Synthesis, Structural and Luminescence Studies of Europium Doped Borate Glasses

Different concentration of (0.1, 0.2, 0.3, 0.4, 0.5, 1.0 & 1.5) mol% of europium doped lead borate lithium glasses were prepared by melt quenching method. The properties of thermal, photoluminescence & structural were analyzed using DTA, FTIR and emission spectra. Emission spectra was used to evaluate the JuddOfelt (JO) parameters. Radiative parameters like stimulated emission cross-sections (σe), effective band width (∆λeff), transition probabilities (A), optical gain bandwidths (σe x ∆λeff), radiative lifetime (τrad) and optical gain (σe x τrad) values were evaluated for the transition D0→Fj (j=1, 2, 3 and 4) of Eu3+ ions. The outcome of transition D0→F4 at its highest value of branching ratios and stimulated emission cross-section are evaluated with the literature. Hence we can could the prepared host glass doped with Eu3+ ions are good fibre amplifiers and it can be used as a red laser.


Introduction
Glasses doped with RE ions are of larger interest due to their use in infrared-to-visible up-converters, solidstate lasers, bulk lasers, waveguide lasers, planar waveguides, temperature sensors, flat panel displays, field emission displays, optical fiber amplifiers, high-density memory devices, high-density frequency domain optical data storage, color display devices and electroluminescent devices [1][2][3]. Among all RE ions, europium is a unique component because of the divalent or trivalent valence state. Photonic applications are used Eu 3+ ion due to its property of red light emission, which was observed during the transition 5 D0 -----7 F2 state and it is very sensitivity to minute changes when surrounded by chemicals. The emission from non-degenerate 5 D0 to 7 Fj (j = 0-6) levels gives site symmetry around active ions and number of sites available for active ions [4][5][6][7].
Archimedes principle were used to calculate the densities of the prepared glass samples. From room temperature to 900 °C DTA measurement were carried out in nitrogen atmosphere with a resolution of 1ml min−1 using Perkin Elmer STA 6000. From KBR pellet technique FTIR spectra (350 -3500) are recoded using Thermo Nicolet Avata 370. Using Shimadzu spectrophotometer UV-1800 (250 -1200 nm), the optical absorption spectra were recorded. Photoluminescence and time decay characterization are done using Horiba Fluorolog spectrofluorometer with 450Wxenon lamp as excitation source.

Optical studies
The optical absorption spectra were shown in Fig. 1 it consists of three peaks the transitions are from 7 F0 to 5 L6, 5 D2 & 5 D1 at 393, 464 and 525 nm wavelengths. All these transitions are well agreeing with the pervious literature [8,9]. Compare to all transitions 7 F0 → 5 L6 is more intense because of well population region. As the Eu 3+ concentration increases, the absorption peak intensity is increasing and no peak shift is observed. This is evidence for the RE dissolvable in the glass matrix. By Mott and Davis calculation the indirect and direct energy band gaps were expected [10] (ℎ ) = �ℎ − � (1) where α represents absorption coefficient, hυ represents IPE (incident photon energy), B represents a constant, Eopt represents OEBG (optical energy band gap) and n represents the number which expresses the transition processes. Irregular variation is observed in both indirect and direct energy band gap values with the increase of Eu2O3 concentration, this may be due to the changes occurring in bonds and also due to the photon-lattice interaction [11]. Optical parameters which are calculated in this paper are tabulated in table 1 and the variation are shown in

Thermal analysis
The characteristic temperatures of any glass like melting temperature, crystallization temperature and transition temperature are evaluated by Differential thermal analysis [13]. The ELPB0 and ELPB1 glasses, DTA curves obtained at the heating rate of 10 0 C/min are depicted in Figs. 4 (a) and (b). All thermal parameters were determined and are given in table 2, in the range of 442-470 0 C are the glass transition temperature of the prepared glass samples. From the onset crystallization (Tc) and glass transition temperature (Tm) of the glasses, the thermal stability was determined [14]. GFA (Glass forming ability) K is determined by the relation given by Hruby [15]. The thermal stability is determined based on the relation provided by Saad and Poulin [16]. To summarize this samples output are recommended for fibre fabrication due to its phenomenon thermal stability.

