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Upconverted luminescence under 800 nm laser diode excitation in Nd3+-activated fluoroaluminate glass
Optical Materials 28 (2006) 129–136 www.elsevier.com/locate/optmat
Upconverted luminescence under 800 nm laser diode excitation in Nd3+-activated fluoroaluminate glass Cz. Koepke
b
a,*
, K. Wisniewski a, L. Sikorski a, D. Piatkowski a, K. Kowalska a, M. Naftaly b
a Institute of Physics, N. Copernicus University, Grudziadzka 5/7 87-100 Torun, Poland Department of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK
Received 28 September 2004; accepted 20 October 2004 Available online 16 June 2005
Abstract We report on the upconverted luminescence in neodymium-activated fluoroaluminate glass obtained with 800 nm diode laser excitation. Several anti-Stokes emissions: at 588, 607, 720 and 750 nm are observed and appropriate transitions are assigned. For both latter emissions we observe strong dependence on temperature: the 720 nm emission intensity decreases with temperature, whereas the 750 nm emission increases. Interpretations are presented in terms of the influence of oxygen-affected sites on the radiationless transitions and multiphonon anti-Stokes excitation. The models provide reasonable fits to the experimental data. 2005 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 42.70.Ce; 78.40.Ha; 78.55.Hx Keywords: Laser materials; Upconversion; Spectroscopy; Neodymium; Fluoride glasses
1. Introduction Glass hosts are very attractive for laser applications because of their cheapness and relatively easy fabrication in various geometries (especially in the form of fibres). Moreover, when activated by rare earths (RE), glasses offer a variety of dopant sites with strong ion– host interactions, with the result that the emission and absorption lines are Stark split [1] and inhomogeneously broadened. This property manifests in the relatively large widths of emission and absorption lines (200– 400 cm1), which, together with the possible partial overlap, makes RE doped glasses good candidates for tunable and ultrafast laser medium. In this context one of the most interesting RE ions is Nd3+ whose lasing and light amplifying in the glass host has been known *
Corresponding author. Tel.: +48 56 611 3239; fax: +48 56 622 5397. E-mail address: [email protected] (Cz. Koepke).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.10.034
and exploited for years (e.g. [2,3]). Neodymium-glass lasers are employed to produce ignition in nuclear fusion research; high-power lasers have found numerous applications in materials processing, and femtosecond mode-locked lasers have recently become commercially available. All these types of Nd-glass lasers use fluorophosphate glass and operate at 1050–1070 nm. Due to the commercial and technological importance of Ndglass lasers, a large number and variety of Nd-doped glasses have been investigated over the years. A new insight into the behaviour of the Nd3+ ion in a glass host has the potential to advance the science and technology of Nd-glass lasers. The energy diagram of the Nd3+ ion is very ‘‘dense’’ in terms of the number of possible states, so that various types of transitions between them, including upconversion, take place. Although a large number of emission lines have been observed to date only the 1.06 lm band has found widespread commercial applications.
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
4
S3/2 ,4F7/2
4 4
D1/2, 4D3/2, 4D5/2
4
2
0.1
G9/2 , G11/2, D3 / 2
G7/2 ,2K13/2
F3/2
4
2 2
200
400
4 F H11/2 9/2
2
600
800
1000
800
1000
absorbance (-log10T)
wavelength (nm)
0.2
0.1
200
400
b
600 wavelength (nm)
Fig. 1. Absorption spectra of the Nd3+-activated fluoroaluminate glasses; Glass 1 (a) and Glass 2 (b). The dashed lines show the wavelength used for excitation when examining anti-Stokes emissions.
3. Spectroscopic measurements
Excitation: EXC=514.5 nm detection: OMA system intensity (arb.units)
The absorption spectra for both examined glasses are illustrated in Fig. 1, and show the characteristic Nd3+ lines due to the 4G5/2, 4D3/2, 4F3/2,5/2,7/2 states, and weaker lines due to the 2P1/2, 4G11/2, 4F9/2 or 2H11/2 states. Also notable is the very high lying conduction band of the glass, typical of fluoride glasses, the material being transparent in the UV up to 230 nm. In addition, Glass 2 has a distinct absorption band at 250 nm, which is due to the phosphate component in the glass. The emission detected using the 514.5 nm excitation line is very similar in both glasses and an example is presented in Fig. 2, where two distinct bands spreading from 850 to 940 nm and from 1030 to 1090 nm dominate the spectrum. Fig. 3 presents an example of a luminescence excitation spectrum monitored at k = 860 nm. In Fig. 4a
F5/2 ,2H9/2
G5/2 ,2G7/2
a
and the respective indices of refraction were: nd = 1.392 and nd = 1.428. Fabrication of these glasses is described in [3]. The glass was doped with 1 mol% of NdF3 which substituted for AlF3 (Glass 1) or YF3 (Glass 2). The standard spectroscopic characteristics such as absorption and emission spectra were measured by an OMA-type spectrometer (ORIEL InstaSpec II). In addition a Perkin–Elmer Lambda 2 UV–VIS Spectrometer was used to observe the absorption spectra, and a Jobin Yvon FLUOROLOG-3 Spectrofluorometer system recorded the luminescence excitation spectra. The measurements of low-temperature emission spectra and their temperature dependence were performed using a closed-cycle helium refrigerator (APD Cryogenics Inc. HC-2D-1/DE-202 system) and the ORIEL InstaSpec II/MultiSpec 1/8 m fibre spectrometer.
4
4
The Nd3+-doped fluoroaluminate glass has been developed and produced at the University of Leeds, UK. The composition of the glass samples used in the present work was: Glass 1: 39AlF3, 6MgF2, 22CaF2, 6SrF2, 6BaF2, 10NaF, 10LiF, 1NdF3 Glass 2: 37AlF3, 12MgF2, 15CaF2, 9SrF2, 6BaF2, 14YF3, 6NaPO3, 1NdF3
I9/2 → to:
0.2
P1/2 , D5 / 2
2. Material preparation and experimental setup
4
4
In this paper we present spectroscopic results in Nddoped fluoroaluminate glass, which due to its structure may offer an insight into the interactions of neodymium ions residing in fluorine and oxide sites, as explained below, with potential implications for commercial fluorophosphates glasses.
absorbance (-log10T)
130
750
nd
2 order (laser)
800
850
900
950
1000
1050
1100
1150
wavelength (nm) Fig. 2. Emission spectrum observed using argon ion laser excitation (514.5 nm) (Glass 1).
