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Evaluation of spectroscopic properties of Yb3+ in tetraphosphate glass
Journal of Non-Crystalline Solids 292 (2001) 108±114
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Evaluation of spectroscopic properties of Yb3 in tetraphosphate glass Long Zhang a,*, Hefang Hu b a
Institut f ur Physikalische Chemie, Westfalische Wilhelms-Universitat Munster, Schlossplatz 4/7, D-48149 Munster, Germany b Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, People's Republic of China Received 28 May 2001; received in revised form 3 July 2001
Abstract The tetraphosphate glasses with various Yb3 concentrations were prepared. The absorption and ¯uorescence emission spectra were measured. The spectroscopic properties and laser performance parameters of Yb3 in tetraphosphate glasses were calculated. The result shows that Yb3 -doped tetraphosphate glass exhibits excellent spectroscopic properties. The disagreement between the reciprocity method (RE) and Fuchtbauer±Ladenburg formula in calculations of Yb3 spectroscopic properties was discussed using the radiation trapping eect. The eects of OH contents on emission properties of Yb3 were investigated. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.55; 42.70; 33.50; 42.55
1. Introduction Recently, Yb-doped materials have attracted great interest as potential diode-pumped laser gain media [1±6]. Since there are only two manifolds in the Yb3 energy level scheme, the 2 F7=2 ground state and the 2 F5=2 excited state, it is commonly believed that concentration quenching and multiphonon relaxation should not aect the lasing or the excitation wavelength [1]. The absorption band is located around 970 nm with a large cross-section, which enables ecient pumping by high power III±V diode lasers now available. The Yb3 ion is of interest in lasers for next-generation nu-
* Corresponding author. Tel.: +49-251 832 9184; fax: +49-251 832 9159. E-mail address: [email protected] (L. Zhang).
clear fusion [2] and as a sensitizer for energy transfer for infrared-to-visible upconversion and infrared lasers. The tetraphosphate LiLaP4 O12 host has received some attentions for the miniature solid lasers because of the high rare earth ion dopant, low concentration quenching, high gain, and low threshold [7]. Recently, Tokai University reported the single-frequency output of LiNdP4 O12 crystal microchip laser [8]. Quite recently, we systematically investigated tetraphosphate glasses doped with Er3 , Yb3 =Er3 , Yb3 , and Nd3 ions. Experimental results showed that tetraphosphate glasses will be a hopeful host for optical component [9±11]. In this paper, the spectroscopic and laser performance parameters of Yb3 in tetraphosphate glass were calculated. The eect of OH on IR emission of Yb3 was analyzed.
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 8 4 - 5
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
2. Experimental procedure 2.1. Sample preparation A series of glasses with LiYbx La 1 x P4 O12 formula have been synthesized with 99.99% purity oxides or carbonate. Each raw material was weighted to an accuracy of at least 0.1% of the raw material weight. 50 g batches were fully mixed and moved into platinum crucible and melted in the air for 1 h at 1200±1300 °C. After glass liquid was bubbled with oxygen to remove OH , the glass melts were cast into a heated aluminum mold, and annealed for 1 h near the transition temperature. Samples for optical and spectroscopic properties measurements were cut and polished by the size of 20 15 3 mm3 except for the samples for IR measurement cut by the size of 20 15 1 mm3 . 2.2. Spectroscopic property measurements The absorption spectra were measured at room temperature using a double-beam spectrophotometer within the wavelength region of 800±1200 nm. The errors in these measurements are estimated to be <10%. The emission spectra were obtained by exciting the samples with InGaAs laser diode (LD) at 970 nm. The light was chopped at 80 Hz and focused 20 15 mm2 face of samples. The emission from the sample was focused onto a monochromator at room temperature and detected by a Ge detector. The signal was intensi®ed with a lock-in ampli®er and processed by a computer. The relative errors in these emission measurements are estimated to be <10%. Reabsorption of the ¯uorescence emission was minimized by focusing the excitation beam immediately below the surface of the samples so that the emitted light traveled only a short distance inside the sample. The ¯uorescence lifetime were measured by exciting the samples with the same LD as above and detected by a S-1 photomultiplier tube. The ¯uorescence decay curves were recorded and averaged with a computer-controlled transient digitizer. To reduce the eect of the radiation trapping due to the self-absorption, the samples used in the lifetime measurements were cut and
109
polished 20 15 1 mm3 . The errors in these measurements are estimated to be <15%. IR transmission spectra were measured with an infrared spectrophotometer in the frequency range 400±4000 cm 1 . 2.3. Sample evaluation 2.3.1. Spectral analysis The absorption cross-section rabs , integrated absorption cross-section Rabs , the spontaneous emission probability Arad were calculated by the following equations [12,13]: 2:303 log I0 =I ; NL Z rabs k dk;
rabs
1
Rabs
2
2
Arad
32pcn kp Rabs ; 3k4p
3
