Terahertz optical properties of 1% Nd-doped KGW

Abstract

We report terahertz optical properties of 1% Nd-doped potassium gadolinium tungstate (KGW) monocrystals studied by terahertz time-domain spectroscopy in the range of 0.25–2.1 THz. KGW refractive index and absorption coefficient are measured for waves polarized parallel to all three axes of optical indicatrix Ng, Nm, Np. Typical values for the refractive indices are ng = 3.75, nm = 3.37, np = 3.40 at 1 THz corresponding to rather large birefringence of ng – nm ≈ 0.38. We find that KGW exhibits dispersion in the studied spectral range with the refractive index ng, for example, significantly increasing by the value of 0.5. We approximate the dispersion of the refractive index in the form of Sellmeier equations. Absorption coefficient behaves similarly for all three optical axes and it is less than 5 cm-1 for the frequencies below 1 THz. It rises significantly up to 50 cm-1 at higher frequencies where we also observe dichroism of more than 10 cm-1. Observed properties can be attributed to vibrational modes at 2.58 and 3.39 THz found by Raman and IR spectroscopy in previous studies. However, we find no particular features in the spectral range under investigation in contrast to Raman and IR spectroscopy results. Relatively high cubic nonlinearity of KGW crystal, its small low-frequency terahertz absorption and high birefringence indicate an opportunity for its usage for optical-to-optical and optical-to-terahertz conversion.