Optical spectroscopy of monovalent copper-doped borate glasses

1. Phys. Printed

Chcm.

Solids

Vol.

51.

No.

8, pp.

933-939.

OOZZ-3697190

1990

Q 1990

in Great Britain.

13.M)

+ 0.00

PcrgamooPress pk

OPTICAL SPECTROSCOPY OF MONOVALENT COPPER-DOPED BORATE GLASSES J. C. ZHANG,~ B. MOINE,~ C. PEDRINI,~C. PARENTS and G. FLEM$ 7U.A. 442 CNRS, Universite C. Bernard Lyon I, 69622 Villeurbanne Cedex, France, SLaboratoire de Chimie du Solide do CNRS, Universite Bordeaux I, 33405 Talence Cedex, France (Received 30 October 1989; accepted 3 January 1990)

Abstract-The introduction of copper ions into borate glasses induces in the visible optical region a strong broad fluorescence which is shown to be characteristic of Cu+ ions occupying a large variety of sites. Keywords: Fluorescence, monovalent

copper, borate glasses, multi-sites.

1. INTRODUCTION

measurements and time-resolved spectroscopy. All the experimental results support a multi-site structure of the Cu+ impurity ions. No particular sites have been detected, contrary to the case of Cu+-doped silica glass where octahedra1 and tetragonal sites seem to have been identified [9].

Cu+ ions, when embedded in solids, usually give rise to fluorescence which depends on the host material. The Cu+ impurity centers at known lattice sites in alkali halide crystals induce fluorescence in the nearUV range. These simple systems have been studied extensively in the past because they can be considered as model systems for understanding ions in crystals. Numerous experimental (spectroscopy) and theoretical (molecular orbital calculations) investigations have led to a good understanding of fluorescence processes in these Cu+-doped materials. Excellent reviews have been devoted to this field by Fowler [l] for the early works and by McClure [2] for more recent researches. The Cu+ ions when embedded in glasses usually exhibit strong broad-band luminescence extending over all the visible range, making these systems of considerable interest for obtaining solid state laser materials. Such a potential was first shown by Deshazer [3,4] and then reported in more detail by Kruglik et al. [5,6]. Of course, these potential applications have recently been the cause of many spectroscopic investigations of Cu+-doped inorganic glasses ([7-91 and references cited therein) and also on Cu+-/?“-alumina single crystals [IO, 1I] which exhibit similar fluorescence. The Cu+ centers in amorphous media occupy usually unknown and multiple sites and the band assignments are difficult. It should be noted that even in the simpler cases of Cu+-doped alkali halide single crystals, the interpretation of the spectra was only recently completely solved when two-photon spectroscopy was applied [2]. Therefore, it is false to hope to reach a quite detailed understanding of the Cu+-doped vitreous systems. Here we report the results of our study on the optical properties of Cu+-doped borate glasses. The optical spectroscopy of our systems is shown to be characteristic of Cu+ ions embedded in solids and the fluorescence dynamics is analyzed through lifetime

2. EXPERIMENTAL PROCEDURES The investigated glasses have the composition 2A1,03, 67B*O, 9 lOBa0, (1 S-w)Li,O, 5La,O,, lAs303, WC&O. As,03 was introduced in order to avoid any possible oxidation of Cu+. These materials were prepared from alkali and alkali-earth carbonates (Merck 99.5%) and from B,03 (Merck 99.5%), La,03 (Rhone-Poulenc 99.99%), Al,03 (Cerac 99.5%), As203 (Merck 99.5%) and Cu,O (Prolabo 99.5%) oxides. The dry starting materials were carefully mixed in stoichiometric proportions, introduced in a vitreous carbon crucible and decarbonated at 500°C for 15 h. The temperature was then raised up to 1200°C and held constant at this value for 1 h. The molten glasses were poured into a preheated brass mold. A specific device allowing melting and pouring under a controlled nitrogen flow was used at this step of the preparation process. Lastly, the bulk samples were annealed at 510°C for 5 h and cooled down to room temperature at 0.5”Cmin-I. No divalent copper could be detected by EPR measurements. The basic characteristics of the corresponding undoped glasses have been reported previously [12]. Copper concentrations were carefully controlled using the mass spectroscopy technique. In the context of the method of preparation, the Cu doping densities are limited to 0.14 mol% (6 x lOI ions cme3). Finally two samples were examined containing 0.014 and 0.14 mol% Cu,O. Optical properties of the Cu+-doped borate glasses were measured by a variety of techniques. The absorption spectra were recorded using a VARIAN 933

