Multiphonon relaxations in chloro-fluoride glasses

J O U R N A L OF

ELSEVIER

I]IH

Journal of Non-Crystalline Solids 184 (1995) 119-123

Multiphonon relaxations in chloro-fluoride glasses J.L. Adam *, M. Matecki, J. Lucas Laboratoire des Verres et C&amiques, URA CNRS 1496, Campus de Beaulieu, Universit~ de Rennes I, Avenue du Gdn~ral Leclerc, 35042 Rennes cddex, France

Abstract Multiphonon relaxation rates are investigated for an Er3+-doped cadmium chloro-fluoride glass. They are found to obey the empirical energy-gap law, and compare favourably with those reported for other halide or oxide glasses. Cd-F vibrations are found to be responsible for the relaxation processes.

I. Introduction Since the demonstration of optical amplification at 1.3 Ixm with pr3+-doped fluorozirconate glass fibers [1], intensive research has been carried out worldwide on low-phonon-energy glasses such as barium-indium-gallium (BIG) fluoride glasses [2,3], chloro-fluoride glasses and chalcogenide glasses [4,5]. For those rare-earth optical transitions which are phonon-sensitive, the quantum efficiencies are increased with low-phonon-energy glasses by comparison with silica for instance. The use of such hosts would result in more favourable operating conditions for praseodymium optical amplifiers, provided that reasonably low-loss fibers are available. The present work deals with glasses based on heavy metal chlorides and fluorides whose transparency ranges from the UV to 10 txm in the infrared, that is 4 Ixm and 2 p~m beyond the multiphonon edge of ZBLAN ( Z r - B a - L a - A 1 - N a fluoride glass) and BIG, respectively. We have deter-

* Corresponding author. Tel: + 3 3 99 28 62 62. Telefax: + 3 3 99 28 16 00. E-mail: [email protected].

mined the optical properties of Er 3÷ ions in that host, both experimentally and theoretically. Er 3÷ was chosen because of its phonon-sensitive visible emissions. From experimental data, the multiphonon relaxation rates are found and compared with those of pure fluoride glasses.

2. Experimental procedure The base glass used in this work was of the following molar composition: 33 C d C 1 2 , 17 CdF2, 34 NaF, 13 BaF2, 3 KF. It will be referred to as CNBK in the text. The sample was prepared from Baker reagent grade CdCI 2 and from Heraeus 99.99% purity CdF 2. KF was a Suprapur product from Merck and both NaF and BaF 2 were Fluortran fluorides from BDH. Erbium ions were incorporated as ErF 3 obtained by fluorination in a platinum crucible of 99.99% purity E r 2 0 3 from Rh6ne-Poulenc. A batch containing 0.15 mol% ErF 3 was prepared, ErF 3 being substituted for CdF2. It should be noted that glasses containing > 0.15 mol% are highly unstable towards vitrification. The sample was prepared by melting the batch in a platinum crucible under a dry argon

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 6 0 4 - 0

120

J.L. Adam et al. /Journal of Non-Crystalline Solids 180 (1995) 119-123

atmosphere, the liquid being poured into a cold brass mould after refining. Because of its hygroscopicity, the glass had to be dry-polished and was stored under a dry argon atmosphere. However, measurements could be carried out for several hours in normal conditions with no evidence of surface hydrolysis. Details on the physical properties of CNBK glass are given in Refs. [4,6]. For t h e 483/2 a n d 4F9/2 levels, lifetimes were measured with excitation light from a 75 W xenon lamp. The excitation wavelength was selected through a monochromator (Jobin-Yvon model H25). The light emitted at right angles from the sample was focused on the slit of a monochromator (JobinYvon HR1000) and then detected by a photomultiplier tube (Hamamatsu R9288). The signal was sent to a boxcar-averager (SRS model 280) triggered by a variable-speed chopper in the excitation beam. Experimental errors on lifetimes are estimated to be _+4%. Low-temperature optical measurements were carried out by means of a cryogenerator (LeyboldHeraeus model ROK 10-300) controlled by a temperature regulator (Variotemp HR1). Above room temperature, the sample was mounted on a brass holder, the temperature of which was controlled by a resistance heater. In practice, temperatures ranging from 10 to 370 K could be achieved. For t h e 2H9/2 level, the lifetime of which is too short to be obtained with the equipment described above, Er 3+ ions were excited by means of a dye laser (Lumonics model Hyperdye 300) pumped by an excimer laser (Lumonics model 500). Decay times were measured by a multichannel scaler (SRS 430) triggered by the excimer laser at a frequency of 10 Hz.

