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Fluorescence of Cr3+ doped alumina optical waveguides prepared by pulsed laser deposition and sol–gel method
Journal of Luminescence 87}89 (2000) 1087}1089
Fluorescence of Cr> doped alumina optical waveguides prepared by pulsed laser deposition and sol}gel method A. Pillonnet , C. Garapon *, C. Champeaux, C. Bovier, H. Ja!rezic, J. Mugnier Laboratoire de Physico-Chimie des Mate& riaux Luminescents, CNRS-UMR 5620, Universite& Lyon I, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Laboratoire de Science des Proce& de& s Ce& ramiques et Traitements de Surface, CNRS-UMR 6638, Universite& de Limoges, 123, av. Albert Thomas, 87060 Limoges Cedex, France De& partement de Physique des Mate& riaux, CNRS-UMR 5586, Universite& Lyon I, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Laboratoire Ingienerie et Fonctionnalisation des Surfaces, CNRS-UMR 5621, Ecole Centrale de Lyon, 69130 Ecully, France
Abstract The #uorescence properties of Cr> in optical waveguides of doped alumina prepared by sol}gel or pulsed laser deposition (PLD) are compared with those of Cr-doped alumina powders obtained by sol}gel method and having c, h and a crystalline phases. Cr> is located in c crystallites in sol}gel "lms but the three phases are present in PLD "lms. 2000 Elsevier Science B.V. All rights reserved. Keywords: Cr : Al O ; Fluorescence; Sol}gel method; Pulsed laser deposition
1. Introduction
2. Sample preparation
Preparation of alumina thin "lms of high optical quality, which could be used as optical waveguides, raises the question of the relation between their structural characteristics and the growth parameters. The aim of this work is to use the #uorescence properties of Cr> doping ions to analyze the structural and optical properties of alumina optical waveguides, prepared by two di!erent methods, sol}gel or pulsed laser deposition, and to compare them with those of the various crystalline phases of bulk alumina. Although the #uorescence properties of Cr>-doped a-Al O (ruby laser crystal) are well known, only few studies have been devoted so far to those of Cr> in other alumina phases [1}3] or alumina waveguides [4]. This paper reports on our preliminary results of room temperature Cr> #uorescence properties in relation to X-rays di!raction.
Three kinds of samples, with the same Cr> concentration (0.04 or 1%) were studied:
* Corresponding author. Fax: #33-4-72-43-11-30. E-mail address: [email protected] (C. Garapon)
(1) Alumina powders were obtained by the sol}gel (SG) procedure as described by Yoldas [5]. The Cr doping was provided by addition of Cr(NO ) to the sol. These powders experienced cumulative 90 mn annealing from 100 to 14003C by 1003 steps. (2) Thin "lms elaborated by dip-coating: the sol was prepared as described by Nass and Schmit [6]. Cr ions were introduced as described previously. The alkoxide concentration, the withdrawal speed and the heat treatment process were adjusted as to obtain "lms with as high refractive index as possible without crack. At least nine successive coatings were required to provide waveguides [7]. The samples were then annealed at different temperatures between 700 and 11003C. (3) Thin "lms obtained by pulsed laser deposition (PLD). The ablation of rotating targets, prepared by sintering the previous sol}gel powders, is achieved by a KrF laser with a 3 J/cm #uence, under vacuum
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 4 9 - 9
1088
A. Pillonnet et al. / Journal of Luminescence 87}89 (2000) 1087}1089
(5;10\ mbar) or 0.1 mbar oxygen pressure. The substrate temperature was 7903C. Films were deposited on SiO /Si substrates [8]. 3. Structural properties For 1% Cr-doped alumina powders, the X-ray di!raction (XRD) spectra, registered with a h}2h di!ractometer show the usual evolution [9]. The structure is going from boehmite to a-Al O through c- and h-phases, depend ing on the annealing temperature. The lines are broad and do not enable to distinguish the d-phase, generally observed between the c- and h-phases. According to grazing incidence X-ray di!raction (GRXD), lightly doped alumina PLD "lms appear as amorphous when deposited at 0.1 mbar oxygen pressure and crystallized into c alumina at vacuum [8]. For 1% Cr-doped alumina "lms, no XRD lines could be observed. The study by transmission electron microscopy (TEM) is underway. For 1% or lightly doped alumina SG "lms annealed at 7003C, there is no detectable crystallization by GXRD. However, TEM experiments indicated that very small c crystallites are observed, their sizes growing as the annealing temperature increases [7]. XRD experiments must thus be completed by other techniques, more sensitive to the eventual presence of small crystallites.
