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Thermal red and blue shifts of Nd ion spectral lines in laser garnet crystals
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 60&6I
1994) 231) 232
Thermal red and blue shifts of Nd ion spectral lines in laser garnet crystals Xuesheng Chena*, John Collinsa, B. Di Barto1o’~ ~J)eparoiicii1
0/
P/iis~.s 00/ 1 sIrolio,,ii Ii /ieoio,i (//ci~c, \orlo/i. ill ()2’OA, Phvs0.. Bo.ooii ( O//egc, ( iiesi,iia Hi/I. 1/.1 ().~I6“. .5 4
5Deparliiiciir o/
.5)
Abstract Thermal red and blue shifts of a number of spectral lines related to transitions 4F; and ~1-~ ~ of Nd~ in the laser garnet crystals of Gd 3Sc,Ga~O 2 and CaY,Mg2Ge3O are reported. These line shifts are compared with 3 found by other workers. those in YAG: Nd
The most prominent rare earth ion in laser applications is trivalent neodymium (Nd3 ) which yields emission at the wavelengths of 0.9, 1.06 and 1.35 pm when doped in a variety of different hosts, It has been found that the host crystal with garnet
structure has many properties which are desirable in a laser host: it is chemically stable, mechanically hard and has good thermal and optical properties. We have measured the temperature dependence of the line positions of a number of spectral lines related to transitions 4F 41 32(R)-+ 92(Z)(0.9pm) 4F 3 + in the and 3.2(R) 2(Y)(l.06 pm) of laser crystals with garnet structure: Gd Nd 3Sc2Ga3Oi, (GSGG) and CaY2Mg2Ge3O12 (CYMGG) and compared with those in another garnet, YAG. found by other workers [I]. The samples doubly doped 3~(I%),used were Cr33(0.l°A) and GSGG:Nd CYMGG: Nd33 (2%). ~ (0.5%) garnet crystals in which Cr3 ~ ions act as sensitizers to enhance the
Nd3 emission [2]. All the spectral lines were measured in emission. The samples were excited by a 650W DVY tungsten halogen lamp with proper filters and lenses. The emission from the sample was chopped. focused by a lens onto the entrance slit of a I m McPherson model 2051 scanning monochromator, then detected by a dry-ice cooled RCA
7102 photomultiplier. amplified by a lock-in amplifier. and finally displayed on a chart recorder. The measurements of the line positions of Nd3
in GSGG temperatures were and madeCYMGG for R Y with and Rdifferent Z transitions. —÷
—~
-~
Typical experimental data are shown in Figs. I and 2. As the temperature increases most of the hnes shift towards the red (longer wavelength). hut the R Z~lines shift to the blue (shorter wavelength). The total thermal inblue shiftgarnets, from 78 GSGG to 300 Kand is about 2.2cm both CYMGG. that we examined. It is worth noting that in YAG: Nd3 most lines were found to have red shifts (e.g. R Y~.Z~ R) but the R Z 5 lines showed blue shifts also [I]. The R y lines in YAG presented small blue thermal shifts: our -~
—÷
*
Corresponding author,
0022-231394/507.00 u 1994 Elsevier Science B.V. All rights reserved SSDJ 0022-231 3)93)E0493-H
X. Chen et a!.
Journal of Luminescence 60&6I (1994) 230 232
1
• ••••••••
A
AA~
•
R~-*Y~ Rt~Yi
o
R
10679 -~ 9517~
2 9457~
••.•
~ 94331’
x,<<
~
A
2—~Y3 o
~,
•u.••
,J.
•
°o0
X,<
~
0
•
0
o
~
0
0
~
0
9004
0
0
R—*Y t 2 R —,Y 2 6 R~_*Y~
ooooo~88e
~ 9009’ I
I
100
200
Tablel 3’°in YAG, GSGG and CYMGG 78 blue to 300K Brief summary of the thermal from red and line shifts of Nd
Transitions
YAG [1]
GSGG
CYMGG
R2—*Y1
Red
Red
Red
R~—.Yi R2—oY3 R1 —o
Red Red
Red Red Red
A
~
90681
I
I
300
400
Temperature
°
~ 500
Red Small red Smallred Small blue Small blue
R1—oY3 I
I
600 700
(K)
Fig. I. Temperature 3~in GSGG. dependence of the positions of the sharp lines of Nd
Small blue Smallblue Small blue Small blue Red Red
R2 -. R1—oY6 R~—~ Z6 R2 —o ZZ1 R2 1 —. R1
Red
Small blue -~
effects of the ion—phonon interaction and is given by the following:
10638
When E1
S
10636
•
E~~>>hw~, 7 T \4 f’TOT
—
RI—oZ5/CYMGG
i3E1(cm_i)
I \T~JJ0
=
.
