Optical spectroscopy, 1.5 m emission, and ... - OSA Publishing

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J. Opt. Soc. Am. B / Vol. 24, No. 9 / September 2007

Babu et al.

Optical spectroscopy, 1.5 ␮m emission, and upconversion properties of Er3+-doped metaphosphate laser glasses P. Babu,1,2 Hyo Jin Seo,1,* Kyoung Hyuk Jang,1 R. Balakrishnaiah,3 C. K. Jayasankar,3 Ki-Soo Lim,4 and V. Lavín5 1

Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea 2 Department of Physics, Government Degree College, Wanaparthy 509 103, India 3 Department of Physics, Sri Venkateswara University, Tirupati 517 502, India 4 Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea 5 Departamento de Fisica Fundamental y Experimental, Electronica y Sistemas, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain *Corresponding author: [email protected] Received March 22, 2007; accepted May 17, 2007; posted June 5, 2007 (Doc. ID 81225); published August 15, 2007 Metaphosphate glasses doped with five concentrations of Er3+ ions have been investigated through absorption and emission spectra, decay curves, and upconversion measurements. Judd–Ofelt parameters have been evaluated from absorption spectrum of the 1.0 mol. % Er3+-doped glass, which are in turn used to predict radiative properties of some important luminescence levels of Er3+ ions in these glasses. Gain bandwidths of an optical amplifier have been evaluated and are compared with those of reported Er3+:glass systems. Temperature dependence of the 1.5 ␮m emission has been studied for the 2.0 mol. % Er3+-doped glass from 13 K to room temperature. Lifetimes of the 4I13/2 level were measured and are found to decrease with concentration of Er3+ ions after 0.1 mol. %. Concentration quenching of lifetimes has been analyzed using the theory developed by Auzel et al. [J. Lumin. 94–95, 293 (2001); Opt. Mater. 24, 103 (2003)]. Infrared to visible upconversion was also measured for three concentrations of Er3+-doped metaphosphate glasses with 794 nm excitation. A mechanism, involving excited state absorption and energy transfer upconversion, has been proposed to explain the upconversion process. © 2007 Optical Society of America OCIS codes: 160.2750, 160.5690, 190.7220, 250.4480, 250.5230, 300.6280.

1. INTRODUCTION Glass materials are attractive hosts for rare earth ions 共RE3+兲 because planar waveguides and optical fibers can be fabricated easily with them compared to crystalline materials. Among the RE ions, the Er ion 共Er3+兲 is one of the most popular and efficient ions for obtaining nearinfrared (NIR) to visible upconversion as well as 1.5 ␮m IR emission for lasers and optical amplification at the third telecommunication window [1]. Further, the Er3+ ion has a number of strong absorption bands where the pumping sources are available. The laser at 1.5 ␮m wavelength attracted much attention since it is located in the “eye safe” spectral region and has attractive applications in atmospheric communication systems [2]. Er3+-doped glasses with a broad 1.5 ␮m emission band have been extensively investigated in searching Er-doped fiber amplifiers (EDFAs) with a wide and flat gain spectrum that is required for dense wavelength division multiplexing (DWDM) optical networks [3–5]. Phosphate glasses are regarded as better hosts for Er3+ ions compared to silicate glasses due to their higher phonon energy, more solubility of RE ions, and smaller upconversion coefficient of the 4I13/2 level [6]. Phosphate glasses are nowadays commonly used for bulk laser applications [7]. Metaphosphate-based glasses are used for simultaneous high-energy and high-peak power laser applications as they have excellent energy storage and extraction 0740-3224/07/092218-11/$15.00

characteristics and can be made in large sizes with high optical quality and free of damage causing inclusions [8]. The main disadvantage with the use of phosphate glasses as laser ion hosts is that they have larger thermal expansion and lower fracture toughness than silicates and are more prone to fracture [9]. In spite of these disadvantages, phosphate glasses are preferred as laser hosts as they possess low refractive index, low melting temperature, low glass transition temperature, and good thermooptical performance [10,11]. In the present work, Er3+-doped metaphosphate glasses with five concentrations of Er3+ have been prepared. The base glass composition has been composed from the ranges of different components used in the Ndphosphate laser glass, LG-750 (Schott Glass Technologies, Duryea, Pa.) [10]. As the LG-750 glass host is well tested for Nd3+ laser action, it is of interest to study the optical spectroscopy, infrared emission, and upconversion properties of Er3+-doped glass hosts of its nearing composition. Judd–Ofelt (JO) theory [12,13] has been applied to derive JO intensity parameters that are in turn used to calculate various radiative properties for the luminescent levels of Er3+ ions in these glasses. The peak stimulated emission cross sections have been calculated from both the Fachtbauer–Ladenburg (FL) formula [14] and the McCumber theory [15,16] for comparison. All these results have been compared with those of reported Er3+:glass sys© 2007 Optical Society of America

Babu et al.

tems. Concentration quenching of lifetime of the 4I13/2 level of Er3+ ions has been analyzed using the theory developed by Auzel et al. [17,18]. Near-infrared to visible upconversion was observed in these glasses with 794 nm excitation and a mechanism has been proposed to explain the process based on excited state absorption and energy transfer upconversion.

