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Pr3+-doped ZBLA fluoride glasses for visible laser emission
Optical Materials 33 (2011) 980–984
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Pr3+-doped ZBLA fluoride glasses for visible laser emission Melinda Olivier a, Parastesh Pirasteh a, Jean-Louis Doualan b, Patrice Camy b, Hervé Lhermite c, Jean-Luc Adam a, Virginie Nazabal a,⇑ a
Sciences Chimiques de Rennes, UMR-CNRS 6226, Equipe Verres et Céramiques, Université de Rennes1, 35042 Rennes, France CIMAP, ENSI Caen, 6 boulevard du Maréchal Juin, 14050 Caen cedex 4, France c IETR-Microelectronique, Université Rennes1, 35042 Rennes, France b
a r t i c l e
i n f o
Article history: Received 7 June 2010 Received in revised form 29 November 2010 Accepted 18 December 2010 Available online 13 January 2011 Keywords: Fluorozirconate Glasses Rare earth Laser Waveguide
a b s t r a c t Praseodymium doped Fluoride glasses were studied in order to fabricate compact solid state laser sources emitting in the visible range for lighting application and for quantum information processing. The objective of this study is focused on red, green and orange emissions. ZBLA bulk glasses (57%ZrF4 – 34%BaF2 – (5 x)%LaF3 – 4%AlF3 – x%PrF3) have been synthesized under dry argon atmosphere. Physicochemical and optical properties such as density, glass transition temperature, composition, transmission and refractive index have been investigated. Spectroscopic studies have been performed to optimize the rare earth doping level in order to reach the best compromise respecting a good optical quality, a highest lifetime and efficient emission intensity at 635, 605 and 520 nm. Spectroscopic measurements have been carried out on bulk samples with Pr3+ concentrations ranging from 0.1 to 3 mol% and first characterizations of ion exchanged Pr: ZBLA waveguides are reported. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Lasers or amplifiers are devices of major interest in integrated optics. This study is achieved within the context of a strong demand in visible laser sources. Low phonon energy materials doped with rare earth ions such as Pr3+ can be used to realise compact solid state laser sources emitting in the visible range. This fluoride glasses are attractive for this kind of applications [1], because of their wide optical transmission range from 0.2 lm to 8 lm [2], their theoretical low optical losses and low phonon energy (580 cm 1)[3], and their potential as hosts for active rare earth ion such as Praseodymium. Indeed, the direct pumping of the Pr3+ ion, in which a blue photon is converted to a green or a red photon, is a very attractive solution, since it is a simple way to obtain the population inversion [4]. That way, laser operation was achieved in recent years with fluoride crystal hosts [5,6]. ZBLA glasses (57%ZrF4 – 34%BaF2 – 5%LaF3 – 4%AlF3) have already been used as hosts materials for RE ions and can be fabricated in the form of waveguides by well controlled technique like ion exchange [7,8]. They are an interesting alternative for such visible laser sources. In the present paper, we present a study on Pr3+-doped ZBLA fluoride glasses dedicated to optimize suitable spectroscopic characteristics for visible emission around 640, 606, and 520 nm and to achieve ion exchanged waveguides.
The glass compositions of the sample prepared in this study is the following: 57%ZrF4 – 34%BaF2 – 4%AlF3 – (5 x)%LaF3 – x%PrF3 with 0 < x < 5 (mol%). The glass batches for 7 g were prepared from reagent grade >99.9% fluorides. All chemicals compounds were stored and handled in a dry box to prevent hydroxide and oxide contamination (H2O = 2 ppm, O2 = 0.5 ppm). The different powders were mixed in a covered carbon crucibles and melted at 850 °C for 1 h in a flowing dry nitrogen atmosphere. The melts were subsequently quenched in air during 20 s and annealed during 16 h at 300 °C and finally slowly cooled to room temperature. All samples were then polished for optical measurements. The composition of the different samples was checked by using scanning electron microscopy with an energy-dispersive X-ray analyzer (SEM–EDS, JSM 6400 – Oxford Link INCA). Pr3+ and F ion concentrations in doped-glass samples were determined by inductively coupled plasma mass spectrometry (ICP–MS). Thermal properties are measured by differential scanning calorimetry (DSC 2010 from TA Instruments) with heating rate of 10 K/min (accuracy ± 2 K). The density of some samples was determined using a helium picnometer (Micromeritics, AccuPyc 1330), with ±0.004 g/cm3 accuracy. Transmittance spectra of Pr3+: ZBLA glasses were measured at room-temperature from 250 to 3200 nm using a spectrophotometer (PerkinElmer-Lambda 1050). The refractive indices of the glasses were determined by measuring the critical angle for the
⇑ Corresponding author. Tel.: +33 223235748; fax: +33 223235611. E-mail address: [email protected] (V. Nazabal). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.12.007
981
3. Result and discussion 3.1. Glass synthesis The Table 1 sums up the results for ZBLA glass of the investigated physicochemical and optical properties such as density, glass transition temperature, composition, transmission and refractive index. Pr3+ ions were introduced in different proportion in order to study the influence on physico-chemical and optical properties of ZBLA glasses. The glass is transparent and homogeneous for bulk of 25 mm of diameter. The concentration analyzed by ICP is in good agreement with the theoretical composition. A slight decrease of aluminum can be noticed and should be controlled for next synthesis. The density of samples lightly increases with the Pr3+ introduction and the glass transition temperature (Tg) shows no significant evolution with Pr3+ concentration.
