Eu3+-doped glass materials for red luminescence

Eu3+-doped glass materials for red luminescence

Optics & Laser Technology 33 (2001) 157–160 www.elsevier.com/locate/optlastec Eu3+-doped glass materials for red luminescence I.V. Kityka; ∗ , J. Wa...

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Optics & Laser Technology 33 (2001) 157–160

www.elsevier.com/locate/optlastec

Eu3+-doped glass materials for red luminescence I.V. Kityka; ∗ , J. Wasylakb , D. Doroszb , J. Kucharskib b Department

a Institute of Physics WSP Czestochowa, Al. Armii Krajowej 13=15, Poland of Glass and Ceramics, Academy of Mining and Metalurgy, Al.Mickiewicza 30, Krakow, Poland

Received 2 January 2001; accepted 19 January 2001

Abstract Red luminescence (at wavelength about 622 nm) from Eu3+ ions embedded in PbO–Bi2 O3 –Ga2 O3 –BaO glass hosts is reported for room and liquid helium temperatures. The substantial in9uence of energy transfer processes between the host and Eu 3+ ions is shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength. Only polarized, excited pulsed XeII laser light ( = 714 nm) gives substantial luminescence with e>ciency up to 14.3%. The role of phonon-relaxation subsystem c 2001 Elsevier Science Ltd. All rights reserved. in the observed luminescence is discussed.  PACS: 42.70; 78.60; 71.20E Keywords: Glasses; Luminescence; Eu3+ ions

1. Introduction It has been shown [1–3] that heavy metal oxide (HMO) glasses are very attractive hosts for rare-earth (RE) ions. In particular, PbO–Bi2 O3 –Ga2 O3 –BaO (PBG) glasses [4] are interesting because they possess high transparency (up to 60%) within the 0.6 –8:7 m spectral range. The PbO and Bi2 O3 oxides [4,5] mainly determine their optical properties whilst introduction of Ga2 O3 gives increasing thermostability of the glasses [6,7]. We have found [4] that the optimal transparency of the glasses corresponds to the composition: PbO(0.30) –Bi2 O3 (0:25)–Ga2 O3 (0:36)–BaO(0.09). We have synthesised both bulk-like specimens in the form of parallelepipeds (8 × 8 × 1 mm3 ) as well as Hlm specimens (thickness varying 0.5 –15 m) deposited on quartz substrates. This glass has a glass transition temperature (640 K) and a large thermal expansion coe>cient (123×10−7 K −1 ) with a refractive index 2.154 (for the wavelength 633 nm). All of these features put this glass among the promising materials for the manufacture of optical Hber in the near-infrared (NIR) spectral range. Their wide IR transparency (up to

8:8 m) is a result of a relatively low frequency of maximum of phonon energy (about 700 cm−1 ) (compared, for instance, with oxide glasses) [8]. The latter factor is important for creation of convenient materials for IR and visible electronics and optoelectronics. More information concerning their optoelectronic properties is presented in Refs. [9 –12]. The main criterion for obtaining high e>ciency luminescence of the RE elements in glasses is for the host glass to possess low phonon energies. This occurs in the HMO glasses because of the large atomic elements, Pb and Bi. This also results in the glass having a high refractive index. In this paper, we report the observation of strong near infrared luminescence from Eu3+ ions embedded into the PGB glasses. To the best of our knowledge, NIR luminescence from Eu3+ ions, embedded in a PGB glass has not yet been reported. 2. Experimental details 2.1. Specimen preparation

∗ Corresponding author. Present address: Faculty of Science, Laboratoire de l’Etat Condense, Universite du Maine, ave Olivier Messiaen, 72085 Le Mans Cedex 09, France. E-mail address: [email protected] (I.V. Kityk).

Starting materials of purity about 99.997% were mixed to yield 50 g batches of PbO–Bi2 O3 –Ga2 O3 –BaO taken in the appropriate composition ratios. 99.996% purity Eu2 O3 was

c 2001 Elsevier Science Ltd. All rights reserved. 0030-3992/01/$ - see front matter  PII: S 0 0 3 0 - 3 9 9 2 ( 0 1 ) 0 0 0 1 2 - 3

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Fig. 2. IR absorption () and 9uorescence (•) spectra of the PGB-Eu glass. ().

