5 Chapter 3

Chapter 5

Lasing characteristics of

PbO-Sb2O3-B2O3:Er2O3 glasses

155

5.1 Introduction

Erbium doped glasses have become major research topic because of their

potential use in optoelectronic systems and devices [1-3]. Er3+ ion has the electronic

configuration, 4f11 with 4I15/2 ground state. In its trivalent state, the Er3+ ions shows

an intra 4f shell transition from the first exited state(4I13/2) to the ground state (4I15/2)

which occurs at a wavelength about 1.53μm. In free Er3+ ion, optical intra 4f shell

transitions are practically forbidden. Incorporation of Er3+ ions in a solid state host

induces mixing of states of opposite parity which makes some of states allowed.

These transitions are rather sharp due the fact that the partially filled 4f shell is

shielded by filled 5s and 5p shells. The life time of first exited state can be as long as

several milliseconds. The sharp and temperature independent emission and long

luminescence life time makes the erbium doped glasses ideally suited for the

fabrication of lasers and optical amplifiers operating at 1.53 μm, a standard

telecommunication wavelength. Indeed erbium doped silica fibers are nowadays

being used as optical fibers in long distance optical fiber links [4-5].

Erbium doped glass gives rise to a large number of absorption and emission

transitions in the ultraviolet, visible and near infrared regions. Reisfeld et al. [6]

were the first to report detailed absorption and emission spectral studies of Er3+ ion

in borate, phosphate, germinate and tellurite glasses [7-9]. The transitions4I15/22H11/2 and 4I15/24G11/2 of Er3+ in the absorption spectra are hypersensitive

and their intensities are greatly influenced by the host matrices [9-12]. The dopant

concentration dependence of lifetime of 4S3/2 level for Er3+ in these glasses has also

been reported [9] The decrease in lifetime with increase in concentration of the

active ion is due to cross relaxation involving the levels 4G11/2,4I15/24F7/2,

4I13/2 and2H11/2,

4S3/2,4I15/24I9/2,

4I13/2. Many laser transitions have been identified for Er3+

ion in crystalline hosts over a wide region [7,13]. Laser action at 1.663 m

involving the transition from 4S3/2 4I9/2 level has been observed for Er3+ doped

YAlO3 single crystals [9]. Laser action occurs for the transition 4I13/24I15/2 of Er3+

ion in silicate, phosphate and fluorozirconate glasses at about 1.6 m [13]. Erbium

ion is an attractive source for pulsed solid state lasers at 1.54 m, and this falls well

within the eye-safe region. Four level lasing involving 4S3/24I13/2 transition has

been reported for Er3+ at ambient temperatures in several hosts including YAlO3[7,9]

156

This transition is of interest because of the existence of sensitive photo detection at

around 850 nm where the second harmonic of the above transition occurs.

Continuous wave operation of Er3+ doped fluorozirconate glass fiber laser emitting

at 2.7 m corresponding to the transition, 4I11/24I13/2 is based on the depletion of

the lower level by excited state absorption of the appropriate pump wavelength.

Er3+ doped phosphate glasses have proved to be optical glasses with good

homogeneity and the laser transition 4I13/24I15/2 around 1.5 m is being used for

laser action with the bulk glasses as the host matrix [68].

5.2 Brief review of the previous work on Er3+ doped glasses

Rama Moorthy et al. [14] have studied the absorption and emission

characteristics of Er3+ ions in alkali chloroborophosphate glasses. Laercio Gomes et

al. [15] have reported the energy level decay and excited state absorption processes

in erbium-doped tellurite glass. Subbalakshmi and Veeraiah [16] have studied the

optical absorption and fluorescence properties of Er3+ ion in MO–WO3–P2O5

glasses. Babu et al. [17] have studied the thermal and optical properties of Er3+

doped oxyfluoro tellurite glasses. Gerlas et al. [18] have studied absorption and

emission cross sections of Er3+ in Al2O3 waveguides. Atsunobu Masuno et al. [19]

have reported the broadband infrared fluorescence in Er3+ doped BaO–SiO2 glasses.

Ondracek et al. [20] have done the characterization of erbium doped glass optical

waveguides by a fine tunable semiconductor laser. Marjanovic et al. [21] have done

characterization of new erbium-doped tellurite glasses and fibers. ZHOU Ya-xun et

al. [22] have studied the concentration-dependent luminescence properties in Er3+

doped TeO2- ZnO-La2O3 glasses. Shiqing Xu et al. [23] have studied the effect of

Bi2O3 on spectroscopic properties of Er3+ doped lead oxyfluoro silicate glasses for

broadband optical amplifiers. Pisarski et al. [24] have studied the effect of heat

treatment on Er3+ containing multicomponent oxyfluoride lead borate glass system.

Kaushal Kumar et al. [25] have studied the enhancement of luminescence properties

in Er3+ doped TeO2–Na2O–PbX (X =O and F) ternary glasses. Wilfried Blanc et.al

[26] have studied erbium emission properties in nanostructured fibers. Xiyan Li et

al. [27] have studied the near-infrared luminescent properties and natural lifetime

calculation of a novel Er3+ complex. Dong et al. [28] have done the observation of

micro laser with Er3+ doped phosphate glass coated microsphere pumped by 780 nm.

157

Laxmi Kanth et al. [29] have studied optical absorption, fluorescence and thermo-

luminescence properties of ZnF2–MO–TeO2 (MO =ZnO; CdO and PbO) glasses

doped with Er3+ ions. Hiroki Yamauchi and Yasutake Ohishi [30] have studied the

Spectroscopic properties of Er3+ doped PbO–Ga2O3–GeO2 glass for optical

amplifiers.

