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Role of monovalent alkali ions in the Yb3+ centers of CaF2 laser crystals
Radiation Measurements 45 (2010) 323–327
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Role of monovalent alkali ions in the Yb3þ centers of CaF2 laser crystals Sana Hraiech a, Anis Jouini a, b, Kyoung Jin Kim b, Yannick Guyot a, Akira Yoshikawa b, Georges Boulon a, * a b
Physical Chemistry of Luminescent Materials (LPCML), University of Lyon, Claude Bernard/Lyon 1 University, UMR 5620 CNRS, La Doua, 69622 Villeurbanne, France IMRAM, Tohoku University, 2-1-1, Katahira, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2009 Received in revised form 26 September 2009 Accepted 9 November 2009
Yb3þ and Mþ monovalent alkali ions (Mþ ¼ Liþ, Naþ, Kþ)-co-doped CaF2 cubic laser crystals were grown by the micro-pulling-down method (m-PD) under CF4 atmosphere. Structural and spectroscopic characterizations of Yb3þ in substitution of Ca2þ (absorption, emission and decay curves) were carried out to study the effect of Mþ ions as charge compensators. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: CaF2 Ytterbium Charge compensator Multisites Crystal growth Spectroscopy Laser crystal
1. Introduction During the last decade, with advances of high performance GaAs and InGaAs laser diodes emitting wavelengths between 900 and 1100 nm, interest on trivalent ytterbium (Yb3þ ion)-doped crystals has been renewed for applications in high-efficiency (>50%) and high power (>50 W/cm) diode-pumping laser systems. The trivalent ytterbium is the most promising ion that can be used in nonNd3þ laser in the same range, it have some advantages over the Nd3þ ions due to its very simple energy level scheme, only two levels: the 2F5/2 ground state and the 2F7/2 excited state. There is no excited state absorption reducing the effective laser cross-section, no up-conversion, no cross-relaxation. The intense absorption lines are well suited for laser diode pumping near 980 nm and the small quantum defect between absorption and emission (about 650 cm1 ), much smaller than Nd3þ- doped laser hosts, reduces the thermal loading of the material during laser operation (Boulon, 2008). In order to exploit all these favorable properties we need an efficient high-power laser-diode emission matching the absorption band of the Yb3þ ion. This need was fulfilled by diode lasers based on InGaAs quantum wells emitting in the 900–980 nm region, developed in the 1990s to pump Er3þ-doped fiber amplifiers for
* Corresponding author. E-mail address: [email protected] (G. Boulon). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.11.017
commercial use in the telecommunications industry (Wu et al., 1990). Among several possibilities with broadband absorption suitable for laser diode pumping, we have focused our attention on CaF2 (fluorite structure) which is a well-known fluoride crystal with the advantages of a wide transparent wavelength range (0.15–9 mm), low phonon energy (w320 cm-1) and high thermal conductivity (10 W m-1 K-1) as large as YAG at the same time. In our previous studies, Ca1-xYbxF2þx crystals were grown by either simple melting or by laser heated pedestal growth (LHPG) technique (Ito et al., 2004; Boulon et al., 2006) under Ar atmosphere by growing Yb3þ concentration gradient fiber in order to understand involved concentration quenching mechanisms. In addition, high-quality large size crystals (more than 10 inches) have also been grown by Czochralski technique under CF4 atmosphere (Jouini et al., 2008). Laser performances were obtained as tunable laser operation between 1030 and 1070 nm in samples grown by Czochralski with a slope efficiency of 57% and various transmission output coupler (Jouini et al., 2008). Similar results of diode-pumped CW laser operation were reported with a 5at.% Yb3þ-doped sample: 5.8 W output power has been obtained at 1053 nm for 15 W of incident power at 980 nm and a small-signal gain as high as 1.8 was achieved, confirming the great potential of Yb3þ-doped CaF2 as an amplifier medium for femtosecond pulses (Lucca et al., 2004). Nevertheless, in such samples trivalent Yb3þ activator ion is substituting divalent Ca2þ site and charge compensation is needed. The first idea to keep good crystal quality is to use monovalent
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charge compensator ions like Liþ, Naþ, Kþ alkali ions in substitution of one of the eight nearest neighbour Ca2þ ions along any [1 1 0] direction (Kirton and Mclaughlan, 1967). This is our approach in the present work as a continuation of previous efforts within the framework of collaboration between the Japanese (IMRAM) and the French (LPCML) laboratories on Yb3þdoped single crystals as KY3F10 (Ito et al., 2007), LiYF4 (Bensalah et al., 2007) fluorides or Sc2O3 sesquioxide (Simura et al., 2007), GGG garnet (Guyot et al., 2006), YAP perovskite (Boulon et al., 2008) for laser applications. Co-doped crystals have been grown by a specific technique, the m-PD technique, allowing relatively fast growth of high quality samples with different compositions (Yoshikawa et al., 2007) allowing us to compare results with samples already grown by the classical Czochralski technique (Su et al., 2005a,b). In first hand, our main objective was to know more on the real behaviour of various Yb3þ multisites which might be created in compensated Yb3þ-doped CaF2 and on the tendency of clustering especially in our samples. It has been already established since long time that trivalent rare-earths-doped CaF2 form fluorite solid solutions. Then, such centers might be involved in concentration quenching of luminescence which is a key knowledge for laser crystals (Boulon, 2008; Kaczmarek et al., 2005). This is why we analyze the evolution of the formation of pairs, considered as the simplest rare-earth clusters with the association of two neighbour ions at 3.6 Å, in both Yb3þ-doped CaF2 samples and compensated Yb3þ-doped CaF2, by using an original spectroscopic way based on co-operative phenomena.
