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Observation of avalanche-like behavior in Tm3+ : Y2O3
OURNALOF
LUMINESCENCE ELSEVIER
Journal of Luminescence 60&61 (1994) 668 67)
Observation of avalanche-like behavior in Tm3 + Y203 J.M. Dyson*, S.M. Jaffe, H. Eilers, M.L. Jones, W.M. Dennis, W.M. Yen I) par iou,?! of Phr a s 00(1 1 tronon,
Tin’
,in
er
lii
of (georgia
‘It/u n
(, I ~OO)L’, .5 I
Abstract Asalanche-like up-conversion is observed in 2°o Tm : Y20 crystal fibers at a temperature of 12 K. Tsso crossrelaxation mechanisms are used to model the dramatic population increase of the metastable lesel that is central to the up conversion process. Both transient and steady-state data agree well with the predictions of this model *
1. Introduction The avalanche effect is a phenomenon that has been observed and studied in both rare-earth and transition metal systems. [1,2] Avalanche behavior has several identifying characteristics. Up-converted fluorescence is always generated and, in gen eral, occurs in a system with a high ratio of excited to ground state absorption at the laser wavelength, A dramatic increase in the fluorescence is observed when the pump intensity reaches a critical level. In addition, this fluorescence will have a slow rise time compared to the limiting rate of the up-conversion process. Due to the energy level structure and candito the 3~is a good cross-relaxation processes. Tm date for observing the avalanche process. New studies have been performed in Tm~+ : Y 2O~at 12 K pumping with 653.7 nm light. Steady-state cxperiments were performed using a cw dye laser. Transient data were obtained by chopping this laser at 50Hz with a duty cycle on 0.25.
The data indicate that avalanche behavior ma~ be occurring in the system. For example. we ob served blue up-converted light which has a much slower rise time than the millisecond lifetime of the metastable level, It was also observed that the tern poral evolution of this fluorescence is different at low and high pump powers. Absorption spectra indicated that there was no resonant transition from the ground state when pumping at 651.7 nm~ however, there was some non-negligible ahsorp tion. We have called this process “avalanche like” because of this residual ground state absorption. Starting with the simplest analysis possible, we have used rate equations to model this system. Numerical solutions data have been compared withand the 12K experimental in both transient steady-state regimes. The agreement indicates that the model is appropriate to the system and points towards avalanche-like behavior in the observed up-conversion process.
2. Results and discussion *
Corresponding author
We have investigated the power dependence of the transmission and fluorescence tntensity
0022-2~1394 $0700 c 1994 Flsevier Science B.V. All right. resersed .S’SDI 0022 2~M)9~)E0400 R
f.M. Dyson et a!.
Journal of Luminescence 60&61 (1994) 668 671
monitored at 489 nm for 653.7 nm excitation light. Although our steady-state curve shows only the low-power dependence, the transient data show both the low- and high-power dependence of the system. Our model predicts this behavior in the transients, as well as in the low-power steady-state fluorescence curve. The left side of3 Fig. + : Y 1 shows a partial energy level diagram for Tm 2O3. We have simplified this diagram by using only those levels involved in the energy transfer processes (right side of Fig. 1). The simplified diagram uses three ions to illustrate one possible combination of these processes. Ion A is 3F pumped by L into 3H level 4 ( 2). It decays with a rate of r43 into level 3 ( 4). Rates r42 and r41 have been found to be small compared to r43 and have been
neglected. We have 3F measured the emission lifetimes from levels 2( 4) and 5(tG4). Both of thesearefound to be concentration dependent, suggesting that crossrelaxation processes occur in the decay of both of these levels. Energy released3H from ion 3F A during the transition from level 3 to 2 ( 4 4) with a rate X~3can be 3H 3F used to excite ion3HB from level 1 to 2 ( 6 4). Ions in level 3 ( 4) can also decay 3H to level 1 ( 6) with a rate r31. Rate r32 has been found to be small compared to r3i and X~3and has also been neglected. —+
