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Emission decay times of F3+ and F2 color centers in LiF crystals
Journal of Luminescence 87}89 (2000) 580}582
Emission decay times of F> and F color centers in LiF crystals
G. Baldacchini *, F. De Matteis, R. Francini, U.M. Grassano, F. Menchini , R.M. Montereali ENEA, Dipartimento Innovazione, C.R. Frascati, P.O.Box 65, 00044 Frascati, Rome, Italy Dipartimento di Fisica and INFM, Universita` di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133 Rome, Italy
Abstract F> and F centers in LiF display e$cient broad emissions in the green}red spectral region when excited in their absorption bands located around 450 nm. Recently it has been shown that color centers, impurities and low-dimensionality structures in heavily colored samples in#uence the optical cycles and in particular the lifetimes of the emitting states. In order to determine these e!ects we measured the decay times, which are reported in the literature with values scattered by more than a factor 4. We obtained the radiative lifetime q "(8.1$1.2) ns for F> and q "(15.5$0.8) ns for F centers and we did not observe any temperature dependence in the range 15}300 K. 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium #uoride; Color centers; Emission; Lifetime
The interest in LiF containing color centers as a material for tunable laser systems arose more than 20 years ago [1,2]. The main advantages of this system in comparison to the other alkali halides used for color center lasers are the high emission e$ciency even at room temperature (RT) and the non-hygroscopicity of the material. Lasing at RT and in pulsed regime has been obtained in several spectral ranges: 0.51}0.57 lm (F> centers), 0.65}0.74 lm (F centers), 0.84}1.12 lm (F> centers), 0.86}1.02 lm (F\ centers), and 1.09}1.26 lm (F\ centers) [3,4]. However, all these centers with the exception of F\ su!er from bleaching e!ects, and the detailed know ledge of the kinetics of the conversions among the centers is very important to increase the emission e$ciency and the stability of the laser. To this aim a detailed study on the radiative and nonradiative processes in the optical cycle of the F> has been recently performed [5]. It has been found that a metastable triplet state, TS, plays an important role in the optical cycle of the center (see Fig. 1). This new state
* Corresponding author. Tel.: #39-06-9400-5365; fax: #3906-9400-5400. E-mail address: [email protected] (G. Baldacchini)
decreases the optical e$ciency of the radiative emission by trapping a sizeable fraction of the active centers. The following relations describe the increase and the decrease of the triplet population after switching on and o! the pumping beam: 1 " ; q = #= , (1) q 1 "= q where q and q are the triplet build-up and decay time, respectively, q is the radiative lifetime, ; is the pump rate out of the ground state "xed by the experimental conditions, and = and = are the transition probabil ities to and from the triplet state. The knowledge of = and = is very important to establish the radiative yield of the F> emission, and therefore their values should be known precisely. This is possible only if the radiative lifetime q is well known, but this is not the case as it will be shown later on. In the present paper we are reporting further experiments in colored LiF crystals providing reliable data on the radiative lifetimes of the F> and also of the F centers in the temperature range between 15 and 300 K.
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 3 0 1 - 4
G. Baldacchini et al. / Journal of Luminescence 87}89 (2000) 580}582
The F> and F absorption bands are strongly overlap ping in the region around 450 nm, but their emissions at 532 and 674 nm are mostly resolved. However, for a more clear cut experimental situation, we colored LiF crystals with c-rays of 1.2 MeV from a Co source at di!erent temperatures, in order to be able to produce samples containing mainly one type of centers. In order to increase the F>/F ratio, the coloration of pure LiF
Fig. 1. Energy level diagram of the F> center showing the radiative (*), nonradiative (- - - -) and relaxation ( ) ) ) ) ) ) transitions.
Fig. 2. Time evolution of the normalized emission (circles) at 520 nm, F> centers, and 690 nm, F centers, after a laser pulse excitation (dashed curve) at 460 nm and at two temperatures.