2. EXPERIMENTAL DETAILS Er3+-doped metaphosphate glasses with a composition 共mol. % 兲 of 共59− x / 2兲P2O5-17K2O-共15− x / 2兲BaO-9Al2O3xEr2O3 (PKBAEr) (x = 0.01, 0.1, 1.0, 2.0, and 3.0) were prepared by the melt quenching technique. Approximately 10 g of the batch composition, after being thoroughly mixed in an agate mortar, was taken in a platinum crucible and melted in an electric furnace kept at a temperature of 1075° C for 90 min. The melt was poured onto a preheated brass mold at a temperature of 350° C and annealed at this temperature for 12 h to remove thermal strains. The sample was then allowed to cool to room temperature and polished for measuring optical properties. Refractive index was measured on Abbe refractometer and density was measured by the Archimedes method. The absorption spectrum was measured on a spectrophotometer (Hitachi U-3400). The 488 nm line of Ar+ laser was used as an excitation source for measurements of Stokes luminescence due to the transitions to the lower levels upon resonant excitation. The luminescence was dispersed by a 75 cm monochromator (Acton Research Corp. Pro-750) and observed with a photomultiplier tube (PMT) (Hamamatsu R928). The signal from the PMT was fed to a digital oscilloscope (LeCroy 9310) and then the data were stored in a personal computer. The 1.5 ␮m emission and lifetimes of the 4I13/2 → 4I15/2 transition were measured by exciting at 975 nm using the optical parametric oscillator (OPO) pumped by the second harmonic 共532 nm兲 of a Nd: YAG (yttrium aluminum garnet) laser (Spectron Laser Systems SL802G). The IR signal was detected by an InGaAs detector. Decay curves were obtained using a digital storage oscilloscope interfaced to a personal computer. For upconversion measurements, a pulsed Ti:sapphire laser at 794 nm was used as an excitation source. The laser pulse repetition rate was 82 MHz with a pulse width of 150 fs. The luminescence was dispersed by a 30 cm monochromator and observed with a PMT. The excitation spectra measured in the 780– 820 nm range was obtained by exciting with a cw Ti:sapphire (Spectra-Physics 3900S), pumped by a 10 W Ar+ laser (Spectra-Physics Beamlok), and monitoring the green upconverted luminescence through a 0.19 m singlegrating monochromator (JY Triax 180) with a PMT (Hamamatsu R928). For low-temperature measurements, the sample was cooled using a helium closed-cycle refrigerator (Janis CCS 100).

Vol. 24, No. 9 / September 2007 / J. Opt. Soc. Am. B

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regions, is shown in Figs. 1(a) and 1(b), respectively. A total of 13 absorption bands, corresponding to the transitions from the ground state, 4I15/2, to various excited states, are identified and assigned. The assignments were made by comparison with the energy levels of Er3+:aquo-ion [19], Er3+ : LaF3 crystal [20] and reported Er3+:glass systems [21–25]. The peak positions (barycenters) of the absorption bands (in nanometers), their experimental 共fexp兲 and calculated 共fcal兲 oscillator strengths, obtained using the well-known expressions [12,13,26,27], are shown in Table 1. The small value 共±0.7⫻ 10−6兲 of root mean square (rms) deviation [24] between experimental and calculated oscillator strengths indicates good agreement between them. Experimental oscillator strengths of some of the reported Er3+:glass systems, which include fluoride [22] 关共45− x兲InF3-14ZnF2-19BaF2-17SrF23GaF2-2LaF3 − xErF3兴, lead borate [22] 关共73.5 − x兲PbO-18.5B2O3-5Al2O3-3WO3-xEr2O3兴, oxyfluoride [23] 共50SiO2-50PbF2-3ErF3兲, and tellurophosphate [24] 关50共NaPO3兲6-10TeO2-20AlF3-19NaF-1Er2O3共NaTFP兲兴 are also given in Table 1 for comparison. The absorption bands, 4I15/2 → 4G11/2 and 4I15/2 → 2H11/2 located at 378 and 521 nm, respectively, are most intense and are called hypersensitive transitions (HSTs) [24,26]. The HSTs are sensitive to small changes of environment

3. RESULTS AND DISCUSSION A. Absorption, Judd–Ofelt Analysis, and Radiative Properties The optical absorption spectrum of 1.0 mol. % Er3+-doped PKBAEr glass, measured in the UV-visible (VIS) and NIR

Fig. 1. (Color online) Absorption spectrum of 1.0 mol. % Er3+-doped PKBAEr glass in the (a) UV-VIS and (b) nearinfrared regions. The band assignments are transitions from the ground state, 4I15/2.

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Table 1. Absorption Transitions (From the Ground State, 4I15/2), Absorption Band Positions „␭p… Experimental „fexp…, and Calculated „fcal… Oscillator Strengths „10−6… of 1.0 mol. % Er3+-Doped PKBAER Glass and Experimental Oscillator Strengths of Some Reported Er3+:Glass Systems PKBAEr Level 4 I15/2→ 4

I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4 F5/2 4 F3/2 2 G9/2 4 G11/2 4 G9/2 4

a

关24兴.

b

关22兴.

c

␭p (nm)

fexp

fcal

NaTFPa fexp

1532 977 801 652 544 521 488 451 443 407 378 364

2.48 0.79 0.30 2.41 0.53 11.41 2.11 0.59 0.31 0.76 21.36 3.11

2.20 1.09 0.30 2.69 0.89 12.26 3.17 1.06 0.59 1.23 20.40 1.64

1.59 0.54 0.28 1.88 0.30 8.79 1.73 0.32 0.15 0.53 15.94 1.10

Lead Borateb fexp

Oxyfluoridec fexp

Fluorideb fexp

1.73 0.54 0.34 2.80 0.96 7.70 3.27 1.27d — — — —

1.68 0.51 0.27 1.80 0.31 5.70 1.55 0.56d — 0.37 9.75 —

1.67 0.55 0.37 2.14 0.51 3.44 1.87 0.90d — 0.66 5.22 2.19

关23兴.

d

Combined oscillator strengths of 4F5/2 and 4F3/2.

