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Photoluminescence of lasing color centers in mixed crystals KCl:KBr:O2
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal of Luminescence
72-74
(1997)626-628
Photoluminescence of lasing color centers in mixed crystals KCl:KE3r:02 G. Baldacchini”,
M. Cremonab, R.M. Montereali a,*, R.B. Pode”, A. Scaccod
a ENEA, Dip. INN, Settore Fisica Applicata, CR Frascati, C.P. 65. 00044 Frascati (R&I), Italy b ENEA guest, Pontificia Universidade Catolica do Rio de Janeiro, C. P. 38008, 22453-900 Rio de Janeiro, R. J., Brazil c ENEA guest, Nagpur University, Physics Department, Nagpur 440010, India d Dip. di Fisica. Universita’ La Sapienza. P. le A.Moro 2, 00185 Roma. Italy
Abstract Up to now more than ten color center systems in various host lattices of alkali halides have been successfully developed in tunable lasers; among them are the oxygen stabilized Fc centers. In this work we have studied the optical properties of these laser-active centers in mixed KC1 : KE3r crystals doped with oxygen, having in mind an extended range of tunability with only one laser crystal. We have observed the emission band shifting from 1.7 to 2.0 Frn depending on the mixing ratio of the two halides, and also other luminescences due to more complex centers related to Na impurity. Keywords: Color centers; Alkali halides; Mixed crystals
1. Introduction
2. Results and discussion
In the last two decades many kinds of color centers in alkali halides have been used for achieving tunable infrared lasing action [l]. Among the color center lasers, those based on the Fl : 0; center in NaCl, KC1 and KBr crystals cover the range from 1.5 to 2.1 pm. However, it would be highly desirable to have such a large tuning range with only one crystal, as it has been already done for the FA centers lasing between 2 and 3 pm [2]. As a matter of fact, KC1 and KBr can be mixed in any proportion, and so a detailed study has been performed on Fl centers stabilized by negative oxygen ions in such mixed crystals.
* Corresponding author. [email protected].
Fax:
+39-06-94005400;
e-mail:
0022-23 13/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PIISOO22-2313(96)00151-2
Various mixed crystals doped with KOz (5 mol% in the melt) were grown by the Czochralski method in air atmosphere and additively colored at 680°C. All crystals were quenched for 30 s at 600°C and, then, following the usual procedure [3], exposed to F light irradiation at 260 K for the time required to induce F center aggregation. Fig. 1 shows the absorption spectra at 77 K of a 50 : 50 mixed crystal before and after the conversion. Before the conversion, the peaks at E270,370,570 and 820 mn correspond to 02- vacancy defect pairs, V, F and F2 centers, respectively. After the conversion, the 02- vacancy defect pairs, the F and F2 centers decrease as expected, while the bands due to at least two new centers, (F~)H at 1070 and (F~)H and/or (FZf)*” at Zl400 nm, appear and grow. The shift towards longer wavelengths and the broadening
G. Baldacchini et al. 1 Journal of Luminescence
I.
*
I.,
500
*
x
1
*
*
1000
a.
1.
1500
72-74 (1997)
627
626628
1
Wavelength (nm) Fig. 1. Absorption spectra at 77 K of an 0; doped (5 mol%) KC1 : KEir (50 : 50) mixed crystal 1.6 mm thick, measured after additive coloration (dotted curve) and after optical conversion with F light at 260K (solid curve).
of the F band are a consequence of the formation of some FA centers due to unwanted Na impurities. Na-related centers were observed in other measured crystals as well; in particular, in KC1 where the energy separation between the FAI and FAZ bands is larger than in KBr [4]. The luminescence was measured in the same crystal by exciting into its absorption bands with several laser sources. Various emission bands appear depending on the excitation and, for instance, the F2 absorption resulted to be a composite band, because pumping at different wavelengths within its range appreciably changes the emission spectra. However, all emission spectra display a luminescence at 1940 nm, as shown in Fig. 2, obtained by exciting with an Ar ion laser. This luminescence, due to (Fzf )n and/or (F,’ )M centers, is the lasing emission under study, and its emission shifts from ~1.7 to ~2.0 urn passing from pure KC1 to mixed crystals with different mole ratios and to pure KE3r. Therefore, the original motivation of this work is fulfilled, but it was not possible to obtain a very high number of (F,‘)n centers most probably because of the Na impurities or of the crystal growth method, where KO2 was added to the melt. New Na-free crystals, grown with more suitable
2000
1500
Wavelength
(
2500
nm )
Fig. 2. Emission spectrum at 77K of an 0; doped (5mol%) KC1 : KBr (50 : 50) mixed crystal 1.6 mm thick, measured after optical conversion by pumping with an Ar-ion laser in multiline configuration.
