Photoluminescence properties of Er3+-doped alkaline earth titanium phosphate glasses

Journal of Alloys and Compounds 491 (2010) 349–353

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Photoluminescence properties of Er3+ -doped alkaline earth titanium phosphate glasses D.V.R. Murthy a , A. Mohan Babu a , B.C. Jamalaiah b , L. Rama Moorthy a,∗ , M. Jayasimhadri c , Kiwan Jang c , Ho Sueb Lee c , Soung Soo Yi d , Jung Hyun Jeong e a

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Sree Vidyanikethan Engineering College, Tirupati, 517 102, India Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea d Department of Photonics, Silla University, Pusan 617-736, Republic of Korea e Department of Physics, Pukyong National University, Pusan 608-737, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 7 October 2009 Accepted 22 October 2009 Available online 31 October 2009 Keywords: Glasses Optical absorption J–O parameters Luminescence McCumber theory Decay properties

a b s t r a c t Er3+ -doped alkaline earth titanium phosphate (RTP) glasses with molar composition of 24 (NaPO3 )6 + 30 KH2 PO4 + 25 TiO2 + 20 RCl2 + 1 Er2 O3 were prepared by melt quenching technique. Judd–Ofelt intensity parameters (˝2,4,6 ) were determined from the experimental oscillator strengths (fexp ) of absorption bands. From these parameters spontaneous emission probabilities (AR ), luminescence branching ratios (ˇR ) and radiative lifetimes ( R ) have been calculated. Visible and near infrared photoluminescence spectra has been recorded by exciting the samples at 380 and 970 nm respectively. An intense broad emission band at 1.53 ␮m was observed corresponding to 4 I13/2 → 4 I15/2 transition. McCumber theory has been applied to determine the emission cross-sections ( e ) of the 4 I13/2 → 4 I15/2 transition using the absorption cross-sections ( a ). The lifetimes of 4 S3/2 level were measured for the glasses by exciting the samples at 540 nm wavelength and the quantum efficiencies were also determined. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Erbium-doped glasses have been extensively investigated for the development of optical fiber amplifiers and infrared lasers because of their key role in the optical signal amplification at 1.5 ␮m telecommunication window [1,2]. The wavelength division multiplexing (WDM) in telecommunication system require amplifying materials possessing broad emission spectra. Moreover, an increasing demand for compact optical amplifiers has also been perceived in order to supply low cost optical devices. Among the latest evolutions of telecommunication networks, the research field concerning the behavior of tripositive erbium (Er3+ ) doped glasses are most important [3]. Phosphate glasses are regarded as better hosts for Er3+ ions compared to silicate glasses, because of their higher phonon energy, more solubility of RE3+ ions and smaller upconversion coefficient of the 4 I13/2 level [4,5]. The addition of TiO2 produces further increase of the linear and non-linear refractive indices. The high linear index increases the local field correction at the rare earth ion site leading to large radiative transition probabilities, where as the non-linear index enhances the optical non-linearities [6,7].

∗ Corresponding author. Tel.: +91 877 2289472; fax: +91 877 22252. E-mail address: [email protected] (L.R. Moorthy). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.179

The present work deals with the systematic evaluation of spectroscopic properties of Er3+ -doped alkaline earth titanium phosphate (RTP) glasses. Judd–Ofelt (J–O) theory [8,9] has been applied to determine three intensity parameters ˝2 , ˝4 and ˝6 from the measured absorption spectral intensities. The ˝ ( = 2, 4, 6) values are further used to evaluate the radiative parameters, such as spontaneous emission probabilities, branching ratios and emission cross-sections for various excited levels. From the emission spectra, the important luminescence properties of 4 I13/2 →4 I15/2 transition have been determined. The peak stimulated emission cross-sections have been evaluated using the McCumber’s [10] theory and are compared with the measured emission cross-section values. 2. Experimental Er3+ -doped RTP glasses (in mol%) were prepared by melt quenching technique and as follows: Er:MgTP (Glass A): 24 (NaPO3 )6 + 30 KH2 PO4 + 25 TiO2 + 20 MgCl2 + 1 Er2 O3 Er:CaTP (Glass B) : 24 (NaPO3 )6 + 30 KH2 PO4 + 25 TiO2 + 20 CaCl2 + 1 Er2 O3 Er:SrTP (Glass C) : 24 (NaPO3 )6 + 30 KH2 PO4 + 25 TiO2 + 20 SrCl2 + 1 Er2 O3 The starting chemicals were mixed and grinded thoroughly in an agate mortar and then homogeneous mixtures were melted in an electronic furnace at a temperature range 950–1000 ◦ C for 1 h. The melts were then poured on a preheated brass mould and annealed at 300 ◦ C for about 8 h to remove thermal strains. Then

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D.V.R. Murthy et al. / Journal of Alloys and Compounds 491 (2010) 349–353 Table 1 Experimental (Eexp ) and calculated (Ecal ) energy levels (cm−1 ) along with free-ion parameter values for Er3+ -ions in RTP glasses. Level

Glass A Eexp

4

I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 4 H11/2 4 F7/2 4 F5/2 4

/2

4

H9/2 G11/2  rms EAVG F2 F4 F6 F2 /F4 F2 /F6 Fk ˛ ˇ   4

Fig. 1. Room temperature UV–vis and NIR absorption spectra of Er3+ -doped RTP glasses.

the glass samples were allowed to cool to room temperature and were polished for optical measurements. Thus the glass samples with good optical quality were obtained and also found stable against atmospheric moisture. The refractive indices were measured on an Abbe refractometer at sodium wavelength of 589.3 nm using 1-bromonapthalene as contact liquid. The densities were measured by the conventional Archimedes method using xylene as an immersion liquid. The absorption spectra were measured using Varian Cary 5E UV–vis–NIR spectrophotometer over the spectral range of 360–1600 nm. The visible fluorescence spectra were measured using the Hitachi F-3010 fluorescence spectrophotometer in the wavelength range of 400–725 nm by exciting the samples at 380 nm using xenon lamp as an excitation source. The near infrared fluorescence spectra in the wavelength range 1400–1700 nm were measured by using a computer controlled 970 nm diode laser (SDL-6362-P1) as an excitation source. Decay times of 4 S3/2 level were measured with a pulsed tunable laser (Lambda Physics, Scan Mate OPPO) under 540 nm excitation by monitoring the emission at 546 nm and the signal was collected by using a digital oscilloscope (Le Croy LS 140, 100 MHz).

3. Results and discussion 3.1. Absorption spectra, energy levels and intensity analysis The absorption spectra of Er3+ -doped RTP glasses in the wavelength range 360–1600 nm are shown in Fig. 1. Eleven absorption bands corresponding to the transitions from the ground state 4 I15/2 to various excited states are observed. The energy levels are analyzed in terms of a parametric free-ion Hamiltonian which can be written as a sum of different interactions [11] ˆ FI = EAVG + H



+

i

 k

T i tˆi +

ˆ 2 ) +  G(R ˆ 7) ˆ so + ˛Lˆ (Lˆ + 1) + ˇG(G F k fˆk + 4f A

 k

P k pˆ k +



ˆj Mj m

(1)

j

ˆ so , Lˆ , G, ˆ tˆi , pˆ k , m ˆ j ) represent the angular where the operators (fˆk , A integrals and their associated parameters. While performing freeion calculations, the parameter sets Ti (i = 2, 3, 4, 6, 7, 8); Mj (j = 0, 2, 4) and Pk (k = 2, 4, 6) were fixed to the values of the Er3+ :LaCl3 [12], since the variation of Ti , Mj and Pk parameters in the least squares fit did not show any improvement in root mean square ( rms ) deviations. The fitting between the experimental and calculated energy levels was carried out by the standard least square fit with  rms exp deviations as figure of merit. The experimental (Ei ) and calculated energies (Eical ) obtained by the least squares fit calculations of Er3+ -doped RTP glasses are given in Table 1. The  rms deviations between the experimental and calculated energies are ±65, ±65

0 6527 10,267 12,563 15,385 18,382 19,231 20,576 22,222 22,624 24,631 26,596 ±65 35,600 98,228 71,373 49,085 1.38 2.00 218,686 16.06 −607 1,787 −2,390

Glass B Ecal

Eexp

14 6,581 10,232 12,484 15,345 18,436 19,223 20,589 22,237 22,618 24,722 26,522

Glass C Ecal

0 6,527 10,267 12,563 15,385 18,382 19,231 20,576 22,222 22,624 24,631 26,596 ±65 35,600 98,228 71,373 49,085 1.38 2.00 218,686 16.06 −607 1,787 −2,390