around RE ions [22,26] and follow the selection rules, 兩⌬S 兩 = 0 , 兩⌬L 兩 ⱕ 2, and 兩⌬J 兩 ⱕ 2 [26]. From Table 1, it is interesting to note that oscillator strengths of these HSTs are larger in PKBAEr glass than those of the Er3+:glass systems that are compared. This is an indication for the lower site symmetry [26] (higher asymmetry) of Er3+ ions in the PKBAEr glass. The important 4I13/2 → 4I15/2 共⌬J = 1兲 transition is partly magnetic dipole and partly electric dipole (ED) in nature. The magnetic dipole line strength 共Smd兲 is independent of ligand fields and is a characteristic of the particular transition determined by the quantum numbers. The electric dipole line strength 共Sed兲 is a function of glass structure and composition and can be calculated using the JO theory [12,13,26]. To get broad and flat emission spectrum, usually the ED contribution is increased by modifying the structure and matrix composition [3]. For the 4 I13/2 → 4I15/2 transition of 1.0 mol. % Er3+-doped PKBAEr glass, the calculated values of the Smd and Sed are 69 ⫻ 10−22 cm2 and 356⫻ 10−22 cm2, respectively. The calculated line strength ratio Sed / 共Sed + Smd兲 is found to be 0.838, which is larger than those of Er3+ : NaTFP (0.769), fluoride (0.683), silicate (0.675), phosphate (0.652), and germinate (0.568) glasses [24,28], thereby indicating that PKBAEr glasses are more preferable for device applications. Using the experimental oscillator strengths of various absorption bands and refractive index 共n = 1.549兲, the JO intensity parameters 共⍀␭ , ␭ = 2 , 4 , 6兲 have been derived [12,13] and are given in Table 2. For the calculation of JO parameters, the matrix elements, 储U␭储2, are taken from [29], and computed for the Er3+:lithium borate glass, as they are almost independent of the host matrix. The JO parameters so obtained are ⍀2 = 8.05⫻ 10−20 cm2, ⍀4 = 1.46⫻ 10−20 cm2, and ⍀6 = 2.28⫻ 10−20 cm2. These values are compared with those of reported Er3+:glass systems in

Table 2 that include ZTE 共40ZnF2-10ZnO49TeO2-1Er2O3兲 [2], tellurite 共85TeO2-15Ga2O3-1Er2O3兲 [21], phosphate [25], NaTFP [24], lead borate [22], oxyfluoride [23], and fluoride [22]. From Table 2, it can be seen that the order of JO parameters 共⍀2 ⬎ ⍀6 ⬎ ⍀4兲 is the same for PKBAEr, Er3+ : ZTE [2], phosphate [25], and NaTFP [24] glass systems whereas for other glass systems (Er3+:lead borate [22], tellurite [21], and oxyfluoride [23]), the parameters follow the general trend as ⍀2 ⬎ ⍀4 ⬎ ⍀6. The trend of JO parameters for Er3+: fluoride glass [22] 共⍀6 ⬎ ⍀4 ⬎ ⍀2兲 differs from the other two. This variation in trends of JO parameters arises due to the large and more sensitive values of oscillator strengths of the two HSTs. Table 2. Judd–Ofelt Parameters (⍀␭, 10−20 cm2) and Spectroscopic Quality Factor „␹ = ⍀4 / ⍀6… for 1.0 mol. % Er3+-Doped PKBAEr Glass and for Some of the Reported Er3+:Glass Systems Glass PKBAEr Telluritea Phosphateb NaTFPc Lead borated ZTEe Oxyfluoridef Fluorided a

关21兴.

b

关25兴.

c

关24兴.

d e f

关22兴.

关2兴.

关23兴.

⍀2

⍀4

⍀6

␹ = ⍀4 / ⍀6

8.05 6.46 6.28 5.92 3.31 3.14 2.75 1.47

1.46 1.64 1.03 1.07 1.63 1.19 1.25 1.51

2.28 1.47 1.39 1.44 1.29 1.43 0.76 1.69

0.64 1.12 0.74 0.74 1.26 0.85 1.65 0.89

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The JO parameters 共⍀␭兲 depend on the host matrix and can provide information on the local structure and bonding in the vicinity of RE ions. The ⍀2 parameter has been reported to be related to covalent bonding and/or asymmetry of the RE ion site and ⍀6 parameter to the rigidity of the medium in which the ions are located [30,31]. For PKBAEr glass, the value of ⍀2 is larger than the other two parameters. This tendency is also seen for other oxide glasses shown in Table 2. The larger value of ⍀2 in PKBAEr glass suggests a larger degree of covalency of the Er– O bond and/or asymmetry of the Er3+ sites than other Er3+:glass systems that are compared in Table 2. The ⍀4 parameter is affected by the factors causing changes in both ⍀2 and ⍀6 and is not usually studied for local structure investigations [31]. However, the spectroscopic quality factor ␹共=⍀4 / ⍀6兲, which was initially established only to quantify the IR emission of Nd3+ ions [23], is an important predictor for stimulated emission in a laser active medium [32]. The value of ␹ has been calculated for the PKBAEr glass and is compared with those of reported Er3+:glass systems in Table 2. The value of ␹ for the PKBAEr glass is found to be 0.64, which is within the range of 0.22–1.5 for Nd3+ ions in different hosts [33] and is smaller than those of Er3+:glass systems shown in Table 2. However, it is still much larger than that of the most standard material Er3+ : YAG 共␹ = 0.3兲 [25]. The JO parameters have been used to predict radiative properties (detailed expressions for the calculation are given in our earlier paper [27]) of the important luminescent levels of Er3+, in 1.0 mol. % Er3+-doped PKBAEr glass, such as radiative transition probabilities 共A兲, branching ratios 共␤R兲, and lifetimes 共␶rad兲 and they are presented in Table 3. The radiative decay rates of Er3+ emission transitions are dominated by ⍀4 and ⍀6 since the matrix elements 储U共2兲储 of these transitions are zero or much smaller than 储U共4兲储 and 储U共6兲储 except for the HSTs, Table 3. Emission Transitions „SLJ\ S⬘L⬘J⬘…, Energy Gap „⌬E…, Predicted Radiative Transition Probabilities „A…, Branching Ratios „␤R…, and Lifetimes „␶R… of Some Important Luminescent Levels of 1.0 mol. % Er3+-Doped PKBAEr Glass SLJ 4