5.3 Sample preparation

The molar composition of the glasses under study is 30 PbO-25 Sb2O3-(45-x)

B2O3-x Er2O3 where x = 0, 0.2, 0.4, 0.6 and 1mol%. The samples were labeled as E0,

E2, E4, E6, E8 and E10 respectively. Appropriate amounts of AR grade reagents of

PbO, Sb2O3, B2O3 and Er2O3 powders are thoroughly mixed in agate mortar and

melted in a silica crucible in the temperature range of 900 °C to 950 °C 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 250 °C

for 2 h. The samples prepared were then ground and optical polished to the

dimensions of 1 cm×1 cm×0.2 cm. The detailed composition of the samples is

E0: 30 PbO-25 Sb2O3-45 B2O3-0.0 Er2O3

E2: 30 PbO-25 Sb2O3-44.8 B2O3-0.2 Er2O3

E4: 30 PbO-25 Sb2O3-44.6 B2O3-0.4 Er2O3

E6: 30 PbO-25 Sb2O3-44.4 B2O3-0.6 Er2O3

E8: 30 PbO-25 Sb2O3-44.2 B2O3-0.8 Er2O3

E10: 30 PbO-25 Sb2O3- 44 B2O3-1.0 Er2O3

5.4 Physical properties

Various physical properties like density, molar volume, oxygen mol%,

oxygen packing density, Er3+ ion concentration, inter ionic distance, refractive

index, polaron radius, molar refraction and polarizability of Er3+ doped glasses were

calculated and presented in Table 5.1.

Table 5.1. Physical parameters of PbO-Sb2O3-B2O3 glasses doped with Er3+ ions

Name of the Sample E0 E2 E4 E6 E8 E10

1Average MW(g/mol)

171.16 171.79 172.41 173.04 173.67 174.29

2Density, ρ (g/cc) (±0.001)

4.972 5.079 5.173 5.299 5.417 5.567

3Refractive index, n (±0.001)

1.503 1.644 1.646 1.648 1.649 1.651

4Molar Volume, Vm (MW/ρ) (±0.01)

34.42 33.82 33.33 32.66 32.06 31.31

5Molar Refraction, RM (±0.001)

11.519 12.245 12.098 11.883 11.679 11.434

6Polarizability, α

e(×10-24cm3) (±0.001)

4.568 4.856 4.798 4.712 4.632 4.534

7Oxygen mol % , O (±0.01)

2.4 2.4 2.4 2.4 2.4 2.4

8

Oxygen packing density (gm atom/L)

(±0.01)69.73 70.96 72.01 73.48 74.86 76.65

9Nd3+ ion concentration, Ni (×1021/cc)(±0.01)

0 0.11 0.22 0.33 0.45 0.58

10Inter ionic distance, ri (Å) (±0.001)

- 20.87 16.57 14.47 13.05 11.99

11Average MW(g/mol)

- 0.146 0.158 0.165 0.171 0.176

159

As the concentration of Er3+ ions increased, a considerable increase in the

density and a considerable decrease in the molar volume of samples were observed.

Modification of the geometrical configurations of the glass network, change in

coordination and the variation of dimensions of the interstitial holes can be

considered as responsible for such a variation of density. Oxygen packing density

was also found to be increased with the increase in the concentration of Er3+ ions.

Such an increase indicates an increase in the structural compactness of the samples.

5.5 Characterization

5.5.1 XRD

The X-ray diffraction of all the samples was recorded on Rigaku

diffractometer mini flex with CuKɑ radiation. It was ensured that the samples

prepared were freed from visible inhomogeneous such as inclusions, cracks or

bubbles. Based on the visual examination, the absence of peaks in the X-ray

diffraction pattern we could come to the conclusion that samples prepared are

reasonably free from crystalline nature. The Fig. 5.1 shows the X-ray diffractograms

of erbium doped lead antimony borate glasses. The absence of symmetrical peaks

confirms the amorphous nature of the samples.

Fig. 5.1 X- ray difractograms of PbO-Sb2O3-B2O3 glasses doped with Er3+ ions.

160

5.5.2 Differential scanning calorimetry studies

The glass transition temperatures of these glasses were determined by

differential scanning calorimetry traces that were recorded using DSC Q20 (TA-

Instruments) with a programmed heating rate of 20oC per minute in the temperature

range 50-500 oC (Fig 5.2). All DSC traces indicate typical glass transitions with the

inflection points between 380 oC to 390 oC. Although the inflection points of all the

samples appear to be nearly same, it is interesting that the glass transition

temperature showed increasing trend with increase in concentration of dopant ions.

The glass transition temperature for all the samples were presented in the Table 5.2.

Fig. 5.2 DSC Traces of PbO-Sb2O3-B2O3 glasses doped with Er3+ ions.

Table 5.2 Thermal parameters of PbO-Sb2O3-B2O3 glasses doped with Er3+ ions.

