THz spectra of five borates crystals

Terahertz spectral responses have been studied for five borate crystals Na-5[B2P3O13] (NBP), Zn3BPO (ZBP), SrB4O7 (SBO), Na3La9O3(BO3)(8) (NLBO) and PbB4O7 (PBO). It is found that the samples had good transmission in 0.25-1.5 THz region. Both SBO and NLBO have an absorption coefficient less than 10 cm(-1). Among them, SBO has not only the smallest absorption coefficient but also a very flat dispersion in the frequency region under investigation. Distinct resonance absorption peaks are observed for ZBP at v (1) = 1.4 THz, v (2) = 2.0 THz and SBO at v = 2.4 THz. In the spectrum of PBO, two abnormal dispersions appear in the frequency regions 1.44-1.74 and 2.2-2.5 THz. The absorption coefficients and refraction indices of the five crystals are extracted from the THz time-domain (THz-TDS) spectra in 0.25-2.5 THz region. The properties and origins of the spectral responses are addressed.