934

J. C. ZHAKG er al.

UV-visible-NIR spectrophotometer model 2300, spanning the wavelength range from 185 to 3152 nm, and equipped with a liquid-helium cryostat for low temperature measurements. Excitation spectra were obtained at Lure (University of Orsay) using AC0 synchrotron radiation as a light source, and a homemade vacuum-UV monochromator. For emission spectra, the exciting source was either the third harmonic of a Q-switched Nd:YAG laser (Quantel), or the pumped dye laser beam which could be mixed with the fundamental (1.06 pm) or frequencydoubled and mixed with the fundamental. The excitation wavelength available this way may thus be scanned from 230 to 420 nm with energy up to several tens of millijoules. Fluorescence was analyzed using a Hilger computer-scannable 1m monochromator and the dispersed emission was detected with a cooled EMI 8852 or RCA AsGa PMT followed by an amplifier/discriminator and a photon counter. The apparatus was carefully calibrated and the emission spectra corrected accordingly. Decay time measurements were recorded either with a multichannel analyzer (Intertechnique In90) with a minimum dwell-time per channel of 2 ps for the long lifetimes, or a model 8100 waveform recorder (Biomation) with a minimum sample interval of 10 ns for the short lifetimes. For time-resolved spectroscopy, the signal was sampled by a P.A.R. Model 162/165 Boxcar integrator at different delay times. 3. RESULTS AND DISCUSSIOS Figure 1 shows the absorption spectra of undoped (curve 1) and weakly Cu-doped (O.O14mol%) (curve 2) glasses at room temperature. Owing to the strong absorption of the host glass, the features due to the Cu+ ions alone do not appear, but can be obtained by subtraction of curves 2 and 1. The resulting curve 3 shows a peak around 46,50Ocm( ‘Y 2 I5 nm), a shoulder near 42,000 cm-’ ( z 240 nm)

r00m

Wnpemture

thickness:033

mm

energy

Fig.

(x103cm-')

1. Absorption spectra of undoped and weakly Cu-doped glasses at room temperature.

room

energy

tempemPurc

(IO3 x cm

-1)

Fig. 2. Excitation spectra of glasses containing 0.14 mol% of Cu-. Curve I: blue filter (402nm. Aho z 1400 cm-‘); curve 2: green filter (544 nm, Ahw z 600 cm-‘); curve 3: red filter (> 590 nm).

and a long tail extending up to 27,50Ocm(= 365 nm). Low temperature measurements (4.4 K) give similar spectra and do not lead to a resolved structure in the low energy range. The excitation spectra (Fig. 2) were recorded at room and low temperatures by considering three domains of the broad fluorescence: blue (-400 nm), green (~550 nm) and red (> 590 nm). In all cases, the spectra consist of two main broad bands peaking near 46,500 cm-’ (215 nm) (band C) and 35,70Ocm(= 280 nm) (band A) and a third one which can be estimated to peak around 43,000 cm-‘( = 235 nm) (band B). Bands B and C are well correlated to the absorption peaks. Band A corresponds to the nonresolved absorption tail. One notes that the relative intensity of the excitation bands is completely different. The decrease in excitation efficiency observed as the absorption coefficient increases may be related to non-radiative surface recombination which becomes dominant since the exciting light is absorbed closer to the sample surface. The temperature effect on the spectra is weak: one observes a broadening of 6-12% when the temperature is raised from 10 to 300 K while the maxima shift lightly towards higher wavelengths. Moreover, the excitation bands clearly move to the red with the emission domain considered. This latter trend and the broadening behavior of the spectra support qualitatively the reasonable assumption of the Cu+ multi-site structure in glasses. Since

Monovalent copper-doped

lmilting

borate glasses

935

Icvals

Energy (xlO%m’)

_ -

Fig. 4. Deconvolution of the absorption spectrum (curve 3 of Fig. 1) into three Gaussian bands.

ABCDEFG

Fig. 3. Schematic representation of the energy levels of Cu+ occupying various sites from A to G in borate glasses.

the Cu+ fluorescent ions introduced in amorphous media reside in a large variety of sites, the energies of the various ionic levels may differ quite substantially. This situation is sketched in Fig. 3 where the lowest emitting levels and a hypothetical absorption band are schematically represented for different sites. This crude representation of the impurity ion distribution is helpful to interpret the spectral variations. The red shift observed as the selected emission wavelength increases is easy to understand. Now, concerning the broadening behavior, if it is admitted that the site distribution contributes to the bandwidth, for the same fluorescence spectral interval considered, the maximum bandwidth of the excitation spectra should be observed at the center of the emission band where the density of sites is maximum. The experimental data are in good agreement with this expectation, owing to the spectral interval used and indicated in the caption of Fig. 2. Assignment of the various components of the absorption is difficult because of the strongly inhomogeneous character of the bands. Unlike the cases of Cu+ in alkali halide single crystals where the impurity occupies a well-defined or nearly centrosymmetric position, the d +s transitions have no clear temperature dependence typical of parity-forbidden transitions allowed by coupling with lattice vibrations of odd parity [13, 141. It is therefore not possible to distinguish between vibronic d +s and parity allowed d -+p transitions by measuring the temperature dependence of the absorption bands [15,16]. Moreover, the probable very low symmetry of the sites tends to remove the selection rules and therefore to reduce the difference between the allowed and forbidden transition probabilities. The deconvolution of curve 3 of Fig. 1 into three Gaussian bands in the range 37,000-47,000 cm-’ (Fig. 4) enabled us to calculate the oscillator strengths for the absorption

bands using equation

the

following

f = 1.018 x 10’ x +

modified

Smakula

x&

(1)

where n = 1.62 is the refractive index of the host material, S is the area of the band shape and X/V the number of ions per centimeter cubed. From the results gathered in Table 1, band C can be tentatively assigned to d +p transitions and the other three weaker bands to d + s transitions. Comparison with, for example, the case of Cu+ in LiCl[l6] shows that the oscillator strengths for the d -*p transitions are comparable while those of the d + s transitions are relatively higher in the Cu+-doped borate glasses, as expected. The fluorescence spectrum consists of a very broad band covering the whole visible region. The trichromatic coordinates (X = 0.19; y = 0.19; z = 0.62) are at the limit of the zones characterizing the white and the blue colors according to the diagram of the “Commission Intemationale de 1’Eclairage” (Fig. 5). The comparison of the Cu+ emission intensity with the one of the halophosphate Ca,(PO,),(F, Cl): Sb: Mn (Fig. 6) indicates a high quantum efficiency of our Cut-doped borate glasses of about 57%. The fluorescence spectrum depends strongly on the excitation wavelength as can be seen in Fig. 7. The emission band shifts toward the red and becomes narrower as the excitation wavelength increases. These results are again in good agreement with the multi-site structure of the Cu+-doped glasses. It should be noted that the Table I. Band positions and oscillator strengths for the absorption bands Band c BZ B, A Itail)

Energy (cm-‘) 46,400 43,000 40,200 <3700

f 8.5 x 2.3 x 1.4 x <0.2 x

1O-1 IO-* 10-f 10-z

936

J. C.

&L4NG

et al.

(b)

Wavelength

o-2

I

I

OX

06

(m-J

Fig. 6. Comparison of the fluorescence intensities of the two following samples in identical experimental conditions (A,, = 254 nm, T = 300 K): (a) Cu 0.14 mol% doped borate glass powder: (b) standard phosphor (NBS 1031). I

x 0.8

Fig. 5. Trichromatic coordinate diagram according to the “Commission Intemationale de I’Eclairage”. @ stands for the investigated glasses.

very narrow spectral width laser excitation does not allow the selective excitation of particular Cu+ sites because of the large vibronic coupling and consequently cannot lead to a strong fluorescence line narrowing. The T-dependence of the emission spectrum is illustrated in Fig. 8 where a narrowing of about 12% and a red shift of the emission band upon cooling is observed. The intensity of the fluorescence, measured as the area of the emission spectrum, is nearly temperature independent between liquid helium and room temperatures and is also Cu+ concentration-independent. This means that the radiationless transitions are not effective over the wide range of temperature considered. Furthermore the

absence of self-quenching of the fluorescence for Cu+ concentrations up to O.l4mol% proves that no significant Cu-Cu interactions occur in these weaklydoped glassy systems currently investigated. The lifetime measurements were carried out on various Cu+-doped glasses using different experimental conditions (temperature, excitation wavelength, emission range, excitation density, . . .). In all cases, the decays are non-exponential, even for the low concentration O.O14mol% Cu,O glass, and are not Cu concentration-dependent, confirming the weakness of the Cu-Cu interactions. At first sight, energy transfers between Cu+ impurity ions can therefore be ruled out. The non-exponentiality is likely due to the multi-site structure of the material. In order to quantitatively characterize the decays, we have fitted them by the sum of two exponentials, the time constants of which have no physical meaning in the present 1: EXC 35.5nm 2: EXC 320 nm 3: EXC 290nm ~:EXC 2~6nm

Energy

(x103crn-‘)

Fig. 7. Emission spectra of Cu 0.14 mol% doped borate glass at room temperature after different laser excitations.

Monovalent copper-doped

93-l

1: Tz300K

21

l-

borate glasses

2:T,

10K

c .k 5 t 1 '? s OS> 'I 5 t .fi E : it ? G o-

18

2'2 Energy

26

30

I 31

(x103cmA)

Fig. 8. Temperature dependence of the emission spectrum of Cu 0.14 mol% doped borate glass under laser excitation at 290 nm. case.

Then

time-constant

considered for each decay a T defined as follows:

we have

il I(t).t.dr f= s O

(2)

g I(t) dt

I0

with

I(t) =lo, exp( -t/r,)+I,,exp(-t/r,).

(3)

which represents the mean value of the fast and slow components 7, and TV, may be written as 7,

f=

IO,71

10272 71 + 4,7, + IO272 IO,71+ IO27272

7=%171+X272,

(4) (9

where

The decays also show a strong temperature dependence, as can be seen in Fig. 9. At very low temperature, the value of 7 is about 450~s and is nearly constant around liquid helium temperature. Then 7 decreases very quickly upon heating in the range 4-50 K, and then very slowly above T = 50 K. This temperature dependence of the fluorescence dynamics is typical of a Cu+ center and is usually interpreted by a model involving two close emitting thermalized metastable levels, the lifetime of the lowest one being much longer than that of the upper one [19,20]. In the cases of Cu+- and Ag+-doped cubic single crystals, these two lowest excited states were identified as the spin-orbit components T,* and T,, of the ‘Et state, which is the lowest state in the d9s manifold. By solving the appropriate rate equations, the radiative lifetime of the fluorescence can be written as [21]: 7-,

x,+x2=

1.

1, or x2 can be used as a measure of the non-exponentiality of the decays. The decay time i increases with the selected emission wavelength whatever the temperature. Typically, for the sample containing 0.14 mol% Cu,O excited at 290 nm, 7 = 42,57, and 65 ps at room temperature for A,,,,= 400, 500 and 600 nm, respectively. Furthermore the decay time gradually increases for the longer excitation wavelengths. These trends are usually observed for luminescent centers in disordered media [I 7, 181, and can be related to the multi-site structure. The experimental results can be easily interpreted using Fig. 3 and considering that the lifetimes of subsets of ions increase from the A to the G sites.

2: 4, + AZ,exp(--GT) 1 + exp(-c/kT)

’

(6)

where A,, and A,, are the radiative transition probabilities between the excited states (upper level 2 and lowest level 3) and the ground state (level I). c represents the energy mismatch between the two excited states. A,, is evaluated as the inverse of the experimental lifetime at very low temperature and the best fit to experimental data obtained from expression (6) (solid line in Fig. 9) leads to the following results: A,, = 2242 s-’ A,, c 28300 s-’

c 2: 25.7 cm-‘.

J. C. ZHANG et al.

938

0

$0

100

I

150

260

250

Temperature

300

350

(K)

Fig. 9. Temperature dependence of the time constants i of the fluorescence decays of Cu 0.14 mol% doped borate glass under laser excitation at 290 nm. The value of c is close to those usually obtained in Cu+-doped crystals. We have previously mentioned several experimental results supporting the existence of a multi-site structure of the Cu+ impurity ions in our borate glasses. In particular, the non-exponential decays of the fluorescence can be considered as the superposition of exponential decays typical of well-defined sites. The time-resolved spectroscopy technique provides confirmation of the multi-site structure. For example, normalized time-resolved fluorescence spectra, obtained at low temperature under 290 nm excitation, are represented in Fig. 10. The emission band, time-resolved at a short time (100 ns) after the laser excitation pulse (curve 2), is centered in the blue region and the band shifts towards the red (low

energy) as the gate opening delay increases. It is expected that the time-resolution at long time leads to a real site selection since the sites giving rise to short fluorescence cannot contribute to the long-lived fluorescence (see Fig. 3). A narrowing of the emission band is actually observed when the delay strongly increases, the gate width being constant (comparison of curves 2 and 5 or 6 and 7).