S3/2

⌬E 共cm−1兲

A 共s−1兲

␤R

␶R (ms)

F9/2 I9/2 4 I11/2 4 I13/2 4 I15/2

3059 5903 8153 11,862 18,389

1 85 62 783 1931

⬃0 0.030 0.022 0.274 0.675

0.35

4

I9/2 I11/2 4 I13/2 4 I15/2

2844 5094 8803 15,330

9 117 82 1617

0.005 0.064 0.045 0.886

0.55

4

I11/2 I13/2 4 I15/2

2250 5959 12,486

3 84 121

0.012 0.405 0.584

4.81

4

3709 10,236 6527

40 243 207

0.140 0.860 1.000

3.53

S ⬘L ⬘J ⬘ 4

4

4

F9/2

4

4

I9/2

4

4

I11/2

I13/2 I15/2 4 I15/2 4

4

I13/2

4.82

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2 H11/2 → 4I15/2 and 4G11/2 → 4I15/2. For these transitions ⍀2 plays an important role. The predicted lifetimes of the luminescent levels have been compared with those of reported Er3+:glass systems in Table 4. As can be seen from Table 4, predicted lifetimes in PKBAEr glass decrease of the order of 4I13/2 ⬎ 4I9/2 ⬎ 4I11/2 ⬎ 4F9/2 ⬎ 4S3/2. A similar trend is also found for Er3+:phosphate [25], NaTFP [24], and tellurite [21] glass systems whereas for Er3+:fluoride and lead borate [22] glass systems, the predicted lifetimes decrease continuously with increasing energy of the excited states. Further, the predicted lifetimes in PKBAEr glass are less than those of Er3+:phosphate [25], fluoride [22], and NaTFP [24] glass systems but are slightly more than Er3+:lead borate [22] and much more than Er3+:tellurite [21] glass systems. Figure 2 shows the absorption and emission cross sections of the 4I15/2 → 4I13/2 and 4I13/2 → 4I15/2 transitions, respectively, versus wavelength for the 1.0 mol. % Er3+-doped PKBAEr glass. The absorption cross section has been determined from the absorption spectrum [30]. From Fig. 2, one can see that the emission cross section exceeds that of the absorption cross section. The peak absorption cross section 共␴apeak兲 is 6.74⫻ 10−21 cm2 at 1533 nm, which is smaller than 8.33⫻ 10−21 cm2 at 1529 nm in gallium tellurite glass [21]. The emission cross section has been calculated using the McCumber re兲 lation [15,16] and the peak emission cross section 共␴peak e is 8.02⫻ 10−21 cm2 at 1532.8 nm, which is smaller than 8.54⫻ 10−21 cm2 in gallium tellurite glass [21] but higher than 7.3⫻ 10−21 cm2 in silicate glasses and 7.6 ⫻ 10−21 cm2 in phosphate glasses [34,35].

B. Infrared (1.5 ␮m) Emission and Decay Properties Among the Er3+ emissions, the emission band at 1.53 ␮m arising from the 4I13/2 → 4I15/2 transition is the most important one as it is useful for optical communication and IR laser applications. For the evaluation of spectroscopic parameters of the 1.53 ␮m emission in PKBAEr glasses for different Er3+ ion concentrations, the emission was recorded by exciting the glass samples with 975 nm IR radiation (OPO) and the resulting spectra are presented in Fig. 3. The emission peak positions 共␭p兲, FWHM, and 兲 of the peak stimulated emission cross sections 共␴peak e emission bands and gain bandwidth of an optical ampli⫻ FWHM兲 are determined for all the PKBAEr fier 共␴peak e glasses. These values are presented in Table 5 and are compared with those reported for Er3+:glass systems [3,6,15,24,36]. It is worth noting that for 3.0 mol. % Er3+-doped PKBAEr glass, the ␭p showed a redshift of ⬃11 nm compared to 1.0 mol. % Er3+-doped glass. The redshift of the emission band indicates an increase in covalent bonding between Er3+ ions and the associated ligands [37]. Because of the difference of the emission spectra in different glass hosts, FWHM of the emission band is often used as a semiquantitative indication of the bandwidth. For the present PKBAEr glasses, FWHM of the emission bands have been estimated and they are presented in Table 5 along with those of reported Er3+:glasses. From Table 5 it can be seen that for PKBAEr glasses FWHM value increases from 60 to 81 nm when the concentration of Er3+ ions increases from 0.01 to 3.0 mol. %. It is very

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Table 4. Comparison of Predicted Radiative Lifetimes (ms) of Some Important Luminescent Levels of 1.0 mol. % Er3+-Doped PKBAEr Glass with Some Reported Er3+:Glass Systems Level 4

I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 4

a

关25兴.

b

关22兴.

c

PKBAEr

Phosphatea

Fluorideb

NaTFPc

Lead Borateb

Tellurited

4.82 3.53 4.81 0.55 0.35

9.96 6.14 7.78 0.88 0.56

8.62 6.71 5.85 0.74 0.53

6.35 4.90 6.34 0.75 0.51

4.41 3.26 2.51 0.34 0.30

2.90 2.26 2.50 0.26 0.21

关24兴.

d

关21兴.

Fig. 2. (Color online) Absorption and emission cross sections of the 4I15/2 → 4I13/2 and 4I13/2 → 4I15/2 transitions, respectively, in the 1.0 mol. % Er3+-doped PKBAEr glass at room temperature.

