical polarization of emitted THz wave.