where N is Yb3 ion concentration (ion/cm3 ), L is the sample thickness, kp is the wavelength of the absorption peak, and n kp is the refractive index at kp . The index of refraction was measured at 486.1, 589.3, and 656.3 nm, with a precision Vprism refractometer (made in China) using H2 and Na lamps as spectral source. The Cauchy's equation, n k A B=k2 , was used to determine the refractive index at kp . The emission cross-section, remi , of Yb3 was determined from the absorption cross-section, rabs , using the method of reciprocity [3,12], i.e., Zl remi k rabs k exp Zu
Ezl
hck kT
1
;
4
where Zl , Zu represent the partition functions of the lower and upper levels, respectively; K is the Boltzmann's constant; Ezl is the zero-line energy, which is de®ned as the energy separation between the lowest components of the upper and lower states. Because the zero-line energy Ezl is associated with the strong peak in the absorption spectrum of a Yb-doped glass, it is very easy to determine the value of Ezl . Since the coordination of Yb3 is nearly the same in various glass hosts,
110
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
the values of Zl =Zu do not vary greatly; thus we choose the limit value at room temperature, 4/3, i.e. the degeneracy weighting of the two states [3]. While we have chosen the reciprocity method of Eq. (4) to obtain the emission cross-sections, we will use the Fuchtbauer±Ladenburg equation to calculate them as a method of checking the reasonableness of our results. The form of this equation is [12] raemi k
k4 Arad q k; 8pcn2
5
where q k is the normalized line shape function of 3 the 2 F5=2 ! 2 F7=2 transition of R Yb , which can be represented as q k I k= I k dk; and Arad is the spontaneous emission probability, which can be obtained from Eq. (3). 2.3.2. Laser performance parameters The complete assessment of Yb3 -doped laser glasses involves several important parameters, bmin ; Isat and Imin , which impact laser performance. The parameter bmin , which addresses the resonant absorption loss at laser wavelength klaser is de®ned as the minimum fraction of Yb3 ions that must be excited to balance the gain exactly with the ground-state absorption at klaser [14]. The quantity of bmin can be expressed simply by [3,14] rabs klaser rabs klaser remi klaser 1 1 Zl Ezl hcklaser exp ; 1 Zu kT
bmin
The Imin parameter is the minimum absorbed pump intensity, which is required for threshold to be reached if there is no loss due to absorption and scattering at the laser wavelength klaser . Imin takes into account both the absorption and emission properties and is calculated by the following expression [3]: Imin bmin Isat
hc kpump rabs kpump sf 1 1 Zl Ezl hcklaser 1 exp : Zu kT
8
3. Results Utilizing the above Eqs. (1)±(5) we calculated the spectroscopic properties of Yb3 in the given glasses based on the measured absorption and emission spectra. Figs. 1 and 2 illustrated the absorption and emission spectra of LiYb0:1 La0:9 P4 O12 glass (PLY) and the corresponding absorption and emission cross-section. Table 1 listed the spectroscopic properties of PLY laser glass. For comparison with PLY glass, we also listed in Table 1 the spectroscopic properties of other laser glasses (FP, PNK, PN, ADY, LY, QX) reported in recently published papers [3±6]. It can
6
here we choose the emission wavelength of the subpeak as the laser wavelength klaser . The parameter Isat , which characterizes the pumping dynamics, is the pump saturation intensity. The ease with the ground-state depletion may be accomplished depends on the absorption crosssection at laser pump wavelength, kpump , and the ¯uorescence lifetime, sf , of Yb3 . The form of the relationship is [3] Isat
hc ; kpump rabs kpump sf
where hc=kpump is the pumping energy.
7 Fig. 1. The absorption LiYb0:1 La0:9 P4 O12 glass.
and
emission
spectra
of
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
Emission Intensity/arb. unit
1200
111
0min 15min
1000
60min 800 600 400 200 0 800
900
1000
1100
Wavelength/nm
Fig. 2. The absorption and emission cross-section of LiYb0:1 La0:9 P4 O12 glass.
be seen that PLY glass has large absorption and emission cross-section. We also calculated the laser performance parameters of PLY glass studied in the present work using the Eqs. (5)±(8). In order to compare with PLY glass, the parameters of those laser glasses (FP, PNK, PN, ADY, LY, QX) are also listed in Table 1. It can be seen that the present PLY glass has excellent laser performance parameters. The bmin and Imin are smaller than those of FP, LY and QX glasses, and Isat is smaller than that of FP and PNK glasses. OH groups are known to have great in¯uence on the IR emission [15]. We measured Yb3 ¯uorescence intensities of LiYbx La 1 x P4 O12 glasses with dierent oxygen-bubbling time. Experimental results indicated that, as shown in Fig. 3, the ¯uorescence intensities increased more than four times for 1:82 1021 =cm3 Yb3 -doped tetraphosphate glass through a 60 min oxygen-bubbling
Fig. 3. IR ¯uorescence spectra of 1:82 1021 =cm3 Yb3 -doped tetraphosphate glass with dierent oxygen-bubbling time.
process in melt. The IR transmission spectra are shown in Fig. 4 for these samples. The coecient of the OH vibration band at 3000 cm 1 , aOH , is used as a measure of the OH content [15]. From Fig. 4, we can see that the OH content in the sample without bubbling oxygen is much larger than in the sample bubbled for 60 min. For understanding the eect of OH on ¯uorescence decay rate, we measured the ¯uorescence lifetimes sf ) of dierent Yb3 concentration doped tetraphosphate glass with various OH contents. The results were shown in Fig. 5. It can be seen that the measured decay rate, 1=sf , increases linearly with aOH . When increasing Yb3 concentration with constant of aOH , the decay rate increases. It is because the rate of energy transfer to OH increases with Yb3 concentration owing to the decrease of distance between Yb3 and OH .
