Comments on “Optical properties of borate crystals in the terahertz domain”

We comment on the recent paper by V.D. Antsygin et al. [Opt. Commun. 309 (2013) 333–337], in which for the first time dispersions of refractive indices nx,z in lithium triborate (LBO) were presented for the THz domain. Their claim of “nx>nz”, based on measurements by THz time-domain spectroscopy (TDS), is contrary to the well-known relationship for the maximum transparency region.