4.

The present study shows that the optical properties of Cu-doped borate glasses are due to monovalent copper ions occupying a large variety of sites. The strong fluorescence band, due to the s +d transition of Cu+, and occurring in the visible spectral r1

non-time-resolved Delay=100

Fig. 10. Time-resolved

fluorescence

ns;Gote

widthz5ns ”

=5ns

N

,200ns;

N

Ans

I,

=

b/US;

N

=5ns

II

=500 ns;

//

=1ops

N

=3OO,MSj

/I

-;_10ps

N

Energy

CONCLUSION

,150

ns;

(x103cm’)

spectra of Cu 0.14 mol% doped at 290 nm at T = 5 K.

borate

glass under

laser excitation

Monovalent copper-doped region, is considerably broadened by the multi-site structure and also by the strong coupling between the s electron and the phonons of the medium. These spectroscopic characteristics make this system a possible candidate for tunable glass laser. Further research is considered to optimize the material in terms of Cu+ concentration. Moreover, in order to assess

whether this material is a promising tunable laser medium for the visible range, further experiments are now in progress, in particular amplification tests and gain measurements.

939

borate glasses Shagov A. A. and Zolotareva (USSR)

L. E., Opt. Specrrosc.

59, 439 (198s).

Kruglik G. S., Skripko G. A., Shkadarevich A. P., Ermolenko N. N., Gorodetskaya 0. G., Belokon M. V., Shagov A. A. and Zolotareva L. E., J. Luminesc. 34,343 (1986).

Liu H. and Gan F., J. non-crystalline Soli& 80, 447 (1986).

Skripko G. A., Shkaderevich A. P., Ermolenko N. N., Gorodetskaya 0. G., Belokon M. V. and Shagov A. A., Soviet Phys. solid St. 28, 1021 (1986). Debnath R. and Das S. K.. Chem. Phvs. Lett. 155. 52 (1988).

10. Barrie J. D., Dunn B., Stafsudd 0. M. and Nelson P., J. Luminesc. 37. 303 (1987).

Acknowledgemenls-The authors express their thanks to J. Y. Barraud for mass spectroscopy analysis and to J. M. Dance for EPR measurements.

11. Barrie J. D., Dunn B.,‘Holl&rgsworth G. and Zink J. I., J. Phys. Chem. 93, 3958 (1989).

12. Lurin C., Parent C.. Le Flem G. and Hagenmuller P., J. Phys. Chem. Solirls 46, 1083 (1985).

REFERENCES W. B., Physics of Color Centers, p. 54. Academic, New York (1968). McClure D. S., Tunable Solid State Lasers, Proceedings of the Firsf International Conference, La Jolla, California, 13-15 June 1984 (Edited by P. Hammerling, A. B. Budgor and A. Pinto), p. 172. Springer, Berlin (1985). Deshazer L. G., Laser Focus 17, 22 (1981). Deshazer L. G., Proceedings of the First International Conference on Tunable Solid State Lasers, La Jolla, California, 1984 (Edited by P. Hammerling, A. B. Budgor and A. Pinto), p. 91. Springer, Berlin (1985). Kruglik G. S., Skripko G. A., Shkadarevich A. P., Ermolenko N. N., Gorodetskaya 0. G., Belokon M. V.,

I. Fowler

2.

3. 4.

5.

13. Fussgaenger K., Phys. Status Solidi 34, 157 (1969). 14. Fussaaenaer K.. Phvs. Status Solidi 36. 645 (1969). IS. Nag&akaS., Ikezawa M. and Ueta M., J. phys. Sot. Japan 20, 1540 (1965).

16. Simonetti J. and McClure D. S., Phys. Rev. B16, 3887 (1977). 17. See for example, Breeher C., Riseberg L. A. and Weber M. J., Proceedings of the 12th Rare Earth Research Conference, Vail Colorado U.S.A., Vol. I, p. 351 (1976). 18. Boulon G., Moine B. and Bourget J. C., Phys. Rer. B22, 1163 (1980).

19. Pedrini C., Phys. Status Solidi (b) 87, 273 (1978). 20. Pedrini C. and Jacquier B., J. Phys. C 13, (1980). 21. Moine B. and Pedrini C., Phys. Rev. B30, 992 (1984).