Spectroscopic investigation of Tm 3+ containing Lithium borate glasses

Transcription

1 Sectroscoic investigation of Tm + containing Lithium borate glasses D.D. amteke and H. C. Swart* Deartment of Physics, University of the Free State, P.O. Box 9, Bloemfontein 900, South Africa SwartHC@ufs.ac.za, ddhyvnit@gmail.com Abstract. In the resent study we investigated the sectroscoic roerties of Tm + containing lithium borate glasses as a function of Tm + concentration. The hysical roerties were analyzed by using the density and molar volume of the glasses. The Judd-Ofelt theory was used to analyze the exerimental data. The main focus of the resent study was to study the effect of ion concentration on the emission roerties of the glasses. The emission intensity of the glasses increased with the addition of Tm + ions u to 0.5 mol% and decreased with a further addition due to concentration quenching. 1. Introduction Ongoing rogress in the otical technologies is totally deendent on the rare earth materials in which hotons are maniulated by using the change in the host materials or change in the concentration of singly or multi-doed rare earth materials. Glasses are the excellent host for the rare earth ions and their alications lay an imortant role in hotonics, nuclear waste management, household and other technologies. Among all rare earth ions Thulium (Tm + ) is the best active ion because of its broad emission range. This emission range cover the alications in telecommunication u to the laser technology. Emission from Tm + ions at 2 µm is useful in the area of light detection and ranging (LIDA), remote sensing and in the medical field [1]. The broad emission around 1460 nm is useful for the develoment of an otical amlifier for alications in fiber-otic communication. By using the 1860 nm emission of Tm + it is ossible to build a mid infrared laser [2]. Glass hosts can be utilized to attain both these roerties. Among the available otential hosts, the lithium borate glasses are very interesting due to their easy formability, wide range of comosition and good rare earth solubility [-5]. The resent work was to understand in detail the sectroscoic behavior of Tm + ions in the lithium borate glasses. We reared the 27.5 Li 2O: (72.5-X) B 2O : X Tm 2O glasses and analyzed their hysical and sectroscoic roerties. We also the Judd-Ofelt (J-O) arameters by using the absortion data of the reared glasses. J-O is a theory in hysical chemistry describing the intensity of electron transitions within the 4f shell of rare earth ions in solids and solutions. By using the J-O arameters the estimated values of the radiative transition robabilities, radiative lifetimes of excited states and branching ratios are reorted in the resent study. Luminescence studies have been erformed as a function of mol % of the Tm + ions. SA Institute of Physics ISBN:

2 2. Exerimental The 27.5 Li 2O: (72.5-X) B 2O : X Tm 2O (Tm 2O = 0, 0.25, 0.5, 0.75 and 1 mol%) glasses were reared by the conventional melt quench technique. The aroriate quantity of high urity chemicals were weighed and mixed in an agate mortar by using acetone. The obtained mixture were transferred in an alumina crucible and melted in an electric furnace at 127 K. To achieve the homogeneity of the melt, the melt was allowed to soak for 1 h with intermediate stirring. Samles of the desired shae were obtained by quenching the melt on the brass molds at room temerature. Samles were immediately transferred to an annealing furnace reheated at 57 K and collected the next day. The reared samle were labeled as Tm0, Tm1, Tm2, Tm and Tm4 according to increasing doing concentration. The density of the samles were by the Archimedes rincile using the METTLE TOLO weighing balance. A LAMBDA 950 Ultra violet/visible/near infra-red (UV/Vis/NI) sectrohotometer (PerkinElmer) was used to measure the absortion sectra of the glasses. The excitation and emission sectra of the glasses along with the lifetimes were recorded by using an Edinburgh FLS980 system with a 450 W steady state Xe lam as excitation source. The emission sectra were recorded by using a liquid nitrogen cooled Hamamastu near infrared hotomultilier tube (NI-PMT) with the resonse time of 800 s. The decay analysis were erformed by 60 W ulsed microsecond Xe flash lam and NI-PMT.. esults and Discussion Fig. 1 shows ictures of the glasses with different concentration of Tm 2O. The obtained glasses were colorless with a tint of yellow for the higher concentration Tm 2O glasses and also have good transarency. The absortion sectra were normalized by using the thickness of the samles and concentration of the Tm + ions in the glass matrix and lotted as absortion cross section as shown in Fig. 2 for the 0.75 mol % Tm2O as reresentative. In these sectra five absortion bands centred at 464, 677, 782, 1208 and 1677 nm are assigned to the H 6 1 G 4, H 6 F 2+ F, H 6 H 4, H 6 H 5 and H 6 F 4 transitions of the Tm + ions [6]. The wavelength range of the observed transition, mean wavelength ( λ ) and the integrated cross σ λ λ ) were determined and deicted in Table 1. These values were further used to calculate section ( ( ) d the Judd-Ofelt arameters for the reared glasses. Figure 1. Glasses with different Tm2O content. Table 1. Absortion arameters of 0.75 mol % Tm2O lithium borate glass. Manifold ange (nm) λ nm σ ( λ )d λ (cm 2 nm) 1 G F2, F H H F SA Institute of Physics ISBN:

3 Figure 2. Absortion cross section sectra of 0.75 mol % Tm 2O lithium borate glass. Table 2. Variation of ( S ), ( S intensity arameters Ω t. ) line strengths (X cm 2 ) and J-O Manifold 0.25 mol% Tm 2O 0.5 mol% Tm 2O 0.75 mol% Tm 2O 1 mol% Tm 2O S S S S S S S S 1 G F2, F H H F Ω 2 X Ω 4 X Ω 6 X By using the J-O theory [7,8] it s ossible to calculate the manifold-to-manifold transition robabilities, radiative lifetimes and branching ratios of emission. The accuracy of the J-O arameter deend on accuracy of the absortion measurements. By using the values of integrated cross section, line strength of manifold to manifold electric diole can be by using the following equation: 2 ch(2 J ' + 1) = ( ) S n σ λ dλ 8λ e λ n + 1 (1) where n is the refractive index and J is the total angular momentum, and the other symbols have their usual meaning. By using the values of line strength J-O intensity arameters were obtained by solving SA Institute of Physics ISBN:

4 the equation for five observed transitions. J-O theory also rovides the theoretical exression for line strength which is given by: 2 t [ ] [ ' '] ' (2) S = Ω f SL J U f S L J n t n t= 2,4,6 where Ωt are the Judd Ofelt arameters, t U are the doubly reduced unit tensor oerators, S is the sin oerator and L is the angular momentum oerator for L-S couling scheme for ground state and first excited state [9]. The obtained values are rovided in Table 2. The ( S ) and ( S ) line strengths have values relatively close to each other which justify the quality and accuracy of the fitting use in this study. The values or manifold 1 G 4 has a very large difference between the and exerimental values. This difference may be due to the hyersensitivity of the 1 G 4 transition. As the hyersensitive transition is strongly deendent on the host and surrounding and their redicted and exerimental values may vary. The variation of is Ω2 strongly deends on the variation in the covalent bonding. Change in covalency between the Tm + ions and ligands anions directly affects the values of Ω 2 as it reflects the asymmetry of constituent s ions around the Tm + ions. Ω 4 and Ω 6 values are the measure of rigidity of the structure in which the rare earth ions are situated [10]. The values of Ω 2 were found to decrease with an increase in the concentration of the Tm + ions, which confirms an increase in the ionic nature of the bonding in the glasses. Figure (a). Excitation Sectra of reared 0.75 mol % Tm 2O containing glass. Figure (b). Emission sectra of different mol % Tm 2O glasses. Fig. (a) shows the excitation sectra of 0.75 mol % Tm 2O containing glasses as reresentative at an emission wavelength of 1195 nm. The sectra show three excitation bands at 58, 468, and 596 nm from the ground state H 6 of the Tm + to various excited states. These bands are identified as 1 D 2, 1 G 4 and F 2 after comaring with the literature [2]. Amongst all, the 1 G4 was the most rominent band and it was selected to study the emission roerties of the Tm + ion containing glasses. Fig. (b) shows the emission sectra of glasses for different mol % of Tm + ions. These sectra contain a rominent band at 1195 nm along with a broad hum at around 1544 nm. These emission bands corresond to the H 4 H 5 and H 4 F 4 transitions of the Tm + ions. Fig. 4 (a) gives the intensity variation of the H 4 H 5 and H 4 F 4 transitions as function of comosition. It is observed that the emission intensity increased u to 0.5 mol % Tm 2O and decreased with further addition of the Tm + ions. This effect was due to concentration quenching and came in to effect because of the increase number of Tm + ions in the glass matrix [11]. SA Institute of Physics ISBN:

5 Figure 4 (a). Emission intensity variation with mol % Tm 2O. Figure 4 (b) Florescence decay rofile of the H 5 state of the 0.75 mol % Tm + ion containing glass. Table. Emission band ositions (λ, nm), effective bandwidths ( λeff, cm -1 ), radiative transition robabilities (A, s -1 ), eak stimulated emission cross-sections ( σ x10-22 cm 2 ), branching ratios ( ) and lifetime (τ ex µs) of H 4 state of Tm 2O containing glasses. SN Transitions Parameter Tm1 Tm 2 Tm Tm 4 1 H 4 H 5 λ λeff A σ (ex) (cal) H 4 F 4 λ λeff A σ (ex) (cal) Lifetime (τex µs) By using the emission sectra of the reared glasses the values of the branching ratio ( ), stimulated emission cross-section ( σ ) and effective line width of the transition ( λ ) were by using the following relations [12, 1]: (, ' ') A( ΨJ, Ψ ' J ') ΨJ Ψ J = A ΨJ T P ( ) 4 λp A σ P = (4) 2 8π cn λeff where A is the transition robability of the uer excited state, AT is the total transition robability, n is the refractive index, λ P eak wavelength and λeff is the effective line width of the transition given by 1 λeff = I( λ) dλ I (5) max eff () SA Institute of Physics ISBN:

6 I is the fluorescence intensity and I max is the maximum intensity. The obtained values are resented in Table. From the table it is clear that the emission cross section ( σ ) has the highest values for the 0.5 mol % Tm2O containing glasses. The H4 F4 transition has the highest values of σ and branching ratio among the observed transitions. A big difference between the theoretical and exerimental values was observed, the theoretical calculations, however, does not consider all the rocesses involved in the excitation and emission. These values are useful in justifying the alication of these glasses for lasers. Thus the 0.5 mol % Tm + ions containing glasses are the best for laser alication and telecommunication. The decay curves of the H 4 transition were recorded at room temerature under 468 nm excitation by monitoring the emission at 1195 nm and are showed in Fig. 4(b) for different mol % of Tm 2O. On analyzing these curves it was observed that they were non-exonential in nature after 0.5 mol % Tm + containing glasses. The lifetimes of the H 4 state are deicted in Table for the different concentrations of the Tm + ions. It is clear that the decay times decreased with an increase in Tm + ion concentration, the shorting of the life time and deviation from the exonential behavior was mainly due to the concentration quenching in the life time of the H 4 state. It is therefore clear that there was an energy transfer amongst the Tm + ions at higher concentrations. Conclusions The structural and sectroscoic roerties of lithium borate glasses with different concentration of Tm2O were successfully studied. J-O intensity arameter were for all reared comositions. Ω 2 intensity arameter decreased with Tm + ion concentration showing an increase in ionic bonding in the glasses. A decrease in Ω 4 and Ω 6 arameter showed that the rigidity of the glasses decreased with Tm + ion concentration. Emission sectra of the reared glasses showed four rominent bands which belongs to the F2, H4, H5 and F4 transitions. The emission intensity of the glasses increased u to 0.5 mol % Tm2O and decreased with further addition. Acknowledgements This research is suorted by the South African esearch Chairs Initiative of the Deartment of Science and Technology and the National esearch Foundation of South Africa (grant 84415). The financial suort from the University of the Free State is also acknowledged. The Edinburgh Instruments FLS980 system used in this study was funded by the National esearch Foundation of South Africa (Grant EQP ). eferences [1] Gebavi H, Milanese D, Liao G, Chen Q, Ferraris M, Ivanda M, and Taccheo S 2009 J. Non-Cryst. Solids 55(9) 548. [2] Walsh M, Barnes N, eichle D and Jiang S 2006 J. Non-Cryst. Solids [] amteke D and Gedam S 2014 J. are Earth [4] amteke D, Annaurna K, Deshande V and Gedam S 2014 J. are Earth 2 (12) [5] amteke D, Swart H and Gedam S Physica B [6] Carnall W, Fields P and ajnak K 1968 J. Chem. Phys. 49 (10) [7] Judd B 1962 Phys. ev. 127, [8] Ofelt G 1962 J. Chem. Phys. 7, 511. [9] Walsh M, Barnes N and Bartolo B 1998 J. Al. Phys [10] Venkatramu V, Babu P, Jayasankar C, Troster T, Sievers W and Wortmann G 2007 Ot. Mater [11] amteke D, Kumar V and Swart H 2016 J. Non-Cryst Solids [12] Dousti M, Ghoshal S, Amjad, Sahar M, Nawaz F and Arifin 201 Ot. Commun [1] Seshadri M, Venkata ao K, ao J and atnakaram Y 2009 J. Alloys Comd [14] Suhasini T, Suresh Kumar J, Sasikala T, Jang K, Lee H, Jayasimhadri M, Jeong J, Yi S and Moorthy L 2009 Ot. Mater SA Institute of Physics ISBN: