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On the existence of a reabsorption between excited states of “ns2” ions in alkali halides
0038-1098/81/200765-05 $02.00/0
Solid State Communications, Vol. 38, pp. 765-769 Pergamon Press Ltd. 1981. Printed in Great Britain.
ON THE EXISTENCE OF A REABSORt~ION BETWEEN EXCITED STATES OF "ns 2'' IONS IN ALKALI HALIDES M. Billardon and J.M. Ortega* Laboratoire d'Optique Physique, EPCI, I0, rue Vauquelin, 75231 Paris Cedex 05, France (Received 11 S e p t e m b e r 1980; in revised f o r m 10 D e c e m b e r 1980 b y M. Balkanski)
The behaviour of several ns 2 ions doped alkali halides has been studied under an intense ultraviolet laser excitation. From the apparition of a strong F-band coloration and from previous gain measurements, we conclude that an intense reabsorption between excited states occurs, which restricts the possibility of making new laser materials with those systems. COLOR CENTERS lasers have been demonstrated to work efficiently in the visible and infra-red ranges [ 1]. Recently the F ÷ center in CaO has been shown to exhibit gain [2], when pumped with a pulsed nitrogen laser, and, more recently, lasing action has been obtained [3] in the 3 6 0 - 3 9 0 nm range with this system. The advent of high power sources in the ultraviolet region, namely the excimer lasers, has given the possibility of extending the range of tunable laser sources towards this region. Among the wide variety of color centers in alkali halides some interest has been paid [2] to the so-called ns 2 ions (or thallium-like ions), whose luminescent properties are well-known [4], as candidates for tunable lasers in the 2 5 0 - 3 5 0 n m region. In this paper, we report studies on some of these systems, aimed at obtaining new laser materials. Besides the F ÷ center in CaO, Duran et al. [2] had made gain measurements with the CaF2 : Eu 2÷, which is not an ns 2 ion (its ground state is a 4 f and its excited state a 4fa5d level) and the KC1 : Bi 3÷ systems, when pumped with a nitrogen laser. They found a negative gain (i.e. absorption) in the first case and a zero gain in the second, although its theoretical value was much greater than the experimental error. Our first purpose was to perform gain measurements with systems in which fluoresence has a good quantum efficiency and which can be excited with an excimer laser. In fact we have not performed any such measurement since we have observed that for every system we have explored, except in KI, irradiation defects (essentially F centers) were created very efficiently by the excirner laser light. As a result, quick degradation of the samples occurred. We have then concluded that a strong reabsorption takes place between the relaxed excited state and a higher excited state of those systems. The crystals have been grown with the Bridgman *E.R. 5 du CNRS.
method and doped so that the resulting optical density in the excitation bands was about l mm- t at the excimer laser wavelength: The Au and Ag impurities have been reduced electrolytically in order to obtain Au- and Ag- ions. All the irradiations have been performed at 77 K, temperature at which those systems were expected to constitute reliable laser materials. Irradiations at 4 K were not possible under our present experimental conditions. Let us now examine the different systems we have explored. We have measured the radiative lifetimes, r n , and, by comparison with a BiBuQ dye [6], the quantum efficiency, Q, of the ultraviolet luminescence of the A band of the following crystals: KBr : Au-
Q ~ 0.2
rn ~
80 nsec
(X = 318 nm, AX = 6 nm),
RbCI : Au-
Q-~ 0.5
rR -- 110nsec
(X = 317nm, AX --~ 5 nm).
This gives for the cross-section of stimulated emission [2] respectively 2 x 10 -18 and 5 x 10-1Scm 2 (for comparison it is 5 x 10-17cm 2 for coumarin 307, which lases easily in the optical cavity made by a simple quartz cell, when pumped by a pulsed laser). These systems were excited in the "A" absorption band [4, 5] near 3080A by a pulsed dye laser (Rhodamin B laser, excited by a pulsed N2 laser, doubled by a KDP crystal) at relatively low power: 1 - 1 0 kW Gain measurements need a higher power: 0.1-1 MW [1, 6]. However, when excited with an XeCI laser (3080 A), those systems exhibit a strong coloration, due to F centers (Fig. 1), which appears after a few laser shots. The same effect occured in KCI : Au-. The XeC1 laser energy (4.02 eV) is well below the absorption gap of KBr, KCI and RbC1 [7]. Moreover we have verified by irradiating, in exactly the same conditions, undoped crystals of the same materials, that no coloration
765
766
REABSORPTION OF " n s : " IONS IN ALKALI HALIDES
Vol. 38, No. 8
1
0.5 ¢-
(.} Q.