Fig. 3. (Color online) Emission cross sections of the 4I13/2 → 4I15/2 transition for different concentrations of Er3+-doped PKBAEr glasses at room temperature 共␭ex = 975 nm兲.

interesting to note that the emission band for the 3.0 mol. % Er3+-doped PKBAEr glass has the highest FWHM value 共81 nm兲, which is comparable to that of Er3+:ZBLAN glass [38] 共⬃80 nm兲 and Er3+:bismuth glass [39] 共79 nm兲, and significantly larger than that of Er3+:tellurite glass [3] 共60 nm兲. The larger inhomogeneous broadening of the IR emission band in the higher concentration Er3+-doped sample is mainly due to greater variation of environment and coordination numbers surrounding the Er3+ ions [37]. The addition of more Er3+ ions modifies the structure with a greater variety of dopant sites. The local crystal field generated in the higher concentrated sample seems to be mainly responsible for the larger broadening of the IR emission band in the present PKBAEr glasses. Further work on structural investigations is definitely needed to clearly explain the phenomenon. The peak stimulated emission cross sections 共␴peak 兲 e have been calculated for the emission 4I13/2 → 4I15/2 transition for all the PKBAEr glasses using the McCumber relation [15,16] and FL formula [14,27]. The resulting values are presented in Table 5 (the values shown in parentheses are calculated using the FL formula). These values are compared with those of reported Er3+: glass systems; namely, TBLE3 共80TeO2-10BaO10La2O3-Er2O3兲 [3], NaTFP [24], PGG 共60PbO-10Ga2O3-

30GeO2-Er2O3兲 [36], phosphate [6], and silicate [15] in Table 5. It is worth noting that the magnitudes of cross sections calculated with the FL formula are greater than those obtained with the McCumber relation. The differences between the values calculated by two methods are attributed to the reabsorption caused by the spectral overlap of the emission and absorption bands of Er3+ at 1532 nm [14]. Hence, care must be taken while comparing the cross section values. As can be seen from Table 5, the values of ␴peak for PKBAEr glasses are comparable with e those reported for commercial Er3+:phosphate glasses [40], TBLE3 [3], bismuth [39], and PGG [36] glasses: less than Er3+:NaTFP [24] glass but more than Er3+:phosphate [6] and silicate [15] glasses. Generally, transitions with large stimulated emission cross sections exhibit low threshold and high gain laser operation. Hence, the PKBAEr glasses with relatively larger stimulated emission cross section values are better hosts for broadband amplifiers and laser applications in the NIR region. ⫻ FWHM for the 4I13/2 Generally, the product ␴peak e → 4I15/2 emission band is used to measure the gain bandwidth of an optical amplifier. The larger the product, the better the property is. Table 5 lists the ␴peak ⫻ FWHM vale ues for different concentrations of Er3+-doped PKBAEr

Babu et al.


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Table 5. Emission Peak Positions „␭p…, Effective Linewidths (FWHM), and Peak Stimulated Emission Cross Sections „␴epeak… of the 4I13/2 \ 4I15/2 Transition and Gain Bandwidth of an Optical Amplifier „FWHMÃ ␴epeak… for Different Concentrations of Er3+-Doped PKBAEr Glasses and for Some Reported Er3+:Glass Systems Concentration

共mol. % 兲 PKBAEr 0.01 0.1 1.0 2.0 3.0 Reported TBLE3b NaTFPc PGGd Silicatee Phosphatef

ions/ cm3 共1020兲

␭p (nm)

FWHM (nm)

0.024 0.248 2.42 4.85 6.84

1533.0 1531.6 1532.8 1532.4 1543.8

60 61 67 78 81

1531.0 1534.0 1535.0 — 1535.0

60 39.3 51.4 45 37

a

Values shown in parentheses are calculated using the FL formula 关14,27兴.

b

关3兴.

c

f

8.06 7.90 8.02 7.79 6.74 9.97 11.70 6.80 5.50 6.40

(10.53) (10.36) (9.43) (8.10) (7.80)

Gain bandwidtha 共FWHM⫻ ␴epeak兲 483.6 481.9 537.3 607.6 545.9

(631.8) (632.0) (631.8) (631.8) (631.8)

598.3 459.8 349.5 247.5 236.8

关24兴.

d e

␴epeak a 共10−21 cm2兲

关36兴.

关15兴.

关6兴.

glasses as well as for some reported Er3+:glass systems. As can be seen from Table 5, the gain bandwidths for the present glass systems are comparable to Er3+:TBLE3 [3], slightly more than Er3+:NaTFP [24], but far better than Er3+:PGG [36], phosphate [6], and silicate [15] glass systems. The gain bandwidth increases with Er3+ ion concentration, reaches maximum for 2.0 mol. %, and then decreases for 3.0 mol. %. It is interesting to note that the gain bandwidths (shown in parentheses in Table 5) calculated using the ␴peak values obtained from the FL formula e are higher and nearly the same for all the PKBAEr glasses. Hence, the present PKBAEr glass systems, particularly those with higher Er3+ concentration (1.0, 2.0,

and 3.0 mol. %), can be used as host materials for potential broadband optical amplifier WDM. To know the effect of temperature on the emission bandwidth for a particular concentration, temperature dependence of the 4I13/2 → 4I15/2 emission was measured, from 13 K to room temperature, for the 2.0 mol. % Er3+-doped PKBAEr glass. The resulting spectra, after normalization, are presented in Fig. 4. From Fig. 4 one can see that the overall emission bandwidth increases with temperature. The emission bandwidth is approximately three times larger at room temperature than at 13 K. The variation of the shape of the emission spectra at higher temperatures is mainly caused by the onset of new

Fig. 4. (Color online) Emission spectra of the 4I13/2 → 4I15/2 transition in the 2.0 mol. % Er3+-doped PKBAEr glass at different temperatures. The spectra are normalized with respect to maximum of the peak 共␭ex = 975 nm兲.