581
crystals (impurities (1 ppm) was performed at low temperature (!603C) and followed by a RT bleaching with an excimer laser (XeCl, 308 nm) for about 1 h. Indeed, the UV bleaching destroys the F centers through the following reaction: FJ F>#e\, F P
(2)
and the centers di!usion at RT produces an increase of the F> defects: F>#F P F>. (3) On the contrary, a high concentration of F centers was obtained with a RT coloration followed by a thermal annealing at 2003C for 20 min and by a quenching. The annealing at high temperature preferentially destroys the larger aggregate centers, thus favoring the survival of F defects. The decay times of the F> and F centers emissions have been measured at di!erent temperatures (15}300 K) and wavelengths within the emission spectra by means of a fast memory oscilloscope (HP 54510A) triggered by the exciting laser pulse derived from an excimer#dye laser system. The dye laser pulse at 460 nm has a half width of approximately 8 ns and a repetition rate variable from 2 to 200 Hz. The experimental results are presented in Fig. 2a and b for the F> and F centers, respectively. The dashed line represents the laser excitation pulse and the experimental data (circles) have been deconvoluted using the laser pulse shape and a single exponential decay, which is also observed in the above "gures after the laser pulse. The obtained values for the decay times are plotted in Fig. 3 for the two centers as a function of temperature. Changing the detection wavelength in the range 500}570 nm for the F> and 640}720 nm for the F center emission does not in#uence the evaluated data, con"rming that the emission bands are well resolved and that a single decay process takes place.
Table 1 Radiative lifetime of the F> and F centers in LiF as found in the literature and measured in the present work Sample
Temperature (K)
F> centers q (ns)
LiF LiF LiF LiF LiF LiF : Mg LiF : Mg,Na,OH LiF LiF LiF LiF
4.2 80}200 RT RT RT RT 77}300 RT RT RT 15}300
8$2 11.5$1
F centers q (ns)
+17 12 +18 9.61$0.25 8.76$0.16 10 4 +11 8.1$1.2
17.9$0.32 18.5$0.47
15.5$0.8
Reference [6] [7] [7] [8] [9] [10] [11] [12] [13] [14] Present work
582
G. Baldacchini et al. / Journal of Luminescence 87}89 (2000) 580}582
it will be possible to measure more accurately the e!ects on q of impurities or of other color centers, which could be the main reason for the di!erent values reported in Table 1. References
Fig. 3. Radiative decay time (,lifetime) as a function of temperature for the F> (squares) and F (circles) color centers.
For an overall comparison, the average values of our measurements are reported in Table 1 together with a few of the numerous ones known from the literature. The results for the F> lifetimes are quite scattered around our values. The slight di!erences measured on changing the impurities [10,11] are almost within our experimental errors. It is likely that the Mg, Na and OH impurities in#uence the lifetime of F> centers. The large value of 18 ns [9] is given approximately and without experimental details. The small one of 4 ns [13] has been measured with a phase and modulation technique, which is as accurate as the more common pulsed method only when the phase shift is big enough, which is not this case. For the F centers all the previous data are slightly larger than ours and therefore it is possible that the optical gain of this center is larger than previously evaluated. In conclusion, the lifetimes of the F> and F centers are now better known especially in a wider temperature interval. Indeed, our results indicate that they are constant in the temperature range 15}300 K. At this point
[1] Yu.L. Gusev, S.I. Marennikov, V.P. Chebotaev, Sov. Tech. Phys. Lett. 3 (1977) 124. [2] Yu.L. Gusev, S.I. Marennikov, S.Yu. Novohzilov, Sov. J. Quant. Electr. 5 (1978) 960. [3] T.T. Basiev, S.B. Mirov, in: Room Temperature Tunable Color Center Lasers, Harwood Academic, Switzerland, 1994. [4] V.V. Ter-Mikirtychev, T. Tsuboi, Prog. Quant. Electr. 20 (1996) 219. [5] G. Baldacchini, M. Cremona, G. d'Auria, R.M. Montereali, V. Kalinov, Phys. Rev. B 54 (1996) 17508. [6] H. MoK dl, F. Lanzl, W. von der Osten, W. Waidelich, Int. Symp. on Color Centers in Alkali Halides, Rome, Italy, 1968, p. 199. [7] L. Bosi, C. Bussolati, G. Spinolo, Phys. Lett. 32A (1970) 159. [8] T. Kulinski, F. Kaczmarek, Opt. Commun. 28 (1979) 101. [9] L. Zheng, L. Wan, Opt. Commun. 55 (1985) 277. [10] T. Kurobori, Y. Imai, N. Takeuchi, Opt. Commun. 64 (1987) 259. [11] T. Kurobori, T. Kanasaki, Y. Imai, N. Takeuchi, J. Phys. C 21 (1988) L397. [12] T.T. Basiev, S.B. Mirov, V.V. Osiko, IEEE J. Quant. Electr. 24 (1988) 1052. [13] E. Martins, Tese de Mestrado, Univ. de Sa o Paulo, Instituto de Fisica, Sa o Paulo, 1990, p. 85. [14] A.G. Bazylev, V.S. Kalinov, S.A. Mikhonov, S.I. Ovseichuk, L.K. Skavarda do Karmo, J. Appl. Spectrosc. 57 (1992) 894.