4. Fluorescence properties Time-resolved #uorescence spectra were obtained by excitation with a pulsed dye laser. We used an AsGa photomultiplier and a photon counting system in the visible range and, for the IR domain, a germanium photodiode and a boxcar integrator. Fluorescence decays were recorded using a multichannel analyzer. Phase selective excitation spectra were recorded by using a chopped Xe lamp light and a lock-in ampli"er. More details may be found in Ref. [10]. 4.1. Al2O3 powders Time-resolved #uorescence spectra, recorded after excitation into the T band at 580 nm, change as the annealing temperature increases. These spectra are constituted by the long-lived EPA emission (R lines) of Cr> located in strong crystal "eld sites. Each alumina phase, c, h and a, has its speci"c lines. Above 12003C the well-known narrow R lines of ruby at 693.2 and 694.7 nm are observed with their vibronic side-band and a 3 ms life time. From 900 to 11003C we observe two lines, located at 684 and 687.2 nm, which we attribute, as in Refs. [2,3], to the R lines of Cr> in the h-phase. From 400 up to 10003C annealing temperature, we observe a broad
Fig. 1. Emission spectra of Cr> : Al O powders annealed at 6003C: (a) exc 580 nm, delay 4 ms, gate 4 ms; (b) exc 647 nm, delay 2 ls, gate 10 ls.
Fig. 2. Excitation spectra for di!erent modulation frequency F and emission j of Cr> : Al O powders annealed at (a) 14003C (j 694.7 nm, F 30 Hz); (b) 10003C (j 684 nm, F 30 Hz); (c) 6003C (j 700 nm, F 30 Hz); (d) 6003C (j 800 nm, F 2 kHz).
asymetric line peaking at about 690 nm (Fig. 1). The #uorescence decay is strongly non-exponential with an asymptotic time constant of about 14 ms. These unresolved, inhomogeneously broadened, R lines are due to Cr> in octahedral sites of the c-phase with a strong crystal "eld and a very disordered environment [1}3]. For the c-phase, in addition, using a longer excitation wavelength (647 nm) and a short time scale, we observe a short-lived IR broad band (Fig. 1), extending from 700 to 1300 nm with a maximum at about 820 nm. The #uorescence decay is non-exponential with time constant in the tens of microseconds range. This band is thus attributed to the T PA emission of Cr> ions located in weak crystal "eld sites. Excitation spectra of the characteristic emissions of each phase enable to con"rm these attributions (Fig. 2). Monitoring short wavelength emission around 690 nm and choosing low modulation frequency (30 Hz), we select the long-lived EPA emission of strong "eld sites. The maximum of the corresponding A PT excita tion band is located at 554, 562 and 576 nm for the a-, hand c-phases respectively. Monitoring longer wavelength (800 nm) and using high modulation frequency (2 kHz), the maximum is shifted to longer wavelength (607 nm) in agreement with the attribution of the short-lived IR band to weak "eld sites.
A. Pillonnet et al. / Journal of Luminescence 87}89 (2000) 1087}1089
1089
Fig. 3. Emission spectra of SG "lms: (a) exc 410 nm, delay 2 ls, gate 10 ms; (b) exc 647 nm, delay 2 ls, gate 10 ls.
Fig. 4. Emission spectra of PLD "lms (exc 647 nm): (a) delay 10 ls, gate 10 ms; (b) delay 2 ls, gate 10 ls.
The spectroscopic properties of Cr> in c-Al O ap pear as very similar as those observed for the nonstoichiometric spinel MgO-2.6 Al O [10]. Both crystal have a distorted spinel structure with a well-de"ned oxygen arrangement and cation vacancies in the octahedral sites [9]. The origin of the IR broad band in c-Al O is probably related to Cr> located nearby cation vacancies as it is in the non-stoichiometric spinel.
5. Conclusion
4.2. Sol}gel xlms For both concentrations (0.04 and 1%) and for 700 or 10003C annealing temperature we observe the same spectra (Fig. 3). They are very similar to those obtained for the sol}gel c-Al O powders annealed at 6003C: besides an inhomogeneously broadened EPA line at 690 nm, we observe also a short-lived T PA broad band emission in the near IR, which is due to Cr> ions located in sites of weak crystal "eld strength (this band is seen only partially on the spectra due to the photomultiplier cut-o!). The position of the A PT maximum in the excitation spectrum is the same as for the c-phase. 4.3. PLD xlms Inspite of the lower deposition temperature (7903C or less), we already observe the characteristic narrow EPA lines of Cr> in a-Al O , together with the R lines of the h-phase. They are superimposed on the broad EPA line at 690 nm and the T PA broad band of the c-Al O phase (Fig. 4). The excitation spec trum is very similar to that of a alumina. The study of the relation between the proportion of the di!erent phases and the deposition parameters (oxygen pressure and substrate temperature) is underway.
