Terahertz (THz) technology is becoming increasingly important in both science and technology. THz timedomain spectroscopy (THz-TDS) and THz quantum cascade lasers (QCLs) provide a powerful tool to explore the nature of various kinds of materials for a low energy excitation and ultrafast dynamics; the THz imaging is expected to be applicable to security, factory testing and so on. Thus THz-TDS and QCLs have triggered the research field of THz-technology-based basic research and application research. In addition to those activities, the development of ultrafast devices and logic circuits in the fields of THz photonics and THz electronics has accelerated the growth of the THz research field such as THz wireless communication, THz network system, ultrafast ADCs and so on. This paper reviews the future prospect of such THz-technology-based research fields.

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Inter- and intramolecular vibrations of biomolecules, originating from weak bonds such as hydrogen bonds or van der Waals' force, are known to exist in teraher tzrange. Therefore, tunable sources of coherent terahertz wave can be used for selective excitation of vibrational modes and stimulation of chemical reactions, as well as for recognition of molecules. Two systems for high-power terahertz wave generation are introduced. They are based on difference-frequency generation in Ga Pcrystal. A high-resolution OPO-source system enables measurement of fine structures in spectra, even those resulting from defects in molecules. A conventional Cr: Forsterite-source system is simple, but provides sufficient resolution for measurement at room temperature. Operation principles and construction of the instruments are described in details. Spectrometers which employ these terahertz wave generators were applied to measure absorption spectra of biomolecules. The results demonstrate superiority of these spectrometers in comparison to FTIR and TDS, in range, resolution and sensitivity.

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This article reviews the status of Free-Electron Lasers (FELs) working in the terahertz frequency range. At this time, three operating FEL facilities exist that are able to supply powerful and tunable terahertz radiation for various application programs. This article describes the performance of FELs in these facilities and also presents novel radiation sources based on synchrotron radiation. Recent progress in accelerator technology, particularly in the energy recovering linac and electron beam compression technology, opens up the possibility of a terahertz radiation source that produces terahertz waves with MW peak power and KW average power.

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Photonic generation of millimeter- and sub-millimeter-waves up to THz range using uni-traveling-carrier photodiodes (UTC-PDs) is described. The UTC-PD utilizes only electrons as the active carriers and this unique feature is the key to achieving excellent high-speed and high-output characteristics simultaneously. The obtained performance includes record output powers of over 20mW at 100GHz and more than 10μW at 1 THz. Promising applications, such as fiber radio communications systems, imaging systems, local signal supply for radio telescope, and measurement systems, are also overviewed.

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At present, there is no established technique to detect illicit drugs concealed in mail envelopes in a nondestructive way. Therefore, such mail easily passes through the domestic post or even over the international borders. We studied the nondestructive detection of chemicals (including illicit drugs) hidden in envelopes, and developed an effective inspection system using terahertz waves in a two-step procedure: First, scattering of the terahertz waves is an indicator of the presence of powders in the envelope; second, the identification of the chemicals is done by spectral fingerprinting. Furthermore, we applied the diffuse reflectance spectroscopy for the case of thicker targets and tested whether powders can be identified using spectra obtained in diffuse reflection.

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We review the terahertz time domain spectroscopy in biomolecules using ultrashort optical pulses, which allows us to obtain the complex dielectric function of the sample directly. We measured the temperature dependence of terahertz spectra in dry arginine crystals, and discussed unharmonicity in intermolecule interaction. We also demonstrated the time-domain attenuated total reflection measurement in a diluted arginine solution, which is sensitive to small change in refractive index in a strong absorption medium, such as water. By the analysis with considering local field, it is clear that there was intermolecular action between amino acid molecules and water molecules. This technique will open to the field of terahertz spectroscopy in hydrated biomolecules.

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This paper shows some results on the characterization of the industrial materials by THz spectroscopy. The applications for bio-materials, dielectric thin films, polymers, nanocomposites, and semiconductors were discussed.

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Inspection and failure analysis of large-scale integrated circuits (LSI) have become a critical issue, as there is an increasing demand for quality and reliability in LSIs. Recently, we have proposed a laser THz emission microscope (LTEM) for inspecting electrical failures in LSIs, which detects the THz emission from LSIs by scanning them with femtosecond laser pulses. We successfully obtained the THz emission image of MOSFETs without bias voltage. We also measured MOSFETs in which the connection lines from the electrodes to the p-n junctions were interrupted, and found that the polarity of the temporal waveform of the THz emission from the pn junction in damaged MOSFETs has the opposite sign compared to that in normal MOSFETs. This result indicates that the LTEM can be used for the inspection of MOSFETs without bias voltage, which is attractive for the testing of semiconductor devices during the manufacturing process.

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We have used quantum-cascadlea sers operating at 4.7THz, 3.5THz, and 2.3THz to perform THz magnetoopticals tudies of semiconductors. We observedc yclotronr esonance in In As and In Sb quantumw ellsf rom liquid-heliumt emperaturesto room temperature. This representst he first spectroscopica pplicationof THz quantum-cascadela sers. Results showt hat these compact lasers are convenient and reliable sources with adequatep ower and stability for this type of far-infraredm agneto-opticaslt udyo fsolids. Their compactness promisesi nterestingf uture applicationsi nsolid-states pectroscopy. As an example, the developmento fan ultracompacot ptically-detected THz resonances pectroscopys ystemwill be described.

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A new system on transient temperature measurement with a Mach-Zehnder interferometer employing Polarization Maintaining Fibers (PMF) has been successfully developed. Two PMF fibers, one as the reference arm that avoids externally added disturbances and another as the sensor arm that monitors temperature change, are employed. When temperature changes, the length and phase constant of the sensor arm changes, resulting in a phase difference at the mixing point of two optical waves after passing through both arms, forming interference fringes of concentric circles that move quickly in a radial direction. This system enables us to measure the transient state of temperature change with a resolution of (6/1000)°C and time resolution of 0.1 s.

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Transient reflectivity measurements following ultrafast laser excitation reveal that the decay time of photoexcited carriers in semiconductor materials decreases as defect density increases due to recombination through deep impurity levels. We demonstrated microscopic imaging of defect density distributions for compound semiconductors (semi-insulator GaAs, epitaxially lateral overgrown In GaP, and GaN) with high sensitivity by plotting reflectivity at a fixed time after the pump pulse, which corresponds to carrier lifetime.

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