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
intensity (arb.units)
80000 OBS
= 860 nm
60000
40000
20000
0 300
400
500 600 700 wavelength (nm)
800
Fig. 3. Luminescence excitation spectrum monitored at 860 nm. The dashed line in the region 415–450 nm is inserted to replace the lamp line seen in the second grating order.
24000
0.03
intensity (arb.units)
0.02
20000
0.01 0.00 400
500
600
700
800
4. Discussion and interpretations 4.1. Upconverted emission in the range 570–620 nm
8000 500
550
a
600
650
700
wavelength (nm) wavelength (nm) 20000
630
600
570 λEXC= 800 nm
15000
intensity (arb.units)
and b we show the room-temperature (RT) upconverted luminescence using a 800 nm 500 mW laser diode as an excitation source. In the inset of Fig. 4a is shown upconverted luminescence at 570–620 nm with a local, distinct emission peak at 750 nm, which appears on the anti-Stokes side of excitation. Fig. 4b compares the RT and low temperature (LT, 10 K) upconverted luminescence at 570–620 nm. Fig. 5a and b depicts the results of more systematic luminescence measurements focusing attention on the region of 680–760 nm. We have obtained a series of emission spectra for two different glass samples measured at various temperatures using excitation into the 4 F5/2 state at 800 nm (diode laser). A distinct decrease with temperature of the 720 nm luminescence is observed for both samples, while starting from T = 300 K a new emission peak appears around 750 nm. A more detailed view of the 720 nm emission decrease in shown in Fig. 6. Fig. 7 shows a series of the anti-Stokes emission bands peaking around 750 nm when varying the temperature in the range 350–600 K. Notably, the decrease of the 720 nm emission and the increase of the 750 nm emission occur at two different ranges of temperature, so that they appear to be quite independent tendencies.
16000
12000
T = 300 K
10000 T = 10 K
We have carried out a very careful inspection of the absorption and emission spectra obtained with sufficient spectral resolution to create a detailed energy diagram of Nd3+ in our fluoroaluminate glass. This is illustrated in Fig. 8 together with an assignment of transitions responsible for the observed spectral lines. In assigning these transitions we have taken into account the fact that the spectrum seen in Fig. 4a and b consists of two lines of quite different widths. Hence we assume that we deal with transitions which probably originate not from one state, but from entirely different states, in this case: 4G7/2 + 2K13/2 and 4G5/2 + 2G7/2. Moreover, from the inspection of the diagram in Fig. 8 we conclude that the final states for these transitions should also be different: 4I11/2 and 4I9/2 respectively.
5000
4.2. Decrease of the 720 nm upconverted emission 0 15500
b
131
16000
16500 17000 wave number (cm-1)
17500
Fig. 4. Upconverted emission spectra at 585 nm and 607 nm obtained with 800 nm laser diode excitation; inset in (a) shows the scale in comparison with the anti-Stokes emission at 750 nm and (b) shows the emission at two temperatures: 300 K and 10 K.
We have assigned the 720 nm emission to transitions from the 2H11/2 state to two terminal Stark sublevels of the 4I11/2 state. The double character of the terminal state of the 720 nm emission can be deduced from Fig. 6 (only two Gaussians fit the spectra perfectly). We have tested several models that could explain the observed decrease of emission with temperature, especially models
132
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
wavelength (nm)
wavelength (nm) 750
720
690
750
660
720
690
660
50 K
10 K
100 K 150 K 200 K 250 K 300 K
50 K 100 K 150 K 200 K 250 K 300 K
intensity (arb.units)
intensity (arb.units)
10 K
= ~ 800 nm
= ~ 800 nm
EXC
EXC
13500
14000
14500
15000
wave number (cm-1)
a
13000
13500
14000
14500
wave number
b
15000
(cm-1)
Fig. 5. Decrease of the 720 nm emission intensity with temperature and the partial increase of the 750 nm emission intensity for two materials: Glass 1 (a) and Glass 2 (b). Excitation wavelength for both cases is 800 nm (laser diode).
wavelength (nm) 750
720
EXC
wavelength (nm)
690
660
750
= ~ 800 nm
720
690
600 K 550 K
10 K 50 K 100 K
EXC
intensity (arb.units)
intensity (arb.units)
660
150 K 200 K 250 K 300 K
= ~ 800 nm
500 K
450 K 400 K 350 K
13500
14000
14500
15000
wave number (cm-1) Fig. 6. A more detailed view of the 720 nm emission decrease; a steep increase of the 750 nm emission is seen at T = 300 K.
based on Boltzmann distribution of inhomogeneously broadened densities of states and the exponential energy gap law, and have found that none of these explain the observed results. It appears that the only model capable of explaining the interesting behaviour of the 720 nm emission is an approach based on the assumption that an additional state exists which enables additional paths for radiationless transitions selectively from the 2H11/2
13000
13500
14000
14500
15000
-1)
wave number (cm
Fig. 7. Increase of the 750 nm emission intensity with temperature. The excitation wavelength is 800 nm (laser diode).
state. Such a situation can be qualitatively illustrated by a configuration coordinate diagram as presented in Fig. 9. The origin of the introduced new state associated with ‘‘FRE-O site’’ is in unintentional oxygen ions that become incorporated in the glass during processing and interfere in regular fluorine-coordinated sites. Since both examined glasses contain a substantial amount of such
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
Fig. 8. Energy levels diagram of the upconverted transitions in the 570–620 nm range.
