Concentration dependent enhanced luminescence and its quenching effect in erbium incorporated heavy metal oxide—borate glasses

Erbium incorporated zinc-lead-bismuth-borate glasses are prepared through melt-quenching technique and their concentrated dependent structural, thermal and spectroscopic properties are evaluated. Prepared glasses are thermally stable and amorphous nature and the density and refractive indices of the prepared glasses increases with Er2O3 concentration. UV–vis-NIR spectra exhibits multiple transitions accompanied by hyper sensitive transition of 4I15/2 → 2H11/2. Theoretical Judd-Oflet calculations were performed and correlated with experimental results. JO and bonding parameters reveal the covalence nature of the prepared glasses and radiative probabilities shows highest branching ratio for 4I15/2 → 2H11/2 transition. Luminescence results exhibit luminescence enhancement up to 1 mol% of erbium and thereafter luminescence quenches due to the existence of cross-relaxation channels. Using McCumber’s theory, absorption and emission cross-section values are estimated and are in agreement with each other and as well as with the experimental stimulated cross-sectional values. About 40% population inversion is achieved for the prepared glasses. Therefore, the investigated results suggest that, the prepared glasses are suitable candidates for amplifier applications.


Introduction
In recent years, erbium doped fiber amplifiers are drawing much demand as a potential candidate for various efficient devices for technological applications such as optical amplifiers, fiber networks and photonic devices [1]. The transition spectra of Er ion consist of very strong intensities and sharp spectral characteristics in 4f transitions [2]. In general, the optical transitions of rare earth ion depend on the type of host material. Numerous reports are available on the utilization of the different host to achieve high optical parameters. Selecting suitable host is a vital task to attain minimum loss, high luminescence properties [3]. It is reported that, borate is cost effective, structurally deformable former and it has high phonon energy. In contrast, heavy metal oxides are having high polarizability capacity and low phonon energy. In addition, heavy metal oxides are suitable candidates for enhancing optical properties due to their larger linear refractive index, high nonlinear response, smaller cut off phonon energy (500-700 cm −1 ) and good physio-chemical properties [4,5]. When borate is replaced by heavy metal oxides, the non-radiative losses can be reduced and intern the luminescence properties can be enhanced [6]. Apart, the amplification of Erbium doped fiber amplifiers (EDFA) at 1550 nm is reported [7]. However, erbium and other rare-earth ions incorporated matrix exhibits narrow gain bandwidths (about 100 nm flat gain) [8,9]. For the transmission of high bit rate information, narrow region ranging 1530-1610 nm spectra will be utilized, which is based on EDFA systems. Bismuth borate glasses are the most suitable host candidates for achieving efficient luminescence in the rare earth ions, such as, erbium ion, since the luminescence efficiency is strongly dependent on the phonon energy of the host matrix. Incorporation of Er ions at Bismuth site, reduces the phonon energy, The density of glasses is estimated by Archimedes's principle using toluene as an immersion liquid. Refractive index is determined using Abbe refractometer, mono-bromo-naphthalene as contact liquid. X-ray diffraction measurements are carried out using RIGAKU, ULTIMA IV x-ray diffractometer using Cu K α radiation of wavelength 1.5418 Å to investigate the amorphous nature of the prepared glass samples. DTA studies are carried out in the temperature range 100°C-800°C by Perkin Elmer, USAA, Diamond TG-DTA instrument using Alumina crucibles of having 45 μl volume in the N 2 atmosphere with heating rate of 10°C min −1 having sensitivity of 0.06 μV. FTIR spectroscopic studies are carried out by KBr pellet method using Thermo Nicolet 6700 instrument in the range 500 to 1800 cm −1 . Optical studies are carried using UV-vis-NIR Double beam Spectrophotometer using Perkin-Elmer Lambda 750 instrumentin the range 200-2000 nm for absorption with a UV-vis resolution of 0.17 to 5 nm and NIR 0.2 to 20 nm. Further, luminescence and time decay measurements are carried out using EDINBURGH FLS 980, using an excitation of 980 nm laser diode to record IR range with sensitivity >25 000:1 standard with life time range of 100 ps to 50 μs and with a pulse width <1 ns.

Physical parameters
The density of the prepared glasses is determined by Archimedes principle using toluene as an immersion liquid. The refractive indices are estimated by digital Abbe refractometer instrument. Using the experimentally obtained density and refractive index values different physical properties such as, molar volume, molar refractivity, molar polarizability, polaron radius, inter ionic distance, field strength, dielectric constant, reflection loss and erbium ionic concentrations are estimated using suitable expressions available in the literature [13][14][15][16][17] and are summarized in table 1. It is observed that, the density and refractive index exhibits non-linear variation with erbium concentration. The other physical properties of the prepared glasses vary with increase in erbium ions concentration, indicates the distorted environment around the doped RE ion in the glass matrix. Figure 1 depicts the variation of different physical parameters with concentration of erbium in the host glasses. Incorporation of erbium ions in to the host glass matrix, result in losing the bonds which in turn results in the creation of non-bridging oxygen's. The field strength found increasing with erbium concentration, while inter ionic distance decreases. Substitution of erbium ions causes change in oxygen to boron ratio, which induces BO 4 units, results in the closely packed structure of the glasses, as evidences in the decrease in the molar volume values. The observed variations in the physical parameter results confirm the possibility of increase in the glass transition temperature in the prepared glass samples.

X-ray diffraction studies
The XRD patterns of the prepared glass samples are depicted in figure 2. It is observed from figure 2, that, there are no distinct crystalline peaks except two broad humps in the 2θ region 20°-40°and 40°-60°within the detection limit of x-ray diffraction. The observed two humps are due to the short range order of atoms of the material and are the characteristics of amorphous nature of the glasses, which confirms the prepared glasses are in amorphous in nature.

Thermal studies
The DTA plots of the prepared glasses are depicted in the figure 3. The DTA graphs exhibits an exothermic peak in the range 650°C-670°C, which represents onset of crystallization temperatures (T x ) and an endothermic peak in the range 430°C-450°C, which represents the glass transition (T g ) temperature respectively. From DTA graph, the T g and T x are determined by taking inflection of dips (endo-down) and humps (exo-up) of the graph respectively. Having determined vaules of T g and T x , further, the thermal stability factor, ΔT=T x -T g is determined by taking the difference of two values. For BE0 T g , T x and ΔT found to be 439°C, 657°C and 218°C. Similarly for BE1 and BE3 the values of T g , T x and ΔT found to be 441°C, 670°C, 229°C and 456°C, 688°C, 232°C respectively. The estimated values of, T g , T x and ΔT are found increases with erbium concentration, indicates the stability of the glasses. That is the stability of the prepared glasses increases with increase of Er concentration in the host glass matrix. It is reported that, the stability of glass plays a vital role in deciding the suitability of glasses for photonic applications. Greater the stability, higher is the re-heating capacity [16]. Therefore, ΔT must be maxima to accomplish fiber drawing glass, which will result in low optical absorption and scattering losses over long optical path lengths. Since, the prepared glasses exhibit greater thermal stability with increase in the Er concentration could be potential candidates for optical fiber applications [17,18].

Fourier transform infra-red spectroscopy
The composition dependent structural changes and the structural groups are investigated using Fourier transform infrared spectroscopy. The figures 4(a) and (b) shows the transmittance and absorbance spectra of BE series glasses. It is observed from the figure 4 that, three major bands are observed and the corresponding peak positions are summarized in the table 2 [7,19]. The bands observed around 700 cm −1 are attributed to

