CRYSTAL GROWTH AND OPTICAL PROPERTIES OF LaMgAl11O19 : Cr, Nd

Transcription

1 CRYSTAL GROWTH AND OPTICAL PROPERTIES OF LaMgAl11O19 : Cr, Nd C. Wyon, J. Aubert, D. Vivien, A. Lejus To cite this version: C. Wyon, J. Aubert, D. Vivien, A. Lejus. CRYSTAL GROWTH AND OPTICAL PROPER- TIES OF LaMgAl11O19 : Cr, Nd. Journal de Physique Colloques, 1987, 48 (C7), pp.c C < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1987 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C7, suppl6ment au n012, Tome 48, decembre 1987 CRYSTAL GROWTH AND OPTICAL PROPERTIES OF LaMgAlllOlg: Cr, Nd C. WYON, J.J. AUBERT, D. VIVIEN* and A.M. LEJUS" DLETI, IRDI-CEA, CENG, BP 85 X, F Grenoble Cedex, France "ENSCP, 11, Rue Pierre et Marie Curie, F Paris Cedex 05, France Single crystals of chromium and chromium-neodynium codoped lanthanum-magnesium hexaluminates LaMgAlllOlg have been pulled through the Czochralski method up to diameters 25 mm and length 75 mm. I. INTRODUCTION For some years lanthanum-magnesium hexaluminates LaMgAll1019 with the magnetoplumbite structure (space group P63/mmc) are well known as remarkably good laser hosts for neodynium ions [1,2]. LaMgAll1019: Nd rods show stimulated emission at 1054 and 1081 nm [3] with a 20 % higher yield than YAG: Nd near 1060 nm 143. Chromium ions have already been introduced in this matrix to be studied as potential tunable solid state lasers in the nm range [5]. To improve laser performance of LaMgAll1019: Nd crystals through Cr sensitization as in GSGG: Cr, Nd [6], the crystal growth of chromium and neodynium doped lanthanum-magnesium hexaluminates has been studied. 2. EXPERIMENTAL Single crystals of Lal-xNdxMgA111-yCry019 (0 < x < 0.15, 0 < y < 0.055) have been produced through the Czochralski technique using RF heated iridium crucibles under nitrogen or argon atmospheres with the following conditions : melting point : 1870 t 20 C - 1 crystallization rate : 0.7 mm.hr rotation rate gas flow : rpm - 1 : 21 l.min Pink ~ d doped ~ + and brown cr3+ and cr3+-~d3+ doped boules are pulled from the melt + according to. the preferential growth a axis up to diameters 25 mm and length 75 mm with very good transparency: no scattering centers can be observed. The weak (001) -+ cleavage plane keeps more difficult the pulling according to the optical c axis. According to the optical measurements, the segregation coefficients of ~ d and ~ + 3+ Cr respectively are and , which are in good agreement with Shinn's results [7]. These two nearly unity segregation coefficients give a good homogeneity of neodynium and chromium along the single crystals, and allow higher 3+ pulling rates than in YAG: Nd. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE 7. RESULTS 3.1 Chromium doped crystals Figure 1 shows the room temperature polarized transmission spectra in the nm range of an undoped sample and of a chromium doped sample T% - e = 1.6 mm Figure 1 : Room temperature transmission spectra of LaMgAlll-xCr,Olg (x = 0.055) and of undoped (LaMgAlllOlg. Transmission spectra of LaMgAll1019: cr3+ shows the two characteristics broad bands of cr3+ transitions: a strong W absorption band near 220 nm m a residual broad polarized absorption from 850 to 1700 nm, with a maximum near 1380 nm. This band intensity increases with an increase in chromium amount in the melt. The transmission spectrum of the undoped sample reveals a W absorption band at A = 255 nm which seems to be characteristics of the I?e3+ charge transfer [8] as in undoped YAG [9]. The intense W absorption band observed in LaMgAlllOlg: Cr samples must be correlated to the undoped sample one, and may be due to complex charge trans- 2- Eer between 0 + ~ e and/or ~ + cr3+ ions. The very broad infrared absorption band centered near 1380 nm is only observed in chromium dopedsamp1es.assuming that this band arises from an impurity ion in the initial melt, the impurity concentration has to obey to the following relationship: Cs(g) = Co k(l-g) k- 1 where g : mass percentage of the pulled melt Co : initial impurity concentration in the melt CS(~): impurity concentration in the crystal at g k : segregation coefficient of the impurity The k value calculated from the absorption coefficients at 1380 nm is less than 0,

