Effect of 0.015 mol % cobalt (Co) doping on the broadband terahertz optical properties of congruent LiNbO3 single crystal

Abstract

Incorporating transition elements into a Non-Linear Optical (NLO) material offers a substantial potential for efficient broadband terahertz (THz), photorefractive, and NLO applications. Cobalt, a versatile transition metal, is particularly influential in modifying the nonlinear optical properties of efficient nonlinear optical single crystals. The undoped Congruent Lithium Niobate (LiNbO₃, LN) and 0.015 mol % CoO-doped Congruent Lithium Niobate (Co:LN) single crystals are grown using the Czochralski technique along the z-direction in ambient air. The Powder X-ray diffraction (PXRD) studies confirm the crystalline perfection with different orientations of the diffraction planes. Raman spectroscopy confirmed the impact of cobalt doping on the lattice vibrational modes, evidenced by the altered intensity and full width at half maximum (FWHM) values, without shifting the vibrational peak positions of the observed vibrational mode. Further, to probe the terahertz-material interaction, a THz analysis is carried out on both crystals by employing the terahertz time-domain spectroscopy (THz-TDS) in transmission mode operated in the ambient air condition at room temperature. The terahertz optical parameters are derived from the transmission spectra and phase shift between the reference and sample signal. The 0.015 mol% Co doping significantly influences the THz optical parameters of the undoped LiNbO₃ single crystal. In addition, a theoretical Lorentz’s Damped Harmonic Oscillator (DHO) model is employed, which accurately describes the obtained THz response of both the crystals in the THz region.