Table 1 The spectroscopic properties and laser performance parameters of Yb3 ion in some laser glass Glasses
System
rabs kp pm2
remp pm2
sf ms
bmin
Isat kw=cm2
Imin kw=cm2
PLY FP[4] PNK[3] PN[3] ADY[6] LY[6] QX[5]
Phosphate Fluorophosphate Phosphate Phosphate Aluminate Silicate Phosphate
0:99 0:05 0.43 0.68 1.00 0.70 0.55 0.50
1:32 0:07 0.50 1.09 1.35 1.03 0.80 0.70
1:21 0:18 1.20 1.70 1.09 1.58 1.68 2.00
0.1111 0.1597 0.0935 0.0596 0.0984 0.1670 0.1946
17.05 20.91 15.55 9.90 11.38 11.68 10.79
1.89 3.34 1.45 0.59 1.12 1.95 3.34
112
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114 100 60min 0min
Transmission(%)
80
60
40 20
0 4000
3500
3000 2500 -1 Wavenumber/cm
2000
150
Fig. 4. The IR transmission spectra of glass sample having 1 mm thickness.
Fig. 5. Decay rate, 1=sf , as a function of the absorption coef®cient, aOH , of the OH vibration band at 3000 cm 1 , and as a function of the Yb3 concentration for the tetraphosphate glass.
4. Discussion 4.1. Comparison between reciprocity method (RE) and F±L equation in calculation of Yb3 spectroscopic properties From Fig. 2, it can be seen that the emission spectrum calculated using reciprocity method is approximately in agreement with that calculated using F±L equation. But there are many dierences between the two methods in the emission cross-sections of primary peak, remp , the ratio of the primary-to-secondary peak emission crosssection, rems =remp , and eective bandwidth, Dkeff .
Utilizing the Eqs. (4) and (5), we calculated the emission cross-section of Yb3 in tetraphosphate glasses based on measured absorption and emission spectra. The results are listed in Table 2. It can be seen that the emission cross-sections, remp or rems , decrease with increasing Yb3 concentration. We suppose that such concentration eect is relative with the Yb3 ions cluster or ions pair [10]. If remp RE represents the value of emission cross-section calculated by reciprocity method, and remp FL represents the value calculated by F±L equation from Table 2, it can be seen: (1) For remp , remp RE is larger than remp FL for various Yb3 concentrations. Furthermore, remp RE = remp FL is approximately equal to Dkeff FL = Dkeff RE , i.e. the reciprocity of the ratio of their eective bandwidths. (2) For rems =remp , the values calculated by reciprocity method, rems =remp RE , are smaller than those by FL, rems =remp FL . (3) rems =remp FL increases with Yb3 concentration; on the other hand, rems =remp RE is approximately unchanging with Yb3 concentration. We think these facts can be attributed to the eect of radiation trapping. Radiation trapping arises from the spectral overlap of the emission and absorption bands of Yb3 and increases with sample size, refractive index, and spectral overlap between ¯uorescence and absorption [16]. As shown in Fig. 1, there is a most serious spectral trap in primary emission peak. The strong ¯uorescence trapping will decrease the measured value of remp , i.e. remp FL , and increase the Dkeff FL . With the increase of the Yb3 concentration, the ¯uorescence trapping increases and leads to the rems =remp FL increase. The eect of radiation trapping can be described by the parameter rems =remp FL rems = remp RE = rems =remp RE . The larger value of this parameter re¯ects more serious trapping. From the parameters rems =remp FL rems =remp RE = rems = remp RE listed in Table 2, it can be found that the eect of radiation trapping of Yb-doped glasses is serious, especially in high Yb3 concentration. Because the reciprocity method, which is calculated from absorption spectrum, avoids the eect of ¯uorescence trapping, it is a simple and ecient method in calculating emission cross-section of Yb3 -doped (especially highly doped) glasses.