where
10634 cut o~
231
9558
R2-,Y1ICYMGG
150
(1)
200
250
300
5(
+
[~<~e~ei>~2
when E,
£
9556 100
dx,
4 3
£
50
e—l
3 (kT~’\ 4itpvc~h
£
9560
.~3 ~‘
—
~
<
350
or 1
—
<~ei~v~et>](la)
hwD, 7 T’\2
CT,
.~3
0:T
Temperaiure (K)
~5E1(cm_i)=
fl(s) ~ dx
Fig. 2. Temperature 3’°in dependence CYMGG. of R2 —. Y~and R~—~ Z0 line positions of Nd
results indicate that these lines in GSGG undergo very small red shifts. Table 1 summarizes briefly the thermal red and blue line shifts observed in the three garnet laser crystals, Nd3’1’ doped YAG, GSGG and CYMGG, for the temperature range 78-300 K. The thermal shift of a spectral line is the subtraction of the shifts of the two levels involved in the transition. From the existing theory [1,3], the shift of the ith energy level E is due to the stationary
~
><(T)2’
~
jO
e
—
(2)
where —
13j~ =
3w3 2~ ~
(2a)
In the above equations: TD is the Debye temperature of the crystal, p the mass density of the crystal, v the average velocity for sound waves in the crystal and c the speed of light. V~and ~ are ion-phonon coupling parameters. ~ is the wave
.1. (‘lien of oi/.
232
Journal of I.ionino’~ooHio’ 60&6 /
function of the ion in the ith energy level, = (E 1 E1),/k. w11 = (E1 E1),/Ii. and P denotes the principle value of the integral, The thermal shift of an energy level can also he caused by thermal expansion due to thermal changes of the lattice parameters. Johnson et al. [4] and Paetzold [5] have concluded from a set of experiments that the effect of thermal expansion 3 in is hard ionic crystals on the energy level of Nd negligible. comparing with the effect due to the ion—phonon coupling described in Eqs. (I) (2(. 3 and in the The large thermal red line shifts of Nd GSGG and CYMGG were fitted very well to Eq. (I) by treating ~ and 7~as adjustable parameters: 3’1 [I]. Thus, we this was also the case in YAG : Nd can conclude, as have by others, that these red line shifts are dominated by the Raman processes described by Eq. (1). The reason that the shift is to the red is usuallythe explained the following Considering ith levelinwith an energy manner. E 1 (e.g. levels the R. Y, or manifolds) which is v~ involved in the in transitions, theZ coupling coefficient in Eqs. (l)and (Ia) is believed to be negative because there —
—
+
are many more energy levels (E1’s) in manifolds above the ith level than energy levels in manifolds below toit.account thus the shiftsreddownward. In order for ith the level observed line shift, the
,~
1994 . 230 232
the initial level R with increasing temperature. Thus, the reasoning used above for the observed red line shifts cannot explain the observed blue line shifts of the R Z3 lines, In addition, the blue line shifts could not be fitted well to Eq. (1). So, it seems that the Eq. (I) by itself cannot explain the blue shifts of the R —o Z5 transitions and that there should be that some causes additional to us yet, the Zeffect, which is not clear 5 level to move downwards faster as temperature increases. Using Eq. (2). we tried the effect from one-phonon processes ontotheinclude Z 5 level due to the Z4, Z3. to see if we could explain the blue shift, but we found that the Z4, Z, red shift repelled Z5 Rup, Z which can only cause a larger of the 5 line. So that including the effect from one-phonon processes does not explain the observed blue line shift. Kushida observed3 a [I], blue and shifttentatively of the same R —o Z5 attributed lines in YAG: Nd it to a pushing effect of the nearby. higher-lying 3I manifolds ~l 13 2). manifolds However, closer in YAG : Nd the R levels(4[have2. higher in energy than the Z 5. so it is not clear why the R level would not get pushed down faster. It should also he pointed out that the R Y6 lines 3were [I], foundand to have blue shifts in YAG : Nd R 1 —o Y,,, R Y5 and R1 —o Y4 showed clear blue 3I shiftsWe overfound somethat temperature [4]. R —o Y ranges in LaF3 : Nd 10 has small red shifts in GSGG. It isred interesting to note that the blueforshifts and small shifts were observed only the transitions from the R levels to the upper levels within the Z and Y manifolds. ...