2 -21
3 3 H4 P2
12 3 3 3 H4 P1, I6
10 8
3 3 H4 P0
6
3 1 H4 D2
4 2 0 400
57 ZrF4 – 34 BaF2 – (5 x) LaF3 – 4 AlF3 – x PrF3 316 °C ± 2 °C 4.600 ± 0.004 g cm 3 0.2–7 lm 1.517 ± 0.001 n = 1.506 + 5.55 103 k 2 4.10 108 k 4
500
550
600
650
Fig. 1. Absorption cross section of a ZBLA glass doped with 0.5 mol% of Pr3+.
The absorption cross section of a Pr3+-doped ZBLA glass in the visible range is reported in the Fig. 1. The transitions from the ground 3H4 level to the 3P0, 1I6/3P1, 3P2 and 1D2 manifolds are identified in the Fig. 1. It is interesting to point out the relatively broad, most intense, band of the absorption spectrum around 442 nm, which is suitable for a direct pumping with a GaN blue laser diode. The Fig. 2 shows the emission spectrum obtained for a Pr3+ concentration of 0.5 mol% by pumping into the 3P2 level with a 50 mW Nichia laser diode operating at 445 nm. The typical emission transitions of the Praseodymium ions in the visible can be observed around 636, 604, 536, 521 and 480 nm. These emission lines are associated with transitions from the thermalized 3P1, 1I6 and 3P0 levels to the 3H5, 3H6 and 3F2 levels, around 521/536, 604, and 636 nm, respectively. The superimposing of absorption and emission spectra revealed the likely reabsorption at the lower wavelength side of emission transition bands which occurs at 478 nm and a possible cross-relaxation effect around 600 nm involving the (3P0 ? 3H6) and (3H4 ? 1D2) transitions. Fluorescence decays of 3P0 level measurements were performed on ZBLA glasses with several Pr3+ concentrations ranging from 0.1 to 3 mol%. The results, reported on the Fig. 3, shows that the lifetime clearly decreases when increasing the concentration. The values for the fluorescence lifetimes measured at room temperature are found between 43 ± 2 ls and 12 ± 1 ls and the longest lifetime is obtained for a Pr3+ concentration of 0.2 mol%. Such behaviour of the fluorescence decay with the concentration is likely due to energy transfer between Pr3+ in 3P0 excited state level and 3H4 ground state. The most important mechanism is the cross-relaxation between active ions giving rise to a non-radiative relaxation of the
1.4 3 P0
Absorption (cm-1)
1.2
0.10
3 H4
1.0
0.08
0.8
0.06
0.6
3 P0
0.4 3 3 3 P0, P1 H5
ZBLA Composition Tg Volumic mass Transmission n (633 nm) Cauchy distribution
450
0.2 0.0 400
450
500
550
3 H6 3 3 P0 F2
0.04
Emission (a.u.)
Table 1 Physicochemical and optical properties of ZBLA glasses.
14
Wavelenght (nm)
3.2. Spectroscopic measurements Spectroscopic studies have been performed to optimize the rare earth doping level in order to reach the best compromise respecting a good optical quality, a highest lifetime and efficient emission intensity at 635, 605 and 520 nm.