2 at% and MF (M ¼ Na, Li, K) of 0, 10 and 20 at% in the starting materials have been used. Graphite crucible is surrounded by refractory carbon and inductively heated using a radio-frequency generator. The starting materials were thoroughly mixed and put into the crucible. The chamber was evacuated to 104 Torr and the crucible was heated to 600 C for 1 h to remove oxygen impurities. During this baking procedure, the chamber is further evacuated up to 10-5 Torr. After the baking, the recipient was filled with high purity CF4 (6N) until ambient pressure and the crucible was heated up to the melting temperature of about 1030 C. Crystal growth was carried out using an a-axis seed of un-doped CaF2 crystal. The growth rate was 0.1–0.15 mm/min. Controlling power and pulling rate during the growth process constantly maintained the crystal diameter. The crystals obtained were transparent colorless crystals. Their size was a few centimeters and they were cut and polished for optical experiments. 2.2. Phase characterization To identify the obtained phase, powder X-ray diffraction analysis was carried out in the 2q range from 10 to 70 using a RIGAKU diffractometer (RINT2000). The X-ray source was CuKa. The chemical composition was analyzed by electron probe microanalysis (EPMA) using a JEOL JXA-8600L analyzer. The distribution of Yb in the single crystal was measured along the growth axis using an electron probe of 10 mm in diameter and a step of 250 mm.
2. Experimental procedure 2.3. Optical measurements 2.1. Growth using the m-PD method CaF2 single crystals were grown by the micro-pulling-down (mPD) method (Yoshikawa et al., 2007). YbF3 dopant concentration of
Absorption spectra were measured with a Lambda 900 spectrophotometer equipped with a continuous flow helium refrigerator. Emission spectra were acquired by exciting the samples with
0.25 2
7
(O )
0.15
h
1 -> 6
6
0.10
5
4 3 2
F7/2
σ abs x 10-20 (cm2)
0.05
2
+ 2.91 % Na
1 -> 5
1 -> 7
0.20
F5/2
0.00 1 -> 5
0.6
(C3v )
0.4
1 -> 7
1 -> 6
0.2 0.0 1 -> 5
0.6
1 Absorption
+ 0 % Na
(C3v )
Emission
Yb3+
+ 0.67 % Na
0.4 1 -> 7
0.2
1 -> 6
0.0 880
920
960
1000
1040
Wavelength (nm) þ
Fig. 1. Na concentration dependence of the 12 K absorption spectra in 2% Yb
3þ
-doped CaF2. If Na concentration increases, Yb3þ ions occupy Oh site symmetry instead of C3v. þ
S. Hraiech et al. / Radiation Measurements 45 (2010) 323–327 3+
refrigeration system (SMC, TBT air–liquid) was used for the low temperature (12 K) measurements. The visible emission was detected by a ORIEL spectrometer coupled with a gated intensified CCD camera (ANDOR/ORIEL INTASPEC V ICCD). The decay curves were recorded with a Lecroy LT 342 digital oscilloscope.
+
CaF2 : 2%Yb - 2.91 % Na C4v vibronic line
O
h
Oh
C3v
325
Intensity (a. u.)
3. Results and discussion
C3v
3+
5 -> 1 5 -> 2
Due to the strong evaporation of NaF, LiF and KF from the melt at the temperature above 1400 C, the concentration of Mþ alkali cations incorporated into the crystals are much lower than those in the starting materials. As an example, in initial 20%Naþ–2%Yb3þ codoped CaF2, the measured concentrations are 2.91% Naþ–2.13%Yb3þ co-doped CaF2. The same tendency was observed at any Naþ concentration and also for Liþ and Kþ ions.
vibronic line 5 -> 3
Oh C2v
C3v
5 -> 4
5 -> 2 5 -> 1
3+
CaF2 : 2%Yb
3.2. Evidence of Yb3þ multisites from compensator alkali ions
5 -> 3
C2v
960
3.1. Evaporation of alkali cations
+
CaF2 : 2%Yb - 0.67 % Na
5 -> 4
980
1000
1020
1040
1060
The creation of several symmetry sites with rare earth trivalent ions like Yb3þ ions has been studied a long time ago. Just to be short, multisite structure is induced by several types of charge compensating due to interstitial F ions and various clusters. The main symmetry sites which have been characterized are the neighbour (C4v), next nearest neighbour (C3v) or (C2v) and also high symmetry cubic site (Oh). The last paper on the refined analysis of the luminescent centers in the Yb3þ:CaF2 laser crystal has been published by Petit et al. (2007). We will use it as a reference for the position of emission transitions in the compensated samples we are analysing. In this article, we choose to relate data directly in figures as a condensate way for some selected samples.
1080
Wavelength (nm) Fig. 2. Na concentration dependence of the 12 K emission spectra in 2% Yb3þ-doped CaF2. If Naþ concentration increases, Yb3þ ions occupy Oh site symmetry instead of C3v and C2v.The 0-phonon line assigned as the resonant 5 / 1 transition of Oh site symmetry is clearly observed at 964 nm. þ
a QUANTEL Nd3þ: YAG pumped dye laser using a mix of LD700 and DCM dye. The dye laser light was converted to the 930–980 nm region by way of a H2 Raman shift cell. The infrared emission was detected in a perpendicular direction to the excited beam with a Jobin-Yvon HRS1 monochromator equipped with a 1000 nm blazed grating, with a cooled Ge-detector, and the signal was processed with a STANDFORDS boxcar SRS 250. A closed circuit helium
3.2.1. Infrared (IR) absorption properties of Yb3þ ions As an example Fig. 1 shows absorption spectra in the nearinfrared region at low temperature (12 K). They correspond to the Yb3þ ion transitions from the 2F7/2 ground state to the 2F5/2 excited state which have been already detailed in literature. We did not note any change of absorption spectra in Yb3þ- (Liþ/Kþ)-co-doped
1
λ exc = 926 nm
Intensity (u. a.)
