We study the red-to-green upconversion in Er3+ doped fluoride crystals. We show that the dominant upconversion process depends on the excitation wavelength: a two-step absorption between 631 and 634 nm and a two-ion interaction between 641 and 647 nm for an Er3+ doped cadmium fluoride at 77 K. An efficient IR (803 nm)-to-green upconversion via three-step absorption is also reported, for the first time, in Er3+ doped strontium fluoride at 300 K. A selection of sites exhibiting two or three transitions which energetically coincide with an appropriate laser excitation wavelength could optimize the upconversion efficiencies. 1 Introduction. Er3+ upconverted fluorescence can appear following two processes: the two-step absorption proposed by Bloembergen [I] and the two-ion interaction observed by Auzel[2]. The two-step absorption becomes only efficient for laser excitation wavelengths which coincide [3] with that of the transitions between the excited states of the Er3+ ion. During the past few years, the two-step absorption has beeasuccessfully used to develop upconversion lasers [4] . We believe that the multi-step absorption efficiency can be optimized by improving the energy coincidence between the transitions and by selecting an appropriate excitation wavelength. Such a plan can be tested with the fluorite-type crystals (MF2 with M = Cd, Ca, Sr, Ba) in which, the Er3+ ions are distributed in a variety of non-equivalent sites. 2 Site multiplicity In the fluorite-type crystals, the Er3+ ion occupies a hQ+ site, thus requiring a charge compensation. The symmetry and the strength of the crystal field acting on the Er3+ ion depend on the compensator and on its position relatively to the Er3+ ion. A large number of Er3+ sites has been observed. The crystal field splits the energy levels of the Er3+ free ion, into several components. Each distinct surroundings of the Er3+ ion gives rise to its own splitting, therefore to its own absorption or fluorescence spectrum. The Er3+ ions can also be paired. In Cd% and CaF2, the pairs occur at 0.01 mol % but their relative importance decreases as the size of the host cation increases across the Cd-Ca-Sr-Ba series. Notice that the concentration of one site may be considerably enhanced through appropriate preparation. As the energy coincidence behaves as a critical parameter, we expect large changes in the upconversion efficiency when the Er3+ surroundings is modified. This goal can be reached by the choice of a charge compensator [3]; another way consists in growing mixed crystals. 3 Experimental We use the selective laser excitation of the fluorescence to study the upconversion processes. A tunable laser beam is focused within the crystal. By adjusting the excitation wavelength, it is possible to excite only one erbium site at one time (optimum concentration 0.1 mol %). We select the fluorescence wavelength hf with a spectrometer and the excitation wavelength he with the tunable laser. The excitation spectra are obtained by monitoring the fluorescence as a function of the excitation wavelength. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991789 C7-336 JOURNAL DE PHYSIQUE IV The experiments were performed at 77 K , between 630 and 650 nm (by means of a Spectra-Physics 375 dye laser), on the one hand, on a CdF2 crystal double-doped with 0.1 rnol % Er3+ and 0.1 rnol % Na+ in which most of the Er3+ ions are present in the form of Er single ions of C2 symmetry, on the other hand on a CdF2 crystal single-doped with 0.1 rnol % Er in which the Er3+ ions are paired. One SrF2 crystal doped with 1.3 rnol % Er3+ has also been studied at 300 K, following 4bI2 excitation around 800 nm with a Spectra Physics 3900 Ti sapphire laser. 4 Results and interpretation 4-1 Upconversion via two-step absorption We have studied the green emission from single Er3+ ions of CdF2: 0.1 rnol % Er3+ 0.1 mo1 % Na+ following their excitation by a red dye laser in a spectral range free from electronic levels. We establish that this fluorescence is as intense as that arising from Er3+ pairs. By comparing the excitation spectrum (fig 1) to the energy level structure, we show that the mechanism for exciting the 4S312 level is a two-photon process involving two sequential excitations (fig 2), with 4113/2 acting as the intermediate level. The upconversion efficiency is not very high because of a bad coincidence between the ground state absorption and the excited state absorption. As the concentration increases (1 rnol %), the sharp structure observed in the excitation spectrum is replaced by a large structure which coincide with the 'Ilsn --> 4F9,2 absorption spectrum, proving that the two-ion interaction prevails over the two-step absorption (fig 2) when the Er3+ concentration exceeds 1 rnol %. 0' P vibronic .+ sideband