Yb3+ co-doped fluorophosphate glass

Solid State Communications 135 (2005) 449–454 www.elsevier.com/locate/ssc

Broadband amplification and upconversion luminescence properties of Er3C/Yb3C co-doped fluorophosphate glass Liyan Zhanga,b,*, Hongtao Sunb, Shiqing Xua, Kefeng Lib, Lili Hub a

School of Information Engineering, Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou 310018, China b Shanghai Institute of Optics and Fine Mechanics, Chineses Academy of Science, 390 Qinghe Road, Jiading District, Shanghai 201800, China Received 6 April 2005; received in revised form 27 April 2005; accepted 12 May 2005 by K.-A. Chao Available online 31 May 2005

Abstract Broadband and upconversion properties were studied in Er3C/Yb3C co-doped fluorophosphate glasses. Large U6 and Sed/(SedCSmd) values and the flat gain profile over 1530–1585 nm indicate the good broadband properties of the glass system. And a premise of using U6 as a parameter to estimate the broadband properties of the glasses is proposed for the first time to our knowledge. Results showed that fluorescence intensity, upconversion luminescence intensity, the intensity ratio of red/green light (656 nm/545 nm) are closely related to the Yb3C:Er3C ratio and Er3C concentration, and the corresponding calculated lifetime of 4 F9/2 and 4S3/2 states for red and green upconversion samples proves this conclusion. The upconversion mechanism is also discussed. q 2005 Elsevier Ltd. All rights reserved. PACS: 78.20.Ke; 42.70.Ce; 32.70.Cs; 42.70.Hj Keywords: D. J–O parameters; D. Broadband; D. Effective gain cross section; D. Upconversion

1. Introduction Er3C doped glasses are fine gain materials for lasers and amplifiers in the eye-safe region around 1530 nm for applications in telecommunication, medicine and meteorology [1,2]. Broadband EDFA and ultrashort pulse lasers needed for wavelength-division-multiplexing and soliton transmission system requires gain media with a flat gain profile over a wide frequency band [3,4]. Host material is an important factor to obtain flat and wide amplification profile. Therefore, fluorophosphate glass which has large inhomogeneous

* Corresponding author. Address: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, 390 Qinghe Road, Jiading District, Shanghai 201800, China. Tel.: C86 21 59910994; fax: C86 21 39910393. E-mail address: [email protected] (L. Zhang).

0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.05.019

broadening and performs better broadband and flatness properties is a promising candidate for Er3C-doped lasers and amplifiers [5]. In this work, Yb3C is used as sensitizer of Er3C to improve the pump absorption and the quantum efficiency, and we deduced the Judd–Ofelt parameters from the absorption spectra to access theoretical quantities such as transition probabilities, branching ratio and radiative lifetime, and U6 and Sed/(SedCSmd) values are discussed to estimate the broadband properties. The fluorescence spectra and the effective gain cross-section that reflect the broadband amplification properties were also studied. Because upconversion luminescence of Er3C is also considered as one of the promising solutions to obtain efficient visible lasers which have many potential applications in the fields of color display, optical data storage, biomedical diagnostics, sensors, and undersea optical transmission [6,7], upconversion luminescence and its mechanism are explored based on the spectra and the energy-matching conditions.

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2. Experiments All glasses were prepared by mixing appropriate quantities of reagent grade phosphates, alkaline-earth fluorides and YbF3 according to the corresponding glass compositions. The mixtures were then melted at 1000– 1100 8C in platinum crucible. After refining, melts were cast into a steel mould, annealed to room temperature with a cooling rate 1 8C/10 min. After that, samples were cut and polished for property measurements. Absorption spectra were recorded with Perkin–Elmer Lambda 900 UV/VIS/IR Spectro-photometer over a range of 300–2000 nm. Fluorescence and upconversion luminescence spectra were measured with Triax 550 Spectrofluorimeter under 980 nm LD excitation. Fluorescence lifetime of 4I13/2 level of Er3C was detected by a HP546800B100-MHz oscilloscope. Refractive indices and densities were obtained through V-prism method and Achimedes method, respectively. All the measurements were taken at room temperature.