12 K
0,1
0,01
λ ém = 962 nm
τ = 12.14 ms (Oh )
λ ém = 980 nm
τ1 = 5.09 ms (C4v)
τ2 = 10.52 ms (Oh) λ ém = 1022 nm τ = 12.18 ms (Oh) 0,000
0,005
0,010
0,015
0,020
0,025
0,030
Time (s) þ
Fig. 3. Emission wavelengths dependence of the 12 K decays in 2.91% Na -2%Yb
3þ
co-doped CaF2 under 926 nm pumping. We characterize Oh and C4v site symmetries.
S. Hraiech et al. / Radiation Measurements 45 (2010) 323–327
CaF2. But after introducing NaF, the differences among absorption spectra are clearly seen. With the addition of 0.67% NaF in Yb: CaF2, the strongest absorption band peaking at 981 nm assigned as the 0-phonon line of the trigonal site (C3v) shift to shorter wavelengths (981 / 980 nm). This line disappears completely after introducing 2.91 at.% NaF associated to the absence of Yb3þ ions into C3v site. In this case Yb3þ ions occupy the cubic Oh site with 1 4 5 resonant transitions lines located at 964 nm. The room temperature absorption spectra of (Yb3þ–Naþ) codoped CaF2 crystals have also been studied. We observe broad absorption lines, due to the well-known multisite effect. Increasing the concentration of Naþ as charge compensator to 2.91 at% caused the absorption bands to be very narrow and clearly resolved. The intensity of the C3v zero-phonon line located at 980 nm decreases and becomes very weak, contrary to the shoulder observed at 965 nm which intensity increases. This peak is assigned to the 1 / 5 transition characteristic of cubic Oh site. Then as well at RT as 12 K, this indicates that Yb3þ-doped CaF2 crystal co-doped with high NaF amount reveals high symmetries sites. The shape of the absorption spectra as well as the cross sections values at RT are comparable with those given by Su et al. on crystals grown by Czochralski technique (Su et al., 2005a,b). 3.2.2. IR emission properties As an example, Fig. 2 shows the fluorescence spectra of the crystals Naþ–Yb3þ co-doped CaF2 under excitation at 920 nm recorded at low temperature (12 K). Three samples were measured under the same conditions to compare the luminescence intensity. The 1 4 5 resonant zero-phonon line of the tetragonal C4V site located at 969 nm, are not detected above 2%Yb3þ. This site is only observed for the lowest concentration (Petit et al., 2007). Addition of 0.67% Naþ reveals at low temperature a line at 963 nm associated to the 5 / 1 zero-phonon line attributed to the Oh cubic site. The spectrum of the sample doped by 2.91% Naþ the highest concentration, is different from the two other ones and present two additional lines located towards 969 nm and 1026.5 nm corresponding to the C4v site. The shoulder located at 1028 nm characterises also of the Oh site (Petit et al., 2007). The electronic transitions 5 / 4 and 5 / 3 observed at 1050 nm and 1033 nm respectively on the spectrum of 2% Yb3þ-doped CaF2 decrease in intensity with the increase of Naþ until disappearance. This is explained by the high symmetry for which electronic transitions become prohibited. According to our results, one have noted that the charge compensation by Liþ and Kþ highlights two sites of Yb3þ ions: C3v and C2v whereas co-doping by Naþ controls 4 specific symmetries of the Yb3þ ions: Oh, C4v, C3v and C2 v respectively. 3.2.3. Decay time properties All decay times of the 7F5/2 metastable levels were measured at different wavelengths. They are exponential except the one of 2.91% Naþ which present a double exponential character due to energy transfer between multisites and clusters which should be studied. We have shown in Fig. 3, few examples of decays at 12 K in 2.91% Naþ–2%Yb3þ co-doped CaF2 samples. Depending of the site symmetry, higher the symmetry is (C2v, C3v, C4v, Oh), higher is the decay value in accordance with the transition selection rules. The decay time at 962 and 1022 nm associated to the cubic center Oh are perfectly exponential with high value of 10.5–12 ms in agreement with the high symmetry of this site (Petit et al., 2007). For all other cases we have found around 5–7 ms for C4v symmetry and around 2–3 ms for C3v and C2v symmetries. Due to the resonant transitions between absorption and emission, we can expect strong energy transfer by resonant self-trapping and self quenching mechanisms which affect decay profiles.
3.3. Co-operative emission to probe breakage of Yb3þ pairs by compensator alkali cations in Yb3þ – doped CaF2 Ytterbium co-operative luminescence process involves two Yb3þ excited ions in the near IR at 1 mm which emit a single visible photon by simultaneous de-excitation around 0.5 mm (Nakazawa and Shionoya, 1970). The intensity of the co-operative emission depends on the distance between the ions and is therefore correlated to the degree of Yb3þ clustering in a sample. Co-operative luminescence is a very good signature of pairs constituted by ions at practical distances of less than about 5 Å. It has been shown to be an efficient probe of rare-earth clusters in glasses (Auzel and Goldner, 2001). One needs to obtain signature of such pairs in laser crystals since the spatial extension of only a few Å (3.6 Å in CaF2) would increase tremendously ion–ion interactions. Recognizance of Yb3þ visible co-operative emission spectrum is correlated to IR spectrum convolution which is relatively easy to detect since Yb3þ ions don’t absorb within all this visible spectral range. The co-operative luminescence signal has been described before in Yb3þ-doped CaF2 samples grown either by simple melting (like Czochralski) or by LHPG without any compensator ions (Wu et al., 1990; Hraiech et al., 2007). In this paper we report in Fig. 4 a few experimental data at 12 K of visible co-operative emission spectra on both 2% Yb3þ-doped CaF2 and 2% Yb3þ–x% Mþ (Mþ ¼ Li, Na, K) co-doped CaF2 samples grown by m-PD technique. The convolution spectra superimposed at around 500 nm is the signature of Yb3þ pairs which can be compared with the experimental emission spectra. Only the non-compensated 2% Yb3þ-doped CaF2 sample shows a weak co-operative luminescence signal within this range. In samples containing alkali ion compensators the co-operative luminescence effect has not been detected, especially with Naþ, as a probe of the breakage of Yb3þ pairs by compensator alkali ions.