14 6,581 10,232 12,484 15,345 18,436 19,223 20,589 22,237 22,618 24,722 26,522

Eexp 0 6,519 10,267 12,531 15,385 18,450 19,231 20,576 22,222 22,624 24,631 26,596 ±55 35,615 98,407 71,369 49,103 1.38 2.00 218,879 16.06 −607 1,787 −2,382

Ecal 21 6,563 10,213 12,479 15,352 18,462 19,236 20,596 22,249 22,627 24,708 26,525

and ±55 cm−1 for Er3+ -doped glasses A, B and C respectively. It is observed that full matrix diagonalization procedure leads to good fit between the experimental and calculated energies. The parameter EAVG is the uniform energy shift of the entire configuration, which represents the spherically symmetric electron contribution. The hydrogenic ratios of F2 /F4 = 1.38 and F2 /F6 = 2.00 for all the Er3+ doped RTP glasses indicate that the radial integral parts of the f-orbitals of Er3+ ions remains unchanged due to the shielding of the 4f shells by 5s2 5p6 orbitals. The experimental oscillator strengths (fexp ) for the f–f induced electric dipole transitions can be determined from the area under the absorption band using the expression fexp = 4.32 ×  10−9 ε( ) d , where ε( ) is the molar absorptivity of a band at a wave-number ( ) in cm−1 [13]. According to J–O theory [8,9], the calculated oscillator strengths (fcal ) for an electric dipole transition from the ground state ( J) to an excited state (  J ) is given by fcal =

82 mc (n2 + 2) 9n 3h(2J + 1)

2



˝ | J||U  ||  J  |2

(2)

=2,4,6

where h is the Planck’s constant, J is the total angular momentum of the ground state, (2J + 1) is the degeneracy of the ground state, n is the index of refraction, is the wavenumber of the absorption transition in cm−1 , (n2 + 2)2 /9n is the Lorentz local field correction. ||U || are the doubly reduced matrix elements of the unit tensor operator of rank  = 2, 4 and 6, which are considered to be the independent of the host and are calculated from the intermediate coupling approximation for the transition J →  J . The fexp and fcal values of Er3+ -doped RTP glasses are given in Table 2. Reasonably small ırms deviations of ±0.47, ±0.45 and ±0.48 × 10−6 for Er3+ -doped glasses A, B and C respectively, indicate good fit between the experimental and calculated oscillator strengths. In the present study, the trend of the J–O intensity parameters is in the order ˝2 > ˝4 > ˝6 . The J–O intensity parameters of different Er3+ -doped glasses [14–17] are compared in the Table 3. These ˝ intensity parameters are important for the investigation of the local structure and bonding in the vicinity of RE ions in RTP glasses. In these J–O intensity parameters, especially the ˝2 values are closely related to the glass composition. Normally ˝2 values are sensitive to asymmetry

D.V.R. Murthy et al. / Journal of Alloys and Compounds 491 (2010) 349–353

351

Table 2 Experimental and calculated oscillator strengths (×10-6 ) of Er3+ -doped RTP glasses. Wave-number (cm−1 )

Transition 4 I15/2 →

4

I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4 F5/2 4 F3/2 2 H9/2 4 G11/2 ırms