2
FRE-F site
2
P 1/2
H 11/2 FRE-O site
4
800 nm
F 5/2
133
but activated with Pr3+. As is seen, the FRE-O site can provide paths for radiationless transitions via tunnelling through the barriers of various heights (widths) dependent on the state. It may be assumed, as schematically depicted in Fig. 9, that the suitable barrier corresponding to the 2H11/2 state is the lowest one, selectively providing for this state an effective path for radiationless transitions. On the other hand, in one of our glasses (Glass 2) a NaPO3 phosphate group is added to the composition. An obvious difference due to this modification is seen in the absorption spectrum, where a distinct shoulder appears around 250 nm (Fig. 1b). Further differences are seen in Fig. 5 and especially in Fig. 10, in the way the 720 nm emission decreases with temperature. In the range 200–250 K the emission intensity in Glass 1 undergoes a rather sudden drop whereas in Glass 2 the emission decrease occurs smoothly and gradually. This would suggest that in Glass 1 we deal with a well defined (though relatively small, see Fig. 9) barrier for thermal activation, whereas in Glass 2 such barrier to radiationless transitions is much smaller or even absent altogether. Hence, in the light of Fig. 10 we can say that our model illustrated in Fig. 9 applies to both glasses, but that in Glass 2 there exists an additional pathway for radiationless transitions. In this pathway, the energy barrier for escape from the 2H11/2 state is much smaller than that in Glass 1 (or possibly absent). Moreover, qualitatively identical behaviour is observed for Stokes emission (4F3/2 ! 4I9/2, 4I11/2). The emission intensity drops suddenly between 200 and 250 K in Glass 1, whereas in Glass 2 the intensity decrease is much smoother. This confirms that the effect is of a more universal, unselective character and concerns multiple states (e.g. 2H11/2 and 4F3/2). The occurrence of this effect
720 nm 14000 4
I9/2 „+ ” I11/2
Fig. 9. The proposed model (configuration coordinate diagram) explaining the decrease of the 720 nm emission with temperature.
unintentional oxygen, we can assume that Nd3+ ions can reside in two types of sites, in one (which we call FRE-F) the Nd3+ is in fluorine coordination forming a regular site responsible for all known spectral features of the material. Another type of site (FRE-O) is an oxygenaffected site, where Nd3+ resides in a modified coordination where one fluoride ion is substituted by oxygen. Thus the FRE-O site is characterized by different phonons and plays the role of a ‘‘defect site’’, similar to that which we introduced in [5,6] for the same type of glass
Intensity (arb.units)
4
12000
10000
8000
6000 0
50
100
150
200
250
300
Temperature (K) Fig. 10. An example of fit using Eq. (2) to the experimental data for the 720 nm emission decrease with temperature. Circles correspond to Glass 1, squares to Glass 2.
134
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can be connected with the presence of phosphate groups in the glass. Phosphates are strong network formers, capable of forming molecular chains which thereby create another type of FRE-O sites, most likely to be high phonon energy sites for Nd3+ ions. Examples of such sites are shown in [4, Fig. 1]. The concentration of these sites is quite large (6% NaPO3 in the composition of Glass 2). Thus, in the context of high concentration of phosphate FRE-O sites in Glass 2, an additional path for radiationless transitions which has unselective character independent of the initial excited state, is most likely due to energy transfer from exited states of FRE-F sites to unexcited FRE-O sites. In the situation common for both glasses we can write the following kinetic equations for the states as seen in Fig. 9: dn1 dt dn2 dt dn3 dt dn4 dt
¼ Wn1 þ a41 n4 ¼ Wn1 Wn2 ð1Þ ¼ Wn2 a34 n3 ¼ a34 n3 a41 n4 n4 s expðDE=kT Þ
where ni, (i = 1, 2, 3, 4) are the populations of the ground state and the 4F5/2, 2P1/2 and 2H11/2 states respectively, W is the excitation parameter (pumping rate), and aij are spontaneous transitions rates from jii to jji states. The two parameters in the last equation, which are s—the frequency factor and DE—the height of the barrier between the 2H11/2 and the FRE-O state1, represent the additional path for radiationless transitions. Assuming that N = n1 + n2 + n3 + n4, where N is the total concentration of ions taking part in the described process, we can obtain the following stationary solution: WN n4 ðT Þ ¼ W 2 þ a34 ½a41 þ s expðDE=kT þ W
ð2Þ
Fig. 10 presents an example of curve fits to the experimental points taken from Fig. 5a and b using the above formula and assuming that the emission intensity I is proportional to n4. The fitting parameters are: W = 103 s1, N = 1020 cm3, a34 = a41 = 106 s1, s = 6 · 106 s1, DE = 400 cm1. Circles and squares represent data from different glass samples. Note that for simplicity we use the same set of fitting parameters for both glasses, just to show that the model works in both cases.
4.3. Increase of the 750 nm anti-Stokes emission Fig. 11 illustrates our understanding of the mechanism of temperature dependent excitation of the state emitting at 750 nm. Such multiphonon anti-Stokes excitation mechanism has been proposed by Auzel [9] and exploited at least twice [10,11] to explain the same or similar phenomena as observed here. The kinetics of this system can be expressed by the following set of equations: dn1 ¼ Wn1 þ a31 n3 dt dn2 ¼ Wn1 b23 ðT Þn2 dt dn3 ¼ b23 ðT Þn2 a31 n3 dt where b23 ðT Þ ¼ b023 ðehx=kT 1Þp .