0.67% Na+
Intensity (a. u.)
326
0.374% K+
0.69% Li+
Yb3+ pairs 2% Yb3+
400
500 600 Wavelength (nm)
700
Fig. 4. Visible co-operative emission spectra on both 2% Yb3þ-doped CaF2 and 2% Yb3þ-x% Mþ (Mþ ¼ Li, Na, K) co-doped CaF2. Yb3þ isolated ions are pumped in the infrared at 970 nm. Time-resolved spectroscopy has been used at short time with a gate of 10 ms to resolve mainly pairs among longer energy transfers between Yb3þ ions and other rare earth unwanted impurities like Er3þ and Tm3þ ions. The convolution spectrum as signature of Yb3þ pairs has been superimposed at around 500 nm in each spectrum.
S. Hraiech et al. / Radiation Measurements 45 (2010) 323–327
In addition, other visible emission lines can be seen in Fig. 4 at 410, 480, 540 and 660 nm respectively, as a result of up-conversion non radiative energy transfers by dipole–dipole interactions between Yb3þ ions and both Er3þ and Tm3þ unwanted impurities from the raw materials. This energy transfer is very efficient in all Yb3þ-doped crystals even with few ppm of Er3þ and Tm3þ initial impurities. Concerning the role played by the compensator alkali ions in Yb3þ-doped CaF2, the same conclusion has been obtained recently by chemical approach from characterization of the segregation coefficients K0 ¼ Cs/C0, where Cs is the concentration at the beginning part of the crystal, and C0 is initial concentration in the melt (Su et al., 2005a,b). 4. Conclusion We have characterized optical spectroscopic properties of Yb3þdoped CaF2 fluorite crystals grown by the m-PD technique in which Liþ, Naþ, Kþ alkali ions have been used as monovalent compensators. The Yb3þ resonant zero-phonon lines are useful probes for the site symmetries occupied by this trivalent rare earth ion which is in substitution of divalent Ca2þ. In the case of Naþ–Yb3þ-co-doped CaF2 crystals, IR absorption and emission spectra under tuneable laser pumping have pointed out at least 4 sites of symmetry Oh, C4v, C3v and C2 v. Then, depending on the initial concentration of the compensator, the location of the Yb3þ ions in CaF2 host can be controlled. In addition of the interpretation of the Yb3þ multisite symmetry, we have shown that the co-operative luminescence process, involving two Yb3þ excited neighbour ions in IR, which emit a single visible photon by simultaneous de-excitation is an elegant way to probe that alkali compensators break the tendency of Yb3þ ions pair formation. Such conclusion is important to improve performances of Yb3þdoped CaF2 laser host. References Auzel, F., Goldner, P., 2001. Towards rare-earth clustering control in doped glasses. Opt. Mater. 16, 93–103. Boulon, G., 2008. Why so deep research on Yb3þ-doped optical inorganic materials? J. Alloys Compd. 451, 1–11.
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Boulon, G., Guyot, Y., Canibano, H., Hraiech, S., Yoshikawa, A., 2008. Characterization and comparison of Yb3þ-doped YA1O3 perovskite crystals (Yb:YAP) with Yb3þdoped Y3Al5O12 garnet crystals (Yb:YAG) for laser application. J. Opt. Soc. Am. B 25, 884–896. Boulon, G., Ito, M., Goutaudier, C., Guyot, Y., 2006. Advances in growth of fiber crystal by the LHPG technique. Application to the optimization of Yb3þ-doped CaF2 laser crystals. J. Cryst. Growth 292, 230. Bensalah, A., Ito, M., Guyot, Y., Goutaudier, C., Jouini, A., Brenier, A., Sato, H., Fukuda, T., Boulon, G., 2007. Spectroscopic properties and quenching processes of Yb3þ in fluoride single crystals for laser applications. J. Lumin. 122, 444–446. Guyot, Y., Canibano, H., Goutaudier, C., Novoselov, A., Yoshikawa, A., Fukuda, T., Boulon, G., 2006. Yb3þ-doped Gd3Ga5O12 garnet single crystals grown by the micro-pulling down technique for laser application. Part 2: concentration quenching analysis and laser optimization. Opt. Mater. 28, 1–8. Hraiech, S., Jouini, A., Jin Kim, K., Guyot, Y., Goutaudier, C., Yoshikawa, A., TrabelsiAyadi, M., Boulon, G., 2007. Breakage of Yb3þ pairs by Naþ in Yb3þ-doped CaF2 laser host. Ann. Phys. 32, 59–61. Ito, M., Boulon, G., Bensalah, A., Guyot, Y., Goutaudier, C., Sato, H., 2007. Spectroscopic properties, concentration quenching, and prediction of infrared laser emission of Yb3þ-doped KY3F10 cubic crystal. J. Opt. Soc. Am. B 24, 3023–3033. Ito, M., Goutaudier, C., Guyot, Y., Lebbou, K., Fukuda, T., Boulon, G., 2004. Crystal growth, Yb3þ spectroscopy, concentration quenching analysis and potentiality of laser emission in Ca1-xYbxF2þx. J. Phys. Condens. Matter 16, 1501. Jouini, A., Brenier, A., Guyot, Y., Boulon, G., Sato, H., Fukuda, K., Yoshikawa, A., Fukuda, T., 2008. Spectroscopic and laser properties of the near-infrared tunable laser material Yb3þ-doped CaF2 crystal. Cryst. Growth Des. 8, 808–811. Kirton, J., Mclaughlan, S.D., 1967. Correlation of electron paramagnetic resonance and optical-absorption spectra of CaF2:Yb3þ. Phys. Rev. 155, 279. Kaczmarek, S.M., Tsuboi, T., Ito, M., Boulon, G., Leniec, G., 2005. Optical study of Yb3þ/Yb2þ conversion in CaF2 crystals. J. Phys. Condens. Matter 17, 3771. Lucca, A., Jacquemet, M., Druon, F., Balembois, F., Georges, P., Camy, P., Doualan, J.L., Moncorge´, R., 2004. High-power tunable diode-pumped Yb3þ: CaF2 laser. Opt. Lett. 29, 1879. Nakazawa, Shionoya, 1970. Cooperative luminescence in YbPO4. Phys. Rev. Lett. 25, 171. Petit, V., Camy, P., Doualan, J.