0
QI
I
"--r
"T
500
600
k/nm Fig. 1. Absorption of a sample after exposure at 80 K to a few shots of a XeCI laser. Full line." a RbC1 : Au - sample exhibits a strong F band absorption (the arrow indicates the tabulated F band position at 4 K). The high energy side of the band can be attributed to the so-called "K band" of the F center (see [4]) and to the tail of the UV absorption band of the Au ° remaining in the crystal. Dotted line." a RbCI : Au ° sample (where the Au impurities have not been electrolytically reduced) irradiated exactly in the same conditions does not exhibit any coloration. occured, that is the excimer laser power was weak enough to avoid two photon absorption, which would populate excitonic and conduction band levels directly. Thus the coloration effect is due to the A u impurities only. The KC1 : TI ÷ systems, the fluorescent properties of which are well-known [4, 6], was also strongly colored by irradiation with a KrF laser, after a few laser shots. This corresponds also to the A band absorption of the TI ÷ ion in KC1. In KI : Sn 2÷ and KI : Ag- we have observed n o F center coloration under a XeC1 laser irradiation. However it is known that the F center creation rate is very weak at low temperatures [8]. Thus this does not
exclude a possible excited state reabsorption. We have not attempted gain measurements in those systems since the quantum efficiency of the fast luminescence of interest is very weak in those conditions. In KI : Sn 2 ÷ [9] the luminescence is made of two emission bands in the 4 5 0 - 5 5 0 nm region with different lifetimes: 10 nsec for one band and 1.5 psec for the second. We have found that the quantum efficiency of the emission of interest, the fast one [6], was 10 -3 times the efficiency of the slow emission, leading to no possible gain (in fact, the relative intensities of the two components of the luminescence are strongly dependent upon the wavelength of excitation). In KI : Ag- only the infra-red emission at 1.6 eV has a non-negligible intensity at 77 K [6, 10].
Vol. 38, No. 8
REABSORPTION OF "ns z'' IONS IN ALKALI HALIDES
Excited states f (n+l)Por nD 5
767
• Exciton andJ ~/~. [conduction I ~'~/band levelsJ
Reabsorption 2
4•
} First excited state ( n3 P~)
I
Absor)tion
Emission
5
~ "
I Groundstate (n ISo)
Fig. 2. Sketch of a four level system perturbed by a reabsorption from the relaxed excited state to a 5th level. The waving arrows indicate non-radiative transitions. The levels corresponding to the " A " absorption and emission of a • ns z'" ion are indicated in brackets. We have also studied RbBr : Ag- and CsBr : Ag-. The C band of these systems absorbs at the XeC1 wavelength, but the quantum efficiency of fluorescence is negligible at 77 K. The quantum efficiency of CsBr : Agat 4 K is much higher (Q "" 0.5), but we could not irradiate at liquid helium temperature. It would then be interesting to study the behavior of this system when irradiated at 3080 A. From these results and from those of Duran et aL [2] it can be inferred that a strong reabsorption band occurs between the relaxed excited state and higher excited states of these systems. The reabsorbed light can be either the pumping or the fluorescence light or both, if several excited states can absorb as will be seen below. If the final excited states are high enough in energy, the excitation can be transferred to excitons or electron-hole pairs leading to the creation o f F centers [7] (Fig. 2), as we did observe in our experiments. In this case the energies of two photons are added to produce degradation of the samples. Reabsorption of the fluorescence light may also
"kill" the gain as was observed in CaF2 : Eu :+ and KC1 : Bi 3+ [2]. The absorption coefficient at the frequency v, k(v) can then be written [ I 1 ], with the notations of Fig. 2: k(v) = 8rw----i n 3 - n 4 )
ro
To J
with g(v) = line shape of the 3--4 transition, g'(v) = line shape of the 4 - 5 transition, ro = radiative lifetime of the level 4 and ro = radiative lifetime of the level 5. This formula is restricted for clarity to a three level system, i.e. does not take into account the electronphonon coupling of the ions in solids [ 12]. This would not modify qualitatively our conclusions. Assuming that the non-radiative relaxations are fast ( ~ 10-12 sec) and that the level 5 de-excites non-radiatively (toward the ground state or by an F center creation mechanism) it comes n3 "" n5 "" 0 and
768
REABSORPTION OF " n s : ' " IONS IN ALKALI HALIDES
Singlef
0
S
PD
levels S~
Vol. 38, No. 8
T r i p l e t levels P~
.
, - D .~ Ionizofion
-20000
7
E u
-40000
6 3 P 2 ..~ "
/6 P, 6 Po
W -60000
- 80000 - 8 4 178
So
Fig. 3. Energy level scheme of the Hg atom, whose ground state is a 6s z.
[ c2 g'(v) g(U)]n," =
To
To j
Thus it appears that the sign of this absorption coefficient, negative for a net gain, is independent of the relaxed excited state population, n4, i.e. of the experimental conditions, particularly the pumping light intensity. In fact any gain (or excited state absorption) measurement is proportional to the difference between stimulated emission and (stimulated) absorption between excited states. This shows how a reabsorption phenomenon decreases the gain of such systems. The reabsorption in ns z ions systems can be understood easily. The electronic levels of those ions are considerably broadened by e l e c t r o n - p h o n o n interaction [4]. Thus (Fig. 2) a transition of the type 4 ~ 5 has a good probability of overlapping a 4 ~ 3 transition if the level 5 lies in the proper energy range. It can be seen that some levels lie in this proper energy range in the case of n s 2 ions. Let us consider the energy level scheme of the Hg atom [13] whose ground state is a 5dl°6s2 (Fig. 3). It has been shown [4] that the relative level spacings are roughly the same for n s : ions in substitutional positions in alkali halides. The A band corresponds to the 63/1 level, B to 6 3Pz (weakly allowed) and C to 6 IP1 level. From the 3p1 level, corresponding to the A emission, several reabsorptions may
occur toward the 6D levels at energies comparable to t h e I S o - 3 P l separation. Reabsorption from the 6 IPl level, corresponding to the C emissions, which we did not explore, seems less likely, since it is higher in energy than the 6 3Pl level. However, many higher sin~et levels exist in the atom and may occur in the crystals. at energies corresponding to the reabsorption. The F center-like systems, such the F ÷ center in CaO, have energy levels which resembles the hydrogen atom: their principal transition is an Is -~ 2p transition the energy of which is much greater than half the ionization limit [ 13]. Thus a reabsorption cannot occur in the ls ~ 2p energy range (a reabsorption occurs, in fact, in the infra-red [14]) and the gain cannot be affected by such an effect as shown by various gain and laser experiments [ I - 3 ] . We can conclude that the heavy ions and particularly the n s 2 ions, in alkali halides are not very likely to produce laser emissions in the UV range. However it is possible that some of them exhibit a positive gain. The reabsorption of the emission light can be more or less important: For example the gain has been shown to be negative in CaF~ : Eu 2÷ and about zero in KC1 : Bi 3÷. It could be positive for some systems, provided that the reabsorption is weak enough and does not lead to an excited level higher in energy than the gap of the crystal in which case coloration would occur,
Vol. 38, No. 8
REABSORPTION OF "ns'-" IONS IN ALKALI HALIDES
except in KI and, generally, in iodides at low temperatures. We have shown experimentally that a reabsorption takes place from the 3Pt (A band) level. There is no proof that such a reabsorption would occur from the tPl (C band) level. From the energy level scheme it is not clear, as mentioned before, if a reabsorption should exist or not. Moreover, since excitation in the C band produces generally more or less emission from the A band, the crystal could be damaged by reabsorption from this A band. Thus ns: ions pumped in the C band, which is generally in the range 200-250 nm, and having a very good quantum efficiency for the C emission, could still be candidates for laser action. Acknowledgements - The authors are very indebted to Pr J. ChapeUe and to Dr B. Briat for the growth of the doped crystals, to the CGE research Center for the possibility of using one of their excimer lasers, and to C. Grisolia for experimental help. REFERENCES L.F. Mollenauer & D.H. Olson, AppL Phys. Lett. 24,386 (1974); L.F. Mollenauer, Commun. Int.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
769
Conf. on Color Centers in Ionic OTstals, Sendai, Japan (I 974) (unpublished). J. Duran, P. Evesque & M. Billardon, AppL Phy~ Lett. 33, 1004 (1978). B. Henderson et al. (to be published). W.B. Fowler, Physics o f Color Centers, (Edited by W.B. Fowler), Chapter 2. Academic Press, New York (1968). M. Krause &Fisher, J. Luminesc. 4,335 (1971). M. Billardon & J. Duran, Rapport DGRST No. 75-7-1485 (1979). M. Saidoh & P.D. Townsend, Rad. Effects 27, 1 (1975); Y. Farge, J. dePhys. 34, C9,475 (1973). T.P.P. Hall, D. Pooley & P.T. Wedepohl, Proc. Phys. Soc. 83,635 (1964). A. Fukuda, Phys. Rev. B1,4161 (1970). K. Kojima, S. Shimanuki & T. Kojima, J. Phys. Soc. Japan 33, 1076 (1972). B. di Bartolo, Optical Interactions in SolMs, Chapter 19. Wiley, New York (1968). D.B. Fitchen, Physics of Color Centers, (Edited by W.B. Fowler), Chapter 5. Academic Press, New York (1968). G. Herzberg, Atomic Spectra and Atomic Structure. Dover, New York (1944). Y. Rondo & H. Kanzaki, Commun. Int. Conf. on Color Centers in Ionic Crystals, Sendai, Japan (1974) (unpublished); K. Park & W.L. Faust, Phys. Rev. Lett. 17,137 (1966).
Solid State Communications, Vol. 38, pp. 765-769 Pergamon Press Ltd. 1981. Printed in Great Britain.
ON THE EXISTENCE OF A REABSORt~ION BETWEEN EXCITED STATES OF "ns 2'' IONS IN ALKALI HALIDES M. Billardon and J.M. Ortega* Laboratoire d'Optique Physique, EPCI, I0, rue Vauquelin, 75231 Paris Cedex 05, France (Received 11 S e p t e m b e r 1980; in revised f o r m 10 D e c e m b e r 1980 b y M. Balkanski)
The behaviour of several ns 2 ions doped alkali halides has been studied under an intense ultraviolet laser excitation. From the apparition of a strong F-band coloration and from previous gain measurements, we conclude that an intense reabsorption between excited states occurs, which restricts the possibility of making new laser materials with those systems. COLOR CENTERS lasers have been demonstrated to work efficiently in the visible and infra-red ranges [ 1]. Recently the F ÷ center in CaO has been shown to exhibit gain [2], when pumped with a pulsed nitrogen laser, and, more recently, lasing action has been obtained [3] in the 3 6 0 - 3 9 0 nm range with this system. The advent of high power sources in the ultraviolet region, namely the excimer lasers, has given the possibility of extending the range of tunable laser sources towards this region. Among the wide variety of color centers in alkali halides some interest has been paid [2] to the so-called ns 2 ions (or thallium-like ions), whose luminescent properties are well-known [4], as candidates for tunable lasers in the 2 5 0 - 3 5 0 n m region. In this paper, we report studies on some of these systems, aimed at obtaining new laser materials. Besides the F ÷ center in CaO, Duran et al. [2] had made gain measurements with the CaF2 : Eu 2÷, which is not an ns 2 ion (its ground state is a 4 f and its excited state a 4fa5d level) and the KC1 : Bi 3÷ systems, when pumped with a nitrogen laser. They found a negative gain (i.e. absorption) in the first case and a zero gain in the second, although its theoretical value was much greater than the experimental error. Our first purpose was to perform gain measurements with systems in which fluoresence has a good quantum efficiency and which can be excited with an excimer laser. In fact we have not performed any such measurement since we have observed that for every system we have explored, except in KI, irradiation defects (essentially F centers) were created very efficiently by the excirner laser light. As a result, quick degradation of the samples occurred. We have then concluded that a strong reabsorption takes place between the relaxed excited state and a higher excited state of those systems. The crystals have been grown with the Bridgman *E.R. 5 du CNRS.
method and doped so that the resulting optical density in the excitation bands was about l mm- t at the excimer laser wavelength: The Au and Ag impurities have been reduced electrolytically in order to obtain Au- and Ag- ions. All the irradiations have been performed at 77 K, temperature at which those systems were expected to constitute reliable laser materials. Irradiations at 4 K were not possible under our present experimental conditions. Let us now examine the different systems we have explored. We have measured the radiative lifetimes, r n , and, by comparison with a BiBuQ dye [6], the quantum efficiency, Q, of the ultraviolet luminescence of the A band of the following crystals: KBr : Au-
Q ~ 0.2
rn ~
80 nsec
(X = 318 nm, AX = 6 nm),
RbCI : Au-
Q-~ 0.5
rR -- 110nsec
(X = 317nm, AX --~ 5 nm).
This gives for the cross-section of stimulated emission [2] respectively 2 x 10 -18 and 5 x 10-1Scm 2 (for comparison it is 5 x 10-17cm 2 for coumarin 307, which lases easily in the optical cavity made by a simple quartz cell, when pumped by a pulsed laser). These systems were excited in the "A" absorption band [4, 5] near 3080A by a pulsed dye laser (Rhodamin B laser, excited by a pulsed N2 laser, doubled by a KDP crystal) at relatively low power: 1 - 1 0 kW Gain measurements need a higher power: 0.1-1 MW [1, 6]. However, when excited with an XeCI laser (3080 A), those systems exhibit a strong coloration, due to F centers (Fig. 1), which appears after a few laser shots. The same effect occured in KCI : Au-. The XeC1 laser energy (4.02 eV) is well below the absorption gap of KBr, KCI and RbC1 [7]. Moreover we have verified by irradiating, in exactly the same conditions, undoped crystals of the same materials, that no coloration
765
766
REABSORPTION OF " n s : " IONS IN ALKALI HALIDES
Vol. 38, No. 8
1
0.5 ¢-
(.} Q.
0
QI
I
"--r
"T
500
600
k/nm Fig. 1. Absorption of a sample after exposure at 80 K to a few shots of a XeCI laser. Full line." a RbC1 : Au - sample exhibits a strong F band absorption (the arrow indicates the tabulated F band position at 4 K). The high energy side of the band can be attributed to the so-called "K band" of the F center (see [4]) and to the tail of the UV absorption band of the Au ° remaining in the crystal. Dotted line." a RbCI : Au ° sample (where the Au impurities have not been electrolytically reduced) irradiated exactly in the same conditions does not exhibit any coloration. occured, that is the excimer laser power was weak enough to avoid two photon absorption, which would populate excitonic and conduction band levels directly. Thus the coloration effect is due to the A u impurities only. The KC1 : TI ÷ systems, the fluorescent properties of which are well-known [4, 6], was also strongly colored by irradiation with a KrF laser, after a few laser shots. This corresponds also to the A band absorption of the TI ÷ ion in KC1. In KI : Sn 2÷ and KI : Ag- we have observed n o F center coloration under a XeC1 laser irradiation. However it is known that the F center creation rate is very weak at low temperatures [8]. Thus this does not
exclude a possible excited state reabsorption. We have not attempted gain measurements in those systems since the quantum efficiency of the fast luminescence of interest is very weak in those conditions. In KI : Sn 2 ÷ [9] the luminescence is made of two emission bands in the 4 5 0 - 5 5 0 nm region with different lifetimes: 10 nsec for one band and 1.5 psec for the second. We have found that the quantum efficiency of the emission of interest, the fast one [6], was 10 -3 times the efficiency of the slow emission, leading to no possible gain (in fact, the relative intensities of the two components of the luminescence are strongly dependent upon the wavelength of excitation). In KI : Ag- only the infra-red emission at 1.6 eV has a non-negligible intensity at 77 K [6, 10].
Vol. 38, No. 8
REABSORPTION OF "ns z'' IONS IN ALKALI HALIDES
Excited states f (n+l)Por nD 5
767
• Exciton andJ ~/~. [conduction I ~'~/band levelsJ
Reabsorption 2
4•
} First excited state ( n3 P~)
I
Absor)tion
Emission
5
~ "
I Groundstate (n ISo)
Fig. 2. Sketch of a four level system perturbed by a reabsorption from the relaxed excited state to a 5th level. The waving arrows indicate non-radiative transitions. The levels corresponding to the " A " absorption and emission of a • ns z'" ion are indicated in brackets. We have also studied RbBr : Ag- and CsBr : Ag-. The C band of these systems absorbs at the XeC1 wavelength, but the quantum efficiency of fluorescence is negligible at 77 K. The quantum efficiency of CsBr : Agat 4 K is much higher (Q "" 0.5), but we could not irradiate at liquid helium temperature. It would then be interesting to study the behavior of this system when irradiated at 3080 A. From these results and from those of Duran et aL [2] it can be inferred that a strong reabsorption band occurs between the relaxed excited state and higher excited states of these systems. The reabsorbed light can be either the pumping or the fluorescence light or both, if several excited states can absorb as will be seen below. If the final excited states are high enough in energy, the excitation can be transferred to excitons or electron-hole pairs leading to the creation o f F centers [7] (Fig. 2), as we did observe in our experiments. In this case the energies of two photons are added to produce degradation of the samples. Reabsorption of the fluorescence light may also
"kill" the gain as was observed in CaF2 : Eu :+ and KC1 : Bi 3+ [2]. The absorption coefficient at the frequency v, k(v) can then be written [ I 1 ], with the notations of Fig. 2: k(v) = 8rw----i n 3 - n 4 )
ro
To J
with g(v) = line shape of the 3--4 transition, g'(v) = line shape of the 4 - 5 transition, ro = radiative lifetime of the level 4 and ro = radiative lifetime of the level 5. This formula is restricted for clarity to a three level system, i.e. does not take into account the electronphonon coupling of the ions in solids [ 12]. This would not modify qualitatively our conclusions. Assuming that the non-radiative relaxations are fast ( ~ 10-12 sec) and that the level 5 de-excites non-radiatively (toward the ground state or by an F center creation mechanism) it comes n3 "" n5 "" 0 and
768
REABSORPTION OF " n s : ' " IONS IN ALKALI HALIDES
Singlef
0
S
PD
levels S~
Vol. 38, No. 8
T r i p l e t levels P~
.
, - D .~ Ionizofion
-20000
7
E u
-40000
6 3 P 2 ..~ "
/6 P, 6 Po
W -60000
- 80000 - 8 4 178
So
Fig. 3. Energy level scheme of the Hg atom, whose ground state is a 6s z.
[ c2 g'(v) g(U)]n," =
To
To j
Thus it appears that the sign of this absorption coefficient, negative for a net gain, is independent of the relaxed excited state population, n4, i.e. of the experimental conditions, particularly the pumping light intensity. In fact any gain (or excited state absorption) measurement is proportional to the difference between stimulated emission and (stimulated) absorption between excited states. This shows how a reabsorption phenomenon decreases the gain of such systems. The reabsorption in ns z ions systems can be understood easily. The electronic levels of those ions are considerably broadened by e l e c t r o n - p h o n o n interaction [4]. Thus (Fig. 2) a transition of the type 4 ~ 5 has a good probability of overlapping a 4 ~ 3 transition if the level 5 lies in the proper energy range. It can be seen that some levels lie in this proper energy range in the case of n s 2 ions. Let us consider the energy level scheme of the Hg atom [13] whose ground state is a 5dl°6s2 (Fig. 3). It has been shown [4] that the relative level spacings are roughly the same for n s : ions in substitutional positions in alkali halides. The A band corresponds to the 63/1 level, B to 6 3Pz (weakly allowed) and C to 6 IP1 level. From the 3p1 level, corresponding to the A emission, several reabsorptions may
occur toward the 6D levels at energies comparable to t h e I S o - 3 P l separation. Reabsorption from the 6 IPl level, corresponding to the C emissions, which we did not explore, seems less likely, since it is higher in energy than the 6 3Pl level. However, many higher sin~et levels exist in the atom and may occur in the crystals. at energies corresponding to the reabsorption. The F center-like systems, such the F ÷ center in CaO, have energy levels which resembles the hydrogen atom: their principal transition is an Is -~ 2p transition the energy of which is much greater than half the ionization limit [ 13]. Thus a reabsorption cannot occur in the ls ~ 2p energy range (a reabsorption occurs, in fact, in the infra-red [14]) and the gain cannot be affected by such an effect as shown by various gain and laser experiments [ I - 3 ] . We can conclude that the heavy ions and particularly the n s 2 ions, in alkali halides are not very likely to produce laser emissions in the UV range. However it is possible that some of them exhibit a positive gain. The reabsorption of the emission light can be more or less important: For example the gain has been shown to be negative in CaF~ : Eu 2÷ and about zero in KC1 : Bi 3÷. It could be positive for some systems, provided that the reabsorption is weak enough and does not lead to an excited level higher in energy than the gap of the crystal in which case coloration would occur,
Vol. 38, No. 8
REABSORPTION OF "ns'-" IONS IN ALKALI HALIDES
except in KI and, generally, in iodides at low temperatures. We have shown experimentally that a reabsorption takes place from the 3Pt (A band) level. There is no proof that such a reabsorption would occur from the tPl (C band) level. From the energy level scheme it is not clear, as mentioned before, if a reabsorption should exist or not. Moreover, since excitation in the C band produces generally more or less emission from the A band, the crystal could be damaged by reabsorption from this A band. Thus ns: ions pumped in the C band, which is generally in the range 200-250 nm, and having a very good quantum efficiency for the C emission, could still be candidates for laser action. Acknowledgements - The authors are very indebted to Pr J. ChapeUe and to Dr B. Briat for the growth of the doped crystals, to the CGE research Center for the possibility of using one of their excimer lasers, and to C. Grisolia for experimental help. REFERENCES L.F. Mollenauer & D.H. Olson, AppL Phys. Lett. 24,386 (1974); L.F. Mollenauer, Commun. Int.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
769
Conf. on Color Centers in Ionic OTstals, Sendai, Japan (I 974) (unpublished). J. Duran, P. Evesque & M. Billardon, AppL Phy~ Lett. 33, 1004 (1978). B. Henderson et al. (to be published). W.B. Fowler, Physics o f Color Centers, (Edited by W.B. Fowler), Chapter 2. Academic Press, New York (1968). M. Krause &Fisher, J. Luminesc. 4,335 (1971). M. Billardon & J. Duran, Rapport DGRST No. 75-7-1485 (1979). M. Saidoh & P.D. Townsend, Rad. Effects 27, 1 (1975); Y. Farge, J. dePhys. 34, C9,475 (1973). T.P.P. Hall, D. Pooley & P.T. Wedepohl, Proc. Phys. Soc. 83,635 (1964). A. Fukuda, Phys. Rev. B1,4161 (1970). K. Kojima, S. Shimanuki & T. Kojima, J. Phys. Soc. Japan 33, 1076 (1972). B. di Bartolo, Optical Interactions in SolMs, Chapter 19. Wiley, New York (1968). D.B. Fitchen, Physics of Color Centers, (Edited by W.B. Fowler), Chapter 5. Academic Press, New York (1968). G. Herzberg, Atomic Spectra and Atomic Structure. Dover, New York (1944). Y. Rondo & H. Kanzaki, Commun. Int. Conf. on Color Centers in Ionic Crystals, Sendai, Japan (1974) (unpublished); K. Park & W.L. Faust, Phys. Rev. Lett. 17,137 (1966).