Glass Glass Transition Temp (Tg) C (±0.1)

E2 385

E4 386

E6 387

E8 389

E10 390

-1.5

-1.1

-0.7

60 110 160 210 260 310 360 410 460

Hea

t Flo

w (

W/g

)

Temperature (OC)

E10

E08

E06

E04

E02

161

Fig. 5.3 Variation of glass transition temperature of PbO-Sb2O3-B2O3 glasses with

5.5.3 FTIR spectral studies

Fourier transform infrared spectra of Er3+ doped PbO-Sb2O3-B2O3 glasses is

shown in the Fig. 5.4. The infrared transmission spectra of PbO-Sb2O3-B2O3 :Er2O3

glasses exhibited bands originated from borate groups at 1200 cm-1 to 1400cm-1 due

to asymmetric stretching of trigonal BO3 units, 1050 cm-1 due to stretching of

tetrahedral BO4 units and another band at 688 cm-1 due to bending of B-O-B

linkages in the borate network [35]. The 1 vibrational band of SbO3 units appeared

at 930 cm-1. The 3 vibrational bands of SbO3 units merged with the band due to

bending vibrations of B-O-B linkages and may have formed a common vibrational

band due to B-O-Sb linkages [36]. In addition, a band due to PbO4 structural groups

at about 462 cm-1 [36] is also observed for all the samples. The bands at 1750 cm-1

and 3000 cm-1 may be due to the stretching vibration of hydroxyl (OH) complexes,

which are due to the absorbed water molecules on the surface of the material.

385

386

387

388

389

390

0.2 0.4 0.6 0.8 1

Tem

pera

ture

OC

Concentration of Er3+ ions

Fig. 5.4. FTIR spectra of PbO-Sb2O3-B2O3 glasses doped with Er3+ ion .

400 800 1200 1600 2000 2400 2800 3200

Tra

nsm

itta

nce

%

Wavenumber (cm-1)

E10

E8

E6

E4

E2

E0

PbO4 Units

ν2-Sb2O3 Units

ν1-Sb2O3 Units

BO4 UnitsBO3 Units

OH UnitsOH Units

163

5.6 Optical absorption

The optical absorption spectra of Er3+ doped lead borate glasses recorded at

room temperature in the wavelength region 300-2000 nm has exhibited twelve

absorption bands all from the ground state 4I15/2 and was shown in the Fig. 5.5. These

levels are assigned to the following appropriate electronic transitions by comparison

with energy levels of Er3+ aqua-ion [31].

4I13/2 (1527 nm)

4I11/2 (973 nm)

4I9/2 (797 nm)

4F9/2 (652 nm)

4S3/2 (543 nm)

2H11/2 (522 nm)

4F7/2 (488 nm)

4F5/2 (452 nm)

4F3/2 (443nm)

2G9/2 (408 nm)

4G11/2 (378 nm)

4G9/2 (364 nm)

The spectra obtained for all the glasses showed identical characterization

with the exception of their relative band intensities. The spectral intensities of these

bands are found to increase with the content of Er2O3.

5.6.1 Optical parameters

The optical band gap energy is calculated from the extrapolation of linear

region of graph (α ħω)1/2 against ħω and presented in the Fig. 5.. Optical band gap

energies of the samples were decreasing with increasing the concentration of Nd3+

ions.

Fig. 5.5. Optical absorption spectra of PbO−Sb2O3−B2O3 glasses doped with Er3+ recorded at room temperature. All transitions are from

the ground state 4I15/2.

350 550 750 950

Abs

orba

nce

(a.u

.)

Wavelength (nm)

E10E8E6

E4

E2

4I11/24I9/2

4F9/24S3/2

2H11/2

4F7/2

4F5/2

2H9/2

4G11/2

4I15/24G9/2

4F3/2

1400 1600

4I13/2

165

Fig. 5.6 Tauc plots for evaluation of optical band gap energy of PbO-Sb2O3-B2O3 glasses doped with Er3+ ions

4

6

8

10

12

3.35

3.36

3.37

3.38

3.39 3.

4

3.41

3.42

3.43

3.44

3.45

3.46

3.47

3.48

3.49 3.

5

(αħω

)1/2

cm-1

/2eV

1/2

hν (eV)

E2E4E6E8E10

166

Urbach plots are obtained by plotting ln(α) against ħω. The values of Urbach

energy (ΔE) are calculated by determining the reciprocals of the slopes of linear

region of curve. The optical parameters for all the samples of Er3+ doped lead

antimony borate glasses are presented in Table 5.3.

Table 5.3 The optical parameters of Er3+ doped lead antimony borate glasses.

Name of the Sample E0 E2 E4 E6 E8 E10

1 Cutoff wavelength, λ (nm) (±1) 353 355 356 357 358 359

2 Optical bandgap energy, Eopt

(eV) (±0.001)3.399 3.423 3.421 3.417 3.314 3.312

3 Urbach Energy, ΔE (eV)(±0.001)

0.511 0.526 0.535 0.547 0.555 0.568

5.6.2 Oscillator strengths

The intensities of absorption transitions of optical absorptions are measured in

terms of oscillator strengths (fexp) and determined from the relative areas under the

absorption bands. The experimental oscillator strengths (fexp) and calculated oscillator

strengths are computed by using the equations 1.9 and 1.23 respectively and presented

in Table 5.4. The small value of root mean square (rms) deviations between

experimental and calculated oscillator strengths indicated good agreement between

them.

All the observed transitions, except the 4I15/2→4I13/2 transition are electric

dipole in nature according to the selection ΔL=±1; ΔS=0; ΔL, ΔJ≤ 21 [36].

However the magnetic dipole transitions were not taken into account in the

calculation of oscillator strengths for the transition 4I15/2→4I13/2 as its contribution is

negligible for those transitions. In calculating the magnetic dipole oscillator strength

the selection rules for magnetic dipole transitions must be considered i.e., ΔS=ΔL=

±1, ΔJ=0, ±1 (0↔0).