-L., Moncorge´, R., 2007. Refined analysis of the luminescent centers in the Yb3þ:CaF2 laser crystal. J. Lumin. 122–123, 5–7. Simura, R., Jouini, A., Mun, J.H., Brenier, A., Yoshikawa, A., Boulon, G., Fukuda, T., 2007. Growth and spectroscopic properties of Yb3þ-doped Sc2O3 crystals grown by the micro-pulling-down method. Opt. Mater. 30, 18–21. Su, L., Xu, J., Li, H., Wen, L., Zhu, Y., Zhao, Z., Dong, Y., Zhou, G., Si, J., 2005a. Sites structure and spectroscopic properties of Yb-doped and Yb, Na-codoped CaF2 laser crystals. Chem. Phys. Lett. 406, 254–258. Su, L.B., Xu, J., Li, H.J., Yang, W.Q., Zhao, Z.W., Si, J.L., Dong, Y.J., Zhou, G.Q., 2005b. Codoping Naþ to modulate the spectroscopy and photoluminescence properties of Yb3þ in CaF2 laser crystal. Opt. Lett. 30, 1003–1005. Wu, M.C., Olsson, N.A., Sivco, D., Cho, A.Y., 1990. A 970-nm strained-layer InGaAs/ GaAlAs quantum well laser for pumping an erbium-doped optical fiber amplifier. Appl. Phys. Lett. 56, 221–223. Yoshikawa, A., Nikl, M., Boulon, G., Fukuda, T., 2007. Challenge and study for developing of novel single crystalline optical materials using micro-pullingdown method. Opt. Mater. 30, 6–10.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Role of monovalent alkali ions in the Yb3þ centers of CaF2 laser crystals Sana Hraiech a, Anis Jouini a, b, Kyoung Jin Kim b, Yannick Guyot a, Akira Yoshikawa b, Georges Boulon a, * a b
Physical Chemistry of Luminescent Materials (LPCML), University of Lyon, Claude Bernard/Lyon 1 University, UMR 5620 CNRS, La Doua, 69622 Villeurbanne, France IMRAM, Tohoku University, 2-1-1, Katahira, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2009 Received in revised form 26 September 2009 Accepted 9 November 2009
Yb3þ and Mþ monovalent alkali ions (Mþ ¼ Liþ, Naþ, Kþ)-co-doped CaF2 cubic laser crystals were grown by the micro-pulling-down method (m-PD) under CF4 atmosphere. Structural and spectroscopic characterizations of Yb3þ in substitution of Ca2þ (absorption, emission and decay curves) were carried out to study the effect of Mþ ions as charge compensators. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: CaF2 Ytterbium Charge compensator Multisites Crystal growth Spectroscopy Laser crystal
1. Introduction During the last decade, with advances of high performance GaAs and InGaAs laser diodes emitting wavelengths between 900 and 1100 nm, interest on trivalent ytterbium (Yb3þ ion)-doped crystals has been renewed for applications in high-efficiency (>50%) and high power (>50 W/cm) diode-pumping laser systems. The trivalent ytterbium is the most promising ion that can be used in nonNd3þ laser in the same range, it have some advantages over the Nd3þ ions due to its very simple energy level scheme, only two levels: the 2F5/2 ground state and the 2F7/2 excited state. There is no excited state absorption reducing the effective laser cross-section, no up-conversion, no cross-relaxation. The intense absorption lines are well suited for laser diode pumping near 980 nm and the small quantum defect between absorption and emission (about 650 cm1 ), much smaller than Nd3þ- doped laser hosts, reduces the thermal loading of the material during laser operation (Boulon, 2008). In order to exploit all these favorable properties we need an efficient high-power laser-diode emission matching the absorption band of the Yb3þ ion. This need was fulfilled by diode lasers based on InGaAs quantum wells emitting in the 900–980 nm region, developed in the 1990s to pump Er3þ-doped fiber amplifiers for
* Corresponding author. E-mail address: [email protected] (G. Boulon). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.11.017
commercial use in the telecommunications industry (Wu et al., 1990). Among several possibilities with broadband absorption suitable for laser diode pumping, we have focused our attention on CaF2 (fluorite structure) which is a well-known fluoride crystal with the advantages of a wide transparent wavelength range (0.15–9 mm), low phonon energy (w320 cm-1) and high thermal conductivity (10 W m-1 K-1) as large as YAG at the same time. In our previous studies, Ca1-xYbxF2þx crystals were grown by either simple melting or by laser heated pedestal growth (LHPG) technique (Ito et al., 2004; Boulon et al., 2006) under Ar atmosphere by growing Yb3þ concentration gradient fiber in order to understand involved concentration quenching mechanisms. In addition, high-quality large size crystals (more than 10 inches) have also been grown by Czochralski technique under CF4 atmosphere (Jouini et al., 2008). Laser performances were obtained as tunable laser operation between 1030 and 1070 nm in samples grown by Czochralski with a slope efficiency of 57% and various transmission output coupler (Jouini et al., 2008). Similar results of diode-pumped CW laser operation were reported with a 5at.% Yb3þ-doped sample: 5.8 W output power has been obtained at 1053 nm for 15 W of incident power at 980 nm and a small-signal gain as high as 1.8 was achieved, confirming the great potential of Yb3þ-doped CaF2 as an amplifier medium for femtosecond pulses (Lucca et al., 2004). Nevertheless, in such samples trivalent Yb3þ activator ion is substituting divalent Ca2þ site and charge compensation is needed. The first idea to keep good crystal quality is to use monovalent