—+
669
(t04),
Ions A and B are now both in level 2 (3F4) and can be pumped by M into level 5 There are several different non-radiative cross-relaxation processes possible from level 5. These processes are represented in our model by a single cross-relaxation process X15. The energy released from level 1G 3F 5 to 2 ( 4 3H 4) can 3H be used to excite ion 1GC from level 1 to 3 ( 6 4). Ions in level 5 ( 4) can 3H also decay to level 1 ( 6) with a rate r51. Rates r54, r53, and r52 are found to be small compared to both r51 and X15 and have therefore been neglected. Below are the rate equations for the above processes: —~
—~
dN1/dt
LN~+ r21N2 + r31N3 + r51N5
= —
dN2/dt
—
dN3/dt dN4/dt
= —
X13N1N3
X15N1N5,
MN2 r21N2 + 2X13N1N3 + X15N1N5 —
r43N4 r31N3 X13N1N3, LN, r43N4 + X~5N1N5, —
—
dN5/dt MN2 r51N5 X15N~N5, where N is the population, r is the decay rate, X is the cross-relaxation rate, M is excited state —
—
3~:Y Energy Levels in Tm Tm levels
Avalanche levels N5
2
2O3
N4
-
—
r
—
1
(cm-i)
(rim)
08
20902
478.4
06
i5602
663.9
04
~ ‘: : i: : N1 0 ~
A
B
~
3~ Y Fig. 1. Energy level diagram of Tm 203 (left) and energy levels used in modelling the up-conversion process (right). Two possible cross-relaxation roots are indicated.
0:
Ii
/ /
~
/
I Power (mW)
Fig. 2. 12 K power-dependent fluorescence intensity of level 5 (dotted line) compared with both the theoretical results from our model (solid line) and a quadratic least-squares fit to the data (dashed line). Fluctuations in the data are due to uncertainty in power measurements.
670
J. i.1. Dyson oU ~i1 Journal of Luminescence 60&6 I ( 1994, 668 671
.
0,002
:~:E ~ ~
0.001
0.30075
~
. .
:~~‘t~’
0 03
,
.
....
°::: 0.015 0.01
~
10
12
Fig. 3. Top: Theoretical transient solutions (Intensity versus time in milliseconds) of our model ai low power (left) and high power (right). Bottom: Transient behavior (intensity versus time in milliseconds) of fluorescence from level 5 at low power (left) and high power )right). The data have been normalized to the high power model population. Fluctuations are due to counting statistics
absorption and L is the pumping rate. L M is the ratio between ground state and excited state absorption cross-sections which we have found to be 0.11. Using experimentally determined decay rates and cross-relaxation rates, the equations have been solved numerically. Fig. 2 compares the experimental fluorescence intensity with both our solutions for the powerdependent population of level 5 and for a quadratic least-squares fit. The agreement suggests that for steady-state, the data fit a quadratic model as well as our model. We now focus on the transient data. Fig. 3 shows that the slow rise time observed in the fluorescence is predicted by our model, It also shows that there is a power-dependent change in the shape of the rise times that we also observe in our data. Simple
two-photon upconversion (quadratic model) does not reproduce this effect and therefore can be ruled out.
3. Conclusion We have observed blue emission when pumping at 653,7 nm. Both the power dependence and transient behavior of this emission have been compared with a rate-equation-based model that takes into account two experimentally observed cross-relaxation processes. The agreement between the experimental data and the model indicate that an avalanche-like process is responsible for the observed up-conversion. The sharp threshold behavior typically associated
f.M. Dyson ci a!.
Journal of Luminescence 60&61 (1994) 668 671
with an avalanche process is mitigated by the presence of a finite ground state absorption which has a cross-section 0’g 0.11 Cexc.
671
ported by DARPA contact # N00014-90J-4088 and by NSF grant DMR-9 117077.
=
Acknowledgements We are grateful to S.M. Jacobsen and B.M. Tissue for helpful discussions. This work was sup-
References [1] H. Ni and S.C. Rand. Opt. Lett. 16 (1991) 1424. [2] Ueli Oetliker, Mark J. Riley. P. Stanley May and Hans
GUdel, J. Lumin. 53(1992)553.