3. Theoretical background Judd–Ofelt theory is often used to calculate the spectroscopic parameters of rare earth ions, such as intensity parameters Ut (tZ2, 4, 6), line strengths, oscillator strengths, spontaneous emission probability, branching ratio and radiative lifetime [8–10]. Ut (tZ2, 4, 6) are calculated by the procedure provided in the references, the electric-dipole line strengths Sed (J/J1) are determined by the following expression: X Sed Z Ut jh4f N ðSLÞJkU ðlÞ k4f N ðS 0 L 0 ÞJ 0 ij tZ2;4;6

Z U2 u22 C U4 u24 C U6 u26

(1)

1

In which J and J are the total angular momentum quantum numbers of the initial and final states, respectively, and jh4 f N ðSLÞJkU ðlÞ k4f N ðS 0 L 0 ÞJ 0 ij is the reduced matrix elements, which can be expressed in u22 ; u24 and u26 to different Ut value. Magnetic-dipole line strengths Smd, experimental oscillator strength and the calculated electronic-dipole oscillator ed md strength fexp, fcal , and fcal , spontaneous emission probability A, radiative lifetime tf and branching ratio b are calculated by the procedure in Refs. [8–10]. The energy transfer efficiency h from Yb3C to Er3C is calculated from: h Z1K

tf Yb t0Yb

(2)

tfYb and t0Yb is the lifetime of Yb3C in Er3C–Yb3C co-doped glass and Yb3C single doped glass, respectively. Effective gain cross-section sg(b) is a parameter usually used to evaluate the broadband and tunable properties of the glasses [11]: sg ðbÞ Z bsemi K ð1 K bÞsabs

(3)

where b is the normalized population of the upper laser level, sabs and semi is the absorption and emission cross-section correspondingly.

4. Results and discussion 4.1. Broadband properties Nine absorption bands were used to calculate the J–O parameters. The intensity parameters Ut (tZ2, 4, 6), oscillator strengths and the root-mean-square are shown in Table 1. Table 2 is the U6 and Sed/(SedCSmd) values of fluorophosphate glass and some other glasses. According to the previous studies [12,13], U6 decreases with increasing covalency of chemical bond between rare earth and ligand atoms which can be adjusted by the composition of glass host. Large U6 is a measure of wide emission band. In this opinion, fluorophosphate glass which possesses the largest U6 value should have the largest inhomogeneous broadening and FWHM value than other glasses. In fact, tellurite glass which has small U6 as 0.64 performs very good broadband properties with FWHM (fluorescence width at half maximum) Table 1 J–O intensity parameters and the oscillator strengths of the glasses Absorption

Energy (cmK1)

fexp (10K6)

fcal (10K6)

4

6532 10,246 12,500 15,361 18,450 19,194 20,534 22,222 22,573 24,570 26,455

1.89 – 0.25 2.2 0.45 6.9 1.89 0.42 0.09 0.69 12.56

1.24 (fed)C0.48(fmd) – 0.36 2.1 0.46 7.0 1.92 0.56 0.32 0.69 11.9

I15/2/4I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4 F5/2 4 F3/2 2 H9/2 4 G11/2 4

U 2 Z3.5!10 K20 cm 2 , U 4 Z2.59!10 K20 cm 2 , 10K20 cm2, drmsZ0.12!10K6.

U 6 Z1.30!

Table 2 Line strength ratio Sed/(SedCSmd) in various glasses Glass

U6 (10K20 cm2)

Sed/(SedCSmd)

FWHM (nm)

Fluorophosphate Fluoridea Silicatea Phosphatea Bismutha Telluriteb Germanatea

1.24–1.48 1.1 0.61 0.55 1.1 0.64 0.28

0.721–0.758 0.683 0.675 0.652 0.822 0.694 0.568

51–55 43 40 37 79 57 53

a b

Ref. [18]. Ref. [14].