324
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charge compensator ions like Liþ, Naþ, Kþ alkali ions in substitution of one of the eight nearest neighbour Ca2þ ions along any [1 1 0] direction (Kirton and Mclaughlan, 1967). This is our approach in the present work as a continuation of previous efforts within the framework of collaboration between the Japanese (IMRAM) and the French (LPCML) laboratories on Yb3þdoped single crystals as KY3F10 (Ito et al., 2007), LiYF4 (Bensalah et al., 2007) fluorides or Sc2O3 sesquioxide (Simura et al., 2007), GGG garnet (Guyot et al., 2006), YAP perovskite (Boulon et al., 2008) for laser applications. Co-doped crystals have been grown by a specific technique, the m-PD technique, allowing relatively fast growth of high quality samples with different compositions (Yoshikawa et al., 2007) allowing us to compare results with samples already grown by the classical Czochralski technique (Su et al., 2005a,b). In first hand, our main objective was to know more on the real behaviour of various Yb3þ multisites which might be created in compensated Yb3þ-doped CaF2 and on the tendency of clustering especially in our samples. It has been already established since long time that trivalent rare-earths-doped CaF2 form fluorite solid solutions. Then, such centers might be involved in concentration quenching of luminescence which is a key knowledge for laser crystals (Boulon, 2008; Kaczmarek et al., 2005). This is why we analyze the evolution of the formation of pairs, considered as the simplest rare-earth clusters with the association of two neighbour ions at 3.6 Å, in both Yb3þ-doped CaF2 samples and compensated Yb3þ-doped CaF2, by using an original spectroscopic way based on co-operative phenomena.
2 at% and MF (M ¼ Na, Li, K) of 0, 10 and 20 at% in the starting materials have been used. Graphite crucible is surrounded by refractory carbon and inductively heated using a radio-frequency generator. The starting materials were thoroughly mixed and put into the crucible. The chamber was evacuated to 104 Torr and the crucible was heated to 600 C for 1 h to remove oxygen impurities. During this baking procedure, the chamber is further evacuated up to 10-5 Torr. After the baking, the recipient was filled with high purity CF4 (6N) until ambient pressure and the crucible was heated up to the melting temperature of about 1030 C. Crystal growth was carried out using an a-axis seed of un-doped CaF2 crystal. The growth rate was 0.1–0.15 mm/min. Controlling power and pulling rate during the growth process constantly maintained the crystal diameter. The crystals obtained were transparent colorless crystals. Their size was a few centimeters and they were cut and polished for optical experiments. 2.2. Phase characterization To identify the obtained phase, powder X-ray diffraction analysis was carried out in the 2q range from 10 to 70 using a RIGAKU diffractometer (RINT2000). The X-ray source was CuKa. The chemical composition was analyzed by electron probe microanalysis (EPMA) using a JEOL JXA-8600L analyzer. The distribution of Yb in the single crystal was measured along the growth axis using an electron probe of 10 mm in diameter and a step of 250 mm.
2. Experimental procedure 2.3. Optical measurements 2.1. Growth using the m-PD method CaF2 single crystals were grown by the micro-pulling-down (mPD) method (Yoshikawa et al., 2007). YbF3 dopant concentration of
Absorption spectra were measured with a Lambda 900 spectrophotometer equipped with a continuous flow helium refrigerator. Emission spectra were acquired by exciting the samples with
0.25 2
7
(O )
0.15
h
1 -> 6
6
0.10
5
4 3 2
F7/2
σ abs x 10-20 (cm2)
0.05
2
+ 2.91 % Na
1 -> 5
1 -> 7
0.20
F5/2
0.00 1 -> 5
0.6
(C3v )
0.4
1 -> 7
1 -> 6
0.2 0.0 1 -> 5
0.6
1 Absorption
+ 0 % Na
(C3v )
Emission
Yb3+
+ 0.67 % Na
0.4 1 -> 7
0.2
1 -> 6
0.0 880
920
960
1000
1040
Wavelength (nm) þ
Fig. 1. Na concentration dependence of the 12 K absorption spectra in 2% Yb
3þ
-doped CaF2. If Na concentration increases, Yb3þ ions occupy Oh site symmetry instead of C3v. þ
S. Hraiech et al. / Radiation Measurements 45 (2010) 323–327 3+
refrigeration system (SMC, TBT air–liquid) was used for the low temperature (12 K) measurements. The visible emission was detected by a ORIEL spectrometer coupled with a gated intensified CCD camera (ANDOR/ORIEL INTASPEC V ICCD). The decay curves were recorded with a Lecroy LT 342 digital oscilloscope.
+
CaF2 : 2%Yb - 2.91 % Na C4v vibronic line
O
h
Oh
C3v
325
Intensity (a. u.)