6,527 10,267 12,563 15,385 18,382 19,231 20,576 22,222 22,624 24,631 26,596

Glass A

Glass B

Glass C

fexp

fcal

fexp

fcal

fexp

fcal

2.14 0.79 0.32 4.52 0.29 14.18 2.36 0.72 0.27 0.64 23.33 ±0.47

1.97 0.91 0.73 4.26 0.70 13.58 3.30 0.84 0.48 1.12 23.98

2.19 0.92 0.30 3.57 0.27 11.97 1.98 0.65 0.27 0.58 22.05 ±0.45

2.00 0.95 0.49 3.35 0.75 12.29 3.10 0.91 0.52 1.11 21.71

1.93 0.53 0.51 2.35 0.32 12.42 1.84 0.24 0.17 0.68 19.74 ±0.48

1.71 0.86 0.30 2.40 0.66 11.64 2.53 0.79 0.45 0.94 20.58

of the RE ion sites along with the covalency between rare earth ions and ligand ions, while ˝4 and ˝6 parameters represent the rigidity of medium in which the ions are located [18–20]. Moreover, the larger values of ˝2 in Er3+ -doped RTP glasses suggests larger degree of covalency of Er–O bond and asymmetry of the Er3+ sites compared to other Er3+ -doped glass systems. The spectroscopic quality factor (X = ˝4 /˝6 ) which is an important parameter to predict stimulated emission in a laser active medium was first introduced by Kaminiskii [21]. The evaluated spectroscopic quality factors (X) of Er3+ -doped RTP glasses are 2.07, 1.62 and 1.13 for Er3+ -doped A, B and C respectively which are compared with those of Er3+ -doped lead borate [14], tellurite [15], phosphate [16] and fluorite [17] glasses. The relatively high values of X suggest that Er3+ -doped RTP glasses could be used as laser materials. The J–O parameters have also been used to calculate radiative parameters such as branching ratios and emission crosssections for the important luminescent levels of Er3+ -doped RTP glasses [22]. 3.2. Visible and NIR fluorescence spectra Fig. 2 illustrates the photoluminescence spectra of Er3+ -doped RTP glasses in the visible region. Each spectra consist four emission bands at 454, 523, 546 and 656 nm corresponding to 4F 4 2 4 4 4 4 4 5/2 → I15/2 , H11/2 → I15/2 , S3/2 → I15/2 , and F9/2 → I15/2 transitions respectively. From the emission spectra and J–O parameters, the important luminescence parameters such as the emission peak positions (P ), radiative transition probabilities (AR ) and stimulated emission cross-sections ( e ) are determined. From these values it is observed that the predicted stimulated emission cross-sections and branching ratios are higher for 2 H11/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions, suggesting that these transitions could be most potential for visible laser emission. The NIR luminescence spectra shown in Fig. 3 exhibit broad peak at 1534 nm corresponding to 4 I13/2 → 4 I15/2 transition. The fullwidth at half maxima (FWHM) and stimulated emission crosssections ( e ) are very important to realize broadband amplification and the product (FWHM ×  e ) is often used to evaluate the gain

bandwidth of optical amplifier [23]. The evaluated  e values are 1.00, 1.01 and 0.91 × 10−20 cm2 for Er3+ -doped A, B and C glasses respectively. These predicted  e values are high compared with those of phosphate [5], silicate [11], PGGEr [24] and germinate [25] glasses. According to McCumber theory [26], the stimulated emission cross-section ( e ) of the 4 I13/2 → 4 I15/2 transition of Er3+ can be evaluated from the measured absorption cross-section ( a ), by using the expression e = a exp

 ε − h  KT

(3)

where  e and  a are the stimulated emission and absorption crosssections respectively and ε is the temperature dependent excitation energy. The measured emission cross-sections ( e ) obtained from the emission spectra as well as from the McCumber relation are given in Table 4. As can be seen from Table 4, that the  e values derived from McCumber theory are in good agreement with measured values obtained from the emission spectra. Fig. 4 shows the absorption and emission cross-sections for the 4 I13/2 → 4 I15/2 transition of Er3+ -doped RTP glasses in the NIR region. The  e values of Er3+ -doped RTP glasses are much higher than those of other glass hosts, indicating that the Er3+ -doped RTP glasses possess high gain band width. The bandwidth properties of the optical amplifier can be evaluated from the product (FWHM ×  e ). The larger the product, the higher is the efficiency of an optical amplifier. The gain bandwidth values are 390, 384 and 373 (×10−28 ) for Er3+ -doped A, B

Table 3 Comparison of ˝ parameters (˝ × 10−20 , cm2 ) and spectroscopic quality factors (X) of Er3+ -doped RTP glasses. Glass

˝2

˝4

˝6

Trend

X = ˝4 /˝6

Glass A [present work] Glass B [present work] Glass C [present work] Lead borate [14] Tellurite [15] Phosphate [16] Flourite [17]

7.88 7.52 7.69 3.31 6.46 4.70 2.50

3.73 2.49 1.90 1.63 1.64 1.40 1.50

1.80 1.53 1.68 1.29 1.47 0.80 1.00

˝2 > ˝4 > ˝6 ˝2 > ˝4 > ˝6 ˝2 > ˝4 > ˝6 ˝2 > ˝4 > ˝6 ˝2 > ˝4 > ˝6 ˝2 > ˝4 > ˝6 ˝2 > ˝4 > ˝6

2.07 1.62 1.13 1.26 1.12 1.75 1.50

Fig. 2. Room temperature visible fluorescence spectra of Er3+ -doped RTP glasses under 380 nm excitation.