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
113
Table 2 Comparison between RE and F±L equation in calculation of Yb3 spectroscopic properties of LiYbx La 1 x P4 O12 glass rems =remp FL rems =remp RE rems pm2 rems =remp Dkeff (nm) remp pm2 X N 1021 =cm3 rems =remp RE RE FL RE FL RE FL RE FL 0.10 0.20 0.30 0.45 0.60 0.75
0.41 0.81 1.22 1.82 2.42 3.01
1.32 1.25 1.17 1.04 0.87 0.69
1.09 0.98 0.84 0.70 0.57 0.42
0.72 0.69 0.64 0.59 0.51 0.41
0.75 0.76 0.72 0.65 0.60 0.55
4.2. The eect of OH on spectroscopic properties of LiYbx La 1 x P4 O12 glass It is well known that OH impurities in glass are eective quenchers of the IR radiation. OH vibration frequencies occur in the range 2700±3700 cm 1 , which is higher than other vibration frequencies in glass [17]. As a result, only three or four phonons are required for non-radiative deexcitation of Yb3 IR transition. The quenching by OH can be schematically illustrated in Fig. 6. Considering the rate of energy transfer to OH groups, the total decay rate, 1=sf , of Yb3 IR emission can be given by [18] 1 Arad WOH Wother ; sf
9
where Arad is the sum of all radiative decay of 2 F5=2 level, WOH is the rate of energy transfer to OH groups and Wother is the decay rate caused by other
0.54 0.55 0.54 0.57 0.59 0.59
0.69 0.78 0.87 0.93 1.11 1.31
48 48 49 51 52 54
53 56 61 72 79 87
0.28 0.42 0.61 0.63 0.88 1.22
factors such as multiphonon relaxation, cooperative upconversion, etc. The energy transfer rate to OH groups WOH , which is proportional to the acceptor and donor [19], can be expressed by WOH kOH NYb aOH ;
10
where kOH is a constant, NYb is the Yb concentration (donor concentration), aOH is the measure of the OH content (acceptor concentration). Because of the linear correlation between the total decay rate 1=sf and aOH as shown in Fig. 5, the decay rate can be described as follows: 1 1 mOH aOH ; sf s0
11
where mOH is the slope of the ®tting curve and 1=s0 is the decay rate in the absence of OH groups. The comparison of the Eq. (11) with Eq. (9) leads to WOH mOH aOH ;
12
1=s0 Arad Wother :
13
The comparison of the experimentally found relation Eq. (12) with the relation Eq. (10) for WOH leads to
Fig. 6. Schematic diagram of OH quenching to IR emission of Yb3 .
mOH kOH NYb ;
14
1 1 kOH NYb aOH : sf s0
15
From these considerations, we concluded that kOH is a constant which is determined by the force of interactions between Yb3 ions and OH groups in the case of energy migration, independent of the concentrations of Yb3 and OH .
114
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
From Eq. (15) and Fig. 5, the constants, kOH , for Yb3 in tetraphosphate glasses can be calculated and is 9:1 10 19 cm4 s 1 . 5. Conclusions Aside from the excellent Yb3 solubility, the tetraphosphate glass doped with Yb3 has good spectroscopic properties. In LiYb0:1 La0:9 P4 O12 glass, the measured lifetime, absorption cross-section and excitation cross-section at their peaks, are 1.21 ms, 0:99 pm2 and 1:32 pm2 , respectively, and bmin , Isat , Imin are 0.1111, 17:05 kw=cm2 , 1:89 kw=cm2 , respectively, which is comparable to the laser glasses developed recently. The tetraphosphate glass is a promising host of miniature solid lasers. OH groups in glass have great in¯uence on Yb3 emission. The constant kOH , which represents the force of interaction between Yb3 ions and OH groups in the case of energy migration, were calculated. The result is 9:1 10 19 cm4 s 1 for Yb3 -doped tetraphosphate glass. References [1] L.D. Deloach, S.A. Payne, L.L. Chase et al., IEEE J. Quantum Electron. 39 (1993) 1179.
[2] A. Giesen, H. Hugel, A. Voss et al., Appl. Phys. B 58 (1994) 365. [3] X. Zou, H. Toratani, Phys. Rev. B 52 (1995) 15889. [4] V. Petrov, U. Griebner, D. Ehrt, W. Seeber, Opt. Lett. 22 (1997) 408. [5] B. Peng, T. Izumitani, Rev. Laser Eng. 21 (1993) 1234. [6] R. Koch, U. Griebner, H. Schonnagel, S. Jiang, Opt. Commun. 134 (1997) 175. [7] H.G. Danielmayer, Advances Lasers, vol. IV, Levine, New York, 1975. [8] K. Otsuka, R. Kawai, Y. Asskawa et al., Opt. Lett. 23 (1998) 201. [9] L. Zhang, G. Huang, H. Hu, J. Inorg. Mater. 16 (2001) 1062. [10] L. Zhang, F. Lin, H. Hu, Acta Phys. Sin. 50 (2001) 1378. [11] L. Zhang, H. Hu, C. Qi, F. Lin, J. Opt. Mater. 17 (2001) 371. [12] S.A. Payne, L.L. Chase, L.K. Smith, et al., IEEE J. Quantum Electron. 28 (1992) 2619. [13] W.F. Krupe, IEEE J. Quantum Electron. 80 (1974) 450. [14] L.D. Deloach, S.A. Payne, L.K. Smith et al., J. Opt. Soc. Am. B 11 (1994) 269. [15] H. Ebendor-Heidepriem, W. Seeber, D. Ehrt, J. NonCryst. Solids 163 (1993) 74. [16] M.J. Weber, J.E. Lynch, D.H. Blackburn et al., IEEE J. Quantum Electron. QE19 (1983) 1600. [17] E.G. Bondarenko, E.I. Galant, S.G. Lunter, Sov. J. Opt. Technol. 42 (1975) 333. [18] H. Toratani, T. Izumitani, H. Kuroda, J. Non-Cryst. Solids 52 (1982) 303. [19] S.E. Stokowski, D. Krashkevich, Mater. Res. Soc. Symp. Proc. 61 (1986) 273.