ð3Þ
Here the populations of the states 4I9/2, 4F5/2 and 4F7/2 are represented by ni (i = 1, 2, 3) respectively, W is pumping parameter (rate), aij are respective spontaneous transitions rates, b023 is the stimulated transition rate at T = 0 K, and p is the number of phonons of energy hx taking part in the process. Assuming that N = n1 + n2 + n3, we obtain the following stationary solution of the set (3): n3 ðT Þ ¼ W
WN
a31 b023
p ½ehx=kT 1 þ 1 þ a31
ð4Þ
Note that here we treat the total concentration of ions taking part in the process in the same way as for the process described in Section 4.2. In view of Fig. 5a and b, where the 750 nm emission starts to increase at the temperature where the 720 nm emission has already stopped decreasing, we can say that this simplifying assumption is justified. In trying to fit the curve described by formula (4) to the experimental points taken from Fig. 7 (bearing in mind the assumption that I n3), we need to choose
4
F7/2
3
930 cm-1 4
2
800 nm
F5/2
750 nm
1
We use the simplest standard Mott–Seitz (Arrhenius) approach to describe radiationless transitions just to show that there is some merit in assuming the existence of a state capable of removing some part of population from the emitting (2H11/2) state. A proper approach would use a more sophisticated formula than that used in (1) (see e.g. [7,8]).
1
4I
9/2
Fig. 11. A model showing anti-Stokes multiphonon excitation which leads to the 750 nm emission growth with temperature.
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
the number of phonons p that are ‘‘borrowed’’ by the ion system from the glass matrix in order to complete the excitation. In Fig. 12 we present the fitting results for three phonon numbers: p = 2, 3 and 4. It is seen that p = 3 gives by far the best fit. Moreover, only the p = 3 assumption produces the characteristic inflected curve, as in the experimental data. The remaining parameters for this fit are: W = 103 s1, N = 1020 cm3, a31 = 5 · 105 s1, b023 ¼ 5 101 s1 , hx ¼ 580 cm1. Finally, taking the energy gap between the states 4F7/2 and 4F5/2 to be 930 cm1, as derived from our absorption spectrum, and taking into account their distribution as seen in absorption/emission spectra, we can determine the maximum distance between the tail-ends of the distributions to be 1725 cm1 (Fig. 13). Dividing this value by p = 3 we obtain the maximum phonon energy as hx ¼ 575 cm1. This is in perfect agreement with previ ous estimations for the maximum phonon energy in this type of glass (580–600 cm1, [12,13]).
60000
p= 4
Intensity (arb.units)
50000 40000
p
=
3
30000 20000
p=2
10000
0
350
400
450
500
550
600
Temperature (K) Fig. 12. An example of a fit using Eq. (4) to the experimental data for the 750 nm emission increase with temperature. Dashed lines show solutions for different numbers of phonons p.
135
5. Conclusions In the present work we have described two different types of anti-Stokes emission, one increasing with temperature (with a peak at 750 nm), the other decreasing with temperature (with a peak at 720 nm). The maxima of these emissions are separated by only 550 cm1, which makes their opposite behaviour even more surprising. Careful transition assignment based on observed spectral characteristics shows that the two emissions originate from the states 4F7/2 and 2H11/2 respectively, and that their respective terminal states are 4I9/2 and 4I11/2. We have proposed models to explain the opposite tendencies in the behaviour of these emissions and obtained satisfactory agreement with experiment. We have also observed upconverted emissions at 588 nm and 607 nm and assigned them to the transitions 4G7/2 + 2K13/2 ! 4I11/2 and 4G5/2 + 2G7/2 ! 4 I9/2 respectively. In addition, we have noted a difference in the emission behaviour of our two glasses. The behaviour was observed in two separate transitions, and was related to the presence of phosphate groups in the glass. The effect may be attributed to the presence of an additional pathway for radiationless transitions in phosphate-containing glass. It is perhaps worth noting that we have obtained all the described results using a relatively cheap source of excitation: a 800 nm laser diode, instead of a sophisticated and expensive Ti-Sapphire laser. There are only a few publications using such a source, of which we should mention e.g. [14].
Acknowledgements We are grateful to Prof. S. Chwirot, Head of the National Laboratory for Atomic, Molecular and Optical Physics, Torun, for rendering possibility of using the Jobin–Yvon Spectrofluorometer to the measurements of the luminescence excitation spectra.
References 4F
7/2
1725 cm-1
930 cm-1
4F
5/2
Fig. 13. A diagram showing how three phonons can span the energy gap between the 4F7/2–4F5/2 states.
[1] L. Del Longo, M. Ferrari, E. Zanghellini, M. Bettinelli, J.A. Capobianco, M. Montagna, F. Rossi, J. Non-Cryst. Solids 231 (1998) 178. [2] M.J. Weber, J. Non-Cryst. Solids 42 (1980) 189. [3] M. Naftaly, A. Jha, J. Appl. Phys. 87 (2000) 2098. [4] D. Ehrt, Curr. Opin. Solid State Mater. Sci. 7 (2003) 135. [5] Cz. Koepke, K. Wisniewski, M. Naftaly, J. Phys.: Condens. Matter 14 (2002) 6785. [6] Cz. Koepke, K. Wisniewski, P. Turbak, A. Sobocinska, M. Naftaly, Influence of the oxygen-affected sites on decay times in Pr3+-activated fluoroaluminate glass, J. Lumin. (2005), in press.
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[7] Cz. Koepke, A. Lempicki, A.J. Wojtowicz, Phys. Stat. Sol. (b) 179 (1993) 233. [8] B. Henderson, R.H. Bartram, Crystal-Field Engineering of SolidState Laser Materials, Cambridge University Press, Cambridge, 2000, p. 188. [9] F. Auzel, Phys. Rev. B 13 (1976) 2809. [10] A.S. Oliveira, E.A. Gouveia, M.T. de Araujo, A.S. GouveiaNeto, Cid B. de Arau´jo, Y. Messaddeq, J. Appl. Phys. 87 (2000) 4274.
[11] L. de S. Menezes, G.S. Maciel, Cid B. de Arau´jo, Y. Messaddeq, J. Appl. Phys. 90 (2001) 4498. [12] M. Naftaly, C. Batchelor, A. Jha, J. Lumin. 91 (2000) 133. [13] R. Rolli, S. Ronchin, M. Montagna, E. Moser, C. Duverger, V.K. Tikhomirov, A. Jha, M. Ferrari, J. Non-Cryst. Solids 280 (2001) 269. [14] A.T. Stanley, E.A. Harris, T.M. Searle, J.M. Parker, J. NonCryst. Solids 161 (1993) 235.