3. Results and discussion
C3v
3+
5 -> 1 5 -> 2
Due to the strong evaporation of NaF, LiF and KF from the melt at the temperature above 1400 C, the concentration of Mþ alkali cations incorporated into the crystals are much lower than those in the starting materials. As an example, in initial 20%Naþ–2%Yb3þ codoped CaF2, the measured concentrations are 2.91% Naþ–2.13%Yb3þ co-doped CaF2. The same tendency was observed at any Naþ concentration and also for Liþ and Kþ ions.
vibronic line 5 -> 3
Oh C2v
C3v
5 -> 4
5 -> 2 5 -> 1
3+
CaF2 : 2%Yb
3.2. Evidence of Yb3þ multisites from compensator alkali ions
5 -> 3
C2v
960
3.1. Evaporation of alkali cations
+
CaF2 : 2%Yb - 0.67 % Na
5 -> 4
980
1000
1020
1040
1060
The creation of several symmetry sites with rare earth trivalent ions like Yb3þ ions has been studied a long time ago. Just to be short, multisite structure is induced by several types of charge compensating due to interstitial F ions and various clusters. The main symmetry sites which have been characterized are the neighbour (C4v), next nearest neighbour (C3v) or (C2v) and also high symmetry cubic site (Oh). The last paper on the refined analysis of the luminescent centers in the Yb3þ:CaF2 laser crystal has been published by Petit et al. (2007). We will use it as a reference for the position of emission transitions in the compensated samples we are analysing. In this article, we choose to relate data directly in figures as a condensate way for some selected samples.
1080
Wavelength (nm) Fig. 2. Na concentration dependence of the 12 K emission spectra in 2% Yb3þ-doped CaF2. If Naþ concentration increases, Yb3þ ions occupy Oh site symmetry instead of C3v and C2v.The 0-phonon line assigned as the resonant 5 / 1 transition of Oh site symmetry is clearly observed at 964 nm. þ
a QUANTEL Nd3þ: YAG pumped dye laser using a mix of LD700 and DCM dye. The dye laser light was converted to the 930–980 nm region by way of a H2 Raman shift cell. The infrared emission was detected in a perpendicular direction to the excited beam with a Jobin-Yvon HRS1 monochromator equipped with a 1000 nm blazed grating, with a cooled Ge-detector, and the signal was processed with a STANDFORDS boxcar SRS 250. A closed circuit helium
3.2.1. Infrared (IR) absorption properties of Yb3þ ions As an example Fig. 1 shows absorption spectra in the nearinfrared region at low temperature (12 K). They correspond to the Yb3þ ion transitions from the 2F7/2 ground state to the 2F5/2 excited state which have been already detailed in literature. We did not note any change of absorption spectra in Yb3þ- (Liþ/Kþ)-co-doped
1
λ exc = 926 nm
Intensity (u. a.)
12 K
0,1
0,01
λ ém = 962 nm
τ = 12.14 ms (Oh )
λ ém = 980 nm
τ1 = 5.09 ms (C4v)
τ2 = 10.52 ms (Oh) λ ém = 1022 nm τ = 12.18 ms (Oh) 0,000
0,005
0,010
0,015
0,020
0,025
0,030
Time (s) þ
Fig. 3. Emission wavelengths dependence of the 12 K decays in 2.91% Na -2%Yb
3þ
co-doped CaF2 under 926 nm pumping. We characterize Oh and C4v site symmetries.
S. Hraiech et al. / Radiation Measurements 45 (2010) 323–327
CaF2. But after introducing NaF, the differences among absorption spectra are clearly seen. With the addition of 0.67% NaF in Yb: CaF2, the strongest absorption band peaking at 981 nm assigned as the 0-phonon line of the trigonal site (C3v) shift to shorter wavelengths (981 / 980 nm). This line disappears completely after introducing 2.91 at.% NaF associated to the absence of Yb3þ ions into C3v site. In this case Yb3þ ions occupy the cubic Oh site with 1 4 5 resonant transitions lines located at 964 nm. The room temperature absorption spectra of (Yb3þ–Naþ) codoped CaF2 crystals have also been studied. We observe broad absorption lines, due to the well-known multisite effect. Increasing the concentration of Naþ as charge compensator to 2.91 at% caused the absorption bands to be very narrow and clearly resolved. The intensity of the C3v zero-phonon line located at 980 nm decreases and becomes very weak, contrary to the shoulder observed at 965 nm which intensity increases. This peak is assigned to the 1 / 5 transition characteristic of cubic Oh site. Then as well at RT as 12 K, this indicates that Yb3þ-doped CaF2 crystal co-doped with high NaF amount reveals high symmetries sites. The shape of the absorption spectra as well as the cross sections values at RT are comparable with those given by Su et al. on crystals grown by Czochralski technique (Su et al., 2005a,b). 3.2.2. IR emission properties As an example, Fig. 2 shows the fluorescence spectra of the crystals Naþ–Yb3þ co-doped CaF2 under excitation at 920 nm recorded at low temperature (12 K). Three samples were measured under the same conditions to compare the luminescence intensity. The 1 4 5 resonant zero-phonon line of the tetragonal C4V site located at 969 nm, are not detected above 2%Yb3þ. This site is only observed for the lowest concentration (Petit et al., 2007). Addition of 0.67% Naþ reveals at low temperature a line at 963 nm associated to the 5 / 1 zero-phonon line attributed to the Oh cubic site. The spectrum of the sample doped by 2.91% Naþ the highest concentration, is different from the two other ones and present two additional lines located towards 969 nm and 1026.5 nm corresponding to the C4v site. The shoulder located at 1028 nm characterises also of the Oh site (Petit et al., 2007). The electronic transitions 5 / 4 and 5 / 3 observed at 1050 nm and 1033 nm respectively on the spectrum of 2% Yb3þ-doped CaF2 decrease in intensity with the increase of Naþ until disappearance. This is explained by the high symmetry for which electronic transitions become prohibited. According to our results, one have noted that the charge compensation by Liþ and Kþ highlights two sites of Yb3þ ions: C3v and C2v whereas co-doping by Naþ controls 4 specific symmetries of the Yb3þ ions: Oh, C4v, C3v and C2 v respectively. 3.2.3. Decay time properties All decay times of the 7F5/2 metastable levels were measured at different wavelengths. They are exponential except the one of 2.91% Naþ which present a double exponential character due to energy transfer between multisites and clusters which should be studied. We have shown in Fig. 3, few examples of decays at 12 K in 2.91% Naþ–2%Yb3þ co-doped CaF2 samples. Depending of the site symmetry, higher the symmetry is (C2v, C3v, C4v, Oh), higher is the decay value in accordance with the transition selection rules. The decay time at 962 and 1022 nm associated to the cubic center Oh are perfectly exponential with high value of 10.5–12 ms in agreement with the high symmetry of this site (Petit et al., 2007). For all other cases we have found around 5–7 ms for C4v symmetry and around 2–3 ms for C3v and C2v symmetries. Due to the resonant transitions between absorption and emission, we can expect strong energy transfer by resonant self-trapping and self quenching mechanisms which affect decay profiles.