L. Zhang et al. / Solid State Communications 135 (2005) 449–454

over 55 nm [14] or even 75 nm [15], and the same phenomenon was observed in bismuth glass [16]. U6 is definitely a parameter which is related to the composition and structure of the glass host, and can be a reflection for the broadband properties, but this evaluation is not unlimited and a precondition should be proposed. But all the previous studies [12,13,17] did not give out a reasonable application premise of using U6 as a parameter for broadband evaluations which induces a confusion in the broadband estimation. According to the original studies of Werber [8], the measured oscillator strength fmea is used to calculate the intensity parameters Ul: g X U h4f N ½aSL
JkU l k4f N ½a 0 S 0 L 0
J 0 i2 fed Z 2J C 1 even l

(4)

Where fed can be derived from the measurement of the integrated absorption spectrum via the formula: ð mc mdn (5) fZ 3 pe N m is the absorption coefficient (cmK1) and others have the same meaning as the above equations. When the rare earth ions are embedded in a medium of refractive index n, the right-hand side of Eq. (5) must include a multiplicative factor n (E0/Eeff)2, where E0 is an average field in the medium and Eeff is the field at the ions site effective in inducing transition [8]. This means that the refractive index of the glass is a factor when calculating the intensity parameters Ul. Therefore, if using U6 as a parameter to evaluate the broadband property of the glasses, the refractive index of glasses should be taken into account, high refractive index glass group such as tellurite, bismuth and geminate (ndO1.85) and low refractive index glasses, fluoride, phosphate, fluorophosphate and silicate (nd!1.65) should be compared among their own group separately, which means that this evaluation is valid and meaningful for the glasses with the same refractive index grade. Under this premise, compared with phosphate, silicate and fluoride glasses, fluorophosphate glass exhibits large U6 value and large inhomogeneous broadening (FWHMZ53 nm) as well as better broadband properties, while to the tellurite, bismuth and germinate glasses with refractive index over 1.9, bismuth glass is outstanding in U6 and FWHM value and germinate is the lowest one. Line strength ratio Sed/(SedCSmd) of 4I13/2/4I15/2 transition is another factor to evaluate the broadband property of the glasses, because Smd is independent of the ligand fields and is characteristic determined by the quantum numbers, while Sed is function of glass structure and composition. In order to obtain broadband and flat emission spectrum, improving the contribution of Sed is an effective way [18]. Larger ratio will induce broad and flat emission spectra but to the same reason as U6, refractive index grade is a premise. The Sed/(SedCSmd) ratio in various glasses shows that fluorophosphate glass has relatively

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larger line strength ratio among the glasses with same refractive index grade. Fig. 1 is the effective gain cross-section of the fluorophosphate glass and phosphate glass calculated from formula (3). The figure shows that fluorophosphate glass exhibits a flat gain profile between 1530 and 1585 nm while phosphate glass has a peak at 1535 nm, illustrating that Er3C/Yb3C co-doped fluorophosphate glass has much better broadband properties and is a good candidate for using as broadband amplifier medium. 4.2. Fluororescence and upconversion process In order to illustrate the influence of Yb3C on the fluorescence and upconversion properties of Er3C distinctively, various ErF3 and YbF3 concentrations in sample 1–6 are shown in Table 3. ErF3 has two doping amounts, 0.1 and 0.2 mol%, and YbF3 varies from 1 to 3 mol%, respectively. Fig. 2 shows the fluorescence intensity (300 mw 980 nm pumping) increases with the increasing Yb3C concentration. Glass 3 with the largest Yb:Er ratio (3Z30) has the strongest fluorescence intensity although the ErF3 concentration is only 0.1 mol%, and it also has the largest upconversion luminescence intensity (Fig. 3). In Table 3, the energy transfer efficiency from Yb3C to Er3C enhances with the increasing Yb3C amount, and samples with 0.2 mol% ErF3 have higher efficiency than 0.1 mol% ErF3 ones, indicating that too lower Er3C concentration decreases the energy transfer efficiency from Yb3C ions to Er3C.

Fig. 1. Effective gain cross-section of Er3C in fluorophosphate glass (a) and phosphate glass (b).

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Fig. 3 is the upconversion spectra of the glasses. During the experiments, we found an interesting phenomenon as depicted in Table 3. The intensity ratio of 656 nm/544 nm clearly showed that from sample 1 to 6, this ratio changes from 1.5 to 0.74 to 1.27 and the upconversion luminescence varies correspondingly from red to green then red again. As can be seen from Fig. 3, in sample 1, the red light around 656 nm is stronger than the green light at 544 nm; in sample 2, this two band almost have the same intensity; but from sample 3 on, green light is obviously stronger than red light until to sample 5. In sample 6, the red emission intensity exceeds the green again. Upon the simplified Er3C and Yb3C energy level diagram in Fig. 4, energy transfer process from Yb3C to Er3C is clearly showed, and from the references on upconversion process [19–24], the possible upconversion mechanism of the green light and red light of all the six samples can be depicted as:

Fig. 2. Fluorescence spectra of Yb3C:Er3C co-doped fluorophosphates glasses.