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Evaluation of spectroscopic properties of Yb3 in tetraphosphate glass Long Zhang a,*, Hefang Hu b a
Institut f ur Physikalische Chemie, Westfalische Wilhelms-Universitat Munster, Schlossplatz 4/7, D-48149 Munster, Germany b Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, People's Republic of China Received 28 May 2001; received in revised form 3 July 2001
Abstract The tetraphosphate glasses with various Yb3 concentrations were prepared. The absorption and ¯uorescence emission spectra were measured. The spectroscopic properties and laser performance parameters of Yb3 in tetraphosphate glasses were calculated. The result shows that Yb3 -doped tetraphosphate glass exhibits excellent spectroscopic properties. The disagreement between the reciprocity method (RE) and Fuchtbauer±Ladenburg formula in calculations of Yb3 spectroscopic properties was discussed using the radiation trapping eect. The eects of OH contents on emission properties of Yb3 were investigated. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.55; 42.70; 33.50; 42.55
1. Introduction Recently, Yb-doped materials have attracted great interest as potential diode-pumped laser gain media [1±6]. Since there are only two manifolds in the Yb3 energy level scheme, the 2 F7=2 ground state and the 2 F5=2 excited state, it is commonly believed that concentration quenching and multiphonon relaxation should not aect the lasing or the excitation wavelength [1]. The absorption band is located around 970 nm with a large cross-section, which enables ecient pumping by high power III±V diode lasers now available. The Yb3 ion is of interest in lasers for next-generation nu-
* Corresponding author. Tel.: +49-251 832 9184; fax: +49-251 832 9159. E-mail address: [email protected] (L. Zhang).
clear fusion [2] and as a sensitizer for energy transfer for infrared-to-visible upconversion and infrared lasers. The tetraphosphate LiLaP4 O12 host has received some attentions for the miniature solid lasers because of the high rare earth ion dopant, low concentration quenching, high gain, and low threshold [7]. Recently, Tokai University reported the single-frequency output of LiNdP4 O12 crystal microchip laser [8]. Quite recently, we systematically investigated tetraphosphate glasses doped with Er3 , Yb3 =Er3 , Yb3 , and Nd3 ions. Experimental results showed that tetraphosphate glasses will be a hopeful host for optical component [9±11]. In this paper, the spectroscopic and laser performance parameters of Yb3 in tetraphosphate glass were calculated. The eect of OH on IR emission of Yb3 was analyzed.
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 8 4 - 5
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
2. Experimental procedure 2.1. Sample preparation A series of glasses with LiYbx La 1 x P4 O12 formula have been synthesized with 99.99% purity oxides or carbonate. Each raw material was weighted to an accuracy of at least 0.1% of the raw material weight. 50 g batches were fully mixed and moved into platinum crucible and melted in the air for 1 h at 1200±1300 °C. After glass liquid was bubbled with oxygen to remove OH , the glass melts were cast into a heated aluminum mold, and annealed for 1 h near the transition temperature. Samples for optical and spectroscopic properties measurements were cut and polished by the size of 20 15 3 mm3 except for the samples for IR measurement cut by the size of 20 15 1 mm3 . 2.2. Spectroscopic property measurements The absorption spectra were measured at room temperature using a double-beam spectrophotometer within the wavelength region of 800±1200 nm. The errors in these measurements are estimated to be <10%. The emission spectra were obtained by exciting the samples with InGaAs laser diode (LD) at 970 nm. The light was chopped at 80 Hz and focused 20 15 mm2 face of samples. The emission from the sample was focused onto a monochromator at room temperature and detected by a Ge detector. The signal was intensi®ed with a lock-in ampli®er and processed by a computer. The relative errors in these emission measurements are estimated to be <10%. Reabsorption of the ¯uorescence emission was minimized by focusing the excitation beam immediately below the surface of the samples so that the emitted light traveled only a short distance inside the sample. The ¯uorescence lifetime were measured by exciting the samples with the same LD as above and detected by a S-1 photomultiplier tube. The ¯uorescence decay curves were recorded and averaged with a computer-controlled transient digitizer. To reduce the eect of the radiation trapping due to the self-absorption, the samples used in the lifetime measurements were cut and
109
polished 20 15 1 mm3 . The errors in these measurements are estimated to be <15%. IR transmission spectra were measured with an infrared spectrophotometer in the frequency range 400±4000 cm 1 . 2.3. Sample evaluation 2.3.1. Spectral analysis The absorption cross-section rabs , integrated absorption cross-section Rabs , the spontaneous emission probability Arad were calculated by the following equations [12,13]: 2:303 log I0 =I ; NL Z rabs k dk;
rabs
1
Rabs
2
2
Arad
32pcn kp Rabs ; 3k4p
3
where N is Yb3 ion concentration (ion/cm3 ), L is the sample thickness, kp is the wavelength of the absorption peak, and n kp is the refractive index at kp . The index of refraction was measured at 486.1, 589.3, and 656.3 nm, with a precision Vprism refractometer (made in China) using H2 and Na lamps as spectral source. The Cauchy's equation, n k A B=k2 , was used to determine the refractive index at kp . The emission cross-section, remi , of Yb3 was determined from the absorption cross-section, rabs , using the method of reciprocity [3,12], i.e., Zl remi k rabs k exp Zu