—~
-
—+
I
--o
downward must shift be of faster the initial (upper) levelterminal of the transition than that of the (lower) level with increasing temperature. If one 3’1 in a crystal examines thesum energy diagram Nd that the upper [3] and the in Eq. (Ia), theoffact level ( R~)of the transition is closer to the manifolds above it than the lower level (Y 1 or Z1( is to the manifolds above it could make the coupling coefficients (71’s) i’norc negative for the R1 levels than for the Y1 or Z1 levels. Hence, the upper level of the transition would shift faster with increasing tem— perature than the lower level, resulting in a red line shift. However, blue line 3shifts + doped were GSGG. found forCYMGG R —o Z5 transitions in the Nd and YAG (see Table I), apparently indicating that the terminal level Z 5 moves downwards faster than
References
II] ‘I Kushida. Phys [21 V.G. Ostroumov.
Rev ISS \u.S.
969) 5(5)
Privis. V.A. Smirnov ~nd
I’S
Shcherhakov, J. Opt. Soc. Amer. B 3)1986) 8).
[31 B. Di Bartolo. Optical Interactions in Solids (Wiles, Ness 1968) p. 341 (for the iheorv) and p. 472 (for the Nd” 1~1\ork. S A Johnson, H 6 Fr’cie, Al, Schawlow and W.M ‘sen. i energy diagram). Opt. Soc. Amer. 57 (1967) 734.
[5] H.K. Paetiold, Z. Physik, 139 (1951) 129.
LUMINESCENCE ELSEVIER
Journal of Luminescence 60&6I
1994) 231) 232
Thermal red and blue shifts of Nd ion spectral lines in laser garnet crystals Xuesheng Chena*, John Collinsa, B. Di Barto1o’~ ~J)eparoiicii1
0/
P/iis~.s 00/ 1 sIrolio,,ii Ii /ieoio,i (//ci~c, \orlo/i. ill ()2’OA, Phvs0.. Bo.ooii ( O//egc, ( iiesi,iia Hi/I. 1/.1 ().~I6“. .5 4
5Deparliiiciir o/
.5)
Abstract Thermal red and blue shifts of a number of spectral lines related to transitions 4F; and ~1-~ ~ of Nd~ in the laser garnet crystals of Gd 3Sc,Ga~O 2 and CaY,Mg2Ge3O are reported. These line shifts are compared with 3 found by other workers. those in YAG: Nd
The most prominent rare earth ion in laser applications is trivalent neodymium (Nd3 ) which yields emission at the wavelengths of 0.9, 1.06 and 1.35 pm when doped in a variety of different hosts, It has been found that the host crystal with garnet
structure has many properties which are desirable in a laser host: it is chemically stable, mechanically hard and has good thermal and optical properties. We have measured the temperature dependence of the line positions of a number of spectral lines related to transitions 4F 41 32(R)-+ 92(Z)(0.9pm) 4F 3 + in the and 3.2(R) 2(Y)(l.06 pm) of laser crystals with garnet structure: Gd Nd 3Sc2Ga3Oi, (GSGG) and CaY2Mg2Ge3O12 (CYMGG) and compared with those in another garnet, YAG. found by other workers [I]. The samples doubly doped 3~(I%),used were Cr33(0.l°A) and GSGG:Nd CYMGG: Nd33 (2%). ~ (0.5%) garnet crystals in which Cr3 ~ ions act as sensitizers to enhance the
Nd3 emission [2]. All the spectral lines were measured in emission. The samples were excited by a 650W DVY tungsten halogen lamp with proper filters and lenses. The emission from the sample was chopped. focused by a lens onto the entrance slit of a I m McPherson model 2051 scanning monochromator, then detected by a dry-ice cooled RCA
7102 photomultiplier. amplified by a lock-in amplifier. and finally displayed on a chart recorder. The measurements of the line positions of Nd3
in GSGG temperatures were and madeCYMGG for R Y with and Rdifferent Z transitions. —÷
—~
-~
Typical experimental data are shown in Figs. I and 2. As the temperature increases most of the hnes shift towards the red (longer wavelength). hut the R Z~lines shift to the blue (shorter wavelength). The total thermal inblue shiftgarnets, from 78 GSGG to 300 Kand is about 2.2cm both CYMGG. that we examined. It is worth noting that in YAG: Nd3 most lines were found to have red shifts (e.g. R Y~.Z~ R) but the R Z 5 lines showed blue shifts also [I]. The R y lines in YAG presented small blue thermal shifts: our -~
—÷
*
Corresponding author,
0022-231394/507.00 u 1994 Elsevier Science B.V. All rights reserved SSDJ 0022-231 3)93)E0493-H
