Spectroscopic Properties and Energy Transfer in Lead Bismuth Gallium Borate Glasses Codoped with Tm3+ and Yb3+

Spectroscopic Properties and Energy Transfer in Lead Bismuth Gallium Borate Glasses Codoped with Tm3+ and Yb3+

K Krishna Murthy Goud*, Dept. of Physics, UCE (A), Osmania University, Hyderabad-500007 (T.S), India.

Ch Ramesh

Dept. of Physics, M G University, Nalgonda (T.S), India.

B Appa Rao

Dept. of Physics, Osmania University,

Hyderabad-500007 (T.S), India.

Abstract Lead bismuth gallium borate (GTY) glasses codoped with Tm3+/Yb3+ were prepared by melt quenching technique. The glasses were characterized by X-ray diffraction spectra. Optical absorption, FTIR and photoluminescence spectra of these glasses have been studied. The optical absorption spectra exhibits a band at 980 nm due to transitions from the ground states 4I15/2 and 2F7/2 to excited states of Yb3+ . The other absorption bands at 658 nm (3H6 3F2), 686 nm (3H6 3F3), 792 nm (3H6 3H4), 1211 nm (3H6 3H5) and 1663 nm (3H6

3F4) are attributed to 4f-4f transitions of Tm3+ ions, respectively.

Judd-Ofelt theory has been applied to the f f transitions for evaluating 2, 4 and 6 parameters. Radiative properties like branching ratio r and the radiative life time R have been determined on the basis JuddOfelt theory. Upconversion emissions have been observed under 980nm laser excitation at room temperature. The spectra exhibited two emission bands centered at 468 nm and 654 nm due to 1G4 3H6 (blue) and 1G4

3H4 (red) transitions, respectively. The results obtained are discussed quantitatively based on the energy transfer between Yb3+ and Tm3+.

Keywords Optical absorption, FTIR, Luminescence

1. INTRODUCTION

   The visible upconversion luminescence of rare earth ions exhibits extensive applications in color display, high density optical data storage and reading, biomedical diagnostics and optical communications, etc. [16]. Among various glass systems, heavy metal oxide based glass systems find potential applications in non-linear optical devices because of their high refractive index and low phonon energy compared with other glasses [7].

   Glasses based on heavy metal oxides (HMO), especially Ga2O3Bi2O3PbO lasses, are becoming important and attractive materials for optical amplifier and planar wave- guide due to their high refractive index and low phonon energies [8-11]. Moreover, their excellent thermal stability and mechanical properties are advantageous for drawing fibers [12, 13]. The reduced phonon energy effectively improves the quantum efficiency of luminescence from excited states of rare-earth (RE) ions in these matrices and provides the possibility of developing more efficient lasers and fiber optics amplifiers at longer wavelengths.

   Among the rare earth ions, the Tm3+ is very attractive since it has two stable excited levels, 1G4 and 3H4. Through 1G43H6 transmission, blue upconversion luminescence near 476 nm can be obtained which can be used

   in color displays, high density optical data storage and reading, biomedical diagnostic, etc. [14-17]. Through 3H43H6 transmission, 794 nm near infrared upconversion luminescence can be obtained which can be applied in the amplifier of the first communication window of quartz optical fiber [18, 19].

   In previous researches, Tm3+/Yb3+ codoped fluoride and tellurite glasses were investigated under 980 nm laser diode (LD) excitation and were shown to be effective blue luminescence materials through the Tm3+:1G43H6 transition [20, 21]. In this investigation, a series of Tm3+/Yb3+ codoped lead bismuth gallium borate glasses have been prepared. The XRD, optical absorption, FTIR and upconversion fluorescence spectra have been measured for these samples and the results are discussed in detail.