Upconverted luminescence under 800 nm laser diode excitation in Nd3+-activated fluoroaluminate glass Cz. Koepke
b
a,*
, K. Wisniewski a, L. Sikorski a, D. Piatkowski a, K. Kowalska a, M. Naftaly b
a Institute of Physics, N. Copernicus University, Grudziadzka 5/7 87-100 Torun, Poland Department of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK
Received 28 September 2004; accepted 20 October 2004 Available online 16 June 2005
Abstract We report on the upconverted luminescence in neodymium-activated fluoroaluminate glass obtained with 800 nm diode laser excitation. Several anti-Stokes emissions: at 588, 607, 720 and 750 nm are observed and appropriate transitions are assigned. For both latter emissions we observe strong dependence on temperature: the 720 nm emission intensity decreases with temperature, whereas the 750 nm emission increases. Interpretations are presented in terms of the influence of oxygen-affected sites on the radiationless transitions and multiphonon anti-Stokes excitation. The models provide reasonable fits to the experimental data. 2005 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 42.70.Ce; 78.40.Ha; 78.55.Hx Keywords: Laser materials; Upconversion; Spectroscopy; Neodymium; Fluoride glasses
1. Introduction Glass hosts are very attractive for laser applications because of their cheapness and relatively easy fabrication in various geometries (especially in the form of fibres). Moreover, when activated by rare earths (RE), glasses offer a variety of dopant sites with strong ion– host interactions, with the result that the emission and absorption lines are Stark split [1] and inhomogeneously broadened. This property manifests in the relatively large widths of emission and absorption lines (200– 400 cm1), which, together with the possible partial overlap, makes RE doped glasses good candidates for tunable and ultrafast laser medium. In this context one of the most interesting RE ions is Nd3+ whose lasing and light amplifying in the glass host has been known *
Corresponding author. Tel.: +48 56 611 3239; fax: +48 56 622 5397. E-mail address: [email protected] (Cz. Koepke).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.10.034
and exploited for years (e.g. [2,3]). Neodymium-glass lasers are employed to produce ignition in nuclear fusion research; high-power lasers have found numerous applications in materials processing, and femtosecond mode-locked lasers have recently become commercially available. All these types of Nd-glass lasers use fluorophosphate glass and operate at 1050–1070 nm. Due to the commercial and technological importance of Ndglass lasers, a large number and variety of Nd-doped glasses have been investigated over the years. A new insight into the behaviour of the Nd3+ ion in a glass host has the potential to advance the science and technology of Nd-glass lasers. The energy diagram of the Nd3+ ion is very ‘‘dense’’ in terms of the number of possible states, so that various types of transitions between them, including upconversion, take place. Although a large number of emission lines have been observed to date only the 1.06 lm band has found widespread commercial applications.
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
4
S3/2 ,4F7/2
4 4
D1/2, 4D3/2, 4D5/2
4
2
0.1
G9/2 , G11/2, D3 / 2
G7/2 ,2K13/2
F3/2
4
2 2
200
400
4 F H11/2 9/2
2
600
800
1000
800
1000
absorbance (-log10T)
wavelength (nm)
0.2
0.1
200
400
b
600 wavelength (nm)
Fig. 1. Absorption spectra of the Nd3+-activated fluoroaluminate glasses; Glass 1 (a) and Glass 2 (b). The dashed lines show the wavelength used for excitation when examining anti-Stokes emissions.
3. Spectroscopic measurements
Excitation: EXC=514.5 nm detection: OMA system intensity (arb.units)
The absorption spectra for both examined glasses are illustrated in Fig. 1, and show the characteristic Nd3+ lines due to the 4G5/2, 4D3/2, 4F3/2,5/2,7/2 states, and weaker lines due to the 2P1/2, 4G11/2, 4F9/2 or 2H11/2 states. Also notable is the very high lying conduction band of the glass, typical of fluoride glasses, the material being transparent in the UV up to 230 nm. In addition, Glass 2 has a distinct absorption band at 250 nm, which is due to the phosphate component in the glass. The emission detected using the 514.5 nm excitation line is very similar in both glasses and an example is presented in Fig. 2, where two distinct bands spreading from 850 to 940 nm and from 1030 to 1090 nm dominate the spectrum. Fig. 3 presents an example of a luminescence excitation spectrum monitored at k = 860 nm. In Fig. 4a
F5/2 ,2H9/2
G5/2 ,2G7/2
a
and the respective indices of refraction were: nd = 1.392 and nd = 1.428. Fabrication of these glasses is described in [3]. The glass was doped with 1 mol% of NdF3 which substituted for AlF3 (Glass 1) or YF3 (Glass 2). The standard spectroscopic characteristics such as absorption and emission spectra were measured by an OMA-type spectrometer (ORIEL InstaSpec II). In addition a Perkin–Elmer Lambda 2 UV–VIS Spectrometer was used to observe the absorption spectra, and a Jobin Yvon FLUOROLOG-3 Spectrofluorometer system recorded the luminescence excitation spectra. The measurements of low-temperature emission spectra and their temperature dependence were performed using a closed-cycle helium refrigerator (APD Cryogenics Inc. HC-2D-1/DE-202 system) and the ORIEL InstaSpec II/MultiSpec 1/8 m fibre spectrometer.
4
4
The Nd3+-doped fluoroaluminate glass has been developed and produced at the University of Leeds, UK. The composition of the glass samples used in the present work was: Glass 1: 39AlF3, 6MgF2, 22CaF2, 6SrF2, 6BaF2, 10NaF, 10LiF, 1NdF3 Glass 2: 37AlF3, 12MgF2, 15CaF2, 9SrF2, 6BaF2, 14YF3, 6NaPO3, 1NdF3
I9/2 → to:
0.2
P1/2 , D5 / 2
2. Material preparation and experimental setup
4
4
In this paper we present spectroscopic results in Nddoped fluoroaluminate glass, which due to its structure may offer an insight into the interactions of neodymium ions residing in fluorine and oxide sites, as explained below, with potential implications for commercial fluorophosphates glasses.
absorbance (-log10T)
130
750
nd
2 order (laser)
800
850
900
950
1000
1050
1100
1150
wavelength (nm) Fig. 2. Emission spectrum observed using argon ion laser excitation (514.5 nm) (Glass 1).