3. Results Briefly, the method consists in calculating the pure radiative rates of a given energy level, on the one hand, and in measuring the total emission rate on the other hand. In the absence of energy transfer between active ions, the multiphonon emission rate is found by subtracting the former from the latter through the equation WMp = (l//'Tmeas) -- (1/'rraO) ,

(1)

Table 1 Measured and calculated oscillator strengths of Er 3÷ ions in CNBK chloro-fluoride glasses; transitions are from the 4115/2 ground state to the levels indicated Level 4113/2 4111/2 419/2 4F9/2 as: 3/2 H1~/2 4F7/2 4F5/2 --4F3/2 2H9/2 4G11/2

Wavelength fmeas.

fcalc.

(_+0.5nm)

( + 5 : < 1 0 -8 ) (10 -8)

1520

152 (e.d.) 51 (m.d.) 61 25 330 69 468 253 100 868

978.6 798.5 654.2 545.0 521.0 487.6

449.7 406.7 378.2

153 (e.d.) 61 54 322 56 488 257 107 88 857

Deviation (10 - s ) - 1 0 -- 29 8 13 -20 -4 -7 11

-02 = 1.80, D 4 = 2.62, .06 = 1.35 (10 2o cm2). 6rms = 17 (10-8). e.d., electric dipole; m.d., magnetic dipole.

where the radiative lifetime, "Trad,is found by applying the Judd-Ofelt theory [7,8]. Thus, the experimental oscillator strengths, fexp, were first determined from the Er 3+ absorption spectrum, for every electronic transition arising from the 4115/2 ground state. Then, a least-squares fitting of experimental and calculated electric dipole oscillator strengths led to the determination of the Judd-Ofelt parameters Ot (t = 2, 4, 6). In the calculation, a constant index of refraction of 1.65 was used. The results are shown in Table 1. For t h e 4115/2 ~ 4113/2 transition, the magnetic dipole contribution was first determined before being subtracted from the measured oscillator strength. The remainder was used as the measured electric dipole oscillator strength in the calculation. For t h e 4F5/2 a n d 4F3/2 levels, which absorption bands strongly overlap, a combination of U ¢t) matrix elements was used [9]. Also it should be noted that t h e 4115/2--> 2 H 9 / 2 absorption was not well defined and was consequently not included in the fitting procedure. The quality of the fit is given by the root-mean-square deviation, 6rms, equal to 17 X 10 -8. This is comparable with deviations obtained with other Er3+-doped halide glasses [10,11]. The phenomenological Judd-Ofelt parameters, which are characteristic of a given ion-host combination, are found to be: ~"~2 = 1.80, ~'~4 = 2.62 and ~'~6 = 1.35 in 10 -20 cm 2 units. They were utilized to calculate the radiative lifetimes which are found to

J.L. Adam et al. /Journal of Non-Crystalline Solids 180 (1995) 119-123

550

4S3/2 (2H11/2) /~ Fit : nine 340 cm-l_phonons ~ 1 1 I ' "r

500

400

~ I~I'4F9;2 (

,

r

a

~

"N~

250 t Fit : seven 395 cm-l-phonons !

200 t 0

, 40

, 80

~' " ~

, , , , , ,20 160 200 240 280 TEMPERATURE(K)

, 320

be 420, 490, 220 and 280 Ixs for the 4F9/2, 483/2, 2Hll/2 a n d 2H9/2 levels, respectively. It is well known that multiphonon relaxations, WMp, increase with temperature. So, for a given energy level, WMp can be determined by investigating the influence of temperature on the experimental decays. The results are shown in Fig. 1 for 483/2 (2Hll/2) and 4F9/2 at temperatures varying from 10 to 360 K and in Fig. 2 f o r 2H9/2 between 10 K and room t e m p e r a t u r e . 4F9/2 was directly excited while 483/2 was populated via 2Hl1/2 which shows a strong absorption band. With a relaxation time estimated to be in the nanosecond range, a fast popula60

