USE OF THE GREEN EMISSION IN Er3+- DOPED CaF2 CRYSTALS FOR THERMOMETRY APPLICATIONS

Abstract

CaF2 has been a subject of great interest for a long time, due to its transparency in a wide spectral domain (0.15-9 µm). Consequently, it has been used in many optical components. Recently, CaF2 doped with certain triply ionized rare earths, viz. Er3+, Tm3+, Pr3+ etc. has been widely studied for laser applications and optoelectronic devices. In addition to its exceptional optical properties, it provides several advantages over other materials used for the same role. First, this material can be easily grown in the form of bulk crystals and thin films with excellent optical quality by using Bridgman-Czokralski techniques, molecular beam epitaxy MBE [1] and liquid phase epitaxy LPE techniques [2]. Second, as CaF2 is a relatively high thermal conductor and low phonon energy system, the heat induced by the laser excitation in the crystal can be evacuated easily. Especially, Er3+-doping for laser materials has attracted much interest because of its emission at the eye-safe wavelength of 1.5 µm occurring between the first excited level 4I13/2 and the ground level 4I15/2 of the Er3+ ions for applications in optical communications on one hand, and their thermally coupled levels 4S3/2 and 2H11/2 which are used in the Fluorescence Intensity Ratio (FIR) technique [3, 4], on the other hand. We report here the use of the green upconversion emissions originating from the thermally coupled levels 2H11/2 and 4S3/2 of the Er3+ ion in CaF2:Er (0.01% at.) for thermometry application in the range 300- 473 K. The mechanism responsible for excitation of the green emitting levels is a sequential two-photon absorption process. The Fluorescence Intensity Ratio (FIR) of the green upconversion emissions at the wavelength of about 519 and 551 nm is studied as a function of temperature in the range 300- 473 K using a 634 nm tunable dye laser as an excitation source. It is found that the logarithm of the FIR varies linearly with the inverse of temperature. The gap between the two thermally coupled levels 4S3/2 and 2H11/2 was determined to be about 721 cm-1. This value is in good agreement with that found by spectroscopic investigations [5]. The calibration curve is established and the temperature is calculated.