Optical spectroscopy of Eu3+ doped zinc borate glasses

Zinc borate binary glasses of composition 4Zn0.3B,03 undoped and doped with 0.5% mol of Eu3' were prepared from the melt giving rise to samples of excellent optical quality. In the present work we use the spectroscopic properties of the Eu3' to obtain information about the structural and dynamic features of our system. Different optical techniques such as site selection, excitation spectroscopy and lifetime measurements as well as Raman spectroscopy were used to study the local environment of the Eu3' ion in these glasses. No energy transfer is observed at our concentration and moreover non-radiative relaxation from the excited 5D, state to the 7F, manifold is not important. A broad distribution of slightly different environments of the Eu3' ion is reflected in the trend of the Stark splittings of the 7F, state and in the crystal field parameters for the individual sites as a function of the excitation wavelength. Phonon sidebands are observed in the excitation and luminescence spectra of the EU~' impurity. We make an attempt to assign these sidebands to coupling of the vibrational modes of the structural units containing the Eu3' ion. The strength of the electron-phonon coupling is evaluated and compared with the result obtained for related systems. Introduction Although vibrational spectroscopic studies on some borate glass systems have been reported [1,2,3], there are no extensive investigations, using the luminescence properties, on the zinc borate binary glasses 4Zn0.3B2O, :EU~'. Nevertheless this system is very easy to prepare and samples of good optical quality can be obtained. In order to use these doped rare earths glasses as optical devices it is important to know the effects of the matrix on the spectroscopic properties of the optically active ion [4,5]. At this aim we have performed excitation spectra, fluorescence spectra and lifetime measurements on the 5~0e7~0 transition of EU~' ions in this glass. For our purposes this is a helpfkl transition. In fact it involves only levels with J=O and thus has no internal structure. Moreover it is relatively strong in our sample. Raman spectra for the doped and undoped glasses have been used to detect possible localized vibrational modes around the Europium. Experimental Emission and excitation spectra were obtained using a FL 2001 Dye Laser with rhodamine 6G, pumped by EMG5O Lambda Physik Excimer laser. The fluorescence was focused onto a model 1401 double Spex monochromator equipped with a cooled RCA C3 1034A02 photomultiplier tube and was detected Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19944114 C4-478 JOURNAL DE PHYSIQUE IV by photon counting. Both emission and excitation measurements were performed in time-resolved spectroscopy. Lifetime measurements were performed on a model SR430 Stanford multichannel using a minimum dwell time of 2.56 ps. An Oxford Instruments cryostat was used for low-temperature measurements. Raman spectra were measured on a double monochromator Ramanor HG 2s JobinYvon, using the 514.5 nm line of a Spectra Physics 166 argon-ion laser, operating at 250 mW. Resolution of the Raman spectra was about 2 cm-I. We used two scattering geometries with the incident beam polarized perpendicular (V) or parallel (H) to the scattering plane. The scattered radiation, measured at an angle of 90" from the incident laser beam, was the sum of the radiation polarized in the both directions (T). We call these geometries as V-T and H-T respectively. Results and Discussion The room temperature Raman spectra of the undoped and doped samples are shown in Figure 1 and Figure 2 respectively. Both geometries V-T and H-T Raman shift (cmI ) Raman shift (cml) Figure 1: Room temperature Figure 2: Room temperature Raman spectra of the undoped Raman spectra of the doped sample sample have been recorded. The low-frequency unpolarized broad band at 63 cm-' is the "boson peak" characteristic of glasses [6]. The features of the spectra at higher energy are similar to those reported for binary metaborate glasses in several papers [2, 7, 81, where the assignment of the vibrational modes of the borate groups is discussed in detail. For the objectives of this work it is enough to observe the strongly polarized structures in the region between 600 and 1100 cm-' and the unpolarized broad band centered at about 1380 cm-'. These bands are assigned to the vibrational modes of borate groups whose energy and intensity are dependent on the amount of ZnO modifier. A completely polarized band appears at about 230 cm-' related to Zn-0 vibrational mode. In this connection we should recall that Raman bands in the region between 100 and 300 cm-' have been observed in Zn(PO,), glasses [9]. The main feature of the Raman spectra for the doped sample, shown in Fig.2, is the polarized H-T band at about 445 cm-'. Its intensity increases with the dopant concentration as shown by the Raman spectra of a sample doped with 2% mol of Europium. Moreover the low 2 . 4 temperature excitation spectrum of the 7Fo+5Do transition, pictured in Fig. 3, displays a phonon sideband at 1720