Fig. 5. (Color online) Decay curves of the 4I13/2 → 4I15/2 transition for different concentrations of Er3+-doped PKBAEr glasses at room temperature 共␭ex = 975 nm兲.

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transitions from several thermally populated higherenergy Stark levels of the 4I13/2 manifold [40]. The luminescence decay curves of the 4I13/2 → 4I15/2 transition have been measured at room temperature by exciting all the PKBAEr glass samples with the 975 nm radiation from OPO and they are shown in Fig. 5. The decay curves are found to be nearly single exponential for all the concentrations of the Er3+ ion. From the decay curves, lifetimes of the 4I13/2 level have been estimated by finding the first e-folding times, and are presented in Fig. 6 as a function of concentration of the Er3+ ions. From Fig. 6, it can be seen that with an increase in Er3+ concentration, the lifetime of the Er3+ level initially increases from 0.01 to 0.1 mol. % and then decreases for higher concentrations. Similar behavior in lifetimes of the 4I13/2 level, with initial increase and then decrease, with increase in Er3+concentration was also noted in tungsten–tellurite glass [41]. The initial rise in the lifetime with an increase in Er3+ ion concentration may be caused by differential site occupancy: that is at lower concentration 共0.01– 0.1 mol. % 兲 Er3+ ions preferentially occupy sites having shorter lifetimes while at higher Er3+ concentrations longer-decay-time sites become occupied. The variations in the ligand fields of Er3+ ions may cause differences in the decay rates. Another explanation for this initial rise in lifetimes is due to radiative trapping [41]. The shortening of lifetimes of the 4I13/2 level at higher 共⬎0.1 mol. % 兲 Er3+ concentration is due to the enhancement of nonradiative energy transfer decay processes [42]. The decrease in lifetimes of the 4I13/2 level with an increase in Er3+ concentration in PKBAEr glasses is attributed to an increase in nonradiative processes such as energy migration among Er3+ ions followed by transfer to recombination centers and interaction between Er3+ ions and the glassy host defects [14]. Quantum efficiency 共␩兲 of the 4I13/2 → 4I15/2 transition can be evaluated by

␩=

␶meas ␶rad

⫻ 100,

共1兲

where ␶meas is the experimentally measured lifetime and ␶rad is the radiative lifetime calculated from the JO theory. The quantum efficiency of the above transition has

Fig. 6. (Color online) Variation of lifetime and quantum efficiency with concentration 共mol. % 兲 of Er3+ ions in PKBAEr glasses.

been evaluated for all the five Er3+-doped PKBAEr glasses and they are also plotted in Fig. 6. As can be seen from Fig. 6, the ␩ value first increases for 0.01– 0.1 mol. % of Er3+ ions and then decreases monotonically with increase in concentration. The decrease in quantum efficiencies with increase in Er3+ concentration can be explained as due to energy transfer processes among Er3+ ions and Er3+ ions and the host. Concentration quenching of lifetime of the 4I13/2 level of Er3+ ions in PKBAEr glasses has been analyzed by the theory developed by Auzel et al. [17,18] to determine whether it is due to a diffusion-limited regime or fast diffusion. In the first case, the order of magnitude for transfer probability between Er3+ ions and Er3+ ions to host (nonradiative sink) is the same. It generally applies to strong self-quenching materials [17]. The second case, where the quenching step is less probable than the diffusion, usually applies to weak self-quenching materials [18]. According to Auzel’s theory, for the diffusion-limited case, assuming an electric dipole–dipole interaction, the self-quenching behavior can be described by [17,18]:

␶w

␶共N兲 =

9 1+

冉冊 N

2

共2兲

,

2␲ N0

where N is the ion doping concentration, ␶w is the measured lifetime at weak concentration, and N0 is the critical sensitizer concentration for self-quenching. Selfquenching by fast diffusion is described by [18]

␶w

␶共N兲 =

N 1 + 1.45

N0ss

冉冊

exp −

␤⌬E

,

共3兲

4

where N0ss is the critical concentration for the diffusion step between active ions, ␤ is the exponential parameter for multiphonon assisted energy transfers in the considered host, and ⌬E is the energy of the considered ion first excited state. In the present analysis, N0ss has been taken to be equal to N0 共3.0⫻ 1020 ions/ cm3兲, calculated from Eq. (2) for 1.0 mol. % of Er3+ ions by taking ␶w = 1069 ␮s, measured lifetime for 0.1 mol. % Er3+-doped PKBAEr glass. The ␤ value has been calculated to be 0.78⫻ 10−3 cm from the equations given in [43] by taking phonon energy 共h␻兲 of the PKBAEr glass host as 1340 cm−1 and the electron– phonon coupling constant 共g兲 as 0.054 [27]. Equations (2) and (3) have been used to simulate the curves showing the variation of lifetimes of the 4I13/2 level for different Er3+ ion concentrations of PKBAEr glasses by substituting the appropriate values. The results are shown in Fig. 7. From Fig. 7 it is clear that concentration quenching is purely diffusion limited for lower concentrations (up to 1.0 mol. %) and then deviates toward a fast diffusion curve for higher Er3+ ion concentrations. This shows a gradual decreasing of the diffusion-limited process and increasing of the fast-diffusion process at higher Er3+ ion concentrations in the present PKBAEr glasses.

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Fig. 7. (Color online) Quenching of the 4I13/2 lifetime with Er3+ ion concentration 共ions/ cm3兲 in PKBAEr glasses. The curves are simulations using Eqs. (2) and (3) for diffusion limited (lower curve) and fast diffusion (upper curve), respectively, along with experimental data (middle curve).

