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Cooperative optical transitions in CaF2:Yb3+
Journal of Luminescence 31 & 32(1984)257-259 North-Holland, Amsterdam
257
3~
COOPERATIVE OPTICAL TRANSITIONS IN CaF
2:Yb
N.Y. ZHANG, R.T. BRUNDAGE and W.M. YEN
Department of Physics, University of Wisconsin, Madison, WI 53706
We have studied cooperative absorption and upconversion processes involving Yb3+ ions in CaF 2:Yb using tunable laser spectroscopy. 3+ enters substitutional ly for Ca2+ in CaF Yb
2, the charge compensation being
achieved by the occurrence of interstitial
For heavily doped 3~ crystals the fluorine interstitial ions tend to cluster and mostfor of the Yb ions occupy sites in these clusters.1 The energy level diagram Yb3~ ions in cubic and cluster sites in CaF Stark levels of the cubic 2F
3+
fluorine ions.
is shown
in Fig. la.
2:Yb
0’ and 0” are the
2F
512 state, is the levels 512 level of the cluster 1 represent higherC’ vibronic of this cluster state. 512 state,transitions and V’. V’ are observed to occur to al I these levels. Luminescence, Absorption 2F
however, occurs only from the cluster sites at 4.2K. By tuning a N 2 laser pumped narrow band dye laser through the region 3~spectral luminescence from 20,000 cm~ to 22,000 cm~ the characteristic infrared Yb was observed. The excitation spectrum is shown in Fig. lb. The peaks in this spectrum correspond closely to the sums of the energies of pairs of excited Yb3+ ions in the combinations shown.
For example, the peak marked C’C’ occurs at
20407 cm~ which is very close to the energy of a pair of Yb3’ C’
level.
ions, each in the
We are observing cooperative absorption by pairs of Yb3+ ions.
Fig. in Energy(cm~) FYb
Cooperative t
0’ of,, 10846
0
I
__
10205
~
20300
0022—2313/84/$03.OO© Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
21300
cm
H. Y Zhang r’t al.
258
/ Cooperatirc
3+
optical transitions in (~I-~ : Yh
Having observed the cooperative absorption we looked for the analogous cooperative pair luminescence transition2 and upconversion processes in this material.
A high power pulsed color center laser was used to excite the sample
at a single ion absorption frequency, and a weak luminescence in the visible was observed in addition to the strong single ion infrared luminescence, as shown Fig. 2.
in
The emission at 480 nm is a candidate for cooperative luminescence,
because its frequency corresponds approximately to twice the frequency of the single ion (cubic sites)
transition.
However, no luminescence was observed from
single ions in cubic sites at 4.2K.
410
~
~ 78~ 680 480
550
soonis Fig. 2
_
_
700nrn
000nrn
Luminescence spectrum of CaF 2:Yb, dotted
temperature.
4.2K intensity scale
is
X50
line
—
4.2K
~ol Id
line
—
room
roan temperature scale in visible
reg ion. We consider the possibil ity that 3~and the visible luminescence at 4.2K is due to Er3+ ions which occur as trace upconversion involving impurities in processes our sample, and we Tm assume that these enter as triply charged ions into the cluster regions of the crystals. at 480 nm can be assigned to the 1G
The luminescence transitions observed
3H 4—
3~,and the 1G 6 transition of Tm
4
level
H. Y Zirang et a!.
/ Cooperative optical transitions in
3+
CaP
259
2 Yb
can be excited by the transfer of three successive Yb3+ excitations as shown Fig. 2b.3
in
The intensity of the 480 nm luminescence is found to vary as the
intensity of the laser beam to the power of 2.7, close to 3, as expected for a 3+ Tm is known to emit
three excitation process as ii lustrated in Fig. 2. luminescence fran the 1G 1G 1
4—
1G 4 level at 650 nm
3H
(
3H 4—
4) and more weakly at 780 nm
5), and such transitions are observed here.