2. EXPERIMENTAL

   For the present study, glasses with the composition [100- (x+y)][0.5PbO-0.25B2O3-0.20Bi2O3-0.05Ga2O3]-xTm2O3-

   yYb2O3 with y = 0 for x = 0, 0.2 and y = 0.2 for x = 0 to 1.0 (step 0.2 mol%) are chosen and the glass samples are labelled

   as GT0Y0, GT2Y0, GT0Y2, GT2Y2, GT4Y2, GT6Y2,

   GT8Y2 and GT10Y2 respectively. Appropriate amounts of AR grade reagents of PbO, B2O3, Bi2O3, Ga2O3, Tm2O3 and Yb2O3 powders were weighed by using digital electronic balance. These chemicals were mixed and thoroughly ground in a mortar to get a homogeneous mixture and melted in a porcelain crucible in the temperature range of 900 to 950 oC in a programmable electrical furnace for thirty minutes until bubble free liquid is formed. The resultant melt is poured in a brass mould and subsequently annealed at 300 oC for about four hours in order to avoid these internal mechanical stresses. After annealing, both surfaces of the samples were optically polished to the dimensions of 1 cm × 1 cm × 0.2 cm in order to meet the requirements for optical measurements.

   The structure of the samples was tested by means of X- ray diffraction using Rigaku miniflex table top X-ray diffractometer with Cu K radiation at room temperature. The diffractograms have been measured in the range of 10- 80 with a scanning speed of 2.4/min. The optical absorption spectra were obtained with the JASCO Model V-670 UV VISNIR spectrophotometer in the wavelength range 350 1600 nm with a spectral resolution of 0.1 nm. The FTIR spectra of glass samples were recorded on a BRUKER OPTICS, TENSOR-27 infrared spectrometer in the range 4000 400 cm-1. These measurements were made on glass

   powder dispersed in KBr pellets. The visible upconversion fluorescence spectra were recorded using JOBIN YVON Fluorolog-3 spectrofluorimeter in the wavelength range 300- 700 nm under the excitation of 980 nm laser diode.

3. RESULTS AND DISCUSSION

   1. X-ray Diffraction

      The short and medium range orders in the prepared samples were tested by means of X-ray diffraction. The obtained diffraction patterns (Fig. 1) have proved the vitreous character of these samples.

      GT10Y2

      counts/sec.

      GT8Y2

      GT6Y2 GT4Y2 GT2Y2

      GT0Y2

      GT2Y0 GT0Y0

      10 20 30 40 50 60 70 80

      2 degrees)

      Fig. 1. XRD spectra of GTY glass system

      Table 1.Values of cut off wavelength, optical band gap and urbach energy of GTY glass system codoped with Tm3+/Yb3+.

      Sample Cut-off Eopt (eV)				E (eV)
      S.No.	code	wavelength (nm)	0.01	0.001
      1	GT0Y0	420	3.07	0.147
      2	GT2Y0	423	3.06	0.149
      3	GT0Y2	426	3.05	0.152
      4	GT2Y2	430	3.05	0.153
      5	GT4Y2	439	3.03	0.157
      6	GT6Y2	457	2.99	0.164
      7	GT8Y2	454	3.00	0.162
      8	GT10Y2	441	3.02	0.159

      The Judd-Ofelt theory helps in the analyzation of the radiative transitions within in the 4f N configuration of a rare earth ion. The Judd-Ofelt parameters 2, 4 and 6 [27, 28] can be determined by obtaining the experimental ground state oscillator strengths of the absorption transitions via an integration of the absorption coefficints for each band. The values of Judd-Ofelt intensity parameters are found to be 2

      = 1.86×10-20 cm2, 4= 1.09×10-20 cm2 and 6 = 1.14×10-

      20cm2, respectively. The Judd-Ofelt theory has often been

      used to calculate the spectroscopic parameters, such as radiative lifetime, oscillator strength and branching ratios (r) using standard equations [29-34]. The results are summarized in Table 2 and Table 3.