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
intensity (arb.units)
80000 OBS
= 860 nm
60000
40000
20000
0 300
400
500 600 700 wavelength (nm)
800
Fig. 3. Luminescence excitation spectrum monitored at 860 nm. The dashed line in the region 415–450 nm is inserted to replace the lamp line seen in the second grating order.
24000
0.03
intensity (arb.units)
0.02
20000
0.01 0.00 400
500
600
700
800
4. Discussion and interpretations 4.1. Upconverted emission in the range 570–620 nm
8000 500
550
a
600
650
700
wavelength (nm) wavelength (nm) 20000
630
600
570 λEXC= 800 nm
15000
intensity (arb.units)
and b we show the room-temperature (RT) upconverted luminescence using a 800 nm 500 mW laser diode as an excitation source. In the inset of Fig. 4a is shown upconverted luminescence at 570–620 nm with a local, distinct emission peak at 750 nm, which appears on the anti-Stokes side of excitation. Fig. 4b compares the RT and low temperature (LT, 10 K) upconverted luminescence at 570–620 nm. Fig. 5a and b depicts the results of more systematic luminescence measurements focusing attention on the region of 680–760 nm. We have obtained a series of emission spectra for two different glass samples measured at various temperatures using excitation into the 4 F5/2 state at 800 nm (diode laser). A distinct decrease with temperature of the 720 nm luminescence is observed for both samples, while starting from T = 300 K a new emission peak appears around 750 nm. A more detailed view of the 720 nm emission decrease in shown in Fig. 6. Fig. 7 shows a series of the anti-Stokes emission bands peaking around 750 nm when varying the temperature in the range 350–600 K. Notably, the decrease of the 720 nm emission and the increase of the 750 nm emission occur at two different ranges of temperature, so that they appear to be quite independent tendencies.
16000
12000
T = 300 K
10000 T = 10 K
We have carried out a very careful inspection of the absorption and emission spectra obtained with sufficient spectral resolution to create a detailed energy diagram of Nd3+ in our fluoroaluminate glass. This is illustrated in Fig. 8 together with an assignment of transitions responsible for the observed spectral lines. In assigning these transitions we have taken into account the fact that the spectrum seen in Fig. 4a and b consists of two lines of quite different widths. Hence we assume that we deal with transitions which probably originate not from one state, but from entirely different states, in this case: 4G7/2 + 2K13/2 and 4G5/2 + 2G7/2. Moreover, from the inspection of the diagram in Fig. 8 we conclude that the final states for these transitions should also be different: 4I11/2 and 4I9/2 respectively.
5000
4.2. Decrease of the 720 nm upconverted emission 0 15500
b
131
16000
16500 17000 wave number (cm-1)
17500
Fig. 4. Upconverted emission spectra at 585 nm and 607 nm obtained with 800 nm laser diode excitation; inset in (a) shows the scale in comparison with the anti-Stokes emission at 750 nm and (b) shows the emission at two temperatures: 300 K and 10 K.
We have assigned the 720 nm emission to transitions from the 2H11/2 state to two terminal Stark sublevels of the 4I11/2 state. The double character of the terminal state of the 720 nm emission can be deduced from Fig. 6 (only two Gaussians fit the spectra perfectly). We have tested several models that could explain the observed decrease of emission with temperature, especially models
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wavelength (nm)
wavelength (nm) 750
720
690
750
660
720
690
660
50 K
10 K
100 K 150 K 200 K 250 K 300 K
50 K 100 K 150 K 200 K 250 K 300 K
intensity (arb.units)
intensity (arb.units)
10 K
= ~ 800 nm
= ~ 800 nm
EXC
EXC
13500
14000
14500
15000
wave number (cm-1)
a
13000
13500
14000
14500
wave number
b
15000
(cm-1)
Fig. 5. Decrease of the 720 nm emission intensity with temperature and the partial increase of the 750 nm emission intensity for two materials: Glass 1 (a) and Glass 2 (b). Excitation wavelength for both cases is 800 nm (laser diode).
wavelength (nm) 750
720
EXC
wavelength (nm)
690
660
750
= ~ 800 nm
720
690
600 K 550 K
10 K 50 K 100 K
EXC
intensity (arb.units)
intensity (arb.units)
660
150 K 200 K 250 K 300 K
= ~ 800 nm
500 K
450 K 400 K 350 K
13500
14000
14500
15000
wave number (cm-1) Fig. 6. A more detailed view of the 720 nm emission decrease; a steep increase of the 750 nm emission is seen at T = 300 K.
based on Boltzmann distribution of inhomogeneously broadened densities of states and the exponential energy gap law, and have found that none of these explain the observed results. It appears that the only model capable of explaining the interesting behaviour of the 720 nm emission is an approach based on the assumption that an additional state exists which enables additional paths for radiationless transitions selectively from the 2H11/2
13000
13500
14000
14500
15000
-1)
wave number (cm
Fig. 7. Increase of the 750 nm emission intensity with temperature. The excitation wavelength is 800 nm (laser diode).
state. Such a situation can be qualitatively illustrated by a configuration coordinate diagram as presented in Fig. 9. The origin of the introduced new state associated with ‘‘FRE-O site’’ is in unintentional oxygen ions that become incorporated in the glass during processing and interfere in regular fluorine-coordinated sites. Since both examined glasses contain a substantial amount of such
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
Fig. 8. Energy levels diagram of the upconverted transitions in the 570–620 nm range.