16
absorption cross-section (10
sample/prism interface (accuracy ±0.001) using different laser beam wavelengths (633, 825, 1311 and 1551 nm) (Metricon2010 instrument). The refractive effective indices of the propagation modes were measured by means of the M-lines prism coupling configuration for both TE and TM polarization. A lock-in amplifier, a monochromator and a photomultiplier tube were used to detect emission spectra for which a blue laser diode operating at 445 nm was employed as excitation source. Fluorescent decays were carried out using the pulsed excitation of a YAG:Nd-pumped optical parametric oscillator laser emitting at around 445 nm. During ion exchange process, the samples are set in an alumina tube which is subject to an inert gas flow (Ar) before and after the treatment. The samples are exposed to an Argon (2 l h 1) diluted HCl gas flow (2 l h 1) during the exchange process. Thin film SiO2 (200 nm thick) was deposited by radio frequency (RF) magnetron sputtering system (MRC 822 Sputtering System) on ZBLA substrates [9]. The quality of the silicon dioxide thin film is characterized on a regular basis, by capacitance measurements of aluminum/RF sputtered SiO2/single crystalline silicon wafer MIS structures. The substrates were then patterned using optical lithography. A S1818 positive photoresist (Rhom & Haas) was spin-coated onto the SiO2-coated ZBLA substrates. The photoresist were then exposed to UV radiation (MBJ3 broad-band mask aligner) with a predefined pattern using a chrome mask for 30 s. The SiO2 thin film was dry etched in a RIE system (Microsys 400, Roth & Rhau AG) with pure CF4 plasma at 50 W of RF power, 10 sccm of CF4 flow rate and 1 mT of partial pressure leading to a highly anisotropic profile and negligible contamination of the ZBLA material.
cm )
M. Olivier et al. / Optical Materials 33 (2011) 980–984
0.02
600
650
700
0.00 750
wavelength (nm) Fig. 2. Emission and absorption bands of a ZBLA glass doped with 0.5 mol% of Pr3+.
M. Olivier et al. / Optical Materials 33 (2011) 980–984
50
Fluorescence decay (µs)
45 40 35 30 25 20 15 10 0,0
0,5
1,0
1,5
2,0
2,5
3,0
3+
Pr concentration (%) Fig. 3. Variation of lifetime with Pr3+ concentration in ZBLA glasses.
3
P0 level. This energy transfer between two ions involves in particular the following levels (3P0 ? 1G4, 3H4 ? 1G4), (3P0 ? 1D2, 3 H4 ? 3H6) or (3P0 ? 3H6, 3H4 ? 1D2). The variation law of this quenching can be reflected by the evolution of the inverse of the measured fluorescence decay (1/smeas) as a function of the concentration. The 1/smeas seems to be proportional to the concentration in the range of 0.5–3 mol%, which could be explained by crossrelaxation mechanisms mentioned previously. Some other effects like migration between excited ions or rare earth clustering can reduce the 3P0 lifetime when the concentration increases more than 1 mol%, for which the measured lifetime smeas become inversely proportional to the square of the concentration. As a result, the visible fluorescence in the green and orange ranges is quenched at high Pr3+ doping levels. For laser application, we selected a Pr3+ concentration of 0.5 mol% to have enough pump absorption on several millimeters samples and a lifetime of about 37 ls which compares well with Pr3+: crystals.
ifications of the glass surface, namely by ion implantation or ion exchange. Due to its simplicity and its flexibility in suiting the characteristics of the resulting waveguides, the latter method is the most powerful and is chosen in this case to exchange F by Cl ions [8]. In order to produce a light-guiding structure, the ion exchange process should lead to a refractive index increase at the surface of the sample. For fluoride glasses, the ion exchange with Cl ions process occurs in the experimental set-up illustrated in Fig. 4. Up to now, two parameters have been studied during the ion exchange process: temperature and time. Refractive index and thickness have been measured on the exchanged planar waveguides (Fig. 5). For the same treatment time, thickness of the exchanged layer increases with the temperature, indeed, a higher temperature allows a better diffusion of chloride ions in the vitreous matrix. Refractive index and number of guided modes at 633 nm are determined by the M-lines method and the results are shown in Table 2. The refractive index variation (Dn) obtained is about 0.08. It can be noticed that the refractive index of exchanged layer is always about 1.595. It is worth noting that if we compared with the refractive index of the ZBLA substrate before ion exchange, which is around 1.517, the variation is large enough to achieve optical waveguiding in the visible range. It can also be
3.5 t = 5h t = 7h
3.0
Thickness (µm)
982
2.5 2.0 1.5
3.3. Ion exchange process
1.0 240
Glass waveguides are considered to be prime candidates for many integrated optics applications. These devices usually exhibit graded-index structures which can be achieved by chemical mod-
245
250
255
265
270
Fig. 5. Variation of layer thickness with temperature and time during ion exchange.
Sample Furnace
Flowmeter
Oil bath
NaOH + phenolphtalein
NaOH + phenolphtalein
260
Temperature (°C)
Soda lime
Fig. 4. Experimental setup for ion exchange.
