Multiphonon relaxation and infrared-to-visible conversion of Er3+ and Yb3+ ions in barium-thorium fluoride glass

Abstract

The upconversion of infrared radiation into green and red fluorescence has been studied for Er3+ and Yb3+ ions in BaF2/ThF4 fluoride glass over a wide temperature range and several dopant concentrations. At room temperature an upconversion efficiency of 3.3×10−5 has been obtained for the green emission from the glass with 1 mol % ErF3 and 19 mol % YbF3 pumped by 973-nm radiation with intensity of 16.5 mW/cm2. For an absorbed intensity of 10 W/cm2 the efficiency for green upconversion emission is predicted to be 2%. Radiative transition rates for the excited states of Er3+ were calculated using Judd–Ofelt theory and intensity parameters obtained from measured integrated absorption coefficients. The calculated values of the multiphonon relaxation and the upconversion fluorescence intensity, obtained from the rate equation model of Wright, are discussed in light of the experimental results.

Frequency upconversion inEr3+-doped fluoroindate glasses pumped at 1.48 μm

“…Because of the small energy gap between those levels ͑ϳ750 cm Ϫ1 ͒, the state 2 H 11/2 may be populated from 4 S 3/2 by thermal excitation and a quasithermal equilibrium occurs between the two levels. This temperature behavior is consistent with previous reports for fluoride glasses, 10 but in the present case the frequency upconversion process is so efficient that the fluoroindate glass could be used to build a sensitive temperature sensor. 15 …”

“…state 4 I 9/2 was not measured, but on the basis of the known results for other host materials, we expect that its magnitude is Ϸ50 s. The lifetimes are mainly determined by the multiphonon relaxation rates, which are small because of the small phonon energies associated with the fluoroindate matrix. 4,10 Thus, considering that the states 4 I 9/2 and 4 I 11/2 are likely to participate in the upconversion processes, we conclude that the large values observed for r and d provide favorable evidence for the relevance of the ET mechanism.…”

“…The radiative decays are obtained using Judd-Ofelt theory, 20 and the nonradiative relaxation rates were obtained from fluorozirconate glass measurements. 10 The actual values used are depicted in Table II. The software package MATHEMATICA was used to solve numerically the system of equations described above and to compare the results with the temporal behavior for xϭ1 and 3.…”

“…The ET process, in which an excited ion nonradiatively transfers its energy to an already excited neighbor, is one of the most efficient mechanisms and has been observed in a large number of systems including fluoroindate glasses. [8][9][10][11][12][13][14] This mechanism can arise from electric multipole or exchange interactions, and its rate occurrence depends on the RE concentration due to the ion-ion separation. In the present case, we expect that ET is the dominant process because of the large Er 3ϩ concentration in our samples and because the intermediate ESA step ( 4 I 13/2 → 2 H 11/2 ) is a two-photon transition with small probability to occur due to the laser frequency detuning for intermediate states and the weak laser intensity used.…”

“…8,9 Erbium ions are appealing for spectroscopic investigation due to their energy level regular spacing which facilitates frequency upconversion via energy transfer or multistep pump absorption using a single excitation wavelength. Accordingly, studies of upconversion in Er 3ϩ -doped fluoride, 10 borate, 11 tellurite and gallate, 12 and fluorophosphate glasses 13 have been reported. Presently, it is well known that the upconversion efficiency is larger for fluoride glasses because the multiphonon emission rates are much lower than the rates for the same levels of Er 3ϩ in other glasses.…”