Spectroscopic and upconversion properties of erbium-doped potassium lithium tantalate niobate crystals under 800 nm femtosecond laser excitation

Abstract

The single crystals of tungsten bronze-type potassium lithium tantalate niobate doped with three concentrations erbium with good quality are grown using top-seed solution growth method. The unpolarized ultraviolet-visible-near infrared optical absorption spectra of these single crystals are measured at room temperature. The green and red upconversion emission properties are analyzed with respect to steady-state spectra and decay behaviors under 800 nm femtosecond laser excitation. The spectroscopic properties of Er3+ ions are studied on the basis of the Judd–Ofelt theory. 12 Er3+ ions absorption bands are observed ranging from 350 to 1650 nm, the lower limit is also the ultraviolet absorption edge of these crystals. The fluorescence branching ratio and radiative lifetime are calculated to support the upconversion mechanism. The relationship of the Judd–Ofelt parameters Ω4<Ω6 of Er3+ in these crystals is different from the general trend Ω2>Ω4>Ω6. Experimental results indicate that upconversion emission is a two-photon process. It follows that the shortening of lifetime and the increasing of intensities of red emission are due to the increase in Er3+ ions cluster sites. Excited state absorption and energy transfer processes lead to the green emission while cross relaxation process produces the red emission.