C. Upconversion The RE3+-doped glasses have shown an important capability to obtain luminescence with energy higher than that of the pumping laser photon, a process known as upconversion. Two possible mechanisms can be responsible for the population of the excited emitting level: excitedstate absorption (ESA) and energy transfer (ET). The former involves only one optically active ion successively promoted to the upper levels by the resonant absorption of two or more laser photons, whereas the latter is a many-body nonradiative mechanism involving two or more nearby interacting optically active ions. Both mechanisms are not mutually exclusive and can coexist to effectively contribute to the population of the high excited emitting level, although usually one of them dominates the upconversion. Infrared-to-visible upconversion luminescence was measured at room temperature by exciting the glass samples at 794 nm and the resulting spectra for three concentrations (0.1, 1.0, and 2.0 mol. %) of Er3+-doped PKBAEr glasses, measured under similar conditions, are shown in Fig. 8. The upconversion spectra consists of three emission bands peaked at 526, 545, and 657 nm, which correspond to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4 F9/2 → 4I15/2, transitions, respectively. The assignation of the transition for each band has been carried out by taking into account the energy level diagram and the emission energies, together with the comparison with results reported in other matrices. To analyze the mechanism of upconversion emission, the dependence of the 545 nm upconverted emission intensities on the pump power after IR excitation has been obtained for 1.0 and 2.0 mol. % Er3+-doped PKBAEr glasses. The pump power dependence curve has the same slope of 1.74 for both the Er3+ concentrations and hence only the curve of 2.0 mol. % Er3+-doped glass is shown in the inset of Fig. 8. The upconversion emission intensity 共Iem兲 depends on the incident pump power 共Ppump兲 according to the relation Iem ⬀ 共Ppump兲n, where n is the number of photons involved in the pumping mechanism. As a conclusion, the nearly quadratic dependency indicates that a


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Fig. 8. (Color online) Upconversion spectra for different concentrations of Er3+-doped PKBAEr glasses 共␭ex = 794 nm兲 at room temperature. The spectra were measured under identical conditions. Inset shows the pump power dependence of the 545 nm green upconversion emission for 2.0 mol. % Er3+-doped PKBAEr glass. The red emission band (for 2.0 mol. % Er3+-doped glass) is shown with ten fold magnification.

two-photon or a two-Er3+ ion upconversion process is responsible for the green and red bands. Although a strictly quadratic dependency may be expected, the decrease in the slopes of the upconverted luminescence intensities versus pump power experimentally observed may occur when upconversion rates are comparable to the intrinsic lifetimes [44], reaching a linear dependency in the limit of infinitely large upconversion rates. Moreover, excitation measurements in the 770– 840 nm range monitoring the green 共 4S3/2 → 4I15/2兲 emission wavelength have been carried out. Excitation spectra of the 1.0 and 2.0 mol. % Er3+-doped PKBAEr glasses look similar and hence excitation spectrum for the 2.0 mol. % Er3+-doped glass, in the range of 770– 840 nm, is shown in Fig. 9 along with absorption spectrum for the 4I9/2 level for comparison. If similar profiles are found for both spectra then the dominant upconversion mechanism would be an ET upconversion (ETU), since this mechanism is pro-

Fig. 9. (Color online) Absorption 共Er3+ : 1.0 mol. % 兲 and excitation 共Er3+ : 2.0 mol. % 兲 spectra of the 4I15/2 → 4I9/2 transition in PKBAEr glasses. Excitation spectrum was measured by monitoring the 545 nm upconversion emission.

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portional to the square of ground-state absorption (GSA) 2 cross section 共␴GSA 兲. However, if changes are appreciable, the upconversion rate is dependent on the product of the ground- and the excited-state absorption cross sections 共␴GSA , ␴ESA兲 at the laser photon energy, since an Er3+ ion in the ground state is successively promoted to the upper level by the absorption of laser photons. For the Er3+-doped PKBAEr glass appreciable changes in the excitation spectrum with respect to the absorption spectrum in some parts of the spectrum are observed for the 4I15/2 → 4I9/2, transition. Hence, it can be inferred that the dominant mechanism of upconversion is ESA in the present PKBAEr glasses. However, they are not clear enough to draw a definite conclusion so also ETU mechanisms may be involved in the upconversion processes after IR excitation at this transition [45]. Stokes emission spectra were also measured for the above three samples using excitation at the 4F7/2 level with 488 nm line of Ar+ laser and the spectra obtained (after normalization with respect to the maximum intensity of the emission 4S3/2 → 4I15/2 band) are shown in Fig. 10. They show that the excited Er3+ ions nonradiatively relax to the 2H11/2 + 4S3/2 levels and in turn to the 4F9/2 level resulting in green and very weak red emissions at exactly the same wavelengths as those of upconversion emissions (see Figs. 8 and 10). The Stokes emission spectra, similar to upconversion spectra, consist of three bands peaked at 526, 545, and 657 nm corresponding to the 2H11/2 → 4I15/2, 4 S3/2 → 4I15/2, and 4F9/2 → 4I15/2, transitions, respectively. The relative intensities of 526 and 545 nm emissions in the Stokes emission spectrum of 0.1 mol. % Er3+-doped PKBAEr glass are 0.26 and 0.74, respectively, which are similar to the 0.27 and 0.73 found for the upconversion spectrum of the same glass (an intensity of 657 nm emission is not taken into account as it is very small). The similarity of Stokes emission and upconversion spectra indicates that the 2H11/2 + 4S3/2 levels are populated upon 794 nm excitation. A possible mechanism, involving ESA in single ions and/or ETU involving two excited ions, has been proposed to explain the above upconversion process. Figure 11

Fig. 10. (Color online) Stokes emission spectra under direct excitation 共␭ex = 488 nm兲 for different concentrations of Er3+-doped PKBAEr glasses at room temperature. The spectra are normalized with respect to maximum of the 4S3/2 → 4I15/2 band.

