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Frequency Upconversion in Er3+/Yb3+ Codoped Lead Bismuth Gallium Borate Glasses

K Krishna Murthy Goud1*, Ch Ramesh2 and B Appa Rao3

1Dept. of Physics, UCE (A), Osmania University, Hyderabad-500007 (T.S), India.

2Dept. of Physics, M G University, Nalgonda (T.S), India.

3Dept. of Physics, Osmania University, Hyderabad-500007 (T.S), India.

 

Corresponding author Email: krishnamurthy.phy@gmail.com

ABSTRACT:

Lead bismuth gallium borate (GEY) glasses codoped with Er3+/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 Er3+ and Yb3+ respectively. The other absorption bands at 488nm, 521 nm, 545 nm, 652 nm, 798 nm and 1510 nm are attributed to transitions from the ground state 4I15/2 to excited state of Er3+. Judd-Ofelt theory has been applied to the f « f transitions for evaluating Ω2, Ω4 and Ω6 parameters. Radiative properties like branching ratio br and the radiative life time τR have been determined on the basis Judd–Ofelt theory. Upconversion emissions have been observed under 980nm laser excitation at room temperature. Green and red up-conversion emissions are centered at 525, 545 and 660 nm corresponding to 2H11/2 ®4I15/2, 4S3/2 ®4I15/2 and 4F9/2 ®4I15/2 transitions of Er3+ respectively. The results obtained are discussed quantitatively based on the energy transfer between Yb3+ and Er3+.

KEYWORDS: glasses; Optical absorption; FTIR; Photo luminescence

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Goud K. K. M, Ramesh Ch, Rao B. A. Frequency Upconversion in Er3+/Yb3+ Codoped Lead Bismuth Gallium Borate Glasses. Mat.Sci.Res.India;14(2)


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Goud K. K. M, Ramesh Ch, Rao B. A. Frequency Upconversion in Er3+/Yb3+ Codoped Lead Bismuth Gallium Borate Glasses. Mat.Sci.Res.India;14(2). Available from: http://www.materialsciencejournal.org/?p=5842


Introduction

Recently, there has been great interest in the conversion of infrared light to visible light through energy upconversion in rare-earth doped glasses, due to the possibility of infrared-pumped visible lasers and the potential applications in areas such as optical data storage, lasers, sensors, and optical displays [1-3]. Among different oxide glass compositions, lead borate glasses doped with rare earth ions seems to be very attractive systems for applications in optical devices of laser technology. The absorption and emission properties of rare earth ions in lead borate glasses are well reported in the literature [4-6]. 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].

Ga2O3 is a heavy metal oxide and when it is introduced in the glass matrix, is expected to alter the physical properties like refractive index, thermal expansion coefficient, chemical resistance and glass transition temperature, infrared transmittance and the insulating strength of the glasses spectacularly and makes the glasses suitable for the applications like in infrared windows, ultra-fast optical switches, optical isolators and other photonic devices [8, 9]. Among various rare-earth ions that emit upconversion fluorescence, Er3+ is the most widely studied as an active ion and gives rather high efficiency. In general, the most efficient are energy transfer upconversion and excited state absorption processes in the case of the Er3+ ion [10]. The sensitization of Er3+ doped materials with Yb3+ ions is a well-known method for increasing the optical pumping efficiency because of the efficient energy transfer from Yb3+ to Er3+ ions [11]. In this investigation, a series of Er3+/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.

Experimental

For the present study glasses with [100-(x+y)][0.5PbO-0.25B2O3-0.20Bi2O3-0.05Ga2O3]-xEr2O3-yYb2O3with 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 GE0Y0, GE2Y0, GE0Y2, GE2Y2, GE4Y2, GE6Y2, GE8Y2 and GE10Y2 respectively. Appropriate amounts of AR grade reagents of PbO, B2O3, Bi2O3, Ga2O3, Er2O3 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.

The optical absorption spectra were obtained with the JASCO Model V-670 UV–VIS–NIR spectrophotometer in the wavelength range 350–2000 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. For IR measurements, the glasses were pulverized and mixed with KBr in order to obtain thin pellets with a thickness of about 0.3mm. 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.

Results and Discussion

Figure 1 shows optical absorption spectra of all prepared glass samples. The spectra exhibits intense absorption band at 980 nm due to 4I15/2 4I11/2 and  2F7/22F5/2  transitions of Er3+ and Yb3+,  respectively. The other absorption bands are attributed to 4f-4f transitions of  Er3+ ions from the ground (4I15/2) state to the excited states at 488 nm (4F7/2), 521 nm (2H11/2), 545 nm (4S3/2), 652 nm (4F9/2), 798 nm (4I9/2) and 1510 nm (4I13/2) [12, 23]. The intensities of all bands was found to increase with increase in the concentration of Er3+ ions. There is no significant shift is observed in the band positions.

Figure 1: Optical absorption spectra of GEY glass system.

Figure 1. Optical absorption spectra of GEY glass system.
Click on image to enlarge

 

From the spectra it was observed that cut-off wavelength value increases upto 0.6 mol% (GE6Y2) of Er3+ ions and decreases further with increase in the concentration of Er3+ ions. Using standard relations the values of optical band gap and the Urbach energy are calculated [13, 14]. From the data (Table 1) the value of Eopt was found to decrease upto 0.6 mol% (GE6Y2) of Er3+ ions and increases further with increase in the concentration of Er3+ ions.

Table 1: Values of cut off wavelength, optical band gap and urbach energy GEY glass system doped with Er3+/Yb3+.

S.No.

Sample code

Cut-off

wavelength (nm)

Eopt (eV)

±0.01

ΔE (eV)

±0.001

1

GE0Y0

400

3.02

0.152

2

GE2Y0

408

2.96

0.148

3

GE0Y2

409

2.95

0.148

4

GE2Y2

419

2.89

0.144

5

GE4Y2

425

2.85

0.139

6

GE6Y2

431

2.80

0.135

7

GE8Y2

427

2.83

0.138

8

GE10Y2

423

2.86

0.141

 

The decrease in the optical band gap with the increase in the concentration of Er2O3 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 Er2O3. 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. 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 Ω2, Ω4 and Ω6 [15, 16] can be determined by obtaining the experimental ground state oscillator strengths of the absorption transitions via an integration of the absorption coefficients for each band. The Judd-Ofelt theory has often been used to calculate the spectroscopic parameters, such as radiative lifetime, oscillator strength and branching ratios (βr) using standard equations [17-22].  The results are summarized in Table 2 and Table 3.

Table 2: Radiative life time (τR) and branching ratios (βr) of Er3+ in GEY glass.

Transitions

br (%)

τR (ms)

2H11/24I15/2

14

0.74

4S3/24I15/2

16

0.80

4F9/24I15/2

52

2.76

 

Table 3: Experimental and calculated oscillator strength of Er3+ in GEY glass.

Transition from 4I15/2

fexp(x10-6)

fcal(x10-6)

4I13/2

0.982

0.976

4I11/2

0.549

0.558

4I9/2

0.446

0.441

4F9/2

1.207

1.219

4S3/2

0.328

0.312

2H11/2

4.435

4.442

4F7/2

1.163

1.171

r.m.s. deviation

±0.091

 

According to literature [23, 24], Ω2 is related with the symmetry of the rare earth site while Ω6 is inversely proportional to the covalency of Er-O bond.  The Er-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 βr value above 50% becomes a potential laser emission.  Referring to the data on emission transitions in the present glass system, the transition 4F9/24I15/2 has the highest value of βr among various transitions. This transition may therefore considered as a possible laser transition.  The values of Judd-Ofelt parameters it was found to be in the order Ω246.

The FTIR spectra of GEY glass system codoped with Er3+/Yb3+ was shown in figure 2.  A band cited in the region ~482 cm-1 is identified due to bending vibrations of Bi2O3 pyramidal units and also due to the presence of PbO4 structural units [25].  A band cited in the region ~613 cm-1 is identified due to network forming GaO4 tetrahedral groups [26]. 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 ~1250 cm-1 is identified due to asymmetric stretching modes of borate triangles BO3 and BO2O-.

Figure 2: FTIR spectra of GEY glass system.

Figure 2. FTIR spectra of GEY glass system.
Click on image to enlarge

 

Ga2O3 is 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 [27]. From the spectra it was observed that intensity of band corresponding to GaO4 tetrahedral groups increases from 0 mol% of Er3+ ions (GE0Y0) to  0.6 mol% of Er3+ ions  (GE6Y2), beyond this concentration the 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.

Figure 3 represents the upconversion emission spectra of Er3+/Yb3+ codoped GEY glass system in the wavelength range of 500 –700 nm under the excitation of 980 nm laser diode at room temperature. The spectra exhibited three emission bands centered at 525 nm (2H11/2 4I15/2), 545 nm (4S3/24I15/2) and 660 nm (4F9/2 4I15/2). It was observed that the upconversion luminescence intensity of red emission (660 nm) is higher than the upconversion luminescence intensity of green emission (525 and 545 nm). It is also important to point out that the green emission is very weak and red emission is very prominent to be observed by the naked eye at low excitation power for Er3+/Yb3+ codoped GEY glass system at room temperature.

Figure 3: Frequency upconversion emission spectra of Er3+/Yb3+ codoped glasses

Figure 3. Frequency upconversion emission spectra of Er3+/Yb3+ codoped glasses
Click on image to enlarge

 

From the spectra it can be concluded that the intensity of green and red emissions increases with increase in the concentration of Er3+ ions upto 0.6 mol% (GE6Y2) and decreases with further increase in the concentarion of Er3+ ions in Er3+/Yb3+ codoped glass samples. In an upconversion, the emission intensity (Iup) increases in proportion to the nth power of infrared excitation intensity (IIR), i.e.,

Formula

where n is the number of IR photons absorbed per visible photon emitted [28]. A plot of log Iup versus log IIR yields a straight line with slope ‘n’. Figure 4 shows such a plot for the 660 nm emissions under 980 nm excitation. From figure 5 the slope value (n) for the 660 nm emission band was calculated and got around two. The results shows that a two photon upconversion process is responsible for the green (525, 545 nm) and red (660 nm) emissions from the 2H11/2 4I15/2, 4S3/24I15/2 and 4F9/2 4I15/2 transitions, respectively [29].

Figure 4: Dependence of upconversion fluorescence intensity on excitation power under 980 nm excitation

Figure 4. Dependence of upconversion fluorescence intensity on excitation power under 980 nm excitation
Click on image to enlarge

 

The possible upconversion mechanisms for the emisions in the Er3+/Yb3+ codoped GEY glass system under 980 nm excitation are explained based on the energy matching conditions and the quadratic dependence on 980 nm pump intensity as illustrated in Fig. 5 [30]. The energy transfer mechanism for the green emissions as follows (Process-I) : in the first step, the 4I11/2 level is directly excited with 980 nm light by ground state absorption (GSA) and/or by the energy transfer (ET) process from 2F5/2 level of Yb3+ : 2F5/2 (Yb3+) + 4I15/2 (Er3+) → 4I11/2 (Er3+) + 2F7/2 (Yb3+). Due to much larger absorption cross-section of Yb3+ than Er3+ in the 980 nm region, the energy transfer (ET) process is dominant to the excitation of 4I11/2 level.

Figure 5: Energy level diagram of Er3+/Yb3+ codoped GEY glass system under the excitation of 980 nm laser diode [35].

Figure 5. Energy level diagram of Er3+/Yb3+ codoped GEY glass system under the excitation  of 980 nm laser diode [35].
Click on image to enlarge

 

The second step involves the excitation processes based on the long-lived 4I11/2 level as follows: excited state absorption (ESA) 4I11/2 (Er3+) + a photon → 4F7/2 (Er3+) and ET 2F5/2 (Yb3+) + 4I11/2 (Er3+) → 2F7/2 (Yb3+) + 4F7/2 (Er3+). For an Er3+ ion in the 4F7/2 excited state the interaction with a nearby Er3+ ion in the ground state would lead to two Er3+ ions at the 4I11/2 level. This process can be represented as follows: 4F7/2 (Er3+) + 4I15/2 (Er3+) → 4I11/2 (Er3+) + 4I11/2 (Er3+). However, the transition probability involved in the above processes can be small, and so the 4F7/2 level is populated. The populated Er3+ 4F7/2 level then relaxes rapidly and non-radiatively to the next lower levels, 2H11/2 and 4S3/2 resulting from the small energy gap between them. Er3+ ions at the 2H11/2 level can also decay to the 4S3/2 level due to multiphonon relaxation process. The estimated energy gap between the 2H11/2 level and the next lower level 4S3/2 is about 800 cm-1. Thus, multiphonon relaxation rate is very large and the 525 nm emission intensity will be reduced [31]. Most of the erbium ions at 4S3/2 level relaxes non-radiatively to the next lower level (4F9/2). Hence the intensity of green emissions centered at 525 nm and 545 nm are very small as observed in the upconversion spectra. In this way the above mechanism is responsible for the green emissions centered at 525 and 545 nm corresponding to the 2H11/2 4I15/2 and 4S3/24I15/2 transitions, respectively.

The energy transfer mechanism for the red emission at 660 nm (4F9/2 4I15/2) as follows (Process-II): excited state absorption (ESA) : 4I13/2 (Er3+) + a photon → 4F9/2 (Er3+)  and cross-relaxation (CR) between Er3+ ions: 4I13/2 (Er3+) + 4I11/2 (Er3+) → 4I15/2 (Er3+) + 4F9/2 (Er3+), and ET from Yb3+ : 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+). The 4I13/2 level is populated owing to the non-radiative relaxation from the upper 4I11/2 level. Besides, the non-radiative process from 4S3/2 level, which is populated by means of the process described previously, to the 4F9/2 level also contributes to the red emission. The experimental evidence of the enhancement of the red emission when compared to the green emission can be explained by the process-II. The number of Er3+ ions in the 4I11/2 level relaxing non-radiatively to the lower 4I13/2 level are much greater than those excited to the upper 4F7/2 level via process-I. The much longer lifetime of 4I13/2 state when compared with the lifetime of the 4I11/2 state [32, 33] makes the process-II dominant over the process-I for the red emission. The green emission cannot be populated by the process-II. 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 Er3+/Yb3+ co-doped glasses the red emission increases more than the green by means of the process described above. The presence of Ga2O3 also contributes to the relative enhancement of the red emission when compared to the green, as previously reported [34].

Conclusions

We have prepared and characterized Er3+/Yb3+ codoped lead bismuth gallium borate (GEY) 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 Ωt (t = 2, 4, 6) intensity parameters, the oscillator strengths, branching ratios (βr), and the radiative lifetimes of Er3+ doped glass. From the values of branching ratio it was found that the transition 4F9/24I15/2 (660 nm) has the highest value of βr among various transitions. This transition may therefore considered as a possible laser transition. The upconversion luminescence was recorded and investigated under the excitation of 980 nm laser diode. The intense red (660 nm) and weak green (525 and 545 nm) emissions are observed at room temperature. The red emission is more influenced than the green emissions and this is due to the fact that the probability of Er3+ ions relaxing non-radiatively from the 4F11/2 level to the lower 4I13/2 level is much higher than the probability of upconversion to the upper 4F7/2 level as a result of the longer lifetime of the 4I13/2 level compared to the lifetime of the 4I11/2 level, which makes the non-radiative relaxation 4I11/2 4I13/2 more easily to occur. The upconversion processes involved a sequential two-photon absorption for the green and red emissions. With increasing in the concentration of Er3+ upto 0.6 mol% (GE6Y2) the intensities of green (525 and 545 nm) increases slightly, while the red (660 nm) emission intensities increases very much more than that of green emissions. The presence of Ga2O3 also contributes and favors the red emission. The intense red uoconversion luminescence of GE6Y2 glass can act as potential materials for developing upconversion optical devices.

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