M. Olivier et al. / Optical Materials 33 (2011) 980–984 Table 2 Refractive index, thickness and number of guided modes obtained at 633 nm for different parameter of ion exchange. T (°C)
e (lm)
n (±0.001) at 633 nm
Number of guided modes
5 5 7 7 7 9
250 260 240 250 260 260
1.44 2.92 1.36 2.24 3.16 3.68
1.596 1.592 1.597 1.598 1.593 1.595
3 5 3 3 6 7
Fluorescence intensity (a.u.)
Time (h)
3 P0
3 H4
ZBLA planar waveguide ZBLA bulk glass 3 3 P0 F2 3 P0
3 H6
3 3 3 P0, P1 H5
460 480 500 520 540 560 580 600 620 640 660
Wavelength (nm) 3+
Fig. 6. Fluorescence from a Pr : ZBLA ion exchange waveguide compared to bulk emission.
noticed that if the temperature is maintained during all night after the treatment, both the diffusion homogeneity and the thickness increase during the annealing. Regarding the refractive index, it decreases and the refractive index variation is about 0.06, instead of 0.08 without this post-annealing treatment. The maximum thickness of the guiding core, near 4 lm, maybe still increased by adjusting the post-diffusion time and will also allow controlling the refractive contrast between the non-exchanged bulk glass and the exchanged layer. Optical measurements were also performed on an exchanged glass sample corresponding to a planar waveguide having a thickness of about 3.7 lm. The 445 nm signal was coupled by the aid of a microscope objective, and the resulting fluorescence was collected at the end of the waveguide with an 800 lm core fiber. The propagated fluorescence was clearly observed at the wave-
983
guide output and detected using an optical spectrum analyzer. The Fig. 6 shows the fluorescence emitted by the planar waveguide compared to the emission spectrum of a bulk sample normalized at 633 nm far from reabsorption phenomenon. A slight broadening, most likely due to the presence of Cl ions in the exchanged layer, can be observed, but the overall spectrum, and its relative intensities are the same.
3.4. Photolithography To obtain ultimately channeled waveguides, a photolithography step is necessary to perform an ion exchange manipulation. This stage includes a first deposit of a thin layer of SiO2 on the glass fluoride substrate that once etched, will be used for an ion exchange mask. A thin film SiO2 (200 nm thick) was successfully deposited. Using a mixture of argon (Ar, 40 sccm) and oxygen (O2, 12 sccm), a pressure of 4 mT and a RF power fixed at 200 W, the deposition rate was 1.5 nm mn 1 at room temperature. The capacitance recorded at high frequency vs. voltage C(V, 1 MHz) and the intensity vs. voltage I(V) curves showed no leakage current and negligible trapped charges inside the SiO2 thin film [9].The thickness and refractive index were determined close to 200 nm and 1.47, respectively from ellipsometry measurements carried out at 632 nm. A photolithography process was used [8] which occurs in the following major steps described in experimental part. The SiO2 layer was fully etched without any over-etching and no damages appeared at the surface and inside the ZBLA glass. First results of silica etching are shown in Fig. 7 which will allow fabricating channel waveguides by ionic exchange process.
4. Conclusion Study of host composition and RE concentration through physicochemical, optical and spectroscopic measurement allowed us to choose the best compromise between doping concentration for laser applications: 0.5 mol% of Pr3+. In this case, lifetime is about 37 ls and we have enough pump absorption to obtain good emission in red, orange and green wavelength. First tests of ionic exchange process are encouraging. We obtained planar waveguide of 3–5 lm thickness and the variation of refractive index between the exchanged layer and the ZBLA bulk glass is about 0.06–0.08. However, some parameters still have to be optimized, like gas flow during treatment. Moreover, first optical characterization of the planar waveguides indicates that the overall structure is preserved.
Fig. 7. SEM pictures of an etched silica layer on the top ZBLA glass (back scattered electrons mode).
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M. Olivier et al. / Optical Materials 33 (2011) 980–984
Improvements on photolithography process to obtain low losses channel waveguides are under progress.
References [1] E. Lebrasseur, B. Jacquier, M.C. Marco de Lucas, E. Josse, J.L. Adam, J. Lucas, Y. Gao, B. Boulard, C. Jacoboni, Y. Gao, J.E. Broquin, R. Rimet, Journal of Alloys and Compounds 275–277 (1998) 716. [2] C.M. Baldwin, R.M. Almeida, J.D. Mackenzie, Journal of Non-Crystalline Solids 43 (1981) 309.