4 which means that the impurity does not come from the melt, if the pulling conditions don't vary during the whole growth. This band may bedue to charge transfer between cr3+ ions and ions coming from the refractory cell zr4+, or from the crucible cr4+. GSGG: Cr, Nd single crystals exhibit a similar absorption band [lo] which is attributed to cr4+ions. Expecting that the gaseous atmosphere just above the melt may change during the growth, this absorption band may be due to complex charge transfer transitions between coupled iron-chromium ion pairs Fe 3+~e2+ + Cr3+cr4+. Annealing under reducing Ar+H2 or oxydizing O2 atmospheres are in course in order to understand the origin of such a band and to decrease the intensity of this absorp- tionbandwhich minimizes the laser efficiency of.cr-nd codoped samples at 1054 nm. 4 Excited in the T level with a doubled YAG: Nd laser = 532 nm, LaMgAlllOlg: Cr 3+ 2 samples emit a broad fluorescence band lying from 650 to 800 nm with a maximum at nm (figure 2) due to 2~ + A transition. The fluorescence decay time is 3.04 ms 2 at 300 K increasing to 6.1 ms at 4 K. This emission band overlaps absorption bands of ~ d in ~ LaMgAlllOlg + (figure 3). I lurb.un.). :' exc.=532nm - T -300 K -... T=/../, I< Figure 2: Fluorescence spectrum in LaMgAlllOlg: Cr. Excitation in the 4~~ level under A = 532 nm excitation. Xlnml 61,O Chromium and neodynium codoped crystals Transmission spectra of chromium and neodynium codoped samples (figure 3) show that cr3+ ions are more efficient absorber of flashlamps photons than ~ d ions ~ + in LaMgAlllOlg crystals, and might allow an increase in neodynium doped lanthanum magne sium hexaluminate laser efficiency. Figure 3: Room temperature transmission spectrum of LaMgAlllOlg: Cr, Nd (x = 0.10, y = 0.035) 2"" ODD,500 Unfortunately these transmission spectra also show the same residual broad infra- red absorption band as singly chromium dopedsampleswhich strongly decreases the laser efficiency at 1054 nm. Theresidual absorption coefficients at 1054 nm of the

5 C7-486 JOURNAL DE PHYSIQUE head and the bottom of a codoped boule are reported in table I. Neodynium ions do not affect this residual absorption band HEAD BOTTOM : 2/12 h: Table I: Evolution of the absorption coefficient at 1054 nm from the head to the bottom of a La0.gNd0.~MgA110.g65Cr g boule. Excited in the nm range with arc xenon lamps, LaMgAlllOlg: Cr, Nd samples emit a fluorescence band induced by the ~d~' 4~ ~1112 transition the nm range with a fluorescence intensity ten times higher than for singly neodynium doped samples (Figure 4) in spite of a strong residual absorption near 1060 nm. I bcv) 6oKl "1 lool,nde 9,MsAlttOw A I. A btm) X (rtmj Figure 4: Fluorescence spectra (4~31Z of ~d~') under excitation in the nm range with an arc Xenon 1amp.Wlth ~r3+ the detector signal is ten times higher. Evolution of the fluorescence decay time of ~ d ions ~ + with the presence of chromium ions in the host has been studied and is presented in an other paper during this con- ference. 4. CONCLUSION Single crystals of chromium doped and chromium neodynium codoped LaMgAlllOlg have been pulled through the Czochralski technique with very good transparency. Codoping LaMgAlllOlg with chromium and neodynium increases the fluorescence intensity of the N ~ 4 1 transition ~ ~ ~ ~ by a ~ factor ~ of ten compared F with ~ singly ~ neo- ~ dynium doped LaMgAlllOlg in spite of a strong residual absorption in the nm range. Post-growth treatments are in course in order to suppress this residual ab- sorption. Stimulated emission measurements have been performed and will be presented else- where. ACKNOWLEDGEMENTS We would like to thank C.Calvat, G.Basset, M.Couchaud, Ph.Hugot, Y.Grange for growing crystals, B.Franfois for cutting and polishing samples, C.Denayer, M.Olivier, N.Mermillod for optical measurement and R.Moncorg6 for Cr fluorescence decay time measurements.

6 REFERENCES [I] J.J. Aubert, C. Wyon and al. - Proc. CLEO, 1986, San Francisco, June [2] M.D. Shinn, J.A. Caird andal. - Proc. IQEC, 1986, San Francisco, June [3] L.D. Schearer, M. Leduc and al. - IEEE J. of Quant. Elec. QE - 22,5 (1986) 713. [4] K.S. Bagdasarov and al., Sov. J. Quant. Elect. 13 (1983) [5] B. Viana, A.M. Lejus and al., 3. Of Solid State Chemistry (to be published). [6] E.V. Zharikov and al., Sov. J. Quant. Elect. 13 (1983) 82. [7] M.D. Shinn, W.F. Krupke and al. - Laser Science Conf. I, Dallas TX 1985, AIP Conf. Proc. n0146 (1986) 216. [8] P.I. Bykocskii, V.A. Lebedev, Zh. Prikl. Spektros (1986) 711. [9] T. Masumoto and Y. Kuwano. Jap. J. Appl. Phys (1985) 546. [lo] S. Stokowski, J. Caird and al. - Laser Science Conf. I, Dallas TX 1985, AIP Conf. Proc. n0146 (1986) 225.