Fig. 1. Typical transmission spectra of the PBG glasses with diPerent compositions; PbO, Ga2 O3 ; Bi2 O3 , BaO. The dotted line shows the transmittance for the PbO(0.30) –Bi2 O3 (0.25) –Ga2 O3 (0.36) –BaO(0.09). All the measurements were done for the specimens with thickness about 1 cm.

added into the starting powders to synthesize glasses doped with 0.5% of Eu2 O3 . For melting at 1050 K for 25 min we have used a platinum crucible. In order to avoid water contamination in glasses we used Ar gas, 9owing at the rate of about 10 l=min. Quenching procedure was done in a brass mold in air and the samples were annealed at 580 K for 2 h. X-ray diPraction analysis shows an absence of crystallization and spatial homogeneous impurity distribution with a precision of about 0.8%. 2.2. Optical spectra The specimens had the form of a parallelepiped with sizes 8 × 8 × 1 mm3 . The polishing surfaces were polished in order to obtain surface roughness better than 0:14 m. Optical transparency was measured using a Perkin-Elmer Lambda 20 spectrophotometer and a Fourier spectrometer Bio-Rad Win-IR within the spectral range of 0.5 –9 m with a spectral resolution of about 1 cm−1 . The transmission spectra of undoped glasses with diPerent compositions are presented in Fig. 1. One can clearly see that the best transparency (up to 59%) is achieved for the PbO(0.30) –Bi2 O3 (0:25)–Ga2 O3 (0.36) –BaO(0.09) glasses. As a consequence, these glasses were chosen as host materials for the RE doping. 2.3. Luminescence measurements As an excitation source we have used diPerent types of pulsed lasers. Among the lasers we have used XeII lasers with power about 0:8 W generating wavelengths of

0:714 m; KrII lasers (0:94 W) with wavelength 0:66 m; Ar II (1:1 W) 0:528 m and nitrogen (0:8 W) 0:337 m lasers. Simultaneously, we have measured IR induced luminescence excited by YAG-Nd pulsed (W = 30 MW;  = 30 ps;  = 1:06 mm) lasers, and an HF 1:2 W IR laser with wavelengths of 2.64, 2.87 and 3:260 m. Excitation laser beams were used both in polarized as well as in unpolarized regimes. In the visible and near IR region we have used LiNbO3 polarisers with polarization degree about 99.98% and alkali halide polarisers for the IR spectral wavelength range higher than 1.3 m. Excitation was performed in the non-switched as well in the Q-switched regimes in order to operate by the excitation time duration and the corresponding power. The 9uorescence measurements were performed using a grating monochromator with spectral resolution varying within the 4 –12 nm=mm. The quantum e>ciency was determined using 9uorescent time. The excited light was incident on the surface with an incident angle varying between 2◦ and 10◦. The measurements of 9uorescence were made at room temperature using Si and Ge detectors and in the IR region using helium-cooled CdHgTe detectors. The apparatus was equipped with a digitizing oscilloscope at chopper frequencies varying between 100 and 500 Hz. The specimens were kept cool in a liquid helium cryostat.

3. Results and discussion We studied bulk glasses using conventional methods [13]. Fig. 2 presents typical IR absorption spectra of the PGB specimens possessing 0.5% of Eu3+ (in weight units). One can see the existence of two absorption maxima at 2060 and 2198 nm (indicated by A and B), a very wide absorption at about 2600 nm and a very strong maxima about 2800 nm (indicated by C). The observed maxima correspond to the optical excitation from the ground 7 F0 state to the excited

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Fig. 4. Luminescence spectra of the PGB-Eu3+ (0.5%) glasses at RT (◦) and LHeT ( ).

Fig. 3. Energy level diagrams of the Eu3+ ions.

electron–phonon 7 F6 and 7 F5 states (see energy-level diagram of Eu3+ ions Fig. 3). The observed intensive band about 2800 nm results from photoexcitation of energy levels below 7 F4 together with corresponding electron–phonon interactions of the ions and host matrices. An interesting feature of the observed maximum indicates that there is a strong electron–phonon interaction in the investigated glasses and this may be the reason why we do not observe the IR luminescence. One can see that the photoluminescence (particular maxima A, B and C are shifted towards shorter wavelengths) is in agreement with the Stokes shift. We have shown that these photoluminescence maxima have relatively low level of quantum e>ciency (below 6%) and decrease slightly with increasing temperature. The emission spectrum due to the Eu3+ 5 D0 →7 F2; 3; 4 transitions, when pumped at 528 nm Ar laser wavelength is shown in Fig. 4. It is necessary to add that excitation by KrII ( = 0:66 mm) as well as by other laser wavelengths gives substantially lower 9uorescent e>ciency (below 2%). The absorption spectra are shifted, compared with the emission spectra, toward the lower wavelengths. The peak emission wavelength corresponds to 621:2 nm at room temperature (RT) and 615:3 nm at liquid helium temperature (LHeT). This slight temperature shift may indicate a relatively low degree of phonon interaction. The positions of the energy levels in the PGB glass are similar to those in other glass and crystal matrices [14 –16]. An interesting feature is the essential dependence of the quantum e>ciency on the polarization of the excited beam. We have found that this