Ezl
hck kT
1
;
4
where Zl , Zu represent the partition functions of the lower and upper levels, respectively; K is the Boltzmann's constant; Ezl is the zero-line energy, which is de®ned as the energy separation between the lowest components of the upper and lower states. Because the zero-line energy Ezl is associated with the strong peak in the absorption spectrum of a Yb-doped glass, it is very easy to determine the value of Ezl . Since the coordination of Yb3 is nearly the same in various glass hosts,
110
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
the values of Zl =Zu do not vary greatly; thus we choose the limit value at room temperature, 4/3, i.e. the degeneracy weighting of the two states [3]. While we have chosen the reciprocity method of Eq. (4) to obtain the emission cross-sections, we will use the Fuchtbauer±Ladenburg equation to calculate them as a method of checking the reasonableness of our results. The form of this equation is [12] raemi k
k4 Arad q k; 8pcn2
5
where q k is the normalized line shape function of 3 the 2 F5=2 ! 2 F7=2 transition of R Yb , which can be represented as q k I k= I k dk; and Arad is the spontaneous emission probability, which can be obtained from Eq. (3). 2.3.2. Laser performance parameters The complete assessment of Yb3 -doped laser glasses involves several important parameters, bmin ; Isat and Imin , which impact laser performance. The parameter bmin , which addresses the resonant absorption loss at laser wavelength klaser is de®ned as the minimum fraction of Yb3 ions that must be excited to balance the gain exactly with the ground-state absorption at klaser [14]. The quantity of bmin can be expressed simply by [3,14] rabs klaser rabs klaser remi klaser 1 1 Zl Ezl hcklaser exp ; 1 Zu kT
bmin
The Imin parameter is the minimum absorbed pump intensity, which is required for threshold to be reached if there is no loss due to absorption and scattering at the laser wavelength klaser . Imin takes into account both the absorption and emission properties and is calculated by the following expression [3]: Imin bmin Isat
hc kpump rabs kpump sf 1 1 Zl Ezl hcklaser 1 exp : Zu kT
8
3. Results Utilizing the above Eqs. (1)±(5) we calculated the spectroscopic properties of Yb3 in the given glasses based on the measured absorption and emission spectra. Figs. 1 and 2 illustrated the absorption and emission spectra of LiYb0:1 La0:9 P4 O12 glass (PLY) and the corresponding absorption and emission cross-section. Table 1 listed the spectroscopic properties of PLY laser glass. For comparison with PLY glass, we also listed in Table 1 the spectroscopic properties of other laser glasses (FP, PNK, PN, ADY, LY, QX) reported in recently published papers [3±6]. It can
6
here we choose the emission wavelength of the subpeak as the laser wavelength klaser . The parameter Isat , which characterizes the pumping dynamics, is the pump saturation intensity. The ease with the ground-state depletion may be accomplished depends on the absorption crosssection at laser pump wavelength, kpump , and the ¯uorescence lifetime, sf , of Yb3 . The form of the relationship is [3] Isat
hc ; kpump rabs kpump sf
where hc=kpump is the pumping energy.
7 Fig. 1. The absorption LiYb0:1 La0:9 P4 O12 glass.
and
emission
spectra
of
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
Emission Intensity/arb. unit
1200
111
0min 15min
1000
60min 800 600 400 200 0 800
900
1000
1100
Wavelength/nm
Fig. 2. The absorption and emission cross-section of LiYb0:1 La0:9 P4 O12 glass.
be seen that PLY glass has large absorption and emission cross-section. We also calculated the laser performance parameters of PLY glass studied in the present work using the Eqs. (5)±(8). In order to compare with PLY glass, the parameters of those laser glasses (FP, PNK, PN, ADY, LY, QX) are also listed in Table 1. It can be seen that the present PLY glass has excellent laser performance parameters. The bmin and Imin are smaller than those of FP, LY and QX glasses, and Isat is smaller than that of FP and PNK glasses. OH groups are known to have great in¯uence on the IR emission [15]. We measured Yb3 ¯uorescence intensities of LiYbx La 1 x P4 O12 glasses with dierent oxygen-bubbling time. Experimental results indicated that, as shown in Fig. 3, the ¯uorescence intensities increased more than four times for 1:82 1021 =cm3 Yb3 -doped tetraphosphate glass through a 60 min oxygen-bubbling
Fig. 3. IR ¯uorescence spectra of 1:82 1021 =cm3 Yb3 -doped tetraphosphate glass with dierent oxygen-bubbling time.
process in melt. The IR transmission spectra are shown in Fig. 4 for these samples. The coecient of the OH vibration band at 3000 cm 1 , aOH , is used as a measure of the OH content [15]. From Fig. 4, we can see that the OH content in the sample without bubbling oxygen is much larger than in the sample bubbled for 60 min. For understanding the eect of OH on ¯uorescence decay rate, we measured the ¯uorescence lifetimes sf ) of dierent Yb3 concentration doped tetraphosphate glass with various OH contents. The results were shown in Fig. 5. It can be seen that the measured decay rate, 1=sf , increases linearly with aOH . When increasing Yb3 concentration with constant of aOH , the decay rate increases. It is because the rate of energy transfer to OH increases with Yb3 concentration owing to the decrease of distance between Yb3 and OH .
Table 1 The spectroscopic properties and laser performance parameters of Yb3 ion in some laser glass Glasses
System
rabs kp pm2
remp pm2
sf ms
bmin
Isat kw=cm2
Imin kw=cm2
PLY FP[4] PNK[3] PN[3] ADY[6] LY[6] QX[5]
Phosphate Fluorophosphate Phosphate Phosphate Aluminate Silicate Phosphate
0:99 0:05 0.43 0.68 1.00 0.70 0.55 0.50
1:32 0:07 0.50 1.09 1.35 1.03 0.80 0.70
1:21 0:18 1.20 1.70 1.09 1.58 1.68 2.00
0.1111 0.1597 0.0935 0.0596 0.0984 0.1670 0.1946
17.05 20.91 15.55 9.90 11.38 11.68 10.79
1.89 3.34 1.45 0.59 1.12 1.95 3.34
112
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114 100 60min 0min
Transmission(%)
80
60
40 20
0 4000
3500
3000 2500 -1 Wavenumber/cm
2000
150
Fig. 4. The IR transmission spectra of glass sample having 1 mm thickness.
Fig. 5. Decay rate, 1=sf , as a function of the absorption coef®cient, aOH , of the OH vibration band at 3000 cm 1 , and as a function of the Yb3 concentration for the tetraphosphate glass.
4. Discussion 4.1. Comparison between reciprocity method (RE) and F±L equation in calculation of Yb3 spectroscopic properties From Fig. 2, it can be seen that the emission spectrum calculated using reciprocity method is approximately in agreement with that calculated using F±L equation. But there are many dierences between the two methods in the emission cross-sections of primary peak, remp , the ratio of the primary-to-secondary peak emission crosssection, rems =remp , and eective bandwidth, Dkeff .
Utilizing the Eqs. (4) and (5), we calculated the emission cross-section of Yb3 in tetraphosphate glasses based on measured absorption and emission spectra. The results are listed in Table 2. It can be seen that the emission cross-sections, remp or rems , decrease with increasing Yb3 concentration. We suppose that such concentration eect is relative with the Yb3 ions cluster or ions pair [10]. If remp RE represents the value of emission cross-section calculated by reciprocity method, and remp FL represents the value calculated by F±L equation from Table 2, it can be seen: (1) For remp , remp RE is larger than remp FL for various Yb3 concentrations. Furthermore, remp RE = remp FL is approximately equal to Dkeff FL = Dkeff RE , i.e. the reciprocity of the ratio of their eective bandwidths. (2) For rems =remp , the values calculated by reciprocity method, rems =remp RE , are smaller than those by FL, rems =remp FL . (3) rems =remp FL increases with Yb3 concentration; on the other hand, rems =remp RE is approximately unchanging with Yb3 concentration. We think these facts can be attributed to the eect of radiation trapping. Radiation trapping arises from the spectral overlap of the emission and absorption bands of Yb3 and increases with sample size, refractive index, and spectral overlap between ¯uorescence and absorption [16]. As shown in Fig. 1, there is a most serious spectral trap in primary emission peak. The strong ¯uorescence trapping will decrease the measured value of remp , i.e. remp FL , and increase the Dkeff FL . With the increase of the Yb3 concentration, the ¯uorescence trapping increases and leads to the rems =remp FL increase. The eect of radiation trapping can be described by the parameter rems =remp FL rems = remp RE = rems =remp RE . The larger value of this parameter re¯ects more serious trapping. From the parameters rems =remp FL rems =remp RE = rems = remp RE listed in Table 2, it can be found that the eect of radiation trapping of Yb-doped glasses is serious, especially in high Yb3 concentration. Because the reciprocity method, which is calculated from absorption spectrum, avoids the eect of ¯uorescence trapping, it is a simple and ecient method in calculating emission cross-section of Yb3 -doped (especially highly doped) glasses.
