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Terahertz electromagnetic waves have a frequency range between infrared light and microwaves: the frequency of 1 THz corresponds to the photon energy of 4.1 meV (33 cm-1) and to the wavelength of 300 μm. It is well known that the terahertz waves have a high sensitivity to the water concentration in materials. For example, Hu and Nuss compared the terahertz-wave transmittance image of the freshly cut leaf with that of the same leaf after 48 hours (Hu & Nuss, 1995). They demonstrated the freshness between the two leaves can be clearly evaluated from the terahertz-wave transmittance images. In addition, the terahertz waves are sensitive to explosive chemical materials (Yamamoto et al., 2004). Accordingly, the terahertz waves are applicable to a security system in airports because conventional xray inspection systems are insensitive to chemical materials. The above-mentioned characteristics of the terahertz wave lead to the reason why terahertz-wave spectroscopy is attractive. We note that the terahertz waves are useful to investigate the vibration of biological molecules, dielectric constant of materials, and so on (Nishizawa et al., 2005). In the present chapter, we focus our attention on the time-domain terahertz-wave measurements based on the femtosecond-pulse-laser technology. Most of the terahertzwave measurement systems employ photoconductive antenna devices (Auston, 1975; Nuss & Orenstein, 1999) as an emitter of terahertz waves. As mentioned later, the antenna-based terahertz emitters, which are categorized into a lateral/planer structure type emitter, have various disadvantages. For the progress in terahertz-wave spectroscopy, it is still required to develop convenient terahertz-wave emitters. Compound semiconductors with a surface electric field, by being irradiated by femtosecond-laser pulses, emit the terahertz wave originating from the surge current of the photogenerated carriers flowing from the surface to the internal side in the surface depletion layer. This phenomenon provides us a convenient terahertz emitter free from a device fabrication for an external applied bias. In the above terahertz emission mechanism, the doping concentration is a major factor determining the depletion-layer width and surface electric field, which are in the relation of trade-off. In order to obtain intense terahertz wave emission, earlier works focused on

Terahertz Electromagnetic Waves from Semiconductor Epitaxial Layer Structures: Small Energy Phenomena with a Large Amount of Information