Observation of a different birefringence order at optical and THz frequencies in LBO crystal

Abstract

THz optical properties of lithium borate (LBO) crystals were measured using time-domain spectroscopy (TDS). The LBO crystal samples were of high optical quality and were cut and polished along the 〈100〉, 〈010〉 and 〈001〉 axes. Two independent measurements were performed in order to confirm the reproducibility and consistency of results. The contradictions in the previously published data on the THz optical properties of LBO were clarified. It was shown that the birefringence order at THz frequencies is nz<nx<ny, whereas at optical frequencies it is known to be nx<ny<nz. It was seen that nz, which has the highest value in the visible, has the lowest value at THz. This is explained in terms of ionic polarizability and is consistent with the fact that the THz absorption coefficient for a wave polarized along the Z-axis is more than an order of magnitude lower than for the X and Y axes. Absorption as low as 0.2 cm−1 was found at frequencies up to 0.5 THz for a wave polarized parallel to the Z-axis. A set of new dispersion equations was designed for the entire transparency range.