Absorption studies
All the BE series glass samples shows similar absorption spectra and number of transitions as well. Further to evaluate the intensity and JO parameters, the absorption spectrum for BE1 sample in UV-vis-NIR region is depicted in figure 5 and the all parameters are evaluated for all the samples. From figure 5, eight absorption peaks corresponds to the transitions from the ground state 4 I 15/2 of rare earth ion (Er 3+ ) are observed in case of BE1 glass. The observed transition (absorption peaks) are compared with standard aqueous Er 3+ -ion [20]. Among all the observed transitions, the transition from 4 I 15/2 to 2 H 11/2 is having highest intensity, considered as the hypersensitive transition, which is characteristic of the host dependent. Using absorption spectrum, the intensity parameters are determined. Further, using Judd-Oflet theory [11,12], the experimentally calculated line strength values are compared with the theoretically obtained values and the estimated values are summarised in the table 3. It is observed that, the JO parameters follows trend Ω 2 >Ω 4 >Ω 6. From the table 3, it is concluded that, Ω 2 is greater, bonding parameter is +ve and the erbium ions have covalency bond nature with the host. Hence compared to previous reported [21][22][23][24] literature, our matrix is having high covalency nature.

Luminescence and life time studies
The Luminescence studies of BE series glasses is carried out in IR range using excitation wavelength of 980 nm, corresponds to 4 I 13/2 → 4 I 15/2 transition and the obtained spectra is depicted in the figure 6(a). From figure 6(a), broad emission spectrum is observed in all samples. It is observed that, the intensity of the emission peak increases with erbium ions concentration up to 1 mol% and there after decreases for further increase in erbium ions concentration. That is, the luminescence enhancement is seen with erbium ions concentration initially till 1 mol% and further increase in the Erbium ions concentration, spectra shows luminescence quenching effect. The observed quenching effect is due to concentration quenching effect [21][22][23][24][25]. The time decay studies are carried out for same 4 I 13/2 → 4 I 15/2 transition and are shown in figure 6(b). From figure 6(b), all the samples, show single exponential behaviour, which makes easier and simpler to estimate the life time of each sample. All the samples life time is fitted and the obtained values are depicted in figure 6(b). The observed decay curves also confirms the concentration quenching effect as result the life time values found decreasing with increase in erbium concentration. Further, the amplification parameters such as, the gain bandwidth (ΔG=Δλ eff ×σ(λ p )) and gain per unit length (G=τ exp ×σ(λ p )) are estimated and the corresponding values are found to be 840×10 −28 cm 3 and 3.989×10 −24 cm 2 s respectively, which are greater than the previous reported values [26].

McCumber's theory and gain coefficient
Using McCumber's theory [27], the absorption (σ abs (λ)) and emission (σ emi (λ)) cross section values are estimated and the corresponding plots are shown in figure 7(a). The estimated values from graph of both σ abs (λ)  and σ emi (λ) respectively are 5.666×10 −21 cm 2 and 6.378×10 −21 cm 2 . Further, for different population inversion, the absorption and emission cross sections are plotted as a function of wavelength to estimate the approximate value of population inversion and the corresponding spectra is shown in figure 7(b). From figure 7(b), it is observed that, there is a 40% population inversion in case of BE1 glass and it originates in the telecommunication C band region [21][22][23][24][25][26][27].
The overall values and deciding factors for application purpose are summarized in the table 4. For 4 I 13/2 → 4 I 15/2 emission transition, the peak wavelength, FWHM, radiative transition probability, stimulated emission crosssection, gain band width, radiative life time, branching ratio, optical gain, absorption and emission cross section values by FL theory is considered. Among the all host, our sample has FWHM 99 nm, which is suitable for optical fiber applications.

Conclusions
In the present investigation, Erbium doped zinc-lead-bismuth-borate glasses are prepared using the conventional melt quenching technique. Results confirm that, the obtained glasses show high density and refractive index due to the presence of heavy metal oxides in the matrix. The prepared glasses are found amorphous in nature and shows high thermal stability. The FTIR spectra exhibits major borate related (BO 4 and BO 3 units) vibration modes and their increase in intensities are observed due to the creation of NBO's after incorporation of erbium in the matrix. The absorption spectra of BE series in the UV-vis-NIR region shows 8 transition states. Among the observed peaks, the peak corresponds to the 4 I 15/2 → 2 H 11/2 is found highest intensity called hyper sensitive transition (HST). Further, the covalency nature of the BE glasses is revealed from JO intensity and bonding parameters. The radiative probabilities shows highest branching ratio for