methods [5], should be studied to verify the previous hypotheses, which are of great importance for obtaining crystals of laser quality. In conclusion, several absorption and emission bands have been measured and connected to other kinds of aggregated color centers, which are of some interest for laser emission. For instance, the emission bands at 1260 and 1800 nm, reported in Fig. 2, are most probably due to FA(Na) and (FT)u centers, respectively. Indeed, in KC1 and KBr crystals the F*(Na) emissions are centered at 1.10 and 1.47 urn, respectively [4]; therefore the luminescence at 1260 nm, lying in the middle of such range, can be assigned to the FA emission. The origin of the less-intense emission at 1800 nm is more doubtful, but its occurrence under excitation with a Nd : YAG laser at 1.06 urn supports the attribution to (F,‘)n defects, which absorb at SlO70 nm. However, owing to the high variety and complexity of the centers produced in mixed crystals containing oxygen, they will be further described in detail elsewhere, once a positive experimental evidence is achieved.
628
G. Baldacchini et al. /Journal
of Luminescence 72-74 (1997)
References [l] W. Gellermann, J. Phys. Chem. Solids 52 (1991) 249-297. [2] K.-P. Koch, G. Liftin and H. Welling, Opt. Lett. 4 (1979) 387-389. [3] D. Wandt, W. Gellermann and F. Luty, J. Appl. Phys. 61 (1987) 864-869.
626628
[4] F. Luty, in: Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York, 1968) ch. 3. [5] M. Diaf, I. Chini, A. Hamaidia and A. El Akrmi, I. Phys. III France 6 (1996) 1-6.
LUMINESCENCE ELSEVIER
Journal of Luminescence
72-74
(1997)626-628
Photoluminescence of lasing color centers in mixed crystals KCl:KE3r:02 G. Baldacchini”,
M. Cremonab, R.M. Montereali a,*, R.B. Pode”, A. Scaccod
a ENEA, Dip. INN, Settore Fisica Applicata, CR Frascati, C.P. 65. 00044 Frascati (R&I), Italy b ENEA guest, Pontificia Universidade Catolica do Rio de Janeiro, C. P. 38008, 22453-900 Rio de Janeiro, R. J., Brazil c ENEA guest, Nagpur University, Physics Department, Nagpur 440010, India d Dip. di Fisica. Universita’ La Sapienza. P. le A.Moro 2, 00185 Roma. Italy
Abstract Up to now more than ten color center systems in various host lattices of alkali halides have been successfully developed in tunable lasers; among them are the oxygen stabilized Fc centers. In this work we have studied the optical properties of these laser-active centers in mixed KC1 : KE3r crystals doped with oxygen, having in mind an extended range of tunability with only one laser crystal. We have observed the emission band shifting from 1.7 to 2.0 Frn depending on the mixing ratio of the two halides, and also other luminescences due to more complex centers related to Na impurity. Keywords: Color centers; Alkali halides; Mixed crystals
1. Introduction
2. Results and discussion
In the last two decades many kinds of color centers in alkali halides have been used for achieving tunable infrared lasing action [l]. Among the color center lasers, those based on the Fl : 0; center in NaCl, KC1 and KBr crystals cover the range from 1.5 to 2.1 pm. However, it would be highly desirable to have such a large tuning range with only one crystal, as it has been already done for the FA centers lasing between 2 and 3 pm [2]. As a matter of fact, KC1 and KBr can be mixed in any proportion, and so a detailed study has been performed on Fl centers stabilized by negative oxygen ions in such mixed crystals.
* Corresponding author. [email protected].