3.3. Co-operative emission to probe breakage of Yb3þ pairs by compensator alkali cations in Yb3þ – doped CaF2 Ytterbium co-operative luminescence process involves two Yb3þ excited ions in the near IR at 1 mm which emit a single visible photon by simultaneous de-excitation around 0.5 mm (Nakazawa and Shionoya, 1970). The intensity of the co-operative emission depends on the distance between the ions and is therefore correlated to the degree of Yb3þ clustering in a sample. Co-operative luminescence is a very good signature of pairs constituted by ions at practical distances of less than about 5 Å. It has been shown to be an efficient probe of rare-earth clusters in glasses (Auzel and Goldner, 2001). One needs to obtain signature of such pairs in laser crystals since the spatial extension of only a few Å (3.6 Å in CaF2) would increase tremendously ion–ion interactions. Recognizance of Yb3þ visible co-operative emission spectrum is correlated to IR spectrum convolution which is relatively easy to detect since Yb3þ ions don’t absorb within all this visible spectral range. The co-operative luminescence signal has been described before in Yb3þ-doped CaF2 samples grown either by simple melting (like Czochralski) or by LHPG without any compensator ions (Wu et al., 1990; Hraiech et al., 2007). In this paper we report in Fig. 4 a few experimental data at 12 K of visible co-operative emission spectra on both 2% Yb3þ-doped CaF2 and 2% Yb3þ–x% Mþ (Mþ ¼ Li, Na, K) co-doped CaF2 samples grown by m-PD technique. The convolution spectra superimposed at around 500 nm is the signature of Yb3þ pairs which can be compared with the experimental emission spectra. Only the non-compensated 2% Yb3þ-doped CaF2 sample shows a weak co-operative luminescence signal within this range. In samples containing alkali ion compensators the co-operative luminescence effect has not been detected, especially with Naþ, as a probe of the breakage of Yb3þ pairs by compensator alkali ions.
0.67% Na+
Intensity (a. u.)
326
0.374% K+
0.69% Li+
Yb3+ pairs 2% Yb3+
400
500 600 Wavelength (nm)
700
Fig. 4. Visible co-operative emission spectra on both 2% Yb3þ-doped CaF2 and 2% Yb3þ-x% Mþ (Mþ ¼ Li, Na, K) co-doped CaF2. Yb3þ isolated ions are pumped in the infrared at 970 nm. Time-resolved spectroscopy has been used at short time with a gate of 10 ms to resolve mainly pairs among longer energy transfers between Yb3þ ions and other rare earth unwanted impurities like Er3þ and Tm3þ ions. The convolution spectrum as signature of Yb3þ pairs has been superimposed at around 500 nm in each spectrum.
S. Hraiech et al. / Radiation Measurements 45 (2010) 323–327
In addition, other visible emission lines can be seen in Fig. 4 at 410, 480, 540 and 660 nm respectively, as a result of up-conversion non radiative energy transfers by dipole–dipole interactions between Yb3þ ions and both Er3þ and Tm3þ unwanted impurities from the raw materials. This energy transfer is very efficient in all Yb3þ-doped crystals even with few ppm of Er3þ and Tm3þ initial impurities. Concerning the role played by the compensator alkali ions in Yb3þ-doped CaF2, the same conclusion has been obtained recently by chemical approach from characterization of the segregation coefficients K0 ¼ Cs/C0, where Cs is the concentration at the beginning part of the crystal, and C0 is initial concentration in the melt (Su et al., 2005a,b). 4. Conclusion We have characterized optical spectroscopic properties of Yb3þdoped CaF2 fluorite crystals grown by the m-PD technique in which Liþ, Naþ, Kþ alkali ions have been used as monovalent compensators. The Yb3þ resonant zero-phonon lines are useful probes for the site symmetries occupied by this trivalent rare earth ion which is in substitution of divalent Ca2þ. In the case of Naþ–Yb3þ-co-doped CaF2 crystals, IR absorption and emission spectra under tuneable laser pumping have pointed out at least 4 sites of symmetry Oh, C4v, C3v and C2 v. Then, depending on the initial concentration of the compensator, the location of the Yb3þ ions in CaF2 host can be controlled. In addition of the interpretation of the Yb3þ multisite symmetry, we have shown that the co-operative luminescence process, involving two Yb3þ excited neighbour ions in IR, which emit a single visible photon by simultaneous de-excitation is an elegant way to probe that alkali compensators break the tendency of Yb3þ ions pair formation. Such conclusion is important to improve performances of Yb3þdoped CaF2 laser host. References Auzel, F., Goldner, P., 2001. Towards rare-earth clustering control in doped glasses. Opt. Mater. 16, 93–103. Boulon, G., 2008. Why so deep research on Yb3þ-doped optical inorganic materials? J. Alloys Compd. 451, 1–11.
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