2
FRE-F site
2
P 1/2
H 11/2 FRE-O site
4
800 nm
F 5/2
133
but activated with Pr3+. As is seen, the FRE-O site can provide paths for radiationless transitions via tunnelling through the barriers of various heights (widths) dependent on the state. It may be assumed, as schematically depicted in Fig. 9, that the suitable barrier corresponding to the 2H11/2 state is the lowest one, selectively providing for this state an effective path for radiationless transitions. On the other hand, in one of our glasses (Glass 2) a NaPO3 phosphate group is added to the composition. An obvious difference due to this modification is seen in the absorption spectrum, where a distinct shoulder appears around 250 nm (Fig. 1b). Further differences are seen in Fig. 5 and especially in Fig. 10, in the way the 720 nm emission decreases with temperature. In the range 200–250 K the emission intensity in Glass 1 undergoes a rather sudden drop whereas in Glass 2 the emission decrease occurs smoothly and gradually. This would suggest that in Glass 1 we deal with a well defined (though relatively small, see Fig. 9) barrier for thermal activation, whereas in Glass 2 such barrier to radiationless transitions is much smaller or even absent altogether. Hence, in the light of Fig. 10 we can say that our model illustrated in Fig. 9 applies to both glasses, but that in Glass 2 there exists an additional pathway for radiationless transitions. In this pathway, the energy barrier for escape from the 2H11/2 state is much smaller than that in Glass 1 (or possibly absent). Moreover, qualitatively identical behaviour is observed for Stokes emission (4F3/2 ! 4I9/2, 4I11/2). The emission intensity drops suddenly between 200 and 250 K in Glass 1, whereas in Glass 2 the intensity decrease is much smoother. This confirms that the effect is of a more universal, unselective character and concerns multiple states (e.g. 2H11/2 and 4F3/2). The occurrence of this effect
720 nm 14000 4
I9/2 „+ ” I11/2
Fig. 9. The proposed model (configuration coordinate diagram) explaining the decrease of the 720 nm emission with temperature.
unintentional oxygen, we can assume that Nd3+ ions can reside in two types of sites, in one (which we call FRE-F) the Nd3+ is in fluorine coordination forming a regular site responsible for all known spectral features of the material. Another type of site (FRE-O) is an oxygenaffected site, where Nd3+ resides in a modified coordination where one fluoride ion is substituted by oxygen. Thus the FRE-O site is characterized by different phonons and plays the role of a ‘‘defect site’’, similar to that which we introduced in [5,6] for the same type of glass
Intensity (arb.units)
4
12000
10000
8000
6000 0
50
100
150
200
250
300
Temperature (K) Fig. 10. An example of fit using Eq. (2) to the experimental data for the 720 nm emission decrease with temperature. Circles correspond to Glass 1, squares to Glass 2.
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can be connected with the presence of phosphate groups in the glass. Phosphates are strong network formers, capable of forming molecular chains which thereby create another type of FRE-O sites, most likely to be high phonon energy sites for Nd3+ ions. Examples of such sites are shown in [4, Fig. 1]. The concentration of these sites is quite large (6% NaPO3 in the composition of Glass 2). Thus, in the context of high concentration of phosphate FRE-O sites in Glass 2, an additional path for radiationless transitions which has unselective character independent of the initial excited state, is most likely due to energy transfer from exited states of FRE-F sites to unexcited FRE-O sites. In the situation common for both glasses we can write the following kinetic equations for the states as seen in Fig. 9: dn1 dt dn2 dt dn3 dt dn4 dt
¼ Wn1 þ a41 n4 ¼ Wn1 Wn2 ð1Þ ¼ Wn2 a34 n3 ¼ a34 n3 a41 n4 n4 s expðDE=kT Þ
where ni, (i = 1, 2, 3, 4) are the populations of the ground state and the 4F5/2, 2P1/2 and 2H11/2 states respectively, W is the excitation parameter (pumping rate), and aij are spontaneous transitions rates from jii to jji states. The two parameters in the last equation, which are s—the frequency factor and DE—the height of the barrier between the 2H11/2 and the FRE-O state1, represent the additional path for radiationless transitions. Assuming that N = n1 + n2 + n3 + n4, where N is the total concentration of ions taking part in the described process, we can obtain the following stationary solution: WN n4 ðT Þ ¼ W 2 þ a34 ½a41 þ s expðDE=kT þ W
ð2Þ
Fig. 10 presents an example of curve fits to the experimental points taken from Fig. 5a and b using the above formula and assuming that the emission intensity I is proportional to n4. The fitting parameters are: W = 103 s1, N = 1020 cm3, a34 = a41 = 106 s1, s = 6 · 106 s1, DE = 400 cm1. Circles and squares represent data from different glass samples. Note that for simplicity we use the same set of fitting parameters for both glasses, just to show that the model works in both cases.
4.3. Increase of the 750 nm anti-Stokes emission Fig. 11 illustrates our understanding of the mechanism of temperature dependent excitation of the state emitting at 750 nm. Such multiphonon anti-Stokes excitation mechanism has been proposed by Auzel [9] and exploited at least twice [10,11] to explain the same or similar phenomena as observed here. The kinetics of this system can be expressed by the following set of equations: dn1 ¼ Wn1 þ a31 n3 dt dn2 ¼ Wn1 b23 ðT Þn2 dt dn3 ¼ b23 ðT Þn2 a31 n3 dt where b23 ðT Þ ¼ b023 ðehx=kT 1Þp .
ð3Þ
Here the populations of the states 4I9/2, 4F5/2 and 4F7/2 are represented by ni (i = 1, 2, 3) respectively, W is pumping parameter (rate), aij are respective spontaneous transitions rates, b023 is the stimulated transition rate at T = 0 K, and p is the number of phonons of energy hx taking part in the process. Assuming that N = n1 + n2 + n3, we obtain the following stationary solution of the set (3): n3 ðT Þ ¼ W
WN
a31 b023
p ½ehx=kT 1 þ 1 þ a31
ð4Þ
Note that here we treat the total concentration of ions taking part in the process in the same way as for the process described in Section 4.2. In view of Fig. 5a and b, where the 750 nm emission starts to increase at the temperature where the 720 nm emission has already stopped decreasing, we can say that this simplifying assumption is justified. In trying to fit the curve described by formula (4) to the experimental points taken from Fig. 7 (bearing in mind the assumption that I n3), we need to choose
4
F7/2
3
930 cm-1 4
2
800 nm
F5/2
750 nm
1
We use the simplest standard Mott–Seitz (Arrhenius) approach to describe radiationless transitions just to show that there is some merit in assuming the existence of a state capable of removing some part of population from the emitting (2H11/2) state. A proper approach would use a more sophisticated formula than that used in (1) (see e.g. [7,8]).
