Quantum cascade lasers (QCLs) are one of the best arenas for wavefunction engineering using ultrathin layers. Such basic properties of QCLs as transition strength, the lifetime of each energy level, and the coupling strength among energy levels are uniquely determined by wavefunction designs in quantum cascade structures. QCLs with identical material systems, such as InGaAs/InAlAs/InP, GaAs/AlGaAs, and InAs/AlSb, cover a broad wavelength: mid-infrared or the terahertz (THz) region. For practical applications in the mid-infrared region, QC lasers based on anti-crossed dual-upper-state (DAU) designs are a promising candidate because of its broad bandwidth and high device performances. Furthermore, for room-temperature THz generation, THz QCL sources based on intra-cavity difference frequency generation (DFG) in dual-wavelength mid-IR QCLs, so-called THz DFG-QCLs, have made dramatic progress in recent years. We also report the performance of THz DFG-QCLs based on the DAU active region.

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Resonant tunneling diode (RTD) is one candidate for compact and coherent room-temperature terahertz sources. In this paper, we introduce our recent results on RTD terahertz oscillators, including RTD materials and structures for high frequency oscillation. Up to now, oscillation at 1.98 THz has been achieved at room temperature. We also described structures for high output power and some applications of RTD oscillators.

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In this paper, focusing on the two-dimensional plasmons localized on the semiconductor heterojunction interfaces and the surface of two-dimensional materials such as graphene, recent research trends and advances on the physics of plasmon resonances for terahertz wave generation and its application to terahertz light source devices are reviewed.

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A photonic crystal, which is composed of dielectric periodic microstructures, can confine electromagnetic waves within it using the photonic bandgap effect with low loss. In this paper, the recent progress in the development and application of photonic crystals in the terahertz frequency band, of which is located between that of radio and light waves, is reviewed. Key Words: Terahertz, Photonic crystal, Photonic bandgap, Waveguide

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Nonlinear optical crystals of LiNbO3 have fascinated industry and fundamental research since their invention. The potential of the crystals continually indicates excellent and novel properties in terahertz (THz)-wave spectral region as well. Using bulk crystal, intense THz-waves of kilowatt-level peaks was demonstrated with wide frequency tunability from sub-THz to 4.5 THz. Additionally, sensitive THzwave detection on the order of atto-joule energy was obtained by up-conversion from THz-wave to nearinfrared light. To produce more effective and practical devices, periodically poled LiNbO3 (PPLN) was designed. A compact device for THz-wave detection was achieved. Especially, backward THz-wave parametric oscillation was obtained in PPLN with wide wavelength tunability and narrow spectral bandwidth with no cavity structure, which is anticipated as a promising THz-wave source because of its simple structure and lack of mode-hopping during wavelength tuning.

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Ultra-highly sensitive terahertz detectors, called charge-sensitive infrared phototransistors (CSIPs), are described. The detectors are fabricated in GaAs/AlGaAs double quantum well (QW) structures, where electrons in an isolated upper QW (UQW) are photoexcited via intersubband transition, corresponding to a wavelength in a range of 8 μm‒46 μm. Photoexcited electrons immediately tunnel out of UQW, causing the UQW to positively charge up. The pile-up positive charge on the UQW (acting as a photosensitive floating gate) is sensed by a conductance change in the lower QW (acting as the sourcedrain channel). Unprecedented sensitivity has been achieved, leading to the first realization of passive terahertz near-field microscopy. Described here are recent efforts of improving the performance of CSIP: One is the careful design of photo-couplers for better efficiency of intersubband excitation utilizing plasmonic antenna, and the other is the improvement of spectral performance, including the development of multicolor detection with CSIP.

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Mid infrared wavelength from 2 to 5-micron devices has attracted for gas sensing and new band fiber optic devices. We developed metal organic vapor phase epitaxy (MOVPE) growth for Sb-contained III-V materials. Conventional Type-I heterostructure using InAsSb/AlGaAsSb is estimated to have large band offset. However, it suffers from impurities such as oxygen and carbon into Al-contained materials. Sb based material should be grown relatively low temperature because of antimony's high vapor pressure. Therefore, it is not easy to suppress carbon and oxygen concentration. Next, we developed Alfree InAs/GaAsSb type-II superlattice to emit and absorb midinfrared light. In case of MOVPE, arsenic is automatically incorporated into GaSb layer and formed GaAsSb layer. Its concentration is depending on the thickness of GaAsSb layer. It can be controlled and easy to match the lattice matched to InAs substrate. Photoluminescence was measured at 2 to 5-micron band using InAs/GaAsSb type-II (broken gap) superlattice.

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A system for measuring the in-plane crystal quality and nonlinear coefficient of a nonlinear optical crystal using the comparative method of out-of-phase-matched second harmonic generation at a wavelength of 1555 nm was established. The system based on wedge measurement enables easy sample preparation, especially for a crystal with a small slope. The system also has a high accuracy for measuring the ratio between tensor components of the nonlinear coefficient on a crystal surface. In this study, we first measured LiNbO3 (MgO5mol% doped) to demonstrate the feasibility of our system. Second, an organic crystal 4-N, N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST) was measured. The in-plain crystal quality of the DAST was clearly shown by second harmonics power mapping. The ratio of d11 DAST/d12 DAST was 6.0. Nonlinear coefficient values of d11 DAST = 296 ± 2 pm/V and d12 DAST = 49.3 ± 0.3 pm/V were obtained.

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