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Luminescent properties of Cr3+-doped LaSr2Ga11O20
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 6O&6l 119941 154 157
Luminescent properties of Cr3 k-doped LaSr2Ga1 1020 1’, G.P. Morgana*, A.A. Kaminskiih C.J. McDonagha, T.J.Department G1ynn,~’, G .F. Imbusch 0/ Phisic.~ nirersirv ( ollege Ga/nat. Irelatul Russian 4cadetn v of Sciences, Leninskii Prospeki 59
%!oscon, Il 7333 Russian hslcration
Abstract
Luminescence and lifetime measurements are presented for LaSr,Ga 3. The room-temperature emission is 1 1020 Cr dominated by a broadband stretching from 670 nm to beyond 1.1 pm. It is suggested that the Cr3~ions substitute in two distinct high-field sites with predominantly octahedral coordination.
I. Introduction
2. Experimental results
There has been much interest in recent years in tunable, room temperature, solid-state lasers based on vibrational transitions of substitutional transition metal ions in inorganic crystalline hosts [1]. Several notable successes, such as the development of the Ti33 : Al 3 : BeAl
Luminescence measurements were carried out on a single crystal of LaSr 33 with dimensions of 5 x2Ga1 5 x 51020 mm3doped whichwith wasCrobtamed from the A.V. Shubnikov Institute of Crystallography, Moscow. This material is monoclinic
204
(space group C~h[3]. The sample was mounted in
(Alexandrite) lasers, have motivated the search for
203 and Cr
a closed-cycle helium refrigerator on a sample
other suitable materials.
While there would
block whose temperature could be varied continu-
appear to be several likely possibilities based on simple spectroscopic measurements, few systems have so far met with success. The effects of excited state absorption and non-radiative decay
ously from 10K to room temperature. The luminescence was excited by the 488 nm line from an argon ion laser, dispersed by a Spex 500 M 0.5 m monochromator, and detected by either a liquid-
from the luminescing level have been regarded as the major impediments in many cases [2]. Accordingly, the search for other suitable laser host materials continues. In this article we report on the basic optical 35. spectroscopy of LaSr2Ga1 1020 doped with Cr
nitrogen cooled Ge photodiode or PMT. Lifetime measurements were made by chopping the exciting laser beam and using photon counting techniques to monitor the luminescence decay. In this latter arrangement the amplified and discriminated photoelectric pulses from a cooled RCA 31034A PMT were recorded by an EG&G ORTEC ACEMCS card mounted in a PC-compatible computer. The technique of phase-sensitive detection was used to separate spectral features with different
*
Corresponding author,
0022-231394/S07.00 C~ 1994 Elsevier Science By. All rights resened SSDJ 0022-231 3(93)E0363-3
C.J. McDonagh ci a!. / Journal of Luminescence 60&61 (1994) 154— 157
155
lifetimes by chopping the laser beam and monitoring the resultant modulated luminescence with
sharp lines (R lines) are attributed to transitions from the split 2E excited state to the 4A2 ground
a Stanford Research SR530 dual phase lock-in amplifier, Laser-induced photoluminescence spectra recorded from several temperatures are presented in the Fig.sample 1. No at correction has been made for the response of the Ge detector. The roomtemperature spectrum is dominated by a struc-
state 4T while 4A the broadband emission is due to a 2 2 transition. A schematic energy level diagram shown inenergy the inset in Fig. When the 3 + ion isis excited, is stored via1.the populaCr tion in the 2E level which decays slowly to the ground state. In thermal equilibrium, the higher energy 4T 2 level can acquire a small population which decays more rapidly since 4A this is a spinallowed transition to the ground 2 state. This energy level arrangement is typical of Alexandrite, 4T 4A for example, and it is this latter 2 2 transition which gives rise to the tunable laser action in this material. 2E level are also two components the split in The thermal equilibrium. of Accordingly, the intensity of the luminescence from the higher-energy level, ‘R2~relative to that from the lower-energy level, ‘Ri~ will depend on the relative population of the two levels and will vary with temperature as ~ A kT R2/ Ri ~ exp A plot of ln(IR2/IR1) versus temperature should yield a straight line whose slope will give a value for the splitting of the levels, A. The luminescence from the individual R lines were deconvoluted from the broadband emission at higher temperatures as in-