Emission decay times of F> and F color centers in LiF crystals
G. Baldacchini *, F. De Matteis, R. Francini, U.M. Grassano, F. Menchini , R.M. Montereali ENEA, Dipartimento Innovazione, C.R. Frascati, P.O.Box 65, 00044 Frascati, Rome, Italy Dipartimento di Fisica and INFM, Universita` di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133 Rome, Italy
Abstract F> and F centers in LiF display e$cient broad emissions in the green}red spectral region when excited in their absorption bands located around 450 nm. Recently it has been shown that color centers, impurities and low-dimensionality structures in heavily colored samples in#uence the optical cycles and in particular the lifetimes of the emitting states. In order to determine these e!ects we measured the decay times, which are reported in the literature with values scattered by more than a factor 4. We obtained the radiative lifetime q "(8.1$1.2) ns for F> and q "(15.5$0.8) ns for F centers and we did not observe any temperature dependence in the range 15}300 K. 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium #uoride; Color centers; Emission; Lifetime
The interest in LiF containing color centers as a material for tunable laser systems arose more than 20 years ago [1,2]. The main advantages of this system in comparison to the other alkali halides used for color center lasers are the high emission e$ciency even at room temperature (RT) and the non-hygroscopicity of the material. Lasing at RT and in pulsed regime has been obtained in several spectral ranges: 0.51}0.57 lm (F> centers), 0.65}0.74 lm (F centers), 0.84}1.12 lm (F> centers), 0.86}1.02 lm (F\ centers), and 1.09}1.26 lm (F\ centers) [3,4]. However, all these centers with the exception of F\ su!er from bleaching e!ects, and the detailed know ledge of the kinetics of the conversions among the centers is very important to increase the emission e$ciency and the stability of the laser. To this aim a detailed study on the radiative and nonradiative processes in the optical cycle of the F> has been recently performed [5]. It has been found that a metastable triplet state, TS, plays an important role in the optical cycle of the center (see Fig. 1). This new state
* Corresponding author. Tel.: #39-06-9400-5365; fax: #3906-9400-5400. E-mail address: [email protected] (G. Baldacchini)
decreases the optical e$ciency of the radiative emission by trapping a sizeable fraction of the active centers. The following relations describe the increase and the decrease of the triplet population after switching on and o! the pumping beam: 1 " ; q = #= , (1) q 1 "= q where q and q are the triplet build-up and decay time, respectively, q is the radiative lifetime, ; is the pump rate out of the ground state "xed by the experimental conditions, and = and = are the transition probabil ities to and from the triplet state. The knowledge of = and = is very important to establish the radiative yield of the F> emission, and therefore their values should be known precisely. This is possible only if the radiative lifetime q is well known, but this is not the case as it will be shown later on. In the present paper we are reporting further experiments in colored LiF crystals providing reliable data on the radiative lifetimes of the F> and also of the F centers in the temperature range between 15 and 300 K.
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 3 0 1 - 4
G. Baldacchini et al. / Journal of Luminescence 87}89 (2000) 580}582
The F> and F absorption bands are strongly overlap ping in the region around 450 nm, but their emissions at 532 and 674 nm are mostly resolved. However, for a more clear cut experimental situation, we colored LiF crystals with c-rays of 1.2 MeV from a Co source at di!erent temperatures, in order to be able to produce samples containing mainly one type of centers. In order to increase the F>/F ratio, the coloration of pure LiF
Fig. 1. Energy level diagram of the F> center showing the radiative (*), nonradiative (- - - -) and relaxation ( ) ) ) ) ) ) transitions.
Fig. 2. Time evolution of the normalized emission (circles) at 520 nm, F> centers, and 690 nm, F centers, after a laser pulse excitation (dashed curve) at 460 nm and at two temperatures.