[3] S. Aasland, T. Grande, A. Grzechnik, P.F. McMillan, Journal of Non-Crystalline Solids 195 (1996) 180. [4] E. Osiac, E. Heumann, G. Huber, S. Kuck, E. Sani, A. Toncelli, M. Tonelli, Applied Physics Letters 82 (2003) 3832. [5] S. Khiari, M. Velazquez, R. Moncorgé, J.L. Doualan, P. Camy, A. Ferrier, M. Diaf, Journal of Alloys and Compounds 451 (2008) 128. [6] P. Camy, J.L. Doualan, R. Moncorgé, J. Bengoechea, U. Weichmann, Optics Letters 32 (2007) 1462. [7] R.S.R.V. Ramaswamy, Journal of Lightwave Technology 6 (1988) 984. [8] E. Josse, G. Fonteneau, J. Lucas, Materials Research Bulletin 32 (1997) 1139. [9] A. Saboundji, N. Coulon, A. Gorin, H. Lhermite, T. Mohammed-Brahim, M. Fonrodona, J. Bertomeu, J. Andreu, Thin Solid Films 487 (2005) 227.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Pr3+-doped ZBLA fluoride glasses for visible laser emission Melinda Olivier a, Parastesh Pirasteh a, Jean-Louis Doualan b, Patrice Camy b, Hervé Lhermite c, Jean-Luc Adam a, Virginie Nazabal a,⇑ a
Sciences Chimiques de Rennes, UMR-CNRS 6226, Equipe Verres et Céramiques, Université de Rennes1, 35042 Rennes, France CIMAP, ENSI Caen, 6 boulevard du Maréchal Juin, 14050 Caen cedex 4, France c IETR-Microelectronique, Université Rennes1, 35042 Rennes, France b
a r t i c l e
i n f o
Article history: Received 7 June 2010 Received in revised form 29 November 2010 Accepted 18 December 2010 Available online 13 January 2011 Keywords: Fluorozirconate Glasses Rare earth Laser Waveguide
a b s t r a c t Praseodymium doped Fluoride glasses were studied in order to fabricate compact solid state laser sources emitting in the visible range for lighting application and for quantum information processing. The objective of this study is focused on red, green and orange emissions. ZBLA bulk glasses (57%ZrF4 – 34%BaF2 – (5 x)%LaF3 – 4%AlF3 – x%PrF3) have been synthesized under dry argon atmosphere. Physicochemical and optical properties such as density, glass transition temperature, composition, transmission and refractive index have been investigated. Spectroscopic studies have been performed to optimize the rare earth doping level in order to reach the best compromise respecting a good optical quality, a highest lifetime and efficient emission intensity at 635, 605 and 520 nm. Spectroscopic measurements have been carried out on bulk samples with Pr3+ concentrations ranging from 0.1 to 3 mol% and first characterizations of ion exchanged Pr: ZBLA waveguides are reported. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Lasers or amplifiers are devices of major interest in integrated optics. This study is achieved within the context of a strong demand in visible laser sources. Low phonon energy materials doped with rare earth ions such as Pr3+ can be used to realise compact solid state laser sources emitting in the visible range. This fluoride glasses are attractive for this kind of applications [1], because of their wide optical transmission range from 0.2 lm to 8 lm [2], their theoretical low optical losses and low phonon energy (580 cm 1)[3], and their potential as hosts for active rare earth ion such as Praseodymium. Indeed, the direct pumping of the Pr3+ ion, in which a blue photon is converted to a green or a red photon, is a very attractive solution, since it is a simple way to obtain the population inversion [4]. That way, laser operation was achieved in recent years with fluoride crystal hosts [5,6]. ZBLA glasses (57%ZrF4 – 34%BaF2 – 5%LaF3 – 4%AlF3) have already been used as hosts materials for RE ions and can be fabricated in the form of waveguides by well controlled technique like ion exchange [7,8]. They are an interesting alternative for such visible laser sources. In the present paper, we present a study on Pr3+-doped ZBLA fluoride glasses dedicated to optimize suitable spectroscopic characteristics for visible emission around 640, 606, and 520 nm and to achieve ion exchanged waveguides.
The glass compositions of the sample prepared in this study is the following: 57%ZrF4 – 34%BaF2 – 4%AlF3 – (5 x)%LaF3 – x%PrF3 with 0 < x < 5 (mol%). The glass batches for 7 g were prepared from reagent grade >99.9% fluorides. All chemicals compounds were stored and handled in a dry box to prevent hydroxide and oxide contamination (H2O = 2 ppm, O2 = 0.5 ppm). The different powders were mixed in a covered carbon crucibles and melted at 850 °C for 1 h in a flowing dry nitrogen atmosphere. The melts were subsequently quenched in air during 20 s and annealed during 16 h at 300 °C and finally slowly cooled to room temperature. All samples were then polished for optical measurements. The composition of the different samples was checked by using scanning electron microscopy with an energy-dispersive X-ray analyzer (SEM–EDS, JSM 6400 – Oxford Link INCA). Pr3+ and F ion concentrations in doped-glass samples were determined by inductively coupled plasma mass spectrometry (ICP–MS). Thermal properties are measured by differential scanning calorimetry (DSC 2010 from TA Instruments) with heating rate of 10 K/min (accuracy ± 2 K). The density of some samples was determined using a helium picnometer (Micromeritics, AccuPyc 1330), with ±0.004 g/cm3 accuracy. Transmittance spectra of Pr3+: ZBLA glasses were measured at room-temperature from 250 to 3200 nm using a spectrophotometer (PerkinElmer-Lambda 1050). The refractive indices of the glasses were determined by measuring the critical angle for the
⇑ Corresponding author. Tel.: +33 223235748; fax: +33 223235611. E-mail address: [email protected] (V. Nazabal). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.12.007
981
3. Result and discussion 3.1. Glass synthesis The Table 1 sums up the results for ZBLA glass of the investigated physicochemical and optical properties such as density, glass transition temperature, composition, transmission and refractive index. Pr3+ ions were introduced in different proportion in order to study the influence on physico-chemical and optical properties of ZBLA glasses. The glass is transparent and homogeneous for bulk of 25 mm of diameter. The concentration analyzed by ICP is in good agreement with the theoretical composition. A slight decrease of aluminum can be noticed and should be controlled for next synthesis. The density of samples lightly increases with the Pr3+ introduction and the glass transition temperature (Tg) shows no significant evolution with Pr3+ concentration.
2 -21
3 3 H4 P2
12 3 3 3 H4 P1, I6
10 8
3 3 H4 P0
6
3 1 H4 D2
4 2 0 400
57 ZrF4 – 34 BaF2 – (5 x) LaF3 – 4 AlF3 – x PrF3 316 °C ± 2 °C 4.600 ± 0.004 g cm 3 0.2–7 lm 1.517 ± 0.001 n = 1.506 + 5.55 103 k 2 4.10 108 k 4
500
550
600
650
Fig. 1. Absorption cross section of a ZBLA glass doped with 0.5 mol% of Pr3+.
The absorption cross section of a Pr3+-doped ZBLA glass in the visible range is reported in the Fig. 1. The transitions from the ground 3H4 level to the 3P0, 1I6/3P1, 3P2 and 1D2 manifolds are identified in the Fig. 1. It is interesting to point out the relatively broad, most intense, band of the absorption spectrum around 442 nm, which is suitable for a direct pumping with a GaN blue laser diode. The Fig. 2 shows the emission spectrum obtained for a Pr3+ concentration of 0.5 mol% by pumping into the 3P2 level with a 50 mW Nichia laser diode operating at 445 nm. The typical emission transitions of the Praseodymium ions in the visible can be observed around 636, 604, 536, 521 and 480 nm. These emission lines are associated with transitions from the thermalized 3P1, 1I6 and 3P0 levels to the 3H5, 3H6 and 3F2 levels, around 521/536, 604, and 636 nm, respectively. The superimposing of absorption and emission spectra revealed the likely reabsorption at the lower wavelength side of emission transition bands which occurs at 478 nm and a possible cross-relaxation effect around 600 nm involving the (3P0 ? 3H6) and (3H4 ? 1D2) transitions. Fluorescence decays of 3P0 level measurements were performed on ZBLA glasses with several Pr3+ concentrations ranging from 0.1 to 3 mol%. The results, reported on the Fig. 3, shows that the lifetime clearly decreases when increasing the concentration. The values for the fluorescence lifetimes measured at room temperature are found between 43 ± 2 ls and 12 ± 1 ls and the longest lifetime is obtained for a Pr3+ concentration of 0.2 mol%. Such behaviour of the fluorescence decay with the concentration is likely due to energy transfer between Pr3+ in 3P0 excited state level and 3H4 ground state. The most important mechanism is the cross-relaxation between active ions giving rise to a non-radiative relaxation of the
1.4 3 P0
Absorption (cm-1)
1.2
0.10
3 H4
1.0
0.08
0.8
0.06
0.6
3 P0
0.4 3 3 3 P0, P1 H5
ZBLA Composition Tg Volumic mass Transmission n (633 nm) Cauchy distribution
450
0.2 0.0 400
450
500
550
3 H6 3 3 P0 F2
0.04
Emission (a.u.)
Table 1 Physicochemical and optical properties of ZBLA glasses.
14
Wavelenght (nm)
3.2. Spectroscopic measurements Spectroscopic studies have been performed to optimize the rare earth doping level in order to reach the best compromise respecting a good optical quality, a highest lifetime and efficient emission intensity at 635, 605 and 520 nm.
16
absorption cross-section (10
sample/prism interface (accuracy ±0.001) using different laser beam wavelengths (633, 825, 1311 and 1551 nm) (Metricon2010 instrument). The refractive effective indices of the propagation modes were measured by means of the M-lines prism coupling configuration for both TE and TM polarization. A lock-in amplifier, a monochromator and a photomultiplier tube were used to detect emission spectra for which a blue laser diode operating at 445 nm was employed as excitation source. Fluorescent decays were carried out using the pulsed excitation of a YAG:Nd-pumped optical parametric oscillator laser emitting at around 445 nm. During ion exchange process, the samples are set in an alumina tube which is subject to an inert gas flow (Ar) before and after the treatment. The samples are exposed to an Argon (2 l h 1) diluted HCl gas flow (2 l h 1) during the exchange process. Thin film SiO2 (200 nm thick) was deposited by radio frequency (RF) magnetron sputtering system (MRC 822 Sputtering System) on ZBLA substrates [9]. The quality of the silicon dioxide thin film is characterized on a regular basis, by capacitance measurements of aluminum/RF sputtered SiO2/single crystalline silicon wafer MIS structures. The substrates were then patterned using optical lithography. A S1818 positive photoresist (Rhom & Haas) was spin-coated onto the SiO2-coated ZBLA substrates. The photoresist were then exposed to UV radiation (MBJ3 broad-band mask aligner) with a predefined pattern using a chrome mask for 30 s. The SiO2 thin film was dry etched in a RIE system (Microsys 400, Roth & Rhau AG) with pure CF4 plasma at 50 W of RF power, 10 sccm of CF4 flow rate and 1 mT of partial pressure leading to a highly anisotropic profile and negligible contamination of the ZBLA material.
cm )
M. Olivier et al. / Optical Materials 33 (2011) 980–984
0.02
600
650
700
0.00 750
wavelength (nm) Fig. 2. Emission and absorption bands of a ZBLA glass doped with 0.5 mol% of Pr3+.
M. Olivier et al. / Optical Materials 33 (2011) 980–984
50
Fluorescence decay (µs)
45 40 35 30 25 20 15 10 0,0
0,5
1,0
1,5
2,0
2,5
3,0
3+
Pr concentration (%) Fig. 3. Variation of lifetime with Pr3+ concentration in ZBLA glasses.
3
P0 level. This energy transfer between two ions involves in particular the following levels (3P0 ? 1G4, 3H4 ? 1G4), (3P0 ? 1D2, 3 H4 ? 3H6) or (3P0 ? 3H6, 3H4 ? 1D2). The variation law of this quenching can be reflected by the evolution of the inverse of the measured fluorescence decay (1/smeas) as a function of the concentration. The 1/smeas seems to be proportional to the concentration in the range of 0.5–3 mol%, which could be explained by crossrelaxation mechanisms mentioned previously. Some other effects like migration between excited ions or rare earth clustering can reduce the 3P0 lifetime when the concentration increases more than 1 mol%, for which the measured lifetime smeas become inversely proportional to the square of the concentration. As a result, the visible fluorescence in the green and orange ranges is quenched at high Pr3+ doping levels. For laser application, we selected a Pr3+ concentration of 0.5 mol% to have enough pump absorption on several millimeters samples and a lifetime of about 37 ls which compares well with Pr3+: crystals.
ifications of the glass surface, namely by ion implantation or ion exchange. Due to its simplicity and its flexibility in suiting the characteristics of the resulting waveguides, the latter method is the most powerful and is chosen in this case to exchange F by Cl ions [8]. In order to produce a light-guiding structure, the ion exchange process should lead to a refractive index increase at the surface of the sample. For fluoride glasses, the ion exchange with Cl ions process occurs in the experimental set-up illustrated in Fig. 4. Up to now, two parameters have been studied during the ion exchange process: temperature and time. Refractive index and thickness have been measured on the exchanged planar waveguides (Fig. 5). For the same treatment time, thickness of the exchanged layer increases with the temperature, indeed, a higher temperature allows a better diffusion of chloride ions in the vitreous matrix. Refractive index and number of guided modes at 633 nm are determined by the M-lines method and the results are shown in Table 2. The refractive index variation (Dn) obtained is about 0.08. It can be noticed that the refractive index of exchanged layer is always about 1.595. It is worth noting that if we compared with the refractive index of the ZBLA substrate before ion exchange, which is around 1.517, the variation is large enough to achieve optical waveguiding in the visible range. It can also be
3.5 t = 5h t = 7h
3.0
Thickness (µm)
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2.5 2.0 1.5
3.3. Ion exchange process
1.0 240
Glass waveguides are considered to be prime candidates for many integrated optics applications. These devices usually exhibit graded-index structures which can be achieved by chemical mod-
245
250
255
265
270
Fig. 5. Variation of layer thickness with temperature and time during ion exchange.