tureless broadband emission extending from i~670nmto beyond 1.1 ~.imwhile two sharp lines, accompanied by several sideband features, are observed near 700 nm. These two dominant sharp lines are separated by 140cm and have halfwidths of approximately 3 nm. The shorterwavelength feature is found to decrease in intensity relative to the longer-wavelength component as the temperature is reduced and is not observed at 55 K. The broadband is also found to decrease in intensity relative to the other features with decreasing sample temperature and is not observed in the 79 K spectrum. The behaviour described above is consistent with the emission from Cr3~ions in high-field sites with predominant octahedral coordination. The two
—~
—~
4T 2
2E
_________
4A
________
dicated by the broken lines in Fig. 1. The resultant semilogarithmic represented by a straight line fit plot from was whichwell a value of 116cm1 was calculated for A, which agreed well with the observed Ri R2 line splitting of 140cm The ratio of the broadband to total R-line emission should also exhibit a Boltzmann dependence as described by Eq. (1). Here A represents the energy separation between the 2E and 4T 2 level. A semilog of the intensity ratios versus temperature was well fitted by a straight line whose slope indicated a value of 450cm~for A. Lifetimes were measured at the peak of the RI line for several temperatures between 32 and 300 K. The temporal evolution of the decaying luminescence was observed to be non-exponential in all cases and was found to be best described by a sum —
300 K
2
I
I
I
I
plot
75 K
~
-~_
~
900 WAVELENGTH
1000 r~rs)
1
3~-doped LaSr Fig. I. Luminescence for Cr 2Ga11O20 at var!ous temperatures as indicated. The spectra are not corrected for the response of the Ge 3 +detector. in a predominately The inset shows octahedrally the lowest-lying coordinated high-field energy levels of Crsite,
~.
.
.
of results two of exponentials the curve fitting of comparable analysis are intensities. summarised The in Fig. 2 where the estimated lifetimes of the two
ISO
(‘.1. ztlcDonagli ci a!. 2.5
I I
A 2.0
•
Journal of Luminescence 60&61
=
300 cm~ 3 s~ 1.3x10 1.7x104 s~ -
=
/994) /54 /57
decay occurs. If it is assumed that the effects of non-radiative relaxation are small at the temperatures of interest. then the lifetime of the coupled 2E and 4T 2 levels can be described [5], to a simple approximation, by
15
I I-’
r
.
1.0
tIH =
1
+ +
exp ( A/k T) 3exp(—A/kT)
3/T
______________.
(2)
.
Here the parameter ‘r denotes the lifetime of the decaying luminescence, while IE and J~-represent .5~. I
I I
0
100
200
300
2E and 4T the 2 relaxation rates. A good fitthetoopen the temporal dependence luminescent centre, as ofrepresented the data by for the fast 400
circles in Fig. obtained forf’~ l0~s of 4s2, is and A 300 cm 1.13 Thex value 1.7 x l0 here is less than the 450 cm ~ value A obtained =
Temperature (K) Fig. 2. Lifetime ‘,alues obtained from the R 1-line decay fitted with two exponentials at several temperatures. The solid curve is a fit of Eq. (2) to the faster lifetime component (open circles) using the parameters listed in the figure. A similar fit of Eq. (2)to the slower components (solid circles) could not be obtained,
exponential contributions are plotted versus ternperature. At all temperatures, the decay consisted of fast and slow components and the lifetimes decreased uniformly temperatures. This would suggestwith thatincreasing overlapping emissions were being detected from at least two distinct Cr centres with different decay rates. A multiplicity of Cr centres (which may be due to non-stoichiometnc effects of the material [4]) is also suggested by the large width (3 nm) of the R lines and may also explain the large bandwidth of the 4T 2 emission. Phase-resolved techniques were used to separate out the spectral contributions of these distinct features. While both components 2Eexhibit 4A overlapping sharp-line features due to 2 transitions, 4T only the faster component has a broadband 2 emission associated with it. We conclude that the Cr centre emitting the slower 4T luminescence is at aabove high-field such that the 2 populated, level is tooeven far the 2Esite level to be thermally —~
at room temperature. The other 4T centre2E isseparalso a high-field site but with a smaller 2— 2E ation. In radiative this case energy canbybe thermally lost from excited the state by decay or 4T transfer to the 2 level from which rapid radiative
=
~.