Fax:
+39-06-94005400;
e-mail:
0022-23 13/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PIISOO22-2313(96)00151-2
Various mixed crystals doped with KOz (5 mol% in the melt) were grown by the Czochralski method in air atmosphere and additively colored at 680°C. All crystals were quenched for 30 s at 600°C and, then, following the usual procedure [3], exposed to F light irradiation at 260 K for the time required to induce F center aggregation. Fig. 1 shows the absorption spectra at 77 K of a 50 : 50 mixed crystal before and after the conversion. Before the conversion, the peaks at E270,370,570 and 820 mn correspond to 02- vacancy defect pairs, V, F and F2 centers, respectively. After the conversion, the 02- vacancy defect pairs, the F and F2 centers decrease as expected, while the bands due to at least two new centers, (F~)H at 1070 and (F~)H and/or (FZf)*” at Zl400 nm, appear and grow. The shift towards longer wavelengths and the broadening
G. Baldacchini et al. 1 Journal of Luminescence
I.
*
I.,
500
*
x
1
*
*
1000
a.
1.
1500
72-74 (1997)
627
626628
1
Wavelength (nm) Fig. 1. Absorption spectra at 77 K of an 0; doped (5 mol%) KC1 : KEir (50 : 50) mixed crystal 1.6 mm thick, measured after additive coloration (dotted curve) and after optical conversion with F light at 260K (solid curve).
of the F band are a consequence of the formation of some FA centers due to unwanted Na impurities. Na-related centers were observed in other measured crystals as well; in particular, in KC1 where the energy separation between the FAI and FAZ bands is larger than in KBr [4]. The luminescence was measured in the same crystal by exciting into its absorption bands with several laser sources. Various emission bands appear depending on the excitation and, for instance, the F2 absorption resulted to be a composite band, because pumping at different wavelengths within its range appreciably changes the emission spectra. However, all emission spectra display a luminescence at 1940 nm, as shown in Fig. 2, obtained by exciting with an Ar ion laser. This luminescence, due to (Fzf )n and/or (F,’ )M centers, is the lasing emission under study, and its emission shifts from ~1.7 to ~2.0 urn passing from pure KC1 to mixed crystals with different mole ratios and to pure KE3r. Therefore, the original motivation of this work is fulfilled, but it was not possible to obtain a very high number of (F,‘)n centers most probably because of the Na impurities or of the crystal growth method, where KO2 was added to the melt. New Na-free crystals, grown with more suitable
2000
1500
Wavelength
(
2500
nm )
Fig. 2. Emission spectrum at 77K of an 0; doped (5mol%) KC1 : KBr (50 : 50) mixed crystal 1.6 mm thick, measured after optical conversion by pumping with an Ar-ion laser in multiline configuration.
methods [5], should be studied to verify the previous hypotheses, which are of great importance for obtaining crystals of laser quality. In conclusion, several absorption and emission bands have been measured and connected to other kinds of aggregated color centers, which are of some interest for laser emission. For instance, the emission bands at 1260 and 1800 nm, reported in Fig. 2, are most probably due to FA(Na) and (FT)u centers, respectively. Indeed, in KC1 and KBr crystals the F*(Na) emissions are centered at 1.10 and 1.47 urn, respectively [4]; therefore the luminescence at 1260 nm, lying in the middle of such range, can be assigned to the FA emission. The origin of the less-intense emission at 1800 nm is more doubtful, but its occurrence under excitation with a Nd : YAG laser at 1.06 urn supports the attribution to (F,‘)n defects, which absorb at SlO70 nm. However, owing to the high variety and complexity of the centers produced in mixed crystals containing oxygen, they will be further described in detail elsewhere, once a positive experimental evidence is achieved.
628
G. Baldacchini et al. /Journal
of Luminescence 72-74 (1997)
References [l] W. Gellermann, J. Phys. Chem. Solids 52 (1991) 249-297. [2] K.-P. Koch, G. Liftin and H. Welling, Opt. Lett. 4 (1979) 387-389. [3] D. Wandt, W. Gellermann and F. Luty, J. Appl. Phys. 61 (1987) 864-869.
626628
[4] F. Luty, in: Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York, 1968) ch. 3. [5] M. Diaf, I. Chini, A. Hamaidia and A. El Akrmi, I. Phys. III France 6 (1996) 1-6.