Self-Referenced Broadband Cryogenic Characterization of THz Refractive Index and Absorption in MgO-Doped Congruent Lithium Niobate

We report measurements of the terahertz (THz) refractive index and absorption in 5 mol% MgO-doped congruent lithium niobate (MCLN) over 15–340 K and 0.25–2.3 THz, using narrowband THz emission from periodically poled lithium niobate (PPLN) crystals. Both THz-phase and optical group refractive indices along the extraordinary axis are extracted with a self-referenced emission-based approach. Temperature- and frequency-dependent absorption coefficients are obtained from both time-domain field decay and frequency-domain analysis; the time-domain method is found to be more robust, particularly at higher absorption. We derive empirical models for n(f,T) and α(f,T); the refractive index model reproduces the full dataset and all measured central frequencies with sub-percent relative error, while the absorption model recovers the measured absorption coefficients with a mean absolute error of about 0.5 cm⁻¹, and both show good agreement with earlier QPM-based measurements. Taken together, the extended dataset and direct benchmarking provide a consistent and reliable foundation for modeling THz propagation, dispersion, and phase matching in MCLN, especially under cryogenic conditions where literature data are sparse or inconsistent.