Sample Furnace
Flowmeter
Oil bath
NaOH + phenolphtalein
NaOH + phenolphtalein
260
Temperature (°C)
Soda lime
Fig. 4. Experimental setup for ion exchange.
M. Olivier et al. / Optical Materials 33 (2011) 980–984 Table 2 Refractive index, thickness and number of guided modes obtained at 633 nm for different parameter of ion exchange. T (°C)
e (lm)
n (±0.001) at 633 nm
Number of guided modes
5 5 7 7 7 9
250 260 240 250 260 260
1.44 2.92 1.36 2.24 3.16 3.68
1.596 1.592 1.597 1.598 1.593 1.595
3 5 3 3 6 7
Fluorescence intensity (a.u.)
Time (h)
3 P0
3 H4
ZBLA planar waveguide ZBLA bulk glass 3 3 P0 F2 3 P0
3 H6
3 3 3 P0, P1 H5
460 480 500 520 540 560 580 600 620 640 660
Wavelength (nm) 3+
Fig. 6. Fluorescence from a Pr : ZBLA ion exchange waveguide compared to bulk emission.
noticed that if the temperature is maintained during all night after the treatment, both the diffusion homogeneity and the thickness increase during the annealing. Regarding the refractive index, it decreases and the refractive index variation is about 0.06, instead of 0.08 without this post-annealing treatment. The maximum thickness of the guiding core, near 4 lm, maybe still increased by adjusting the post-diffusion time and will also allow controlling the refractive contrast between the non-exchanged bulk glass and the exchanged layer. Optical measurements were also performed on an exchanged glass sample corresponding to a planar waveguide having a thickness of about 3.7 lm. The 445 nm signal was coupled by the aid of a microscope objective, and the resulting fluorescence was collected at the end of the waveguide with an 800 lm core fiber. The propagated fluorescence was clearly observed at the wave-
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guide output and detected using an optical spectrum analyzer. The Fig. 6 shows the fluorescence emitted by the planar waveguide compared to the emission spectrum of a bulk sample normalized at 633 nm far from reabsorption phenomenon. A slight broadening, most likely due to the presence of Cl ions in the exchanged layer, can be observed, but the overall spectrum, and its relative intensities are the same.
3.4. Photolithography To obtain ultimately channeled waveguides, a photolithography step is necessary to perform an ion exchange manipulation. This stage includes a first deposit of a thin layer of SiO2 on the glass fluoride substrate that once etched, will be used for an ion exchange mask. A thin film SiO2 (200 nm thick) was successfully deposited. Using a mixture of argon (Ar, 40 sccm) and oxygen (O2, 12 sccm), a pressure of 4 mT and a RF power fixed at 200 W, the deposition rate was 1.5 nm mn 1 at room temperature. The capacitance recorded at high frequency vs. voltage C(V, 1 MHz) and the intensity vs. voltage I(V) curves showed no leakage current and negligible trapped charges inside the SiO2 thin film [9].The thickness and refractive index were determined close to 200 nm and 1.47, respectively from ellipsometry measurements carried out at 632 nm. A photolithography process was used [8] which occurs in the following major steps described in experimental part. The SiO2 layer was fully etched without any over-etching and no damages appeared at the surface and inside the ZBLA glass. First results of silica etching are shown in Fig. 7 which will allow fabricating channel waveguides by ionic exchange process.
4. Conclusion Study of host composition and RE concentration through physicochemical, optical and spectroscopic measurement allowed us to choose the best compromise between doping concentration for laser applications: 0.5 mol% of Pr3+. In this case, lifetime is about 37 ls and we have enough pump absorption to obtain good emission in red, orange and green wavelength. First tests of ionic exchange process are encouraging. We obtained planar waveguide of 3–5 lm thickness and the variation of refractive index between the exchanged layer and the ZBLA bulk glass is about 0.06–0.08. However, some parameters still have to be optimized, like gas flow during treatment. Moreover, first optical characterization of the planar waveguides indicates that the overall structure is preserved.
Fig. 7. SEM pictures of an etched silica layer on the top ZBLA glass (back scattered electrons mode).
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Improvements on photolithography process to obtain low losses channel waveguides are under progress.
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