We prepared Cr-doped alumina powders by sol}gel method with c, h and a crystalline structure. In addition to the R lines emitted by Cr> in strong crystal "eld sites of each of the three phases, we detected a short-lived IR broad band emitted by Cr> in weak crystal "eld sites of the c-phase. We grew optical waveguides of Cr> doped alumina by sol}gel or pulsed laser deposition. The comparison of the #uorescence spectra of "lms and powders show that Cr> is located in c crystallites in sol}gel "lms but that the three phases are present in PLD "lms. Cr #uorescence gives thus information on the crystalline phases present in the "lms, which could not be obtained by X-ray di!raction.
References [1] S.P. Feo"lov, A.A. Kaplyanskii, R.I. Zakharchenya, J. Lumin. 66}67 (1996) 349. [2] G. Carturan, R. DiMaggio, M. Montagna, O. Pilla, P. Scardi, J. Mater. Sci. 25 (1990) 2705. [3] Q. Wen, D.M. Lipkin, D.R. Clarke, J. Am. Ceram. Soc. 81 (1998) 3345. [4] B.J.H. Stadler, M. Oliveria, L.O. Bouthillette, J. Am. Ceram. Soc. 78 (1995) 3336. [5] B.E. Yoldas, Am. Ceram. Soc. Bull. 54 (1975) 289. [6] R. Nass, H. Schmidt, J. Non-Cryst. Solids 121 (1990) 329. [7] A. Pillonnet-Minardi, O. Marty, C. Bovier, C. Garapon, J. Mugnier, Opt. Mater., submitted. [8] A. Pillonnet, C. Garapon, C. Champeaux, C. Bovier, R. Brenier, H. Ja!rezic, J. Mugnier, Appl. Phys. A, in press. [9] A.M. Lejus, Rev. Hautes Temp. 1 (1964) 53. [10] C. Garapon, H. Manaa, R. MoncorgeH , J. Chem. Phys. 95 (1991) 5501.
Fluorescence of Cr> doped alumina optical waveguides prepared by pulsed laser deposition and sol}gel method A. Pillonnet , C. Garapon *, C. Champeaux, C. Bovier, H. Ja!rezic, J. Mugnier Laboratoire de Physico-Chimie des Mate& riaux Luminescents, CNRS-UMR 5620, Universite& Lyon I, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Laboratoire de Science des Proce& de& s Ce& ramiques et Traitements de Surface, CNRS-UMR 6638, Universite& de Limoges, 123, av. Albert Thomas, 87060 Limoges Cedex, France De& partement de Physique des Mate& riaux, CNRS-UMR 5586, Universite& Lyon I, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Laboratoire Ingienerie et Fonctionnalisation des Surfaces, CNRS-UMR 5621, Ecole Centrale de Lyon, 69130 Ecully, France
Abstract The #uorescence properties of Cr> in optical waveguides of doped alumina prepared by sol}gel or pulsed laser deposition (PLD) are compared with those of Cr-doped alumina powders obtained by sol}gel method and having c, h and a crystalline phases. Cr> is located in c crystallites in sol}gel "lms but the three phases are present in PLD "lms. 2000 Elsevier Science B.V. All rights reserved. Keywords: Cr : Al O ; Fluorescence; Sol}gel method; Pulsed laser deposition
1. Introduction
2. Sample preparation
Preparation of alumina thin "lms of high optical quality, which could be used as optical waveguides, raises the question of the relation between their structural characteristics and the growth parameters. The aim of this work is to use the #uorescence properties of Cr> doping ions to analyze the structural and optical properties of alumina optical waveguides, prepared by two di!erent methods, sol}gel or pulsed laser deposition, and to compare them with those of the various crystalline phases of bulk alumina. Although the #uorescence properties of Cr>-doped a-Al O (ruby laser crystal) are well known, only few studies have been devoted so far to those of Cr> in other alumina phases [1}3] or alumina waveguides [4]. This paper reports on our preliminary results of room temperature Cr> #uorescence properties in relation to X-rays di!raction.