   2. Optical Absorption

      Transitio from 3	n H f (x10-6)	f (x10-6)
      3F4	3.987	4.102
      3H5	3.451	3.598
      3H4	5.684	5.578
      3F	6.981	7.115
      3F2	7.284	7.561

      Transitions	r (%)	R(ms)
      1G43H6	63.11	0.118
      1G 3F	28.09	0.197

      The optical absorption spectra of all the prepared glass samples were shown in figure 2. The spectra exhibits an absorption band at 980 nm due to 2F7/2 2F5/2 transition of

      Yb3+ ions. The other absorption bands at 658 nm (3H6 3F2),

      Table 2. Experimental and calculated oscillator strength of Tm3+ in GT2Y0 glass.

      Table 3. Radiative life time (R) and branching ratios (r) of Tm3+ in GT2Y0 glass.

      686 nm (3H6 3F3), 792 nm (3H6 3H4), 1211 nm (3H6

      6 exp

      cal

      3H5) and 1663 nm (3H6 3F4) are attributed to 4f-4f

      transitions of Tm3+ ions, respectively [22-24]. Assignments to the various observed bands made by comparing their band positions with the previously reported transitions of Tm3+ ions in other glass systems. The intensities of all absorption bands attributed to Tm3+ were found to increase with increase in the concentration of Tm3+ ions and no significant shift is observed in the band positions.

      From the spectra it was observed that cut-off wavelength value increases upto 0.6 mol% (GT6Y2) of Tm3+ ions and decreases further with increase in the concentration of Tm3+ ions. Using standard relations the values of optical band gap and the Urbach energy are calculated [25, 26]. From the data (Table 1) the value of Eopt was found to decrease upto 0.6 mol% (GT6Y2) of Tm3+ ions and increases further with increase in the concentration of Tm3+ ions. The decrease in the optical band gap with the increase in the concentration of Tm2O3 up to 0.6 mol% suggests increasing degree of depolymerization or concentration of bonding defects and non-bridging oxygens (NBO) in the glass network up to this concentration of Tm2O3. Probably in this concentration range the gallium ions may take network forming positions with GaO4 structural units and alternate with BO4 units. Such linkages may cause a decrease in the rigidity of the glass network and leads to the decrease in the optical band gap as observed.

      4 4

      3

      r.m.s. deviation ±0.281

      According to literature [35-37], 2 is related with the symmetry of the rare earth site while 6 is inversely proportional to the covalency of Tm-O bond. The Tm-O bond is assumed to be dependent on the local basicity around the rare-earth (RE) sites, which can be adjusted by the composition or structure of the glass hosts. It is well established that an emission level with r value above 50% becomes a potential laser emission. Referring to the data on emission transitions in the present glass system, the transition 1G4 3H6 has the highest value of r among various transitions. This transition may therefore considered as a possible laser transition. From the values of Judd-Ofelt parameters it was found to be in the order 2>6>4.

      GT0Y0 GT2Y0 GT0Y2 GT2Y2 GT4Y2 GT6Y2 GT8Y2 GT10Y2

      Absorbance (a.u)

      3H6 3F2

      3H6 3F3

      2F7/2 2F5/2

      3H6 3H4

      3H6 3H5

      3H6 3F4

      400 600 800 1000 1200 1400 1600 1800 2000

      Wavelength (nm)

   3. FTIR

      Fig. 2. Optical absorption spectra of GTY glass system.

      PbO in addition to participating in the glass network with PbO4 structural units, it may also enter as modifier. Ga2O3 is

      The experimental FT-IR spectra of GTY glass system in

      the range 1600-400 cm-1 are presented in Fig. 3. The FTIR spectra of these glasses shows characteristic bands corresponding to the different vibration modes of the various functional groups present in the glass system [38-41]. The assignment of the absorption bands detected is summarized in Table 4.

      considered to act as a network former if Ga3+ ions take

      preferentially fourfold coordination in oxide glasses. The excess negative charge on GaO4 tetrahedra is compensated either by localization of a modifier ion nearby or by generation of threefold oxygens. The GaO4 tetrahedrons may enter the glass network and alternate with BO4 tetrahedrons. In some of the glass networks, the gallium ions are also found to be in modifier positions with GaO6 structural units [42].