581
crystals (impurities (1 ppm) was performed at low temperature (!603C) and followed by a RT bleaching with an excimer laser (XeCl, 308 nm) for about 1 h. Indeed, the UV bleaching destroys the F centers through the following reaction: FJ F>#e\, F P
(2)
and the centers di!usion at RT produces an increase of the F> defects: F>#F P F>. (3) On the contrary, a high concentration of F centers was obtained with a RT coloration followed by a thermal annealing at 2003C for 20 min and by a quenching. The annealing at high temperature preferentially destroys the larger aggregate centers, thus favoring the survival of F defects. The decay times of the F> and F centers emissions have been measured at di!erent temperatures (15}300 K) and wavelengths within the emission spectra by means of a fast memory oscilloscope (HP 54510A) triggered by the exciting laser pulse derived from an excimer#dye laser system. The dye laser pulse at 460 nm has a half width of approximately 8 ns and a repetition rate variable from 2 to 200 Hz. The experimental results are presented in Fig. 2a and b for the F> and F centers, respectively. The dashed line represents the laser excitation pulse and the experimental data (circles) have been deconvoluted using the laser pulse shape and a single exponential decay, which is also observed in the above "gures after the laser pulse. The obtained values for the decay times are plotted in Fig. 3 for the two centers as a function of temperature. Changing the detection wavelength in the range 500}570 nm for the F> and 640}720 nm for the F center emission does not in#uence the evaluated data, con"rming that the emission bands are well resolved and that a single decay process takes place.
Table 1 Radiative lifetime of the F> and F centers in LiF as found in the literature and measured in the present work Sample
Temperature (K)
F> centers q (ns)
LiF LiF LiF LiF LiF LiF : Mg LiF : Mg,Na,OH LiF LiF LiF LiF
4.2 80}200 RT RT RT RT 77}300 RT RT RT 15}300
8$2 11.5$1
F centers q (ns)
+17 12 +18 9.61$0.25 8.76$0.16 10 4 +11 8.1$1.2
17.9$0.32 18.5$0.47
15.5$0.8
Reference [6] [7] [7] [8] [9] [10] [11] [12] [13] [14] Present work
582
G. Baldacchini et al. / Journal of Luminescence 87}89 (2000) 580}582
it will be possible to measure more accurately the e!ects on q of impurities or of other color centers, which could be the main reason for the di!erent values reported in Table 1. References
Fig. 3. Radiative decay time (,lifetime) as a function of temperature for the F> (squares) and F (circles) color centers.
For an overall comparison, the average values of our measurements are reported in Table 1 together with a few of the numerous ones known from the literature. The results for the F> lifetimes are quite scattered around our values. The slight di!erences measured on changing the impurities [10,11] are almost within our experimental errors. It is likely that the Mg, Na and OH impurities in#uence the lifetime of F> centers. The large value of 18 ns [9] is given approximately and without experimental details. The small one of 4 ns [13] has been measured with a phase and modulation technique, which is as accurate as the more common pulsed method only when the phase shift is big enough, which is not this case. For the F centers all the previous data are slightly larger than ours and therefore it is possible that the optical gain of this center is larger than previously evaluated. In conclusion, the lifetimes of the F> and F centers are now better known especially in a wider temperature interval. Indeed, our results indicate that they are constant in the temperature range 15}300 K. At this point
[1] Yu.L. Gusev, S.I. Marennikov, V.P. Chebotaev, Sov. Tech. Phys. Lett. 3 (1977) 124. [2] Yu.L. Gusev, S.I. Marennikov, S.Yu. Novohzilov, Sov. J. Quant. Electr. 5 (1978) 960. [3] T.T. Basiev, S.B. Mirov, in: Room Temperature Tunable Color Center Lasers, Harwood Academic, Switzerland, 1994. [4] V.V. Ter-Mikirtychev, T. Tsuboi, Prog. Quant. Electr. 20 (1996) 219. [5] G. Baldacchini, M. Cremona, G. d'Auria, R.M. Montereali, V. Kalinov, Phys. Rev. B 54 (1996) 17508. [6] H. MoK dl, F. Lanzl, W. von der Osten, W. Waidelich, Int. Symp. on Color Centers in Alkali Halides, Rome, Italy, 1968, p. 199. [7] L. Bosi, C. Bussolati, G. Spinolo, Phys. Lett. 32A (1970) 159. [8] T. Kulinski, F. Kaczmarek, Opt. Commun. 28 (1979) 101. [9] L. Zheng, L. Wan, Opt. Commun. 55 (1985) 277. [10] T. Kurobori, Y. Imai, N. Takeuchi, Opt. Commun. 64 (1987) 259. [11] T. Kurobori, T. Kanasaki, Y. Imai, N. Takeuchi, J. Phys. C 21 (1988) L397. [12] T.T. Basiev, S.B. Mirov, V.V. Osiko, IEEE J. Quant. Electr. 24 (1988) 1052. [13] E. Martins, Tese de Mestrado, Univ. de Sa o Paulo, Instituto de Fisica, Sa o Paulo, 1990, p. 85. [14] A.G. Bazylev, V.S. Kalinov, S.A. Mikhonov, S.I. Ovseichuk, L.K. Skavarda do Karmo, J. Appl. Spectrosc. 57 (1992) 894.