GSA, ground state absorption; ESA, excited state absorption; CR, cross relaxation; NR, non-radiative relaxation; R, radiative relaxation; ET, energy transfer. Take glass 1 and 3 as examples, we calculated the spontaneous emission probability, branching ratio and radiative lifetime of them, respectively, the results are listed in Table 4 which shows that the radiative lifetime of 4F9/2 and 4S3/2 in glass 1 is 0.75 and 0.68 ms, respectively, indicating the lifetime of red light is longer than that of green light and the red upconversion luminescence was observed. On the contrary, in glass 3, the radiative lifetime of 4F9/2 and 4S3/2 is 0.57 and 0.66 ms correspondingly,

which shows that the 545 nm radiation exceeds the 656 nm one and making the upconversion luminescence performs green. In order to reveal the reason why sample 3 exhibits the strongest green emission, we studied the specific upconversion mechanisms of this sample, Fig. 5 is the log–log plot of the upconversion luminescence as a function of pumping power at 980 nm of sample 3, inset is the upconversion luminescence intensity versus pumping power of it. Slope value reveals different upconversion mechanisms of these three bands. The red light slope at 656 nm is only 1.6, which

Table 3 YbF3 mol%, ErF3 mol%, 3, tfEr, tfYb, t0Yb, h, intensity ratio of 656 nm/545 nm and upconversion luminescence state of the glasses Glass no.

YbF3 mol%

ErF3 mol%

3ZYb3C:Er3C

tfEr (ms)

tfYb (ms)

t0Yb (ms)

h

Intensity ratio 656 nm/545 nm

Up. lumin.

1 2 3 4 5 6

1.0 2.0 3.0 3.0 2.0 1.0

0.1 0.1 0.1 0.2 0.2 0.2

10 20 30 15 10 5

8.2 8.2 8.2 8.3 8.4 8.2

880 620 530 350 400 440

1.7 1.9 2.0 2.0 1.9 1.7

0.48 0.67 0.74 0.83 0.79 0.74

1.50 1.01 0.78 0.74 0.96 1.27

red redZgreen green green green red

L. Zhang et al. / Solid State Communications 135 (2005) 449–454

Fig. 3. Upconversion luminescence spectra of Yb3C:Er3C co-doped fluorophosphates glasses.

Fig. 4. Simplified energy levels of Er3C and Yb3C and the upconversion mechanism.

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means the dominated excitation mechanism is the stepwise two photon absorption, 4I15/2Chn/4I11/2/4I13/2, 4I13/2C hn/4F9/2, 4F9/2/4I15/2Chn (656 nm), less energy transfer 2 process F 5/2 (Yb 3C )C 4 I 13/2 (Er 3C )/ 2 F 7/2 (Yb 3C )C 4 F9/2(ET) as depicted above occurs, while the green light slopes of 525 and 545 nm is 2.3 and 2.2, respectively, indicating an energy-transfer-dominated upconversion mechanisms involved in this two bands, 4I15/2Chn/ 4 4 I 11/2 , I 11/2 (Er 3C )C 2 F 5/2 (Yb 3C )Z 2 F 7/2 (Yb 3C )C 4 3C F7/2(Er ), 4F7/2/4S3/2/4I15/2Chn (545 nm), 4F7/2/ 4 H11/2/4I15/2Chn (525 nm). All the results prove that the upconversion luminescence is closely related to Yb 3C :Er3C ratio 3 and Er3C concentration. In glass 1, although 3 is relatively high but the energy transfer efficiency from Yb3C to Er3C is low, thusly, most of the upconversion particles in 4I15/2 level can not be excited to 4F7/2 level, so the 4I13/2Chn/4F9/2 transition increases. It results stronger 656 nm red emission. With 3 and h increase, 4I11/2(Er3C)C2F5/2(Yb3C)Z2F7/2 (Yb3C)C4F7/2(Er3C) (ET) transition increases and resulting in an almost equivalent intensity of red and green light in sample 2 and a stronger green light in glass 3. Compared with 3Z20 in sample 2, although the 3 of sample 4 is only 15, but higher Er3C concentration makes it exhibit high energy transfer efficiency, which induces a stronger radiation at 545 nm. With the ratio 3 decreases to 10 in sample 5, the 545 nm intensity begins to decrease until in sample 6 with 3Z5, the 656 nm intensity exceeds again. Sample 5 and 6 proved the importance of Yb 3C concentration to the upconversion process. Both of the samples have high energy transfer efficiency, but most of the energy from Yb3C is transmitted to 4F9/2 level of Er3C, rather than 4F7/2 level.