Babu et al.

Fig. 11. (Color online) Partial energy level diagram of Er3+ in PKBAEr glass showing different transitions related to Stokes emission under direct excitation and upconversion emission. GSA, ground-state absorption; ESA, excited-state absorption; ETU, energy transfer upconversion.

shows the partial energy level diagram of Er3+ ions in PKBAEr glass illustrating the mechanisms of upconversion and Stokes emissions. When PKBAEr glasses are excited with 794 nm wavelength radiation, the Er3+ ions in the ground state absorb pump photons and are excited to the 4I9/2 level. This process is known as GSA. Then Er3+ ions relax nonradiatively to the 4I11/2 and 4I13/2, levels, in which part of the excitation energy in the 4I11/2, level further relaxes radiatively and nonradiatively to the 4I13/2 level. From the 4 I11/2 and 4I13/2 levels, the Er3+ ions are further excited to the 4F3/2 and 2H11/2 levels by the pump photons through the process known as ESA (sequential two-photon absorption) because of the good energy matching. In Fig. 11, ESAs 4I11/2 → 4F3/2, and 4I13/2 → 2H11/2 are denoted as ESA1 and ESA2, respectively. Out of these two, ESA2 is more probable due to the longer lifetime of the 4I13/2 level. The Er3+ ions pumped to the 4F3/2 level relax nonradiatively and populate the 2H11/2 level, which is also directly populated by the ESA2 process. Another possibility of excitation is by energy transfer from the 4I11/2 level (shown as ETU1 in Fig. 11) in which two Er3+ ions in the 4I11/2 level interact, and one ion gains energy and reaches the 4 F7/2 level whereas the other loses energy and goes to the ground state. Most of the electrons in the 4F7/2 level then relax nonradiatively to the 2H11/2 level. Another process that depopulates the 4F7/2 level involves the interaction of two Er3+ ions, one of them in the 4I11/2 level and the other in the 4F7/2 level, both populating the 4F9/2 level (shown as ETU2 in Fig. 11). Out of the two ETU processes, ETU2 is less probable because of the fast depopulation of the 4 F7/2 level by multiphonon relaxation. The Er3+ ions at the 2H11/2 state rapidly decay to the 4S3/2 state due to the multiphonon relaxation process because of the small energy gap 共810 cm−1兲 between them. This should lead to reduced emission 共526 nm兲 from the 2H11/2 level. However, the 2H11/2 level is populated due to thermalization effects with the 4S3/2 level [1]. The above processes lead to two

Babu et al.

green emissions, an intense one at 545 nm 共 4S3/2 → 4I15/2兲 and a weaker one at 526 nm 共 2H11/2 → 4I15/2). Balda et al. [45] reported a similar mechanism for green and red upconversion emission in Er3+:GPN glass with 803 nm excitation. The 4F9/2 level is populated by two processes: the first one being multiphonon relaxation from the 4S3/2 level and the second by ETU2. Though there is a considerable energy gap between the 4F9/2 and 4S3/2 levels 共3060 cm−1兲, multiphonon relaxation is still active because of the large phonon energy 共1340 cm−1兲 of the metaphosphate glass host [27]. The Er3+ ions in the 4F9/2 level relaxes rapidly to the 4I9/2 level by multiphonon relaxation because of the smaller energy gap 共⬃2850 cm−1兲 between them resulting in the very small population of Er3+ ions in the 4F9/2level, which decay to the ground state giving very weak red emission 共 4F9/2 → 4I15/2兲 at 657 nm. In higher phonon energy glass hosts, in general, red emission is weaker than green emission [46]. This is because the 4F9/2 level is depleted faster to the 4I9/2 level than its filling from the 4S3/2 level by multiphonon relaxation since the energy gap to its lower level is smaller for the 4F9/2 level than for the 4S3/2 level. The much weaker red emission in the present PKBAEr glasses in both upconversion and stokes spectra (Figs. 8 and 10) may be explained as follows: the energy gap between 4F9/2 and 4I9/2 共⬃2850 cm−1兲 is nearer to the second harmonic 共2680 cm−1兲 of the phonon energy of the host 共⬃1340 cm−1兲 leading to higher probability for multiphonon relaxation for the 4F9/2 level than for the 4S3/2 level compared to other glass hosts of higher phonon energy. On the other hand, in lower phonon energy glass hosts [47], there is an accumulation of Er3+ ion population in the 4F9/2 level because of its lower multiphonon relaxation leading to more intense red emission. It has been reported that the mechanism of ETU increases with increasing Er3+ concentration [48]. This is evident from the upconversion spectra shown in Fig. 8, where the intensity of 545 nm upconversion emission increases with the concentration of Er3+ ions. The intensity of the red emission remains almost independent of the Er3+ ions concentration because of the reasons already mentioned.


and silicate glasses. With an increase in Er3+ ions concentration, lifetimes of the 4I13/2 level first increase and then decrease. Analysis of concentration quenching of lifetimes using the Auzel’s theory indicates that concentration quenching is purely diffusion limited for lower Er3+ ion concentrations and then the quenching gradually deviates towards fast diffusion for higher Er3+ ion concentrations. The PKBAEr glasses show visible upconversion luminescence with 794 nm excitation and the upconversion efficiency increases with the concentration of Er3+ ions. The mechanism of upconversion involves mainly excitedstate absorption and partly energy-transfer upconversion.

ACKNOWLEDGMENTS This work was supported by the Korea Science and Engineering Foundation (grant F01-2006-000-10100-0). C. K. Jayasankar is grateful to Department of Atomic Energy– Board of Research in Nuclear Sciences, Government of India, for the sanction of Major Research project 2003/34/4BRNS/600.

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