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Terahertz radiation emitted by coherent phonons in Te, PbTe, and CdTe has been investigated by using an ultrafast photoconductive sampling detector. Pronounced coherent radiation originating from the longitudinal optical (LO) phonon oscillations of infrared-active modes was observed for all samples, irrespective of the different crystal structures. In addition, spectral dips at the transverse optical (TO) phonon frequencies, which could not be explained by absorption in the emitting volume, were observed for all samples. The model calculations indicate that the emission rate of the radiation into the air to that into the dielectric (semiconductor) side is scaled by 1/{1+(nd2+κd2)nd3} (nd and κd are the real and imaginary part of the complex refractive index, respectively). Thus, the enhanced emission of radiation by the coherent LO phonons and the spectral dips at the TO phonon frequencies can be explained by the respective increase and reduction of the emission efficiency of the radiation to the air due to the small and large value of the dielectric constant |εd(ω)|=nd2+κd2 near the LO and TO phonon frequencies, respectively.

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Transmission and reflection measurements on polyamide nanocomposite films were performed in the THz frequency region, and the dielectric function was calculated using the Debye model to interpret the data. The relaxation time from the signal of orientation polarization due to the permanent dipole moments exhibits intermolecular interaction. In nanocomposites, the extreme increase of the interaction from polymer clay was first observed by THz spectroscopy. The essential interaction was thought to occur between the amide functional (−NH) group in polyamide-6 and the surface of the clay by considering the correlation with the results of infrared spectroscopy.

Silicon samples with and without implanted layers have been imaged with a standard time-domain terahertz ͑THz͒ imaging system. The carrier concentration and mobility of the substrate have been extracted from the frequency dependence of the THz transmittance using a simple model based on the Drude approximation. The carrier concentration of implanted layers could be determined simply from the relative amplitude of the main THz pluse with a spatial resolution of Ϸ1 mm. Both substrates and thin layers of a semiconductor were characterized with the same THz system.