X. Chen et a!.
Journal of Luminescence 60&6I (1994) 230 232
1
• ••••••••
A
AA~
•
R~-*Y~ Rt~Yi
o
R
10679 -~ 9517~
2 9457~
••.•
~ 94331’
x,<<
~
A
2—~Y3 o
~,
•u.••
,J.
•
°o0
X,<
~
0
•
0
o
~
0
0
~
0
9004
0
0
R—*Y t 2 R —,Y 2 6 R~_*Y~
ooooo~88e
~ 9009’ I
I
100
200
Tablel 3’°in YAG, GSGG and CYMGG 78 blue to 300K Brief summary of the thermal from red and line shifts of Nd
Transitions
YAG [1]
GSGG
CYMGG
R2—*Y1
Red
Red
Red
R~—.Yi R2—oY3 R1 —o
Red Red
Red Red Red
A
~
90681
I
I
300
400
Temperature
°
~ 500
Red Small red Smallred Small blue Small blue
R1—oY3 I
I
600 700
(K)
Fig. I. Temperature 3~in GSGG. dependence of the positions of the sharp lines of Nd
Small blue Smallblue Small blue Small blue Red Red
R2 -. R1—oY6 R~—~ Z6 R2 —o ZZ1 R2 1 —. R1
Red
Small blue -~
effects of the ion—phonon interaction and is given by the following:
10638
When E1
S
10636
•
E~~>>hw~, 7 T \4 f’TOT
—
RI—oZ5/CYMGG
i3E1(cm_i)
I \T~JJ0
=
.
where
10634 cut o~
231
9558
R2-,Y1ICYMGG
150
(1)
200
250
300
5(
+
[~<~e~ei>~2
when E,
£
9556 100
dx,
4 3
£
50
e—l
3 (kT~’\ 4itpvc~h
£
9560
.~3 ~‘
—
~
<
350
or 1
—
<~ei~v~et>](la)
hwD, 7 T’\2
CT,
.~3
0:T
Temperaiure (K)
~5E1(cm_i)=
fl(s) ~ dx
Fig. 2. Temperature 3’°in dependence CYMGG. of R2 —. Y~and R~—~ Z0 line positions of Nd
results indicate that these lines in GSGG undergo very small red shifts. Table 1 summarizes briefly the thermal red and blue line shifts observed in the three garnet laser crystals, Nd3’1’ doped YAG, GSGG and CYMGG, for the temperature range 78-300 K. The thermal shift of a spectral line is the subtraction of the shifts of the two levels involved in the transition. From the existing theory [1,3], the shift of the ith energy level E is due to the stationary
~
><(T)2’
~
jO
e
—
(2)
where —
13j~ =
3w3 2~ ~
(2a)
In the above equations: TD is the Debye temperature of the crystal, p the mass density of the crystal, v the average velocity for sound waves in the crystal and c the speed of light. V~and ~ are ion-phonon coupling parameters. ~ is the wave
.1. (‘lien of oi/.