parameter increases from 5.2% for an unpolarised Ar laser beam up to the 14.3% for the polarized beam over the whole (4.2–300 K) temperature range. The measured lifetime was about 1320 s and is not far from such in the crystalline materials. The relatively high quantum e>ciency of the 5 D0 →7 F2; 3; 4 transition is probably caused by an increased multiphonon relaxation due to the low distances between the ground and Hrst excited states. We have also investigated the possible in9uence of the excited state absorption between the 5 D0 and 5 D4; 5 higher excited states. It is necessary to underline that our evaluations performed using the McCumber method [17] give relative contributions of the stimulated emission below 4.3%. Thus one can assume a dominant role of the absorption cross-section from the ground 7 F1 state in the observed optical spectra. In order to evaluate the role of multiphonon relaxation in the emission processes observed, we have used several methods to evaluate the lifetimes and quantum e>ciency of the Eu levels. Particularly we have done calculations using modiHed Judd–Ofelt theory [18,19]. This approach allows changes of the dipole oscillator strength with inclusion of ePective oscillators to be predicted. Unfortunately this approach is unable to take into account photoexcited electron– quasi-phonon anharmonic states that are necessary for the explanation of the polarized photoexcitation. In Refs. [20,21] a new approach for describing the photoexcited origin was developed using the band energy structure approach with inclusion of the photoinduced anharmonic electron–phonon interactions. This approach starts by making pseudopotential calculations of the electron-vibration states of the Eu3+ states. After this, the photoinduced anharmonic electron–phonon interaction was

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Table 1 Calculated (c ) and measured (m ) lifetimes of Eu3+ levels in PBG glasses together with quantum e>ciency calculated by diPerent methods at RT Method of simulation

c (s)

m (s)

 (%)

ModiHed Judd–Ofelt [18,19] With taking into account of electron–phonon anharmonicity [20 –22] Experimental

1890 1395

— —

17.6 14.12



1320

14.3

calculated using Green functions and Dyson equations [22]. Parameters of the quantum e>ciency and luminescence lifetime calculated by diPerent methods are presented in Table 1. From Table 1 one can see that the modiHed Judd–Ofelt method [18,19] gives parameters of the lifetimes substantially higher than the experimental ones. The same can be said about the quantum e>ciency. In our opinion a better agreement between the theory within the framework of the electron–phonon anharmonicity and observed experimental data may indicate that there is a substantial contribution of the photoexcited electron–phonon harmonicity contributing ion to the observed ePects. 4. Conclusions We have observed red luminescence at 621:2 nm at RT and at 615:3 nm at LHeT in the PGB glasses doped with 0.5% of Eu3+ . We have found that only excitation by the polarized photoexcited Ar pulsed laser light ( = 528 nm) gives a substantial increase of the red luminescence with = 1320 s. We have not observed an additional luminescence in the IR region as well as by excitation by other laser wavelengths. The essential role of the photoinduced anharmonicity in the phonon relaxation processes was shown by

measurement of the luminescent kinetics measurements. We have shown the essential drawback of the Judd–Olfet methods, even in the modiHed form, for explaining the observed ePects. One of the advantages of the proposed optical materials for optoelectronics applications is the high thermal stability of the red luminescence. References [1] Thorpe MF, Mitkova MI, editors. Amorphous insulators and semiconductors. Kluwer Academic Press, Dordrecht, 1997. [2] Kityk IV, Golis E, Filipecki J, Wasylak J, Zacharko VM. J Mater Sci Lett 1995;14:1292. [3] Choi YG, Heo J. J Non-Cryst Solids 1997;217:199. [4] Golis E, Kityk IV, Wasylak J, Kasperczyk J. Mater Res Bull 1996;31:1057. [5] Dumbaugh WH, Lapp JC. J Am Ceram Soc 1992;75(99):2315. [6] Dumbaugh WH. Phys Chem Glasses 1986;27:119. [7] Ling Z, Lin H, Chengshan Z. Proceedings of the 9th International Symposium on Non-Oxide Glasses, Hangzhou, 1994. p. 44. [8] Lapp JC, Dumbaugh WH. Key Engin Mater 1994;94 –95:257. [9] Lin H, Dechent LW, Day DE, StoPer JO. J Non-Cryst Solids 1994;171:299. [10] Heo J, Shin YB, Jang JN. Appl Opt 1995;34(21):4284. [11] Jin J, Sakka S, Fukunagam T, Misawa M. J Non-Cryst Solids 1994;175:211. [12] Choi YG, Heo J. J Non-Crystal Solids 1997;217:199. [13] Simons DR, Faber AJ, De Waal H. Opt Lett 1995;20:468. [14] Diecke GH, Crosswhite HM. Appl Optics 1963;2:675. [15] Chang NFJ. Appl Physics 1963;34:3500. [16] O’Connor M. Trans Metal Soc AIME 1967;239:3500. [17] McCumber DE. Phys Rev 1964;136(4A):A954. [18] Kornienko AA, Kaminskii AA, Dunina EB. Phys Status Solidi B 1990;157:267. [19] Medeiros Neto JA, Hewak DW, Tate H. J Non-Crystal Solids 1995;183:201. [20] Makowska-Janusik M, Kityk IV, Berdowski J, Matejec J, Kasik I, Me9eh A. J Opt: Pure Appl Optics 2000;2:43. [21] Wasylak J, Kucharski J, Kityk IV, Sahraoui B. J Appl Phys 1999;85:425. [22] Kityk IV, Sahraoui B. Phys Rev B 1999;60:942.