Further, the decay
characteristics of the 480 nm and 3+ 650ions nm excited lines are We conclude by identical. transfer fran the Yb3+ that ions. these transitions originate on Tm The emissions at 410 nm and 550 nm are similarly assigned to Er3+ ions excited by energy transfer from Yb3+.
The dependences on laser power for these
two transitions are close to the cube and the square, respectively, and the possible upconversion processes are shown
in Fig. 2.
At room temperature the 480 nm luminescence is more intense than at 4.2K, and its intensity varies as the square of the laser power.
Its decay time at this
temperature is 700 5R
=
luminescence
0s, close to half of the lifetime of the infrared 1500 0s, and is consistent with a two ion cooperative process.
At this temperature the 0’ and V’
levels become populated, and the sum of
these energies corresponds 478 can nm. transfer Consequently, we have the possibi I ity 3+toions their energies to Tm3+ according that a pair of excited Yb to the scheme Yb3~(V’)+Yb3~(O’)+Tm3~(3H
3~(2F 6)Yb
3~(2F 7/2) + Yb
3~(1G 712) + Tm
4)
Alternatively, and since we note that the O’V’ combination gives rise to strong 3~ ions in 0’ cooperative absorption (Fig. ib),luminescence we have the which possibil ity be that Yb and V’ are emitting cooperative cannot distinquished from the Tm3~ (1G
3H 4—
6) emission.
Both processes will vary as the square of the
laser power, as observed. ACKNOWLEDGEMENT The authors acknowledge helpful discussions with G.F. lmbusch and assistance from Chris Levey.
CaF2:Yb crystals suppi led by W. Hargraves of Optovac.
This
research was supported by the National Science Foundation under grant DMR—8i—i 6733. REFERENCES 1) Yu K Voronko, V.V. Osiko, and I. A. Shcherbakov, Soy. Phys. JEW 29, 86 (1969). 2) E. Nakazawa and S. Shionoya, Phys. Rev. Letters 25, 1710 (1970). 3) F.
Auzel, Compt. Rend.
263B, 819 (1966).
257
3~
COOPERATIVE OPTICAL TRANSITIONS IN CaF
2:Yb
N.Y. ZHANG, R.T. BRUNDAGE and W.M. YEN
Department of Physics, University of Wisconsin, Madison, WI 53706
We have studied cooperative absorption and upconversion processes involving Yb3+ ions in CaF 2:Yb using tunable laser spectroscopy. 3+ enters substitutional ly for Ca2+ in CaF Yb
2, the charge compensation being
achieved by the occurrence of interstitial
For heavily doped 3~ crystals the fluorine interstitial ions tend to cluster and mostfor of the Yb ions occupy sites in these clusters.1 The energy level diagram Yb3~ ions in cubic and cluster sites in CaF Stark levels of the cubic 2F
3+
fluorine ions.
is shown
in Fig. la.
2:Yb
0’ and 0” are the
2F
512 state, is the levels 512 level of the cluster 1 represent higherC’ vibronic of this cluster state. 512 state,transitions and V’. V’ are observed to occur to al I these levels. Luminescence, Absorption 2F
however, occurs only from the cluster sites at 4.2K. By tuning a N 2 laser pumped narrow band dye laser through the region 3~spectral luminescence from 20,000 cm~ to 22,000 cm~ the characteristic infrared Yb was observed. The excitation spectrum is shown in Fig. lb. The peaks in this spectrum correspond closely to the sums of the energies of pairs of excited Yb3+ ions in the combinations shown.
For example, the peak marked C’C’ occurs at
20407 cm~ which is very close to the energy of a pair of Yb3’ C’
level.
ions, each in the
We are observing cooperative absorption by pairs of Yb3+ ions.
Fig. in Energy(cm~) FYb
Cooperative t
0’ of,, 10846
0
I
__
10205
~
20300
0022—2313/84/$03.OO© Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
21300
cm
H. Y Zhang r’t al.
258
/ Cooperatirc
3+
optical transitions in (~I-~ : Yh
Having observed the cooperative absorption we looked for the analogous cooperative pair luminescence transition2 and upconversion processes in this material.