‘
=
predicted by the temperature dependence of the broadband to sharp-line intensity ratios. However, this may be expected since the R-line intensities have unresolved contributions from the two Cr centres while the broadband emission is due to one centre only. No reasonable fit of Eq. (2) to the temperature dependence of the slower lifetime cornponent could be obtained. This would indicate that some other channel exists which allows non-radi2E level in this ative [I]. relaxation to occur from the case We note that the temperature dependence of the faster luminescent centre is described well by Eq (2) which ignores the effects of non-radiative relaxation. This would suggest that the quantum efficiency (QE) of this Cr site is high at room temperature. The combination of high QE and broadband luminescence features indicate that this material may be suitable as a potential tunable laser host.
3. Conclusion 35 located at two distinct Luminescence from Cr sites in LaSr 2Gai i020 has been reported for the first time. The emission from one site, due to 2E-4A 2 transition only, is by located 700 nm. The other site is characterised sharpnear 2E luminescence features near 700 nm and also by a broadband 4T 2 emission extending from 670 nm to beyond
C.J. McDonagh ci a!. / Journal of Luminescence 60&61 (1994) 154—157
1.1 j.tm and which dominates the spectrum at room temperature.
157
References [1] B. Henderson and G.F. lmbusch, Contemp. Phys. 29 (1988)
Acknowledgements The Ge detector was obtained through a grant from the University College Galway Development Fund. This work was supported in part by an Eolas
Basic Research Grant No. SC/036/89 from the Irish Government and by the US Air Force Office of Scientific Research under grant no. AF0SR88 0355.
235. [2] C. Borel, C. Wyon, ii. Aubert, H. Manaa and R. Moncorgé, J. Lumin. 55 (1993) 95. [3] A.A. Kamjnskii, in: Proc. 1st Int. School on Excited States of Transition Elements, eds. B. Jezowska-Trzebiatowska, J. Legendziewicz and W. Strek (World Scientific, Singapore, 1989) p. 669. [4] S.M. Healy, C.J. Donnelly, Ti. Glynn, G.F. Imbusch and G.P. Morgan, J. Lumin. 44 (1989) 65. [5] MO. Henry, J.P. Larkin and G.F. Imbusch, Phys. Rev. 13 B (1976) 1893.
LUMINESCENCE ELSEVIER
Journal of Luminescence 6O&6l 119941 154 157
Luminescent properties of Cr3 k-doped LaSr2Ga1 1020 1’, G.P. Morgana*, A.A. Kaminskiih C.J. McDonagha, T.J.Department G1ynn,~’, G .F. Imbusch 0/ Phisic.~ nirersirv ( ollege Ga/nat. Irelatul Russian 4cadetn v of Sciences, Leninskii Prospeki 59
%!oscon, Il 7333 Russian hslcration
Abstract
Luminescence and lifetime measurements are presented for LaSr,Ga 3. The room-temperature emission is 1 1020 Cr dominated by a broadband stretching from 670 nm to beyond 1.1 pm. It is suggested that the Cr3~ions substitute in two distinct high-field sites with predominantly octahedral coordination.
I. Introduction
2. Experimental results
There has been much interest in recent years in tunable, room temperature, solid-state lasers based on vibrational transitions of substitutional transition metal ions in inorganic crystalline hosts [1]. Several notable successes, such as the development of the Ti33 : Al 3 : BeAl
Luminescence measurements were carried out on a single crystal of LaSr 33 with dimensions of 5 x2Ga1 5 x 51020 mm3doped whichwith wasCrobtamed from the A.V. Shubnikov Institute of Crystallography, Moscow. This material is monoclinic
204
(space group C~h[3]. The sample was mounted in
(Alexandrite) lasers, have motivated the search for
203 and Cr
a closed-cycle helium refrigerator on a sample
other suitable materials.
While there would
block whose temperature could be varied continu-
appear to be several likely possibilities based on simple spectroscopic measurements, few systems have so far met with success. The effects of excited state absorption and non-radiative decay
ously from 10K to room temperature. The luminescence was excited by the 488 nm line from an argon ion laser, dispersed by a Spex 500 M 0.5 m monochromator, and detected by either a liquid-
from the luminescing level have been regarded as the major impediments in many cases [2]. Accordingly, the search for other suitable laser host materials continues. In this article we report on the basic optical 35. spectroscopy of LaSr2Ga1 1020 doped with Cr
nitrogen cooled Ge photodiode or PMT. Lifetime measurements were made by chopping the exciting laser beam and using photon counting techniques to monitor the luminescence decay. In this latter arrangement the amplified and discriminated photoelectric pulses from a cooled RCA 31034A PMT were recorded by an EG&G ORTEC ACEMCS card mounted in a PC-compatible computer. The technique of phase-sensitive detection was used to separate spectral features with different
*
Corresponding author,
0022-231394/S07.00 C~ 1994 Elsevier Science By. All rights resened SSDJ 0022-231 3(93)E0363-3
C.J. McDonagh ci a!. / Journal of Luminescence 60&61 (1994) 154— 157
155
lifetimes by chopping the laser beam and monitoring the resultant modulated luminescence with
sharp lines (R lines) are attributed to transitions from the split 2E excited state to the 4A2 ground
a Stanford Research SR530 dual phase lock-in amplifier, Laser-induced photoluminescence spectra recorded from several temperatures are presented in the Fig.sample 1. No at correction has been made for the response of the Ge detector. The roomtemperature spectrum is dominated by a struc-
state 4T while 4A the broadband emission is due to a 2 2 transition. A schematic energy level diagram shown inenergy the inset in Fig. When the 3 + ion isis excited, is stored via1.the populaCr tion in the 2E level which decays slowly to the ground state. In thermal equilibrium, the higher energy 4T 2 level can acquire a small population which decays more rapidly since 4A this is a spinallowed transition to the ground 2 state. This energy level arrangement is typical of Alexandrite, 4T 4A for example, and it is this latter 2 2 transition which gives rise to the tunable laser action in this material. 2E level are also two components the split in The thermal equilibrium. of Accordingly, the intensity of the luminescence from the higher-energy level, ‘R2~relative to that from the lower-energy level, ‘Ri~ will depend on the relative population of the two levels and will vary with temperature as ~ A kT R2/ Ri ~ exp A plot of ln(IR2/IR1) versus temperature should yield a straight line whose slope will give a value for the splitting of the levels, A. The luminescence from the individual R lines were deconvoluted from the broadband emission at higher temperatures as in-
tureless broadband emission extending from i~670nmto beyond 1.1 ~.imwhile two sharp lines, accompanied by several sideband features, are observed near 700 nm. These two dominant sharp lines are separated by 140cm and have halfwidths of approximately 3 nm. The shorterwavelength feature is found to decrease in intensity relative to the longer-wavelength component as the temperature is reduced and is not observed at 55 K. The broadband is also found to decrease in intensity relative to the other features with decreasing sample temperature and is not observed in the 79 K spectrum. The behaviour described above is consistent with the emission from Cr3~ions in high-field sites with predominant octahedral coordination. The two
—~
—~
4T 2
2E
_________
4A
________
dicated by the broken lines in Fig. 1. The resultant semilogarithmic represented by a straight line fit plot from was whichwell a value of 116cm1 was calculated for A, which agreed well with the observed Ri R2 line splitting of 140cm The ratio of the broadband to total R-line emission should also exhibit a Boltzmann dependence as described by Eq. (1). Here A represents the energy separation between the 2E and 4T 2 level. A semilog of the intensity ratios versus temperature was well fitted by a straight line whose slope indicated a value of 450cm~for A. Lifetimes were measured at the peak of the RI line for several temperatures between 32 and 300 K. The temporal evolution of the decaying luminescence was observed to be non-exponential in all cases and was found to be best described by a sum —
300 K
2
I
I
I
I
plot
75 K
~
-~_
~
900 WAVELENGTH
1000 r~rs)
1
3~-doped LaSr Fig. I. Luminescence for Cr 2Ga11O20 at var!ous temperatures as indicated. The spectra are not corrected for the response of the Ge 3 +detector. in a predominately The inset shows octahedrally the lowest-lying coordinated high-field energy levels of Crsite,
~.
.
.
of results two of exponentials the curve fitting of comparable analysis are intensities. summarised The in Fig. 2 where the estimated lifetimes of the two
ISO
(‘.1. ztlcDonagli ci a!. 2.5
I I
A 2.0
•
Journal of Luminescence 60&61
=
300 cm~ 3 s~ 1.3x10 1.7x104 s~ -
=
/994) /54 /57
decay occurs. If it is assumed that the effects of non-radiative relaxation are small at the temperatures of interest. then the lifetime of the coupled 2E and 4T 2 levels can be described [5], to a simple approximation, by
15
I I-’
r
.
1.0
tIH =
1
+ +
exp ( A/k T) 3exp(—A/kT)
3/T
______________.
(2)
.
Here the parameter ‘r denotes the lifetime of the decaying luminescence, while IE and J~-represent .5~. I
I I
0
100
200
300
2E and 4T the 2 relaxation rates. A good fitthetoopen the temporal dependence luminescent centre, as ofrepresented the data by for the fast 400
circles in Fig. obtained forf’~ l0~s of 4s2, is and A 300 cm 1.13 Thex value 1.7 x l0 here is less than the 450 cm ~ value A obtained =
Temperature (K) Fig. 2. Lifetime ‘,alues obtained from the R 1-line decay fitted with two exponentials at several temperatures. The solid curve is a fit of Eq. (2) to the faster lifetime component (open circles) using the parameters listed in the figure. A similar fit of Eq. (2)to the slower components (solid circles) could not be obtained,
exponential contributions are plotted versus ternperature. At all temperatures, the decay consisted of fast and slow components and the lifetimes decreased uniformly temperatures. This would suggestwith thatincreasing overlapping emissions were being detected from at least two distinct Cr centres with different decay rates. A multiplicity of Cr centres (which may be due to non-stoichiometnc effects of the material [4]) is also suggested by the large width (3 nm) of the R lines and may also explain the large bandwidth of the 4T 2 emission. Phase-resolved techniques were used to separate out the spectral contributions of these distinct features. While both components 2Eexhibit 4A overlapping sharp-line features due to 2 transitions, 4T only the faster component has a broadband 2 emission associated with it. We conclude that the Cr centre emitting the slower 4T luminescence is at aabove high-field such that the 2 populated, level is tooeven far the 2Esite level to be thermally —~
at room temperature. The other 4T centre2E isseparalso a high-field site but with a smaller 2— 2E ation. In radiative this case energy canbybe thermally lost from excited the state by decay or 4T transfer to the 2 level from which rapid radiative
=
~.
‘
=
predicted by the temperature dependence of the broadband to sharp-line intensity ratios. However, this may be expected since the R-line intensities have unresolved contributions from the two Cr centres while the broadband emission is due to one centre only. No reasonable fit of Eq. (2) to the temperature dependence of the slower lifetime cornponent could be obtained. This would indicate that some other channel exists which allows non-radi2E level in this ative [I]. relaxation to occur from the case We note that the temperature dependence of the faster luminescent centre is described well by Eq (2) which ignores the effects of non-radiative relaxation. This would suggest that the quantum efficiency (QE) of this Cr site is high at room temperature. The combination of high QE and broadband luminescence features indicate that this material may be suitable as a potential tunable laser host.
3. Conclusion 35 located at two distinct Luminescence from Cr sites in LaSr 2Gai i020 has been reported for the first time. The emission from one site, due to 2E-4A 2 transition only, is by located 700 nm. The other site is characterised sharpnear 2E luminescence features near 700 nm and also by a broadband 4T 2 emission extending from 670 nm to beyond
C.J. McDonagh ci a!. / Journal of Luminescence 60&61 (1994) 154—157
1.1 j.tm and which dominates the spectrum at room temperature.
157
References [1] B. Henderson and G.F. lmbusch, Contemp. Phys. 29 (1988)
Acknowledgements The Ge detector was obtained through a grant from the University College Galway Development Fund. This work was supported in part by an Eolas
Basic Research Grant No. SC/036/89 from the Irish Government and by the US Air Force Office of Scientific Research under grant no. AF0SR88 0355.
235. [2] C. Borel, C. Wyon, ii. Aubert, H. Manaa and R. Moncorgé, J. Lumin. 55 (1993) 95. [3] A.A. Kamjnskii, in: Proc. 1st Int. School on Excited States of Transition Elements, eds. B. Jezowska-Trzebiatowska, J. Legendziewicz and W. Strek (World Scientific, Singapore, 1989) p. 669. [4] S.M. Healy, C.J. Donnelly, Ti. Glynn, G.F. Imbusch and G.P. Morgan, J. Lumin. 44 (1989) 65. [5] MO. Henry, J.P. Larkin and G.F. Imbusch, Phys. Rev. 13 B (1976) 1893.