L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
113
Table 2 Comparison between RE and F±L equation in calculation of Yb3 spectroscopic properties of LiYbx La 1 x P4 O12 glass rems =remp FL rems =remp RE rems pm2 rems =remp Dkeff (nm) remp pm2 X N 1021 =cm3 rems =remp RE RE FL RE FL RE FL RE FL 0.10 0.20 0.30 0.45 0.60 0.75
0.41 0.81 1.22 1.82 2.42 3.01
1.32 1.25 1.17 1.04 0.87 0.69
1.09 0.98 0.84 0.70 0.57 0.42
0.72 0.69 0.64 0.59 0.51 0.41
0.75 0.76 0.72 0.65 0.60 0.55
4.2. The eect of OH on spectroscopic properties of LiYbx La 1 x P4 O12 glass It is well known that OH impurities in glass are eective quenchers of the IR radiation. OH vibration frequencies occur in the range 2700±3700 cm 1 , which is higher than other vibration frequencies in glass [17]. As a result, only three or four phonons are required for non-radiative deexcitation of Yb3 IR transition. The quenching by OH can be schematically illustrated in Fig. 6. Considering the rate of energy transfer to OH groups, the total decay rate, 1=sf , of Yb3 IR emission can be given by [18] 1 Arad WOH Wother ; sf
9
where Arad is the sum of all radiative decay of 2 F5=2 level, WOH is the rate of energy transfer to OH groups and Wother is the decay rate caused by other
0.54 0.55 0.54 0.57 0.59 0.59
0.69 0.78 0.87 0.93 1.11 1.31
48 48 49 51 52 54
53 56 61 72 79 87
0.28 0.42 0.61 0.63 0.88 1.22
factors such as multiphonon relaxation, cooperative upconversion, etc. The energy transfer rate to OH groups WOH , which is proportional to the acceptor and donor [19], can be expressed by WOH kOH NYb aOH ;
10
where kOH is a constant, NYb is the Yb concentration (donor concentration), aOH is the measure of the OH content (acceptor concentration). Because of the linear correlation between the total decay rate 1=sf and aOH as shown in Fig. 5, the decay rate can be described as follows: 1 1 mOH aOH ; sf s0
11
where mOH is the slope of the ®tting curve and 1=s0 is the decay rate in the absence of OH groups. The comparison of the Eq. (11) with Eq. (9) leads to WOH mOH aOH ;
12
1=s0 Arad Wother :
13
The comparison of the experimentally found relation Eq. (12) with the relation Eq. (10) for WOH leads to
Fig. 6. Schematic diagram of OH quenching to IR emission of Yb3 .
mOH kOH NYb ;
14
1 1 kOH NYb aOH : sf s0
15
From these considerations, we concluded that kOH is a constant which is determined by the force of interactions between Yb3 ions and OH groups in the case of energy migration, independent of the concentrations of Yb3 and OH .
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L. Zhang, H. Hu / Journal of Non-Crystalline Solids 292 (2001) 108±114
From Eq. (15) and Fig. 5, the constants, kOH , for Yb3 in tetraphosphate glasses can be calculated and is 9:1 10 19 cm4 s 1 . 5. Conclusions Aside from the excellent Yb3 solubility, the tetraphosphate glass doped with Yb3 has good spectroscopic properties. In LiYb0:1 La0:9 P4 O12 glass, the measured lifetime, absorption cross-section and excitation cross-section at their peaks, are 1.21 ms, 0:99 pm2 and 1:32 pm2 , respectively, and bmin , Isat , Imin are 0.1111, 17:05 kw=cm2 , 1:89 kw=cm2 , respectively, which is comparable to the laser glasses developed recently. The tetraphosphate glass is a promising host of miniature solid lasers. OH groups in glass have great in¯uence on Yb3 emission. The constant kOH , which represents the force of interaction between Yb3 ions and OH groups in the case of energy migration, were calculated. The result is 9:1 10 19 cm4 s 1 for Yb3 -doped tetraphosphate glass. References [1] L.D. Deloach, S.A. Payne, L.L. Chase et al., IEEE J. Quantum Electron. 39 (1993) 1179.
[2] A. Giesen, H. Hugel, A. Voss et al., Appl. Phys. B 58 (1994) 365. [3] X. Zou, H. Toratani, Phys. Rev. B 52 (1995) 15889. [4] V. Petrov, U. Griebner, D. Ehrt, W. Seeber, Opt. Lett. 22 (1997) 408. [5] B. Peng, T. Izumitani, Rev. Laser Eng. 21 (1993) 1234. [6] R. Koch, U. Griebner, H. Schonnagel, S. Jiang, Opt. Commun. 134 (1997) 175. [7] H.G. Danielmayer, Advances Lasers, vol. IV, Levine, New York, 1975. [8] K. Otsuka, R. Kawai, Y. Asskawa et al., Opt. Lett. 23 (1998) 201. [9] L. Zhang, G. Huang, H. Hu, J. Inorg. Mater. 16 (2001) 1062. [10] L. Zhang, F. Lin, H. Hu, Acta Phys. Sin. 50 (2001) 1378. [11] L. Zhang, H. Hu, C. Qi, F. Lin, J. Opt. Mater. 17 (2001) 371. [12] S.A. Payne, L.L. Chase, L.K. Smith, et al., IEEE J. Quantum Electron. 28 (1992) 2619. [13] W.F. Krupe, IEEE J. Quantum Electron. 80 (1974) 450. [14] L.D. Deloach, S.A. Payne, L.K. Smith et al., J. Opt. Soc. Am. B 11 (1994) 269. [15] H. Ebendor-Heidepriem, W. Seeber, D. Ehrt, J. NonCryst. Solids 163 (1993) 74. [16] M.J. Weber, J.E. Lynch, D.H. Blackburn et al., IEEE J. Quantum Electron. QE19 (1983) 1600. [17] E.G. Bondarenko, E.I. Galant, S.G. Lunter, Sov. J. Opt. Technol. 42 (1975) 333. [18] H. Toratani, T. Izumitani, H. Kuroda, J. Non-Cryst. Solids 52 (1982) 303. [19] S.E. Stokowski, D. Krashkevich, Mater. Res. Soc. Symp. Proc. 61 (1986) 273.