Fluorescence studies
The emission spectra of ELP5 glass were recorded in the wavelength region 550 -850 nm at 393nm excitation wavelength and it is presented in Fig. 6. In Fig. 6, five emissions are detected and are at 578, 592, 613, 652 & 700 nm and are allocated to the transitions from 5 D0 → 7 F0, 7 F1, 7 F2, 7 F3 & 7 F4 respectively. Among these emissions 613 nm ( 5 D0 → 7 F2) is more intense compare to other emission peaks. Hence it discloses red luminescence emission which can be used in display devices and red lasers [20].

Judd-Ofelt analysis
JO intensity parameters are essential to calculate the radiative parameters. The calculated JO intensity parameters with respect to europium concentration were tabulated in table 3 and also it is comparable with previously reported values. From the integrated areas of ( 5 D0 → 7 F1) to ( 5 D0 → 7 F2), the luminescence intensity ratio was estimated and these values are tabulated in table 3. JO intensity parameters are generally estimated from the absorption spectrum, but in the case Eu 3+ ion, it can be calculated from the luminescence spectra by intensity ratios of electric -dipole transition 5 D0 → 7 FJ (J= 2 and 4) and magnetic -dipole transition 5 D0 → 7 F1 [ 23,24].
where = ( 2 + 2) 2 /9 , n is the refractive index, e is the charge of the electron, h is the Planck's constant, λ is the wavelength of the particular transition, 0 is the permittivity of free space, Ω2 & Ω4 are the J-O intensity parameters used to describe the metal-ligand in the matrix and || || 2 is the square reduced matrix elements of the tensor operator. The experimental coefficients of spontaneous emission ( 0 ), are calculated by the relation where I0J is the maximum intensity of the 2 nd peak, I01 is the maximum intensity of the 1 st peak, ν1 is the wavenumber of the 1 st peak, and ν2 is the wavenumber of the 2 nd peak. J-O parameters Ω2 and Ω4 values are shown in table 3, the Ω2 and Ω4 values were decreases with the europium concentration. At 0.1 Eu 3+ concentration Ω2 value is more and it shows the existence of a covalent bond between the Eu 3+ ion and their neighbouring ligands. Same variation is observed for both Ω2 and intensity ratio with the europium concentration. The radiative parameters were determined from JO theories like stimulated emission cross-sections, radiative lifetime, total transition probabilities, and effective bandwidth using the equations specified in reference [25,26] and these values are tabulated in table 4. The high value of stimulated emission is the most eye-catching features for the design and the growth of low-threshold and high gain laser applications. At 0.2 mol% of europium the stimulated emission cross section value is high for the transition 5 D0 → 7 F4 and it is well suitable lasing for action.
Eu 3+ ion doped lithium lead borate glasses are emitting red colour, this is recognized from the chromaticity diagram and it is shown in the

Conclusion
Different concentration of (0.1, 0.2, 0.3, 0.4, 0.5, 1.0 & 1.5) europium doped lithium lead borate glasses were prepared by melt quenching process. These prepared glass samples were characterized by DTA, FTIR, UVabsorption, luminescence, and time-decay measurements. From the DTA analysis, the glass transition, crystallization, and melting temperatures on the concentration of Eu2O3 were studied. The obtained results show good thermal stability, and it can be used in fibre fabrication. FTIR studies of the prepared glass samples show the functional groups present in the glass and the structural units. UV-optical absorption spectra show the rare-earth solubility, and optical energy bandgaps show the non-linear behaviour with the concentration of Eu2O3 due to the photon-lattice interaction. From luminescence spectra, the J-O parameters and intensity ratio were calculated. The intensity ratio ( R ) and Ω2 are decreasing with the concentration of Eu2O3, and this shows the covalent bond around the Eu 3+ ions. Radiative parameters like stimulated emission cross-section, optical gain bandwidths, optical gain, and a radiative lifetime of the prepared glasses were having been calculated. Among all the concentrations at Eu -0.2 mol% glass found larger stimulated emission cross-section (48.995×10 -22 cm 2 ) for 5 D0→ 7 F4 transition. The lifetime decay shows the tri-exponential nature for all the concentrations of Eu2O3. From the CIE diagram, it shows that the glass samples are emitting red colour, which is very close to standard red colour coordinates, which indicates that the newly prepared glass samples are emitting red colour, and it can be used in lasers.