The capability for actual measurements-not just simulations-of the dynamical behavior of THz electromagnetic waves, including interactions with prevalent 3D objects, has become increasingly important not only for developments of various THz devices, but also for reliable evaluation of electromagnetic compatibility. We have obtained real-time visualizations of the spatial evolution of THz electromagnetic waves interacting with a single metal micro-helix. After the micro-helix is stimulated by a broadband pico-second pulse of THz electromagnetic waves, two types of anisotropic re-emissions can occur following overall inductive current oscillations in the micro-helix. They propagate in orthogonally crossed directions with different THz frequency spectra. This unique radiative feature can be very useful for the development of a smart antenna with broadband multiplexing/demultiplexing ability and directional adaptivity. In this way, we have demonstrated that our advanced measurement techniques can lead to the development of novel functional THz devices.

Measurements of the phase-matching conditions from second-harmonic and sum-frequency generation allowed us to refine the Sellmeier equations of BNA up to the near-infrared and to improve the tuning curve of THz emission from difference-frequency generation.

Near-field coupling plays an essential role in near-field optics, such as the near-field scanning optical microscope (NSOM). Optimized near-field coupling can efficiently improve the system or device performance. In the metamaterial with subwavelength metallic structures, the control of the near-field coupling in between metallic patterns, especially in between difference layers of metallic patterns, can bring novel functionalities to metamaterial devices for optics. In this paper, we analyze the near-field coupling within a unit of double-layer metallic patterns to show a new strategy for metamaterial devices in terahertz (THz) optics. Metamaterials with single layer metallic patterns [1], such as the very popular split ring resonator (SRR), have demonstrated their ability to form several different types of meta-devices among the spectra from microwave, THz, infrared, to visible. While the in-plane near-field coupling in between the metallic patterns of single layer usually limits the engineering capability on the near-field coupling, which results in the limitation of the metamaterial functionality. The out of plane near-field coupling with multilayers of metallic patterns provides additional freedom for the device design [2]. Figure 1 presents a metamaterial unit of double-layer SRRs and its numerical simulations on the surface currents under THz wave incidence with different polarizations. With particular distance designed between the double-layer SRRs to control the near-field coupling, it shows different behaviors under TE and TM modes of polarizations. Specifically, for TE mode, the loop currents associated with LC (inductor-capacitor) resonance on the top and bottom are in phase, while for TM modes, the dipole currents associated with dipole resonance are out of phase. On the other hand, the incident THz wave shows high transmittance with both the two modes, from which, a high performance THz-wave quarter wave plate is expected.

Excitation procedure in metamaterial becomes complex accompanied with the metamaterial design evolving from the simple uniform single layer metallic patterns to ordered or multilayer metallic patterns. This paper describes the excitation procedure in multilayer metamaterial, associated with the electromagnetic wave tunneling effect [1]. Metamaterial of single-layer split ring resonator (SRR) has low transmission (or forbidden status) on the incoming electromagnetic wave at its resonance. However, when the SRR is sandwiched by a pair of metallic patterns [2], it shows improved transmission, which can be explained by electromagnetic wave tunneling effect. This finding has benefits to optical components design with metamaterials by considering the electromagnetic compatibility. Figure 1(a) shows the schematic of the metamaterial unit cell, where the SRR is sandwiched in the middle position by a pair of metallic patterns of identical un-split ring resonators (USRRs) overlapping with each other. In between the two USRRs, dipole resonance associated with magnetic response provides enhanced magnetic field, while electric field is correspondingly reduced. Thus for the SRR sandwiched, the usual electric response of the inductor-capacitor (LC) resonance is shifted from its original frequency (1.12THz, Fig. 1(d)) to lower frequency (1THz). Consequently, the magnetic response shifts the electric response in the multilayer metamaterial. And at the frequency of SRR original resonance (1.12THz), the forbidden status of the electromagnetic wave propagation becomes tunneling (T=0.16 changes to T=0.58). In addition, replacing the SRR into metallic patterns, such as metal block, wire or mesh, it also shows the tunneling.

Stokes polarimeter is developed with high power, tunable injection-seeded Terahertz parametric generator. The purpose in the development is to analyze detailed performance of new, unique devices with complicate polarization property via Mueller matrix measurement. For explicit samples, proper measurements of full elements of Mueller matrix have been demonstrated.