1
4I
9/2
Fig. 11. A model showing anti-Stokes multiphonon excitation which leads to the 750 nm emission growth with temperature.
Cz. Koepke et al. / Optical Materials 28 (2006) 129–136
the number of phonons p that are ‘‘borrowed’’ by the ion system from the glass matrix in order to complete the excitation. In Fig. 12 we present the fitting results for three phonon numbers: p = 2, 3 and 4. It is seen that p = 3 gives by far the best fit. Moreover, only the p = 3 assumption produces the characteristic inflected curve, as in the experimental data. The remaining parameters for this fit are: W = 103 s1, N = 1020 cm3, a31 = 5 · 105 s1, b023 ¼ 5 101 s1 , hx ¼ 580 cm1. Finally, taking the energy gap between the states 4F7/2 and 4F5/2 to be 930 cm1, as derived from our absorption spectrum, and taking into account their distribution as seen in absorption/emission spectra, we can determine the maximum distance between the tail-ends of the distributions to be 1725 cm1 (Fig. 13). Dividing this value by p = 3 we obtain the maximum phonon energy as hx ¼ 575 cm1. This is in perfect agreement with previ ous estimations for the maximum phonon energy in this type of glass (580–600 cm1, [12,13]).
60000
p= 4
Intensity (arb.units)
50000 40000
p
=
3
30000 20000
p=2
10000
0
350
400
450
500
550
600
Temperature (K) Fig. 12. An example of a fit using Eq. (4) to the experimental data for the 750 nm emission increase with temperature. Dashed lines show solutions for different numbers of phonons p.
135
5. Conclusions In the present work we have described two different types of anti-Stokes emission, one increasing with temperature (with a peak at 750 nm), the other decreasing with temperature (with a peak at 720 nm). The maxima of these emissions are separated by only 550 cm1, which makes their opposite behaviour even more surprising. Careful transition assignment based on observed spectral characteristics shows that the two emissions originate from the states 4F7/2 and 2H11/2 respectively, and that their respective terminal states are 4I9/2 and 4I11/2. We have proposed models to explain the opposite tendencies in the behaviour of these emissions and obtained satisfactory agreement with experiment. We have also observed upconverted emissions at 588 nm and 607 nm and assigned them to the transitions 4G7/2 + 2K13/2 ! 4I11/2 and 4G5/2 + 2G7/2 ! 4 I9/2 respectively. In addition, we have noted a difference in the emission behaviour of our two glasses. The behaviour was observed in two separate transitions, and was related to the presence of phosphate groups in the glass. The effect may be attributed to the presence of an additional pathway for radiationless transitions in phosphate-containing glass. It is perhaps worth noting that we have obtained all the described results using a relatively cheap source of excitation: a 800 nm laser diode, instead of a sophisticated and expensive Ti-Sapphire laser. There are only a few publications using such a source, of which we should mention e.g. [14].
Acknowledgements We are grateful to Prof. S. Chwirot, Head of the National Laboratory for Atomic, Molecular and Optical Physics, Torun, for rendering possibility of using the Jobin–Yvon Spectrofluorometer to the measurements of the luminescence excitation spectra.
References 4F
7/2
1725 cm-1
930 cm-1
4F
5/2
Fig. 13. A diagram showing how three phonons can span the energy gap between the 4F7/2–4F5/2 states.
[1] L. Del Longo, M. Ferrari, E. Zanghellini, M. Bettinelli, J.A. Capobianco, M. Montagna, F. Rossi, J. Non-Cryst. Solids 231 (1998) 178. [2] M.J. Weber, J. Non-Cryst. Solids 42 (1980) 189. [3] M. Naftaly, A. Jha, J. Appl. Phys. 87 (2000) 2098. [4] D. Ehrt, Curr. Opin. Solid State Mater. Sci. 7 (2003) 135. [5] Cz. Koepke, K. Wisniewski, M. Naftaly, J. Phys.: Condens. Matter 14 (2002) 6785. [6] Cz. Koepke, K. Wisniewski, P. Turbak, A. Sobocinska, M. Naftaly, Influence of the oxygen-affected sites on decay times in Pr3+-activated fluoroaluminate glass, J. Lumin. (2005), in press.
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[7] Cz. Koepke, A. Lempicki, A.J. Wojtowicz, Phys. Stat. Sol. (b) 179 (1993) 233. [8] B. Henderson, R.H. Bartram, Crystal-Field Engineering of SolidState Laser Materials, Cambridge University Press, Cambridge, 2000, p. 188. [9] F. Auzel, Phys. Rev. B 13 (1976) 2809. [10] A.S. Oliveira, E.A. Gouveia, M.T. de Araujo, A.S. GouveiaNeto, Cid B. de Arau´jo, Y. Messaddeq, J. Appl. Phys. 87 (2000) 4274.
[11] L. de S. Menezes, G.S. Maciel, Cid B. de Arau´jo, Y. Messaddeq, J. Appl. Phys. 90 (2001) 4498. [12] M. Naftaly, C. Batchelor, A. Jha, J. Lumin. 91 (2000) 133. [13] R. Rolli, S. Ronchin, M. Montagna, E. Moser, C. Duverger, V.K. Tikhomirov, A. Jha, M. Ferrari, J. Non-Cryst. Solids 280 (2001) 269. [14] A.T. Stanley, E.A. Harris, T.M. Searle, J.M. Parker, J. NonCryst. Solids 161 (1993) 235.