LUMINESCENCE ELSEVIER
Journal of Luminescence 60&61 (1994) 668 67)
Observation of avalanche-like behavior in Tm3 + Y203 J.M. Dyson*, S.M. Jaffe, H. Eilers, M.L. Jones, W.M. Dennis, W.M. Yen I) par iou,?! of Phr a s 00(1 1 tronon,
Tin’
,in
er
lii
of (georgia
‘It/u n
(, I ~OO)L’, .5 I
Abstract Asalanche-like up-conversion is observed in 2°o Tm : Y20 crystal fibers at a temperature of 12 K. Tsso crossrelaxation mechanisms are used to model the dramatic population increase of the metastable lesel that is central to the up conversion process. Both transient and steady-state data agree well with the predictions of this model *
1. Introduction The avalanche effect is a phenomenon that has been observed and studied in both rare-earth and transition metal systems. [1,2] Avalanche behavior has several identifying characteristics. Up-converted fluorescence is always generated and, in gen eral, occurs in a system with a high ratio of excited to ground state absorption at the laser wavelength, A dramatic increase in the fluorescence is observed when the pump intensity reaches a critical level. In addition, this fluorescence will have a slow rise time compared to the limiting rate of the up-conversion process. Due to the energy level structure and candito the 3~is a good cross-relaxation processes. Tm date for observing the avalanche process. New studies have been performed in Tm~+ : Y 2O~at 12 K pumping with 653.7 nm light. Steady-state cxperiments were performed using a cw dye laser. Transient data were obtained by chopping this laser at 50Hz with a duty cycle on 0.25.
The data indicate that avalanche behavior ma~ be occurring in the system. For example. we ob served blue up-converted light which has a much slower rise time than the millisecond lifetime of the metastable level, It was also observed that the tern poral evolution of this fluorescence is different at low and high pump powers. Absorption spectra indicated that there was no resonant transition from the ground state when pumping at 651.7 nm~ however, there was some non-negligible ahsorp tion. We have called this process “avalanche like” because of this residual ground state absorption. Starting with the simplest analysis possible, we have used rate equations to model this system. Numerical solutions data have been compared withand the 12K experimental in both transient steady-state regimes. The agreement indicates that the model is appropriate to the system and points towards avalanche-like behavior in the observed up-conversion process.
2. Results and discussion *
Corresponding author
We have investigated the power dependence of the transmission and fluorescence tntensity
0022-2~1394 $0700 c 1994 Flsevier Science B.V. All right. resersed .S’SDI 0022 2~M)9~)E0400 R
f.M. Dyson et a!.
Journal of Luminescence 60&61 (1994) 668 671
monitored at 489 nm for 653.7 nm excitation light. Although our steady-state curve shows only the low-power dependence, the transient data show both the low- and high-power dependence of the system. Our model predicts this behavior in the transients, as well as in the low-power steady-state fluorescence curve. The left side of3 Fig. + : Y 1 shows a partial energy level diagram for Tm 2O3. We have simplified this diagram by using only those levels involved in the energy transfer processes (right side of Fig. 1). The simplified diagram uses three ions to illustrate one possible combination of these processes. Ion A is 3F pumped by L into 3H level 4 ( 2). It decays with a rate of r43 into level 3 ( 4). Rates r42 and r41 have been found to be small compared to r43 and have been
neglected. We have 3F measured the emission lifetimes from levels 2( 4) and 5(tG4). Both of thesearefound to be concentration dependent, suggesting that crossrelaxation processes occur in the decay of both of these levels. Energy released3H from ion 3F A during the transition from level 3 to 2 ( 4 4) with a rate X~3can be 3H 3F used to excite ion3HB from level 1 to 2 ( 6 4). Ions in level 3 ( 4) can also decay 3H to level 1 ( 6) with a rate r31. Rate r32 has been found to be small compared to r3i and X~3and has also been neglected. —+