A high power pulsed color center laser was used to excite the sample
at a single ion absorption frequency, and a weak luminescence in the visible was observed in addition to the strong single ion infrared luminescence, as shown Fig. 2.
in
The emission at 480 nm is a candidate for cooperative luminescence,
because its frequency corresponds approximately to twice the frequency of the single ion (cubic sites)
transition.
However, no luminescence was observed from
single ions in cubic sites at 4.2K.
410
~
~ 78~ 680 480
550
soonis Fig. 2
_
_
700nrn
000nrn
Luminescence spectrum of CaF 2:Yb, dotted
temperature.
4.2K intensity scale
is
X50
line
—
4.2K
~ol Id
line
—
room
roan temperature scale in visible
reg ion. We consider the possibil ity that 3~and the visible luminescence at 4.2K is due to Er3+ ions which occur as trace upconversion involving impurities in processes our sample, and we Tm assume that these enter as triply charged ions into the cluster regions of the crystals. at 480 nm can be assigned to the 1G
The luminescence transitions observed
3H 4—
3~,and the 1G 6 transition of Tm
4
level
H. Y Zirang et a!.
/ Cooperative optical transitions in
3+
CaP
259
2 Yb
can be excited by the transfer of three successive Yb3+ excitations as shown Fig. 2b.3
in
The intensity of the 480 nm luminescence is found to vary as the
intensity of the laser beam to the power of 2.7, close to 3, as expected for a 3+ Tm is known to emit
three excitation process as ii lustrated in Fig. 2. luminescence fran the 1G 1G 1
4—
1G 4 level at 650 nm
3H
(
3H 4—
4) and more weakly at 780 nm
5), and such transitions are observed here.
Further, the decay
characteristics of the 480 nm and 3+ 650ions nm excited lines are We conclude by identical. transfer fran the Yb3+ that ions. these transitions originate on Tm The emissions at 410 nm and 550 nm are similarly assigned to Er3+ ions excited by energy transfer from Yb3+.
The dependences on laser power for these
two transitions are close to the cube and the square, respectively, and the possible upconversion processes are shown
in Fig. 2.
At room temperature the 480 nm luminescence is more intense than at 4.2K, and its intensity varies as the square of the laser power.
Its decay time at this
temperature is 700 5R
=
luminescence
0s, close to half of the lifetime of the infrared 1500 0s, and is consistent with a two ion cooperative process.
At this temperature the 0’ and V’
levels become populated, and the sum of
these energies corresponds 478 can nm. transfer Consequently, we have the possibi I ity 3+toions their energies to Tm3+ according that a pair of excited Yb to the scheme Yb3~(V’)+Yb3~(O’)+Tm3~(3H
3~(2F 6)Yb
3~(2F 7/2) + Yb
3~(1G 712) + Tm
4)
Alternatively, and since we note that the O’V’ combination gives rise to strong 3~ ions in 0’ cooperative absorption (Fig. ib),luminescence we have the which possibil ity be that Yb and V’ are emitting cooperative cannot distinquished from the Tm3~ (1G
3H 4—
6) emission.
Both processes will vary as the square of the
laser power, as observed. ACKNOWLEDGEMENT The authors acknowledge helpful discussions with G.F. lmbusch and assistance from Chris Levey.
CaF2:Yb crystals suppi led by W. Hargraves of Optovac.
This
research was supported by the National Science Foundation under grant DMR—8i—i 6733. REFERENCES 1) Yu K Voronko, V.V. Osiko, and I. A. Shcherbakov, Soy. Phys. JEW 29, 86 (1969). 2) E. Nakazawa and S. Shionoya, Phys. Rev. Letters 25, 1710 (1970). 3) F.
Auzel, Compt. Rend.
263B, 819 (1966).