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Fig. 3. Normalized NIR emission spectra of the 4 I13/2 → 4 I15/2 transition of Er3+ doped RTP glasses excited at 970 nm.

and C glasses respectively. These values are significantly larger than that of phosphate [5], silicate [11], PGGEr [24] and germinate [25] glasses. Therefore, one can conclude that Er3+ -doped RTP glasses are favorable materials for a broad band amplifiers. 3.3. Decay properties The luminescence decay curves of the 4 S3/2 → 4 I11/2 transition have been measured at room temperature by exciting the Er3+ doped RTP samples with 540 nm radiation. The decay curves of 4 S3/2 are nearly single-exponential for all the Er3+ -doped RTP glasses as shown in Fig. 5. The measured lifetimes ( m ) of 4 S3/2 level are 360, 338 and 403 ␮s for Er3+ -doped A, B and C glasses respectively. These values are comparable to that of other reported values [27–30]. The computed/radiative lifetimes ( R ) using the J–O theory are 418, 386 and 441 ␮s for Er3+ -doped A, B and C glasses respectively. The variation in the measured ( m ) and calculated ( R ) lifetimes are due to the non-radiative relaxation rates (WNR ), which is estimated Table 4 The peak stimulated emission cross-sections ( e × 10−20 , cm2 ) measured and predicted by McCumber theory, full widths at half maximum (FWHM, nm) and gain bandwidths (FWHM ×  e ) (×10−28 ) for the 4 I13/2 → 4 I15/2 transition in Er3+ -doped RTP glasses. Glass

Glass A [present work] Glass B [present work] Glass C [present work] Phosphate [5] PGGEr [24] Germinate [25]

Emission cross-section

Predicted

McCumber theory [25]

1.00 1.01 0.91 0.64 0.68 0.51

1.12 1.11 0.94 -

FWHM

Gain bandwidth

39 38 41 37 51 42

390 384 373 237 350 239

Fig. 4. Absorption and emission cross-sections for 4 I13/2 → 4 I15/2 of Er3+ -doped RTP glasses calculated by the McCumber theory.

Table 5 Measured lifetimes ( m , ␮s), calculated lifetimes ( R , ␮s), quantum efficiencies ( %) and non-radiative relaxation rates (WNR , s−1 ) for 4 S3/2 → 4 I15/2 transition of Er3+ doped RTP glasses. Parameter

Glass A

Glass B

Glass C

m R

WNR

360 418 86 385

338 386 88 368

402 441 91 220

Fig. 5. The normalized fluorescence decay curves of 4 S3/2 level in Er3+ -doped RTP glasses excited at 540 nm.

D.V.R. Murthy et al. / Journal of Alloys and Compounds 491 (2010) 349–353

according to the formula [31] WNR =

1 1 − m R

(4)

where WNR is the non-radiative relaxation rate (s−1 ). In the present investigation, the evaluated WNR values are 385, 368 and 220 s−1 for Er3+ -doped A, B and C glasses respectively. The quantum efficiency ( ) of the 4 S3/2 emitting level can be calculated using the following expression [32]

=

m × 100 R

(5)

where  m is the measured lifetime and  R is calculated lifetime. The quantum efficiencies are 86%, 88% and 91% for Er3+ -doped glasses A, B and C respectively. The luminescence decay properties such as experimental lifetimes ( m ), calculated lifetimes ( R ), quantum efficiencies ( ) and non-radiative relaxation rate (WNR ) are presented in Table 5. The relatively high values of quantum efficiencies suggest that the 4 S3/2 → 4 I11/2 transition gives intense emission at 546 nm. 4. Conclusions J–O analysis indicates that the covalency and asymmetry of Er–O bond becomes stronger with increase of ˝2 value in Er3+ doped RTP glasses. The radiative parameters such as emission cross-sections, spontaneous emission probabilities and branching ratios for Er3+ -doped RTP glasses have been determined and are found comparable with other reported values. Large gain bandwidths and stimulated emission cross-sections are obtained for the 4I 4 13/2 → I15/2 transition at 1.53 ␮m emission. The peak stimulated emission cross-sections have been predicted from McCumber’s theory and are found comparable with the measured emission cross-section values. Lifetimes and quantum efficiencies for 4 S3/2 level have been determined. These results indicate that the Er3+ doped RTP glasses are good candidates for the fabrication of optical amplifiers and lasers. Acknowledgements One of the authors Prof. L. Rama Moorthy would like to thank Defence Research and Development Organization, New Delhi for the financial support in the form of major research project (No.

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