5. Conclusions

Fig. 5. Log–log plot of upconversion luminescence as a function of pump power at 980 nm of sample 3. Inset is the variation of upconversion emission verses pumping power in sample 3.

In this work, the broadband and upconversion properties of Er3C/Yb3C co-doped fluorophosphate glasses were explored. The J–O parameters and other spectral parameters were calculated. Results showed that the Er3C doped fluorophosphate glass has larger U6 and Sed/(SedCSmd) value compared with other glasses with same refractive index grade, which means a larger inhomogeneous broadening and a flat and broad emission spectra, while the effective gain cross section exhibits a flat and broad gain profile from 1530 to 1585 nm compared with phosphate glass which has a peak at 1535 nm. This reflects that Er3C/Yb3C co-doped fluorophosphate glass has good broadband properties and is promising in using as broadband EDFA glass. Because of the different Yb3C:Er3C ratio 3 and Er3C concentration, the upconversion luminescence performs red to green to red variation. In glasses with 0.1 mol% ErF3, when 3 changes from 10, 20 and 30, the intensity of 656 nm/545 nm (red/green) varies from 1.5 to 1.01 to 0.78,

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Table 4 AJJ 0 , b and tR value of glass 1 and glass 3 Initial level

End level

Glass 1

4

I13/2 4 I11/2 4

I9/2

4

F9/2

4

S3/2

2

H11/2 F7/2 4 F5/2 2 H9/2 4

4

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

Glass 3

AJJ 0 ðs Þ

b

tR (ms)

AJJ 0 ðsK1 Þ

b

tR (ms)

125.2 117.7 24.4 108.9 35.0 1.94 1217.3 62.2 54.5 2.3 980.3 400.8 32.7 51.4 4973 2423.2 1102.3 1114.5 1314 411.3 19.0 28.7

1 0.83 0.17 0.75 0.24 0.01 0.91 0.05 0.04 0.00 0.67 0.27 0.02 0.04 – – – 0.39 0.46 0.14 0.00 0.01

7.98 7.04

132.2 117.8 25.6 161.2 39.6 2.1 1617 83.1 54.8 1.8 1015 410.6 32.7 58.1 4649.5 2799.7 1143.4 1352.8 1405.7 436.3 15.3 32.6

1 0.82 0.18 0.79 0.20 0.01 0.92 0.05 0.03 0.00 0.67 0.27 0.02 0.04 – – – 0.42 0.45 0.13 0.00 0.00

7.56 7.0

K1

indicating the green light becoming stronger, while in the 0.2 mol% ErF3 samples, with 3 decreasing from 15 to 10 and 5, the 656 nm/545 nm ratio changes from 0.74 to 0.96 and 1.27, respectively, showing that higher Er3C concentration and 3 value induce higher green light emission probability in this system. Result also shows that with fixed Er3C concentration, higher Yb3C concentration will result in higher energy transfer efficiency from Yb3C to Er3C. Glass with 3 mol% YbF3 and 0.1 mol% ErF3 performs the strongest fluorescence intensity and upconversion luminescence. The upconversion mechanisms of sample 3 which has the strongest green emission is, 656 nm exhibits a stepwise two photon absorption process while that of 525, 545 nm are energy transfer dominated upconversion process.

Acknowledgements This work is financially supported by the Chinese National Natural Science Foundation (60307004) and (50402007).

6.86

0.75

0.68

– – – 0.35

4.93

0.57

0.66

– – – 0.31

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