232
Journal of I.ionino’~ooHio’ 60&6 /
function of the ion in the ith energy level, = (E 1 E1),/k. w11 = (E1 E1),/Ii. and P denotes the principle value of the integral, The thermal shift of an energy level can also he caused by thermal expansion due to thermal changes of the lattice parameters. Johnson et al. [4] and Paetzold [5] have concluded from a set of experiments that the effect of thermal expansion 3 in is hard ionic crystals on the energy level of Nd negligible. comparing with the effect due to the ion—phonon coupling described in Eqs. (I) (2(. 3 and in the The large thermal red line shifts of Nd GSGG and CYMGG were fitted very well to Eq. (I) by treating ~ and 7~as adjustable parameters: 3’1 [I]. Thus, we this was also the case in YAG : Nd can conclude, as have by others, that these red line shifts are dominated by the Raman processes described by Eq. (1). The reason that the shift is to the red is usuallythe explained the following Considering ith levelinwith an energy manner. E 1 (e.g. levels the R. Y, or manifolds) which is v~ involved in the in transitions, theZ coupling coefficient in Eqs. (l)and (Ia) is believed to be negative because there —
—
+
are many more energy levels (E1’s) in manifolds above the ith level than energy levels in manifolds below toit.account thus the shiftsreddownward. In order for ith the level observed line shift, the
,~
1994 . 230 232
the initial level R with increasing temperature. Thus, the reasoning used above for the observed red line shifts cannot explain the observed blue line shifts of the R Z3 lines, In addition, the blue line shifts could not be fitted well to Eq. (1). So, it seems that the Eq. (I) by itself cannot explain the blue shifts of the R —o Z5 transitions and that there should be that some causes additional to us yet, the Zeffect, which is not clear 5 level to move downwards faster as temperature increases. Using Eq. (2). we tried the effect from one-phonon processes ontotheinclude Z 5 level due to the Z4, Z3. to see if we could explain the blue shift, but we found that the Z4, Z, red shift repelled Z5 Rup, Z which can only cause a larger of the 5 line. So that including the effect from one-phonon processes does not explain the observed blue line shift. Kushida observed3 a [I], blue and shifttentatively of the same R —o Z5 attributed lines in YAG: Nd it to a pushing effect of the nearby. higher-lying 3I manifolds ~l 13 2). manifolds However, closer in YAG : Nd the R levels(4[have2. higher in energy than the Z 5. so it is not clear why the R level would not get pushed down faster. It should also he pointed out that the R Y6 lines 3were [I], foundand to have blue shifts in YAG : Nd R 1 —o Y,,, R Y5 and R1 —o Y4 showed clear blue 3I shiftsWe overfound somethat temperature [4]. R —o Y ranges in LaF3 : Nd 10 has small red shifts in GSGG. It isred interesting to note that the blueforshifts and small shifts were observed only the transitions from the R levels to the upper levels within the Z and Y manifolds. ...
—~
-
—+
I
--o
downward must shift be of faster the initial (upper) levelterminal of the transition than that of the (lower) level with increasing temperature. If one 3’1 in a crystal examines thesum energy diagram Nd that the upper [3] and the in Eq. (Ia), theoffact level ( R~)of the transition is closer to the manifolds above it than the lower level (Y 1 or Z1( is to the manifolds above it could make the coupling coefficients (71’s) i’norc negative for the R1 levels than for the Y1 or Z1 levels. Hence, the upper level of the transition would shift faster with increasing tem— perature than the lower level, resulting in a red line shift. However, blue line 3shifts + doped were GSGG. found forCYMGG R —o Z5 transitions in the Nd and YAG (see Table I), apparently indicating that the terminal level Z 5 moves downwards faster than
References
II] ‘I Kushida. Phys [21 V.G. Ostroumov.
Rev ISS \u.S.
969) 5(5)
Privis. V.A. Smirnov ~nd
I’S
Shcherhakov, J. Opt. Soc. Amer. B 3)1986) 8).
[31 B. Di Bartolo. Optical Interactions in Solids (Wiles, Ness 1968) p. 341 (for the iheorv) and p. 472 (for the Nd” 1~1\ork. S A Johnson, H 6 Fr’cie, Al, Schawlow and W.M ‘sen. i energy diagram). Opt. Soc. Amer. 57 (1967) 734.
[5] H.K. Paetiold, Z. Physik, 139 (1951) 129.