      GaO 4

      PbO & Bi O

      BO

      GT10Y2 3

      BO B-O-B

      4

      4 2 3

      Table 4. Absorption bands and their assignments for FT-IR spectra.

      GT8Y2

      Wave number (cm-1)

      IR assignments

      Transmittance (%)

      GT6Y2

      GT4Y2

      GT2Y2

      GT0Y2

      GT2Y0

      ~490 Bending vibrations of Bi2O3 pyramidal units, PbO4 bending vibrations.

      ~610 Due to network forming GaO4 tetrahedral groups.

      ~707 Vibrations of B-O-B linkages.

      Asymmetric stretching vibrations of B-O bands in

      GT0Y0

      ~930

      BO4 units.

      1600 1400 1200 1000 800 600 400

      Wavenumber (cm-1)

      Fig. 3. FTIR spectra of GTY glass system.

      A band cited in the region ~490 cm-1 is identified due to bending vibrations of Bi2O3 pyramidal units and also due to the presence of PbO4 structural units. A band cited in the region ~610 cm-1 is identified due to network forming GaO4 tetrahedral groups. The band cited at ~707 cm-1 and is attributed to the vibrations of B-O-B linkages. A band cited in the region ~930 cm-1 is assigned to asymmetric stretching vibrations of B-O bands in BO4 units. The band cited in the region ~1280 cm-1 is identified due to asymmetric stretching modes of borate triangles BO3 and BO2O-.

      ~1280 Asymmetric stretching modes of borate triangles BO3 and BO2O-.

      From the spectra it was observed that intensity of band corresponding to GaO4 tetrahedral groups increases from 0 mol% of Tm3+ ions (GT0Y0) to 0.6 mol% of Tm3+ ions (GT6Y2) beyond this concentration trend is reverse. This is due to the fact that Ga3+ ions go into substitutional positions with GaO4 structural units and alter the glass network upto

      0.6 mol%. Within this concentration Ga3+ ions, isolate the

      rare-earth ions from RE-O-RE bonds and form Ga-O-RE bonds. Such declustering effect leads to the larger spacing between RE ions and may contributes for the enhancement of fluorescence emission.

   4. Upconversion

   GT6Y2

   GT10Y2

   Figure 4 represents the upconversion emission spectra of Tm3+/Yb3+ codoped GTY glass system in the wavelength range of 500 700 nm under the excitation of 980 nm laser diode at room temperature. The spectra exhibited two emission bands centered at 468 nm and 654 nm due to 1G4

   n

   I I

   up IR

   where n is the number of IR photons absorbed per visible photon emitted [45].

   GT2Y2 GT4Y2

   3H6

   (blue) and 1G4

   3H4

   (red) transitions, respectively [43, GT8Y2

   44].

   1G4 3H6

   GT6Y2

   Slope = 2.75

   Slope = 2.81

   Slope = 2.89

   Slope = 2.82

   Intensity (a.u)

   Log Iup (a.u)

   Slope = 2.74

   GT8Y2 GT4Y2

   GT10Y2

   GT2Y2

   1G4 3F4

   2.2 2.3 2.4 2.5 2.6 2.7 2.8 .9

   Log IIR (mW)

   Fig. 5. (b). Dependence of upconversion fluorescence intensity of red emission (654 nm) on excitation power under 980 nm excitation.

   400 500 600 700

   wavelength (nm)

   Fig. 4. Frequency upconversion emission spectra of Tm3+/Yb3+ codoped GTY glass system.