Table 5.4. The experimental (fexp×10-6) and calculated (fexp×10-6) spectral intensities of Er3+ doped antimony lead borate glasses. All the

transitions are from its ground state 4I15/2.

Transition

E02 E04 E06 E08 E10

fexp fcal fexp fcal fexp fcal fexp fcal fexp fcal

4I13/2 2.64 2.663 2.763 2.783 2.859 2.875 2.936 2.937 3.069 3.0794I11/2 1.209 1.172 1.241 1.236 1.266 1.272 1.312 1.301 1.371 1.361

4I9/2 0.661 0.688 0.77 0.723 0.83 0.762 0.849 0.773 0.887 0.823

4F9/2 4.247 4.266 4.511 4.47 4.669 4.672 4.764 4.753 5.015 5.026

4S3/2 1.025 1.026 1.115 1.071 1.181 1.105 1.15 1.129 1.223 1.182

2H11/2 10.305 10.228 10.813 10.789 11.099 11.099 11.412 11.34 11.941 11.871

4F7/2 4.223 4.145 4.333 4.378 4.51 4.492 4.568 4.583 4.819 4.817

4F5/2 1.501 1.244 1.4 1.299 1.415 1.34 1.323 1.369 1.456 1.434

4F3/2 1.038 0.724 0.968 0.755 0.978 0.779 0.998 0.796 1.045 0.834

2G9/2 1.239 1.533 1.734 1.602 1.752 1.655 1.639 1.691 1.803 1.773

4G11/2 18.065 18.147 19.117 19.142 19.644 19.66 20.042 20.117 20.987 21.061

4G9/2 2.989 2.861 2.788 2.999 3.051 3.143 3.053 3.195 3.344 3.386

rms deviation 0.155 0.103 0.079 0.083 0.073

168

The transition 4I15/2→4I13/2 (1.53μm) emission is partly magnetic dipole and

partly electric dipole in nature and has the applications in laser and optical

amplification. The magnetic dipole line strength (Smd) is independent of glass host

(ligand field) and is characteristic of a specific transition that can be determined by

the quantum numbers. The electric dipole line strength (Sed) is a function of glass

composition and structure which can be calculated by using JO theory [34-35, 37]. To

make the emission spectrum broad and flat one increases the electric dipole (ED)

contribution by modifying the glass composition and structure [38]. For the antimony

borate glass doped with 1 mol% of Er3+ concentration, the calculated values of sed and

smd for the 4I13/2→4I15/2 transition are 461 and 69 respectively. Hence the calculated

line strength ratio Sed/(Sed+Smd) is 0.87 is greater than the meta phosphate glasses [39].

This indicates that these glasses are suitable for device applications.

The trends of observed intensities of transitions found in all the Er3+ doped

antimony borate glasses were similar. For example, 4I9/2,4S3/2 and 4F3/2 were similar

and found to have relatively lower oscillator strengths among all the transitions. Some

transitions are very sensitive to small changes of environment around rare earth ions

and follows the selection rules ΔS=0, ΔL≤ 2 and ΔJ≤ 2. In present glass system4G11/2 (at about 26,455 cm-1) and 2H11/2 (at about 19,150 cm-1) transitions were

observed to be more intense than all other transitions due to their hyper sensitive

transitions [40]. The transition 2H11/2 found to have the intensity more than half of the

intensity of 4G11/2 transition which is also observed in tungsten phosphate glasses

[16].

5.6.3 Judd–Ofelt parameters

Judd–Ofelt intensity parameters Ωλ for these glasses were evaluated with the

usual procedure i.e., by performing least square fit analysis, the wave numbers(cm-1),

doubly reduced matrix elements of U(λ)2 λ=2, 4, 6 for all the transitions. The

values of the Ω2 are greater than Ω4 and Ω6 for present glasses. This may be due to

the Erbium ions are being in a highly polarized environment [41]. The trend

Ω2>Ω4>Ω6 was followed for all the five glasses. The J-O parameters of present

glasses along with the comparison of various glasses is presented in the Table 5.5.

169

Table 5.5. A comparison table of Ωλ×1020 (cm2) parameters for a number of other

glass systems containing Er3+ ions.

Ω2 Ω4 Ω6 Trend Reference

E2 5.34 2.98 2.48 Ω2 >Ω4 >Ω6 Present work

E4 5.64 3.13 2.59 Ω2 >Ω4 >Ω6 Present work

E6 5.73 3.29 2.66 Ω2 >Ω4 >Ω6 Present work

E8 5.88 3.34 2.72 Ω2 >Ω4 >Ω6 Present work

E10 5.88 3.34 2.73 Ω2 >Ω4 >Ω6 Present work

Borate 11.3 3.6 2.2 Ω2 >Ω4 >Ω6 [42]

Germanate 11.49 3.96 1.89 Ω2 >Ω4 >Ω6 [43]

Phosphate 10.12 3.38 1.94 Ω2 >Ω4 >Ω6 [43]

Ga-La-S 6.54 2 0.7 Ω2 >Ω4 >Ω6 [44]

Pb(PO3)2 glass 6.3 2 1.1 Ω2 >Ω4 >Ω6 [45]

Sr(PO3)2glass 5 1.3 0.9 Ω2 >Ω4 >Ω6 [46]

Li2O–SiO2 4.69 1.29 0.56 Ω2 >Ω4 >Ω6 [47]

Na2O–SiO2 4.37 0.83 0.27 Ω2 >Ω4 >Ω6 [47]

K2O–SiO2 5.02 0.71 0.24 Ω2 >Ω4 >Ω6 [47]