Three kinds of samples, with the same Cr> concentration (0.04 or 1%) were studied:
* Corresponding author. Fax: #33-4-72-43-11-30. E-mail address: [email protected] (C. Garapon)
(1) Alumina powders were obtained by the sol}gel (SG) procedure as described by Yoldas [5]. The Cr doping was provided by addition of Cr(NO ) to the sol. These powders experienced cumulative 90 mn annealing from 100 to 14003C by 1003 steps. (2) Thin "lms elaborated by dip-coating: the sol was prepared as described by Nass and Schmit [6]. Cr ions were introduced as described previously. The alkoxide concentration, the withdrawal speed and the heat treatment process were adjusted as to obtain "lms with as high refractive index as possible without crack. At least nine successive coatings were required to provide waveguides [7]. The samples were then annealed at different temperatures between 700 and 11003C. (3) Thin "lms obtained by pulsed laser deposition (PLD). The ablation of rotating targets, prepared by sintering the previous sol}gel powders, is achieved by a KrF laser with a 3 J/cm #uence, under vacuum
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 4 9 - 9
1088
A. Pillonnet et al. / Journal of Luminescence 87}89 (2000) 1087}1089
(5;10\ mbar) or 0.1 mbar oxygen pressure. The substrate temperature was 7903C. Films were deposited on SiO /Si substrates [8]. 3. Structural properties For 1% Cr-doped alumina powders, the X-ray di!raction (XRD) spectra, registered with a h}2h di!ractometer show the usual evolution [9]. The structure is going from boehmite to a-Al O through c- and h-phases, depend ing on the annealing temperature. The lines are broad and do not enable to distinguish the d-phase, generally observed between the c- and h-phases. According to grazing incidence X-ray di!raction (GRXD), lightly doped alumina PLD "lms appear as amorphous when deposited at 0.1 mbar oxygen pressure and crystallized into c alumina at vacuum [8]. For 1% Cr-doped alumina "lms, no XRD lines could be observed. The study by transmission electron microscopy (TEM) is underway. For 1% or lightly doped alumina SG "lms annealed at 7003C, there is no detectable crystallization by GXRD. However, TEM experiments indicated that very small c crystallites are observed, their sizes growing as the annealing temperature increases [7]. XRD experiments must thus be completed by other techniques, more sensitive to the eventual presence of small crystallites.
4. Fluorescence properties Time-resolved #uorescence spectra were obtained by excitation with a pulsed dye laser. We used an AsGa photomultiplier and a photon counting system in the visible range and, for the IR domain, a germanium photodiode and a boxcar integrator. Fluorescence decays were recorded using a multichannel analyzer. Phase selective excitation spectra were recorded by using a chopped Xe lamp light and a lock-in ampli"er. More details may be found in Ref. [10]. 4.1. Al2O3 powders Time-resolved #uorescence spectra, recorded after excitation into the T band at 580 nm, change as the annealing temperature increases. These spectra are constituted by the long-lived EPA emission (R lines) of Cr> located in strong crystal "eld sites. Each alumina phase, c, h and a, has its speci"c lines. Above 12003C the well-known narrow R lines of ruby at 693.2 and 694.7 nm are observed with their vibronic side-band and a 3 ms life time. From 900 to 11003C we observe two lines, located at 684 and 687.2 nm, which we attribute, as in Refs. [2,3], to the R lines of Cr> in the h-phase. From 400 up to 10003C annealing temperature, we observe a broad
Fig. 1. Emission spectra of Cr> : Al O powders annealed at 6003C: (a) exc 580 nm, delay 4 ms, gate 4 ms; (b) exc 647 nm, delay 2 ls, gate 10 ls.
Fig. 2. Excitation spectra for di!erent modulation frequency F and emission j of Cr> : Al O powders annealed at (a) 14003C (j 694.7 nm, F 30 Hz); (b) 10003C (j 684 nm, F 30 Hz); (c) 6003C (j 700 nm, F 30 Hz); (d) 6003C (j 800 nm, F 2 kHz).
asymetric line peaking at about 690 nm (Fig. 1). The #uorescence decay is strongly non-exponential with an asymptotic time constant of about 14 ms. These unresolved, inhomogeneously broadened, R lines are due to Cr> in octahedral sites of the c-phase with a strong crystal "eld and a very disordered environment [1}3]. For the c-phase, in addition, using a longer excitation wavelength (647 nm) and a short time scale, we observe a short-lived IR broad band (Fig. 1), extending from 700 to 1300 nm with a maximum at about 820 nm. The #uorescence decay is non-exponential with time constant in the tens of microseconds range. This band is thus attributed to the T PA emission of Cr> ions located in weak crystal "eld sites. Excitation spectra of the characteristic emissions of each phase enable to con"rm these attributions (Fig. 2). Monitoring short wavelength emission around 690 nm and choosing low modulation frequency (30 Hz), we select the long-lived EPA emission of strong "eld sites. The maximum of the corresponding A PT excita tion band is located at 554, 562 and 576 nm for the a-, hand c-phases respectively. Monitoring longer wavelength (800 nm) and using high modulation frequency (2 kHz), the maximum is shifted to longer wavelength (607 nm) in agreement with the attribution of the short-lived IR band to weak "eld sites.