—+
669
(t04),
Ions A and B are now both in level 2 (3F4) and can be pumped by M into level 5 There are several different non-radiative cross-relaxation processes possible from level 5. These processes are represented in our model by a single cross-relaxation process X15. The energy released from level 1G 3F 5 to 2 ( 4 3H 4) can 3H be used to excite ion 1GC from level 1 to 3 ( 6 4). Ions in level 5 ( 4) can 3H also decay to level 1 ( 6) with a rate r51. Rates r54, r53, and r52 are found to be small compared to both r51 and X15 and have therefore been neglected. Below are the rate equations for the above processes: —~
—~
dN1/dt
LN~+ r21N2 + r31N3 + r51N5
= —
dN2/dt
—
dN3/dt dN4/dt
= —
X13N1N3
X15N1N5,
MN2 r21N2 + 2X13N1N3 + X15N1N5 —
r43N4 r31N3 X13N1N3, LN, r43N4 + X~5N1N5, —
—
dN5/dt MN2 r51N5 X15N~N5, where N is the population, r is the decay rate, X is the cross-relaxation rate, M is excited state —
—
3~:Y Energy Levels in Tm Tm levels
Avalanche levels N5
2
2O3
N4
-
—
r
—
1
(cm-i)
(rim)
08
20902
478.4
06
i5602
663.9
04
~ ‘: : i: : N1 0 ~
A
B
~
3~ Y Fig. 1. Energy level diagram of Tm 203 (left) and energy levels used in modelling the up-conversion process (right). Two possible cross-relaxation roots are indicated.
0:
Ii
/ /
~
/
I Power (mW)
Fig. 2. 12 K power-dependent fluorescence intensity of level 5 (dotted line) compared with both the theoretical results from our model (solid line) and a quadratic least-squares fit to the data (dashed line). Fluctuations in the data are due to uncertainty in power measurements.
670
J. i.1. Dyson oU ~i1 Journal of Luminescence 60&6 I ( 1994, 668 671
.
0,002
:~:E ~ ~
0.001
0.30075
~
. .
:~~‘t~’
0 03
,
.
....
°::: 0.015 0.01
~
10
12
Fig. 3. Top: Theoretical transient solutions (Intensity versus time in milliseconds) of our model ai low power (left) and high power (right). Bottom: Transient behavior (intensity versus time in milliseconds) of fluorescence from level 5 at low power (left) and high power )right). The data have been normalized to the high power model population. Fluctuations are due to counting statistics
absorption and L is the pumping rate. L M is the ratio between ground state and excited state absorption cross-sections which we have found to be 0.11. Using experimentally determined decay rates and cross-relaxation rates, the equations have been solved numerically. Fig. 2 compares the experimental fluorescence intensity with both our solutions for the powerdependent population of level 5 and for a quadratic least-squares fit. The agreement suggests that for steady-state, the data fit a quadratic model as well as our model. We now focus on the transient data. Fig. 3 shows that the slow rise time observed in the fluorescence is predicted by our model, It also shows that there is a power-dependent change in the shape of the rise times that we also observe in our data. Simple
two-photon upconversion (quadratic model) does not reproduce this effect and therefore can be ruled out.
3. Conclusion We have observed blue emission when pumping at 653,7 nm. Both the power dependence and transient behavior of this emission have been compared with a rate-equation-based model that takes into account two experimentally observed cross-relaxation processes. The agreement between the experimental data and the model indicate that an avalanche-like process is responsible for the observed up-conversion. The sharp threshold behavior typically associated
f.M. Dyson ci a!.
Journal of Luminescence 60&61 (1994) 668 671
with an avalanche process is mitigated by the presence of a finite ground state absorption which has a cross-section 0’g 0.11 Cexc.
671
ported by DARPA contact # N00014-90J-4088 and by NSF grant DMR-9 117077.
=
Acknowledgements We are grateful to S.M. Jacobsen and B.M. Tissue for helpful discussions. This work was sup-
References [1] H. Ni and S.C. Rand. Opt. Lett. 16 (1991) 1424. [2] Ueli Oetliker, Mark J. Riley. P. Stanley May and Hans
GUdel, J. Lumin. 53(1992)553.