2H9/2 ¢

¢

~'

~4o

(~rad = 0.28 ms)

3o 2o

Fit : five 393 cm-l-phonons

,o

0

io

8~

tion of 4S3/2 f r o m 2HIt/2 can reasonably be assumed. The decays could be fitted to single exponentials over three e-folding times at least. For all three levels, the experimental lifetimes are found nearly constant up to 120 K and start decreasing significantly at temperatures above that. This indicates that multiphonon relaxations occur. It should be noted t h a t 2Hll/2 is thermally populated from the 483/2 level [10]. One consequence of this is that 4S3/2 exhibits a temperature-dependent effective lifetime shown in Fig. 1 as the %ff solid line.

, 360

Fig. 1. Temperature dependence of the 4S3/2 and 4F9/2 transition lifetimes. Data are fitted by a combination of Eqs. (1) and (2). The error on the fitted phonon energy is estimated to be 10%.

5O

121

1~0 ,60 2~0 2~0 2~0 3~0 TEMPERATURE(K)

Fig. 2. Temperature dependence of the 2H9/2 transition lifetimes. Data are fitted by a combination of Eqs. (1) and (2). The error on the fitted phonon energy is estimated to be 10%.

4. D i s c u s s i o n

Experimental multiphonon relaxation rates were determined through Eq. (1). The temperature dependence of WMVwas accounted for by a single-phonon energy model [12] expressed as WMp(T ) = WMp(0) [[ exp-(h~--exp(ht°/kT) 1 ]P,

(2)

where WMp(0) is the low-temperature multiphon emission rate and p is the number of phonons of single energy, h w, needed to span the energy gap between the emitting level and the next lower level. A least-squares fitting of the experimental rates to Eq. (2) led to WMp(0) of 57, 350 and 22 130 s- 1 for 483/2, 4 F 9 / 2 a n d 2 H 9 / 2 , respectively. By means of the radiative lifetimes and the fitted multiphonon rates, the calculated lifetimes are found. They are represented by the bold lines in Figs. 1 and 2. A fairly good agreement is obtained for the 4F9/2 and 2H9/12 lifetimes with seven phonons of energy 395 cm involved in the non-radiative transition for the former level, and five phonons of 393 cm 1 for the latter. Even though measured and calculated lifetimes deviate somewhat for 4S3/2, valuable information can still be obtained in this case, as shown by the computed phonon energy of 340 cm -t, a value close to that obtained for the other two levels. It has been previously observed for various materials that non-radiative relaxations involving more than ten phonons had a very weak probability [11, and references therein]. Our results on new chloro-fluoride

J.L. Adam et al. / J o u r n a l of Non-Crystalline Solids 180 (1995) 119-123

122 9O

Z

~ 75 Z < E6O

100 I50 200 250 300 350 400 450 WAVENUMBER (cm-1)

Fig. 3. Far-infrared transmission spectrum of CNBK chloro-fluoride glass.

glasses are in accordance with this statement and confirm its validity. To investigate whether the phonon energies found above are realistic, we have recorded the far-infrared transmission spectrum of CNBK glass powder, diluted in a polyethylene pellet. Portrayed in Fig. 3, the spectrum shows the fundamental vibrations of the host as a broad band peaking at nearly 250 cm- 1. The band full-width at half-maximum is from 160 to