偏光は、光の本質的で重要な特性の一つであり、テラヘルツ周波数帯においても近年様々な偏 光制御素子が開発されている。特にメタマテリアルやバイオテンプレートマイクロコイル等の新 奇なデバイスは、その構造スケールや配向性等の影響で、複屈折や旋光性、二色性、偏光解消等 の偏光特性が混在する可能性がある。これらの偏光特性を分離して詳細に評価する為には、デバ イスのミュラー行列を評価する事が有用である。我々はこれまで、構造性複屈折等を利用したテ ラヘルツ周波数帯の波長板を開発し、ストークスパラメータ計測に基づく偏光計測の有効性を実 証してきた[1]。ストークスパラメータを取り扱う事で、完全偏光のみでなく部分偏光や非偏光成 分も取り扱う事が可能となり、実用上も有用性が高い。現在、ストークスパラメータ計測に基づ いてミュラー行列を評価するシステムを開発し、様々な偏光素子の評価を進めている。

Injection-seeded terahertz (THz)-wave parametric generators (is-TPGs) based on stimulated phonon-polariton scattering in MgO:LiNbO3 crystal are now well established as practical and widely-tunable sources of high-peak-power THz-wave radiation [1]. One of the key characteristics of these THz-wave sources is the parametric gain coefficient in MgO:LiNbO3 crystals. Theoretically, the gain coefficient has been studied by using the physical parameters of A1-symmetry mode at 248 cm-1 [2]. However, the absolute gain characterization still remains as an unsolved issue because no measurement has succeeded so far in directly determining the gain coefficient of stimulated phonon-polartion scattering.

We demonstrated sub-THz generation from a DAST crystal pumped by is-BBO-OPG outputs seeded at 1323.2 nm and 1325.8 nm. The output frequency was measured using Fabry-Perot etalon as 0.47 THz.

Optical frequency up-conversion based on sum frequency generation is a promising technique for efficient single-photon detection because of commercially available detectors with high efficiency and low noise. We extended the frequency range of up-conversion detection based on sum frequency generation to far infrared at terahertz frequency using a periodically poled lithium niobate. Terahertz photon detection based on sum frequency generation was demonstrated. The photon conversion realizes terahertz photon detector operating at room temperature.

In the Large Helical Device (LHD), ECH has been worked as a method of plasma initiation and electron heating. The ECH system has been improved with respect to each experimental campaign. In the recent campaign, nine gyrotrons were operated reliably and steadily. As a di- agnostic objective, a modulated ECH (MECH) was injected together with main ECH power. A Fourier analysis of the induced heat wave gave useful information of not only the heat transport in the plasmas but also precise power deposition layer (2). Several kinds of ECH experiment were performed by using this flexible ECH system. In LHD, electron ITB formation have been observed by using strongly focused ECH in the plasma core (3).Two di erent kinds of improved confinement were realized depending on the direction of tangentially injected NBI. NBI beam driven currents modify the profiles of the rotational transform / 2 , and the existence low order rational surfaces, / 2 = 0.5 in special, a ects the di erence of appearance of the improved confinement states. The MECH method was used to investigate the internal structure of the thermal di usion in such plasmas (4). Another important role of the MECH is the precise determination of the ECH power deposition. Shift of the depo- sition location by changing an injection polarization in the electron Bernstein wave (EBW) heat- ing was clearly demonstrated by the MECH method. Electron cyclotron current drive (ECCD) experiments were proceeded by using a flexible antenna system, which had wide scanning range in both poloidal and toroidal direction. The driven current by ECCD showed a obvious depen- dence of the injection angle of the incident ECH beams. Steady state plasma sustainment is one of the most important objectives in LHD that has all superconducting magnets for the plasma confinement. We demonstrated over 1 hour plasma sustainment by an 84 GHz CW gyrotron and by the improvement of a transmission line such as su cient cooling and e cient evacuation. In this paper, we will outline the recent ECH system in LHD and give some outstanding ECH/ECCD experimental results which were obtained in late years. In the next section we will describe the details of the ECH system. And some recent experimental results related to ECH, such as the transport study, EBW, ECCD experiments and the steady state plasma sustainment, will be given in Section 3. Finally Section 4 will be devoted to the summary.

The frequency characteristics of commercially available pyro-electric detectors have been compared in the range from 0.9 to 2.8 THz by using an intense continuously-tunable terahertz (THz)-wave source. We have found that the THz-wave interference inside the detector has considerable effect on the performance.