Li2O–B2O 5.81 1.84 1.18 Ω2 >Ω4 >Ω6 [47]

Na2O–B2O3 6.04 1.59 0.91 Ω2 >Ω4 >Ω6 [47]

K2O–B2O3 5.38 1.02 0.44 Ω2 >Ω4 >Ω6 [47]

MgSO4+ K2SO4+

ZnSO4 18.68 2.35 7.15 Ω2 >Ω4 >Ω6 [48]

CaSiO3–Ca3(PO4)2 4.5 1.25 0.75 Ω2 >Ω4 >Ω6 [49]

Aqua ion 1.59 1.95 1.9 Ω2 >Ω4 >Ω6 [48]

170

From the values of Ω2 and Ω6 obtained from the absorption spectra, one can

attempt to extract information on the local interactions of Er3+ ions with the

surrounding glass network as a function of glass composition. The Ω2 parameter is

measure of the degree of covalency of the chemical bonds between Er3+ and its

nearest neighbouring atoms [50]. The Ω6 parameter increases with decreasing rigidity

of the medium and Ω6 values are usually higher for glasses than for crystals. The

Judd–Ofelt analysis therefore seems to indicate a higher covalency and lower force

constant of the Er3+ ligand bonds in our glass systems. The relatively high values of

Ω2 indicate the high degree of inhomogeneity in these glasses [13].

5.6.4 Radiative properties

Various radiative parameters such as electric dipole line strength (Sed), magnetic

dipole line strength (Smd), radiative transition probability (AR), total radiative transition

probabilities(AT), the radiative decay times (τR), and branching ratio(βR) of Nd3+

doped lead antimony borate glasses were calculated using the equations

1.14,1.15,1.25,1.26,1.27 and 1.29 respectively. The radiative parameters of lead

antimony borate glasses for 1% of Nd3+ are presented the Table 5.6.

The radiative properties of Re3+ ions depends upon the number of factors such

as network former and modifier of the glass. The branching ratio (βR) characteristics

the lasing power of the a transition. The intense green transition 4S3/2→4I15/2 was

found to have the branching ratio 67% indicates the potential laser transition in Er3+

doped lead antimony borate glasses.

171

Table 5.6. Electric dipole line strength (Sed), magnetic dipole line strength (Smd),

radiative transition probability (AR), branching ratio(βR) and radiative decay times (τR)

of Er3+ doped lead antimony borate glasses.

Transition Wave Number(cm-1)

Sed(×10-22)(cm2)

Smd(×10-22)(cm2)

A (s-1) βR (%)

4G11/2→2H9/2 1946 254.9 9.9 4.8 04F3/2 3882 34.1 0 4.9 04F5/2 4332 35.2 0 7.1 04F7/2 5963 100.7 0 52.8 02H11/2 7298 68.6 13.8 80.5 04S3/2 8039 45.8 0 58.9 04F9/2 11118 274 1 934.2 24I9/2 13908 50.2 0.1 334.6 14I11/2 16177 20.6 0 215.5 14I13/2 19907 228.2 2.5 4502.7 114I15/2 26455 763.7 0 34943.6 85

∑A (s-1)= 41139 τR (μs)= 24

2H9/2→4F3/2 1936 9.9 0 0.2 04F5/2 2386 18 0 0.7 04F7/2 4017 86.7 4.7 17.7 02H11/2 5352 101.5 2.2 47.2 14S3/2 6093 1.3 0 0.9 04F9/2 9172 24.9 19 104.8 14I9/2 11962 12.3 0.2 63.3 14I11/2 14231 86.4 4.3 778.3 114I13/2 17961 185.1 0 3180.6 444I15/2 24509 69.7 0 3043.8 42

∑A (s-1)= 7237 τR (μs)= 138

172

Transition Wave Number(cm-1)

Sed(×10-22)(cm2)

Smd(×10-22)(cm2)

A (s-1) βR (%)

4F5/2→4F7/2 1631 93 37.7 2.9 02H11/2 2966 74.3 0 9.6 04S3/2 3707 5.8 2.2 2.1 04F9/2 6786 185 0 285.7 44I9/2 9576 59.1 0 256.3 44I11/2 11845 35.7 0 292.9 44I13/2 15575 160.9 0 3005.1 414I15/2 22123 63.7 0 3408.6 47

∑A (s-1)= 7263 τR (μs)= 1374F7/2→2H11/2 1335 198.2 0 1.8 04S3/2 2076 2 0 0.1 04F9/2 5155 23.8 24.1 25.5 04I9/2 7945 166.2 6.6 322.4 34I11/2 10214 140 0 553.2 64I13/2 13944 120.6 0 1212.6 134I15/2 20492 230.7 0 7359.4 78

∑A (s-1)= 9475 τR (μs)= 1054S3/2→4F9/2 3079 7.1 0 1.5 04I9/2 5869 99 0 148.4 34I11/2 8138 24.3 0 97.2 24I13/2 11868 97.6 0 1210.1 274I15/2 18416 64.1 0 2970.4 67

∑A (s-1)= 4428 τR (μs)= 2254F9/2→4I9/2 2790 84.4 42.6 8.5 04I11/2 5059 409.3 17.7 164.6 44I13/2 8789 84.1 0 169.3 44I15/2 15337 327.2 0 3500.8 91

∑A (s-1)= 3843 τR (μs)= 260

173

Transition Wave Number(cm-1)

Sed(×10-22)(cm2)

Smd(×10-22)(cm2)

A (s-1) βR

(%)