105

~

~

T=290K ~

'~ 104

~ 103 Z

4F9/~X,~

380 cm ). The absorption peak at 250 cm -1 is characteristic of Cd-CI stretching vibrations as was shown by Jiang et al. through Raman spectroscopy measurements [13]. On the other hand, Cd-F bonds are responsible for the shoulder observed on the high-energy side of the band. Typically, Cd-F vibration energies are located around 370 cm -1 [5]. It is evident that the phonon energies determined by fitting the multiphonon emission rates are consistent with the absorption spectrum. Very reasonably, these energies can be attributed to Cd-F bond vibrations. It is well established that higher-energy phonons, by contrast with the lower-energy ones, result in a smaller number of phonons involved in the nonradiative transition, and, consequently, in an increased probability. Thus, the fact that the highestenergy phonons are found to account for the observed non-radiative processes adds consistency to our results. In Fig. 4, we have plotted on a semilogarithmic scale the room-temperature values of the fitted multiphonon emission rates as a function of the energy gap, AE, to the next lower level, f o r 4 5 3 / 2 , 4F9/2 and 2H9/2. For comparison, data for BIG, ZBLAN and silicate glasses are also shown [6,14]. They logically follow the sequence of the phonon energies: CNBK ( = 370 cm- 1) < BIG ( = 450 cm 1) < ZBLAN ( = 5 0 0 c m - l ) < s i l i c a t e ( = 1100 c m - l ) . With significantly lower non-radiative probabilities, phonon-sensitive optical transitions such as the 1.3 ~m emission of Pr 3+ ions, should benefit from the use of CNBK glass as a host. The linear dependence observed in Fig. 4 indicates that the energy gap law [14], expressed through Eq. (3), is obeyed: WMp = Ce ,~ae,

~

~

4 S 3 / 2 ~~

102

10 1500

.

where C and a are positive constants characteristic of the host. The best fit of the experimental data to Eq. (3) leads to C = 0.44 × 10 9 S- 1 and a = 4.67 × 10 -3 cm.

\ \\ CNBK BIG ZBLAN ,SILICATE

,

2000 2500 3000 ENERGY GAP (cm'l

\ \\ \ \\ , ~ 3500 )

(3)

4000

Fig. 4. Multiphonon emission rates in CNBK chloro-fluoride, in fluoride, and in silicate glasses at room temperature as a function of the energy gap to the next lower level. Data are fitted by Eq.

(3). The error on the fitted parametersis estimatedto be 5%.

5. Conclusion On the basis of our results, we can conclude that multiphonon relaxations in CNBK chloro-fluoride glasses are due to cadmium-fluorine vibrations

J.L. Adam et al. /Journal of Non-Crystalline Solids 180 (1995) 119-123

whose energy is around 370 cm -1. Moreover, they are found to be significantly less probable in CNBK glass than in pure fluoride glasses. This should enhance the quantum efficiency of phonon-sensitive transitions such as the 1.3 p~m emission of Pr 3+ which is of prime interest for telecommunication applications. However, from a material point of view, more research is needed to improve the moisture sensitivity of the glass. This work was supported by Centre National de la Recherche Scientifique and Alcatel Alsthom Recherche, Grant No. GDR 998. The authors are indebted to Dr B. Jacquier of University of Lyon I for making available his laser equipment.

References [1] Y. Miyajima, SPIE 1581 (1991) 304. [2] J. Lucas, I. Chiaruttini, G. Fonteneau, P. Christensen and S. Mitachi, SPIE 1228 (1990) 56.

123

[3] J.L. Adam, F. Smektala, E. D6noue and J. Lucas, SPIE 1513 (1991) 150. [4] M. Matecki and M. Poulain, J. Non-Cryst. Solids 140 (1992) 82. [5] D.W. Hewack, R.S. Deol, J. Wang, G. Wylangowski, J.A.M. Neto, B.N. Samson, R.I. Laming, W.S. Brocklesby, D.N. Payne, A. Jha, M. Poulain, S. Otero, S. Surinach and M.D. Baro, Electron. Lett. 29 (1993) 237. [6] M. Matecki and J. Lucas, J. Non-Cryst. Solids 162 (1993) 51. [7] B.R. Judd, Phys. Rev. 127 (1962) 750. [8] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [9] W.T. Carnall, H. Crosswhite, and H.M. Crosswhite, in: 'Energy level structure and transition probabilities of the trivalent lanthanides in LaF3', Argonn National Laboratory Report, Argonne, ILL (1977). [10] J.L. Adam, N. Rigout, E. D6noue, F. Smektala and J. Lucas, SPIE 1581 (1991) 155. [11] M.D. Shinn, W.A. Sibley, M.G. Drexhage and R.N. Brown, Phys. Rev. B27 (1983) 6635. [12] L.A. Riseberg and H.W. Moos, Phys. Rev. 174 (1968) 429. [13] H. Jiang, H. Sun, F. Gan, and B. Yuan, J. Mater. Sci. Lett. 9 (1990) 1195. [14] C.B. Layne, W.H. Lowdermilk and M.J. Weber, Phys. Rev. B174 (1968) 429.