   From the spectra it was observed that the upconversion luminescence intensity of blue emission (468 nm) is higher than the upconversion luminescence intensity of red emission (654 nm). It is also important to point out that the red emission is very weak and blue emission is very prominent to be observed by the naked eye at low excitation power for Tm3+/Yb3+ codoped GTY glass system at room temperature. From the spectra it can be concluded that the intensity of blue and red emissions increases with increase in the concentration of Tm3+ ions upto 0.6 mol% (GT6Y2) and decreases with further increase in the concentarion of Tm3+ ions in Tm3+/Yb3+ codoped glass samples.

   GT2Y2

   GT4Y2

   GT6Y2

   GT8Y2

   GT10Y2

   Slope = 2.74

   Log Iup (a.u)

   Slope = 2.83

   Slope = 2.94

   Slope = 2.88

   Slope = 2.71

   2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

   Log IIR (mW)

   Fig. 5. (a) Dependence of upconversion fluorescence intensity of blue emission (468 nm) on excitation power under 980 nm excitation.

   In an upconversion, the upconversion emission intensity (Iup) increases in proportion to the nth power of infrared excitation intensity (IIR), i.e.,

   A plot of log Iup versus log IIR yields a straight line with slope n. Figure 5 shows such a plot for the 468nm and 654 nm emissions under 980 nm excitation. From figure 5 the slope value (n) for the 468 nm and 654 nm emission bands was calculated and got around three. The results shows that a three photon upconversion process is responsible for the blue and red emissions from 1G4 3H6 and 1G4 3H4 transitions, respectively.

   Fig. 6. Energy transfer process between Yb3+ and Tm3+ ions in GTY glass system.

   Figure 6 represents the energy transfer mechanism between Yb3+ amd Tm3+ ions. The excitation process for the 1G4 3H6 and 1G4 3H4 transitions can be explained as follows. First Yb3+ ions in the ground level absorbs a photon and excites to the higher level (2F7/2 2F5/2) then Yb3+ transfers their energy to the Tm3+ ions which are in ground level (3H5). This step is involved by energy transfer (ET) mechanism of excited Yb3+ to Tm3+: (Yb) 2F5/2 + (Tm) 3H6 (Yb) 2F7/2 + (Tm) 3H5. Actually the energy gap between (Yb) 2F5/2 and (Yb) 2F7/2 is in a wide range and the low energy edge is near the high energy edge of the energy gap between (Tm) 3H6 and (Tm) 3H5. Thus the energy transfer from Yb3+ becomes efficient [46].

   In the second step, Tm3+ in the (3H5) excited state relaxes nonradiatively to the metastable level (3H4) by the cooperation of the phonons in the alumina lead borate glasses. Tm3+ in the 3H4 level is excited to 3F2,3 level by ET from Yb3+ and absorption a photon. Thus, the population of 3F2,3 level is based on the processes as follows: ET from Yb3+

   : (Yb) 2F5/2 + (Tm) 3H4 (Yb) 2F7/2 + (Tm)3 F2,3 and excited

   state absorption (ESA): (Tm) 3H4 + a photon (Tm) 3F2,3. Then the 3F2,3 states also relaxes by a multiphonon assisted process to the 3F4 level [47]. Finally, Tm3+ in the 3F4 level is excited to 1G4 level by ET from Yb3+ and absorption a photon. Therefore, the population of 1G4 is based on the processes as follows: ET from Yb3+: (Yb) 2F5/2 + (Tm) 3F4 (Yb) 2F7/2 + (Tm) 1G4 and ESA: (Tm) 3F4 + a photon (Tm)

   1G4.

   It might be ascribed to the fact that the relaxations of 3H5 to 3H4 and 3F2,3 to 3F4 need the assistance of multiphonon relaxation, and the density of high energy phonon states is comparatively low in this glass. From the 1G4 level, the Tm3+ ions decay radiatively to the 3H6 ground state, generating the intense blue emission around 468 nm. This is a process that involved three photons. So the 468 nm emission presents nearly a cubic dependence on the excitation power. Though the phonon energy of the matrix is about 1000 cm1, the blue luminescence of Tm3+/Yb3+ codoped samples can be seen by naked eye under excitation power as low as 70 mW. The red emission around 654 nm is due to the radiative decay of Tm3+ ions from the 1G4 level to the 3H4 level. The major contribution to the blue emission is ascribed to the 1G4 3H6 transition. The transition probability involved in the blue emission process is very high compared to red emission process. Hence the observed red emission is weak.

   Intensity of blue emission (a. u)

   From the above results it can be concluded that Tm3+ and Yb3+ showed a dominating role in the energy absorption and energy transfer in visible region. Because three photon upconversion fluorescence is more sensitive to pump energy. Intense blue emission has been observed when the excitation power rise to the value above 70 mW. It proven that commercial 980 nm laser diode is a power full pumping source for upconversion fluorescence in Tm3+/Yb3+ codoped alumina lead borate glasses. Moreover, as mentioned before, the phonon energy also plays an important role and it can affect the upconversion intensity: with the increase of the phonon energy in Tm3+/Yb3+ co-doped glasses the blue emission increases more than the red by means of the process described above.

   0.0 0.2 0.4 0.6 0.8 1.0

   Tm203 Concentration (mol%)

   Fig. 7. Intensity of blue emission as a function of Tm2O3 concentration at fixed Yb2O3 concentration.

   Figure 7 shows the intensity of blue emission as a function of Tm2O3 concentration in GTY glass system. From the figure it was observed that as the concentration of Tm2O3 is increased the blue emission band is observed to grow gradually up to 0.6 mol% and decreases further increasing the concentration of Tm2O3. A similar behavior is also observed in cut-off wavelengths of these glasses. This is due to the fact that Ga3+ ions (like any other III A group elemental ions), isolate the rare-earth ions from RE-O-RE bonds and form Ga- O-RE bonds. Such declustering effect which seems to be dominant in the glasses containing higher concentrations of Tm2O3, leads to the larger spacing between RE ions and contributes for the enhancement of fluorescence emission.

   Above 0.6 mol% of Tm2O3 there is a decrease in the emission intensity which may be due to optical quenching. For the Yb3+/Tm3+ codoped glass, the 468 nm blue luminescence can be quenched by the following process: Tm3+:1G4+Tm3+:3H6Tm3+:3H4+Tm3+:3H5. However, when the concentration of Tm3+ ions is not too high, this quenching process is not notable because the 3H4 and 3H5 energy levels are still the interim states for the blue upconversion emission. Tm3+ ions in 3F4 or 3H4 can be sensitized again to 1G4. It is just like a cycle. However, when the concentration of Tm3+ ions increases 3F4 and 3H4 states are depopulated by the energy transfer and the cycle breaks. Hence, luminescence quenching takes place resulting the decrease in the upconversion intensity.

4. CONCLUSIONS

We have prepared and characterized Tm3+/Yb3+ codoped lead bismuth gallium borate (GTY) glasses. Infrared spectra revealed the presence of various functional groups present in the glass system. With the help of optical absorption spectra and Judd-Ofelt theory, we have calculated the t (t = 2, 4, 6) intensity parameters, the oscillator strengths, branching ratios (r), and the radiative lifetimes of Er3+ doped glass. The upconversion luminescence was recorded and investigated under the excitation of 980 nm laser diode. The intense blue (468 nm) and weak red (654 nm) emissions are observed at room temperature.

The blue emission is more influenced than the red emission. With increasing in the concentration of Tm3+ upto

0.6 mol% (GT6Y2) the intensity of red (654 nm) increases slightly, while the blue (468 nm) emission intensity increases very much more than that of red emission. From the values of branching ratio it was found that the transition G4 3H6 (468 nm) has the highest value of r among various transitions. This transition may therefore considered as a possible laser transition. The intense blue uoconversion luminescence of GT6Y2 glass can act as potential materials for developing upconversion optical devices.

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