4I9/2→4I11/2 2269 65.7 40.1 3.8 14I13/2 5999 207.6 0 132.9 274I15/2 12547 61 0 357.4 72

∑A (s-1)= 494 τR (μs)= 20234I11/2→ 0 0 04I13/2 3730 390.9 77.4 61 154I15/2 10278 130.6 0 350.3 85

∑A (s-1)= 411 τR (μs)= 24314I13/2→4I15/2 6548 460.8 68.8 319 100

∑A (s-1)= 318 τR (μs)= 3135

5.7 Photoluminescence

The luminescence spectra and lifetime measurements were carried out at room

temperature using JOBIN YVON Fluorolog-3 spectrofluorimeter using xenon arc lamp

as radiation source. The inset of Fig. 5.7 shows the excitation spectrum of present

glasses containing four bands centered at 380nm, 405nm, 448nm, 486nm corresponds

to the exited levels 4G11/2, (2G, 4F, 2H)11/2,4F5/2,

4F7/2. The strongest band at 380nm

corresponding to 4G11/2→4I15/2 transition is used for excitation to observe emission

spectrum.

Fig. 5.7. Visible emission spectra of PbO-Sb2O3-B2O3 glasses doped with Er3+ ions recorded at room temperature (λex=380 nm). Theinset shows the excitation spectrum.

0

0.2

0.4

0.6

0.8

1

500 550 600 650 700

Nor

mal

ized

Int

ensi

ty (

A.U

)

Wavelength (nm)

2H11/2 - 4I15/2

4S3/2 - 4I15/2

4F9/2 - 4I15/2

E10

E8

E6

E4

0

0.2

0.4

0.6

0.8

1

360 400 440 480

Inte

nsit

y (A

.U)

Wavelength (nm)

4G11/2

(2G, 4F, 2H)4F5/2

4F

7/2

175

5.7.1 Visible emission

The visible emission spectra shows bands corresponding to the transitions

2H11/2→4I15/2 (526nm),4S3/2 →4I15/2 (547 nm) and4F9/2→4I15/2 ( 665 nm).

The two transitions 2H11/2→4I15/2,4S3/2→4I15/2 corresponds to bright green

emission whereas the transition due to 4F9/2→4I15/2 to a week red band. It should be

noted that the emission at 547nm (4S3/2→4I15/2) considerably broadens towards the

high energy side. It is observed that the photoluminescence intensity increases with

increase in mol% of Erbium ions. The variation of intensity of 4S3/2→4I15/2 transition

is shown in the Fig. 5.8

Fig. 5.8 Variation of intensity of 4S3/2→4I15/2 transition with mol% of Erbium ions

in PbO-Sb2O3-B2O3 : Er2O3 glasses

0.6

0.7

0.8

0.9

1

0.2 0.4 0.6 0.8 1

Inte

nsit

y (A

.U)

Mol % of Er3+

176

5.7.2 Energy gap calculation

Analysis based on a simple three-level system comprised of 4I15/2 (level 0),4S3/2 (level 1) and 2H11/2 (level 2) predicts that the thermalization of the 2H11/2 level can

be expressed by the following relation [33]

kT

E

gA

gA

I

Iexp

111

222

1

2

5.1

where A1 and A2 are the total spontaneous emission rates, hν1 and hν2 are the photon

energies and g1 and g2 are degeneracies (2J+1) of the 4S3/2 and 2H11/2 levels,

respectively. The I terms represent the integrated emission intensities of a given

transition at a given temperature. The ΔE is the energy gap between the 2H11/2 and4S3/2 level and k is the Boltzmann’s constant. The ratio of the degeneracies for 2H11/2

and 4S3/2 is 3 and the ratio of the photon energies is 1.04 for present glass system. The

ratios of the fluorescence intensities (I2/I1) as determined from the emission spectral

profiles (Fig. 5.). The predicted energy gaps (ΔE) between 2H11/2 and 4S3/2 levels are

calculated by using Equation 5.1 and presented in the Table 5.7. These ΔE values are

in good agreement with the average observed energy levels separation of 693 cm−1

between the 2H11/2 and 4S3/2 emission levels (Table 5.1).

Table 5.7. The predicted energy gaps (ΔE) between 2H11/2 and 4S3/2 levels of Er3+

doped lead antimony borate glasses.

Sample Energy gap(ΔE) cm-1

E2 682

E4 683

E6 682

E8 689

E10 672

5.7.3 Stimulated emission cross-section

The stimulated emission cross-section (σp) provides interesting information

about the potential laser performance of a material and it can be easily evaluated from

177

the luminescence spectrum. The stimulated emission cross-section (σp) is an important

parameter and its value signifies the rate of energy excitation from the lasing material.

From the observed emission bands, the stimulated emission cross-sections of all the

bands in emission spectra were calculated using 1.30. The emission cross-section of

different transitions of Er3+ doped lead antimony borate glasses was shown in Table

5.8.

Generally the branching ratios (βR) and the peak emission cross-sections(σem)

are high for a good potential laser transition [51]. The transition 4S3/2 →4I15/2 was

found to have the branching ratio 67% as well as good emission cross section 7.19

×10-21 cm2 indicates potential laser transition. The transition 2H11/2→4I15/2 have

comparatively larger values of emission cross-section, likely to suffer from the non

radiative process because of the small energy gap 672 cm-1 .

The emission cross-sectio σe for E10 glass at 550nm (green wavelength) is

7.19×10-21 cm2 which is more than soda lime silicate glass (σe=2.55×10-21 cm2) [52].

