Time-domain terahertz spectroscopy of (100)(LaAlO₃)_0.3-(Sr₂ AlTaO₆)_0.7 substrate

We measured terahertz (THz) properties of high-T_c superconductor's (HTS's) substrates, (100) MgO and (100) (LaAlO₃)_0.3-(Sr₂ AlTaO₆)_0.7 (LSAT), at frequencies between 0.5 THz and 2 THz using time-domain terahertz spectroscopy (TDTS). The complex refractive index ñ and dielectric function ε̃ of the MgO substrate were independent of frequencies; those of LSAT slightly increased with increasing frequency. It was determined that ñ = 3.17 + i0.004 and ε̃ = 10.1 + i0.06 for the MgO substrate and ñ = 4.8 + i0.02 and ε̃ = 24 + i0.2 for the LSAT substrate at 1 THz. The LSAT values were almost independent of the temperature at these frequencies. The dielectric loss tan δ's of the LSAT and the MgO substrates were less than 0.01. The tan δ's of LSAT were several times larger than those of MgO, and increased with an increase of frequency.