Terahertz (THz) radiation was generated via optical rectification from organic single crystals, i.e. N-substituted (N-benzyl, N-diphenylmethyl, and N-2-naphthylmethyl) derivatives of 2-methyl-4-nitroaniline (MNA), and it was detected by means of the electro-optic sampling method. The intensity and the spectrum of the THz radiation were compared with those of ZnTe and DAST (N, N-dimethylamino-N'-methylstilbazolium p-toluenesulphonate) crystals. The integrated intensity of the THz radiation from the N-benzyl-MNA crystal was two thirds of that for the ZnTe crystal, the best-known practical THz emitter, and was as intense as that for the DAST crystal, the best organic THz emitter ever studied. The spectrum of the ZnTe crystal extended to 3 THz, while that of the N-benzyl MNA and DAST crystals dropped off around 2.5 THz. The THz radiation intensities from all of the organic crystals were compared quantitatively, on the basis of the consideration of molecular arrangements in the crystals. It was suggested that the absorption due to low-frequency phonon modes affected both the intensity and the spectral band shape of the THz radiation.

A single crystal of N-benzyl 2-methyl-4-nitroaniline (BNA) whose dimensions were 8×10 mm (diameter × length) was grown using a vertical Bridgman method with sufficient optical transparency for spectroscopic use. Polarized reflection spectra of the crystal were measured in order to determine its linear optical parameters (refractive indices and extinction coefficients). The relative magnitudes of the d-tensor components of the crystal were determined to be d 333=(17±9)×d 322, d 322>d 311, d 311∼0 from polarization dependencies of the second harmonic generation. These results are in disagreement with previously reported theoretical studies, in which the effects of intermolecular interactions were completely neglected. Here, we have included these intermolecular interactions in the calculations assuming the presence of supra-molecular clusters. We conclude that the anomalous enhancement of d 333 is due to the interactions along the direction of the hydrogen bonds in the crystal and the resonance effect.

We studied terahertz (THz) electromagnetic wave emission and detection using highly purified N-benzyl-2-methyl-4-nitroaniline (BNA) crystals. In THz emission experiments, we compared the efficiencies of THz emission via optical rectification for BNA, ZnTe, and GaP crystals, and confirmed that the BNA crystal is the most efficient THz emitter among these three crystals within the frequency range limits for observation. In the THz detection experiment, we also compared the modulation efficiencies of the BNA and ZnTe crystals, and found that the ZnTe crystal has a higher efficiency than the BNA crystal. However, the BNA crystal has a higher efficiency than the ZnTe crystal in high-frequency regions. Since the BNA crystal is suitable for detecting a high-frequency region even when using thick crystals, it is expected that the BNA crystal as a THz detector will be easy to prepare and handle.

We have measured the refractive index and absorption coefficient of the N-benzyl-2-methyl-4-nitroaniline crystal using terahertz time-domain spectroscopy. We found that the refractive index had a value between 1.5 and 1.9 in the h100i direction and between 1.9 and 2.3 in the h100i direction. We also found that there were resonances at 1.6 and 3.2 THz in the h100i direction, and 1.3, 2.2, and 3.6 THz in the h001i direction within the observed spectral range between 0 and 5 THz. A Lorentztype oscillator model is applicable to explain our experimental observations.

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