Similarly the σe for E10 glasses at 1531nm is found to be 6.94×10-21 cm2 which is

larger than more 4.2×10-21 cm2 for PbO-PbF2-B2O3 glasses [54] but comparable with

6.30 ×10-21 cm2 for soda lime silicate glasses [52]. In general transitions with large

stimulated emission cross-section exhibit low threshold and high gain laser operation

[39]. Hence, Er3+: lead antimony borate glasses with reasonably good stimulated

emission cross-section values are better hosts for laser applications in the green and

NIR regions and broadband amplifiers.

Table 5.8. Emission cross sections of Er3+ doped lead antimony borate glasses for different transitions.

Transition λP Δλ σe (x10-21) cm2

E2 E4 E6 E8 E10 E2 E4 E6 E8 E10

2H11/2 - 4I15/2 458 14.2 14.6 14.9 15.3 16.1 42.93 44.64 45.88 46.81 47.92

4S3/2 - 4I15/2 485 16.7 17 17.3 17.6 18 6.73 6.9 6.99 7.02 7.19

4F9/2 - 4I15/2 577 19.4 19.7 20.1 20.6 21 14.58 15.04 15.14 15.3 15.87

4I13/2 - 4I15/2 666 111 117 120 122 123 6.29 6.53 6.64 6.71 6.94

179

5.7.4 Near infrared emission

Fig. 5.9 shows near infrared luminescence spectra of Er3+ doped lead

antimony borate glasses. The spectral intensities were normalized with respect to the

peak intensities of E10 glass. The increase in fluorescence intensity at the higher Er3+

doped level is the evidence of absence of concentration quenching. In addition to

main peak at 1.53μm, a side peak is observed at 1.55μm. It was also observed that the

relative intensity of peak at 1.55μm increased with the concentration of Er3+ ions. The

fluorescence width at half-maximum (FWHM) increased from 111nm to 123 nm with

increasing the Er3+ ion concentration from 0.2% to 1%.

Because of the overlap of the absorption and emission spectra of Er3+ ions at

1.53 μm, the broadening trend with increasing Er3+ doped concentration should

mainly be attributed to the radiation trapping [55-59]. The radiation trapping

inevitably induces that the emission intensity at the bands located at lower energies

with respect to the peak wavelength of 1531nm increases accordingly, and then it

leads to a broadening of the emission spectrum. The radiation trapping always occurs

in a typical three-level system when the absorption and emission spectra overlap [56-

59].The radiation trapping effect is in proportion to the Er 3+ doped concentration.

180

Fig. 5.9 Normalized fluorescence near infrared spectra of lead antimony glasses withdifferent Er2O3 doping contents. λexc=980 nm.

Fig. 5.10 Cross-section (absorption and emission) of 4I13/2 → 4I15/2 transition of PbO-

Sb2O3-B2O3-Er2O3 glasses for 1 mol% of Er3+ ions

0

0.2

0.4

0.6

0.8

1

1.2

1400 1450 1500 1550 1600 1650 1700

Nor

mal

ized

Int

ensi

ty (

AU

)

Wavelegth (nm)

4I13/2 - 4I15/2

E10E8E6E4E2

0

1

2

3

4

5

1400 1450 1500 1550 1600 1650 1700

Cro

ss-s

ecti

on (

×10

-21

cm2)

Wavelength (nm)

absorption

Emission

181

To obtain the intrinsic 1.53μm emission spectrum free from re-absorption, the

emission spectra of Er3+ ions were calculated according to the reciprocity method of

McCumber- Miniscalco [60-61], which relates the absorption and emission cross-

sections by

Nl

IIa

)/log(303.2 0 5.2

where N is concentration of 1mol% Er3+ ions (0.56x1021 ions/cm3) and l is thickness

of sample in cm and log(I0/I) is optical density or at absorbance at wavelength λ. The

absorption and emission cross sections of the 4I15/2→ 4I13/2 for 1 mol% of Er3+ ions

were shown in the Fig. 5.10. From Fig. 5.10 it is clear that the peaks of absorption and

stimulated emission cross-section of Er3+ are at 1531 nm (λp). At wavelengths longer

than λp, Me is higher than σa and for lower wavelengths M

e is smaller than σa.

5.7.5 Decay curves of 4I13/2 → 4I15/2

Fig. 5.11 shows the infrared fluorescence decay curves at 1.53μm arising from

the 4I13/2→ 4I15/2 transition of Er2O3 doped lead antimony borate glasses. The decay

curves of glasses demonstrated an almost single exponential decay (for lower

concentrations of Er3+ ions) to non-exponential decay (for higher concentration of Er3+

ions). The measured decay times for the 4I13/2 level are smaller than the calculated

radiative lifetimes. The increase in the decay time was observed with increase in the

concentration of Er3+ ions.

182

Fig. 5.11. Luminescent decay curves of 4I13/2→ 4I15/2 transition in Er3+ doped leadantimony borate glasses. λexc = 980 nm.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 1000 2000 3000 4000 5000

Inte

nsit

y (a

rb.u

nit)

Time (μs)

E2

E4

E6

E8

E10

183

5.7.6 CIE chromaticity co-ordinates

The emission intensities of Er3+ doped lead antimony borate glasses were

characterized by using CIE 1931 chromaticity diagram [62]. The tunability of colour

depends upon the concentration of rare earth ions rather than the pump wavelength.

The colour coordinates of Er3+ LAB glasses (0.28, 0.63) are far away from

pure white colour source coordinates (0.33,0.33). This implies that the light emitted

by Er3+doped LAB glasses was not white. The dominant wavelength of the light

emitted was determined by drawing a straight line from pure white light coordinates

i.e., equal energy point (0.33, 0.33) to the colour coordinates of Er3+ ions (0.28, 0.63).