A. Pillonnet et al. / Journal of Luminescence 87}89 (2000) 1087}1089
1089
Fig. 3. Emission spectra of SG "lms: (a) exc 410 nm, delay 2 ls, gate 10 ms; (b) exc 647 nm, delay 2 ls, gate 10 ls.
Fig. 4. Emission spectra of PLD "lms (exc 647 nm): (a) delay 10 ls, gate 10 ms; (b) delay 2 ls, gate 10 ls.
The spectroscopic properties of Cr> in c-Al O ap pear as very similar as those observed for the nonstoichiometric spinel MgO-2.6 Al O [10]. Both crystal have a distorted spinel structure with a well-de"ned oxygen arrangement and cation vacancies in the octahedral sites [9]. The origin of the IR broad band in c-Al O is probably related to Cr> located nearby cation vacancies as it is in the non-stoichiometric spinel.
5. Conclusion
4.2. Sol}gel xlms For both concentrations (0.04 and 1%) and for 700 or 10003C annealing temperature we observe the same spectra (Fig. 3). They are very similar to those obtained for the sol}gel c-Al O powders annealed at 6003C: besides an inhomogeneously broadened EPA line at 690 nm, we observe also a short-lived T PA broad band emission in the near IR, which is due to Cr> ions located in sites of weak crystal "eld strength (this band is seen only partially on the spectra due to the photomultiplier cut-o!). The position of the A PT maximum in the excitation spectrum is the same as for the c-phase. 4.3. PLD xlms Inspite of the lower deposition temperature (7903C or less), we already observe the characteristic narrow EPA lines of Cr> in a-Al O , together with the R lines of the h-phase. They are superimposed on the broad EPA line at 690 nm and the T PA broad band of the c-Al O phase (Fig. 4). The excitation spec trum is very similar to that of a alumina. The study of the relation between the proportion of the di!erent phases and the deposition parameters (oxygen pressure and substrate temperature) is underway.
We prepared Cr-doped alumina powders by sol}gel method with c, h and a crystalline structure. In addition to the R lines emitted by Cr> in strong crystal "eld sites of each of the three phases, we detected a short-lived IR broad band emitted by Cr> in weak crystal "eld sites of the c-phase. We grew optical waveguides of Cr> doped alumina by sol}gel or pulsed laser deposition. The comparison of the #uorescence spectra of "lms and powders show that Cr> is located in c crystallites in sol}gel "lms but that the three phases are present in PLD "lms. Cr #uorescence gives thus information on the crystalline phases present in the "lms, which could not be obtained by X-ray di!raction.
References [1] S.P. Feo"lov, A.A. Kaplyanskii, R.I. Zakharchenya, J. Lumin. 66}67 (1996) 349. [2] G. Carturan, R. DiMaggio, M. Montagna, O. Pilla, P. Scardi, J. Mater. Sci. 25 (1990) 2705. [3] Q. Wen, D.M. Lipkin, D.R. Clarke, J. Am. Ceram. Soc. 81 (1998) 3345. [4] B.J.H. Stadler, M. Oliveria, L.O. Bouthillette, J. Am. Ceram. Soc. 78 (1995) 3336. [5] B.E. Yoldas, Am. Ceram. Soc. Bull. 54 (1975) 289. [6] R. Nass, H. Schmidt, J. Non-Cryst. Solids 121 (1990) 329. [7] A. Pillonnet-Minardi, O. Marty, C. Bovier, C. Garapon, J. Mugnier, Opt. Mater., submitted. [8] A. Pillonnet, C. Garapon, C. Champeaux, C. Bovier, R. Brenier, H. Ja!rezic, J. Mugnier, Appl. Phys. A, in press. [9] A.M. Lejus, Rev. Hautes Temp. 1 (1964) 53. [10] C. Garapon, H. Manaa, R. MoncorgeH , J. Chem. Phys. 95 (1991) 5501.