Comprehensive investigation of the THz response of undoped and 0.03 wt% Fe-doped LiNbO3 single crystals using THz-time domain spectroscopy and Lorentz DHO model

This study presents a comprehensive investigation into the effects of iron (Fe) doping on the optical properties of congruent lithium niobate (LiNbO₃) single crystals in the THz region, which is the most elusive segment of the electromagnetic spectrum. Lithium niobate is a highly dielectric functional optical material with wide applications in photonics, nonlinear optics, and electro-optic devices. Undoped lithium niobate (LiNbO₃) and 0.03 wt% iron-doped LiNbO₃ (Fe:LiNbO₃) z-cut single crystals in congruent composition are grown under identical conditions to ensure consistency in crystal quality. The Czochralski technique is used for growing LiNbO₃ and Fe:LiNbO₃ crystals. Furthermore, the as-grown crystals are subjected to structural investigations using Raman spectroscopy to determine the effect of Fe doping on vibrational modes. Utilizing the terahertz time-domain spectroscopy (THz-TDS), we systematically examine the impact of Fe incorporation on key THz parameters over a broad spectral range of terahertz. Observations indicate that Fe doping significantly modulates its terahertz (THz) response, primarily through modifications in charge carrier dynamics and THz interaction mechanisms. Moreover, the experimental data are successfully modelled using the Lorentz damped harmonic oscillator (DHO) approach. The fitting reveals that the lowest vibrational mode, E(TO1), highly governs the dielectric response of both the undoped and doped crystals in the THz frequency range. These findings provide critical insight into the tunability of LiNbO₃ crystals via transition metal (Fe) doping and provide a valuable reference for the development of advanced THz photonic devices.