The dominant wavelength of the Er3+ions emission in LAB glasses is about is 545 nm.

The colour purity or colour saturation of a particular dominant colour in a test

source is the distance in chromatic diagram between the emission colour coordinates

and the coordinates of equal energy point, divided by the distance between the equal

energy point and dominant wavelength point [62]. The colour purity of test source

should be as low as possible for getting efficient white light. The colour purity of all

the Er3+ doped lead antimony borate glasses were calculated using equation 1.36

The correlated colour temperature (Tcct) is defined by the CIE as the

temperature of the Planckian blackbody whose perceived colour is closest to that of a

given spectrum at the same brightness and under specified viewing conditions [63].

The correlated colour temperature (Tcct) of all the Er3+ doped lead antimony borate

glasses were calculated using equation 1.38 and presented in Table 5.9.

Table 5.9 Colour Coordinates, colour correlation temperature Tcct and colour purity

of Er3+ doped lead antimony borate glasses.

Sample Colour coordinates (x,y) Colour correlationtemperature Tcct ( K)

Colour purity

E2 (0.291, 0.698) 6089 1.26

E4 (0.296, 0.695) 6021 1.26

E6 (0.299, 0.694) 5978 1.22

E8 (0.301, 0.691) 5968 1.15

E10 (0.311, 0.683) 5829 1.09

184

Fig. 5.12 Colour chromatic diagram of lead antimony borate glasses doped with

1 mol% concentration of rare earth ions.

6 Conclusions

Lead antimony borate glasses with different concentrations of erbium were

prepared by the conventional melt quenching method. The amorphous (glass) nature

was confirmed by the characterization techniques such as X-ray diffractograms and

DSC traces.

The various physical parameters of the samples were calculated and analysed.

The density of the glasses increased with increase in the mol% of rare earth ions

almost linearly because; the high dense rare earth ions are replacing low dense B2O3.

In general the density and molar volume show opposite behaviour and the same thing

is reflected here. The molar volume of the glass samples is decreasing with increase in

the erbium concentration. The oxygen packing density of the samples was increased

with increase in the mol% of rare earth ions. Refractive index of the samples

increased with increase in concentration whereas molar refraction and polarizability

are decreased.

185

Fourier transform infrared spectra PbO-Sb2O3-B2O3: Er2O3 glasses exhibited

five conventional bands at 1330 cm-1 (BO3 units), 1050 cm-1 (BO4 units), 688 cm-1

(bending vibrations of B-O-B linkages) , 930 cm-1 (1 vibrational band of SbO3) and

462 cm-1 (PbO4 structural groups).

The optical absorption spectra of Er3+ doped lead antimony borate glasses

have exhibited twelve absorption bands. All the bands were identified according to

Carnall. Cut off wavelength is shifted to higher wavelength with increase in Er3+ ion

concentration. The optical band gap is found to decrease gradually with increase in

the concentration of rare earth ions whereas Urbach energy is increasing. Except the4I15/2→4I13/2, all the transition are electric dipole in nature according to the selection

ΔL=±1; ΔS=0; ΔL, ΔJ≤ 21. The transition 4I15/2→4I13/2 (1.53μm) emission is

partly magnetic dipole and partly electric dipole in nature.

Oscillator strengths (OS) for various transitions have been calculated using

J-O theory. Good agreement between experimental and calculated OS has been

achieved, demonstrating the applicability of JO theory. The Judd-Ofelt parameters

shows the trend Ω2>Ω4>Ω6 for all the samples. The relatively high values of Ω2

indicate the high degree of inhomogeneity in these glasses.

Electric dipole line strength (Sed), magnetic dipole line strength (Smd),

radiative transition probability (AR), branching ratio (βR) and radiative decay times

(τR) were calculated for all the samples. The intense green transition 4S3/2 →4I15/2 is

found to have the branching ratio 67%. The visible luminescence spectra shows three

transitions, 2H11/2 →4I15/2,4S3/2→ 4I15/2 (bright green emission) and 4F9/2 → 4I15/2(week

red emission).

The transition 4S3/2→4I15/2 is found to have the branching ratio 67% as well as

good emission cross section 7.19 ×10-21 cm2 indicates potential laser transition. The

transition 2H11/2→4I15/2 have comparatively larger values emission cross-section, likely

to suffer from the non radiative process because of the small energy gap 672 cm-1.

The near infrared luminescence spectra of Er3+ doped lead antimony borate

glasses showed a band 4I13/2 → 4I15/2 (1.53 μm). Stimulated emission cross section (σE)

and absorption cross section (σa) were evaluated. Peaks of absorption cross section

and stimulated emission cross-section coincides at 1531nm (λp). At wavelengths

186

longer than λp, Me is higher than σa and for lower wavelengths M

e is smaller than σa.

Fluorescence decay curves were recorded for 4I13/2→4I15/2 transition for all the

samples. The measured decay times for the 4I13/2 level are smaller than the calculated

radiative lifetimes.

CIE Chromatic coordinates were calculated for the emission spectra of all the

samples of Er3+ doped lead antimony borate glasses. The colour coordinates of Er3+

LAB glasses (0.28, 0.63) are far away from pure white colour source coordinates

(0.33, 0.33). This implies that the light emitted by Er3+doped LAB glasses are not

white. The dominant wavelength of the Er3+ions emission in LAB glasses is about

545nm (green).

187

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