Spectral and luminescence properties of Cr3+ and Nd3+ ionsin gallium garnet crystals
A. L. Denisov, V. G. Ostroumov, Z. S. Saidov, V. A. Smirnov, and I. A. Shcherbakov
Institute of General Physics, Academy of Sciences of the USSR, Moscow, USSR
Received July 1, 1985; accepted September 30, 1985
The effective peak stimulated-emission cross section of chromium-doped gadolinium-scandium-gallium garnets(GSGG) has been determined to be 8.5 X 1 0 -2i cm2 at room temperature. The values of the energy-gap AE( 2E -4T2) chromium fluorescence lifetime and the chromium - neodymium energy-transfer parameter CDA (Cr-Nd) aredetermined for several gallium garnets. Temperature-dependent absorption and luminescence spectra of neodym-ium-doped GGG and GSGG are reported and discussed in the context of their use as laser materials.
References 1-6 report the success of sensitizing Nd 3+ lumi-nescence by chromium in doubly doped gallium garnet crys-tals and the rapid development of highly efficient neodymi-um lasers on the basis of gadolinium-scandium-gallium-garnet crystals doped by chromium and neodymium(GSGG:Cr,Nd). Because of a considerable increase in thespectral overlap of the absorption bands in the doubly dopedcrystal with the emission spectrum of the flash lamp and theefficient energy transfer from broad Cr3+ absorption bandsto the upper level of Nd3 + ions, efficiency was considerablyimproved in comparison with YAG:Nd lasers. The sensitiz-ing mechanisms and the possibilities of further improve-ment in neodymium lasers because of the energy transferfrom Cr - Nd were discussed in Refs. 4 and 7.
The development of near-infrared tunable lasers, basedon chromium-doped gallium garnet crystals operating on theelectronic-vibronic transition 4T2 -
4A2 at room tempera-ture, was reported in Refs. 7-13. The present paper reportsan investigation of some spectral and luminescence proper-ties of gallium garnets doped by Cr3+ and Nd3+ ions, whichare important for the development of multipurpose lasers.
One of the most important characteristics of the lasermedium is the emission cross section of laser transition .that determines the initial gain coefficient, threshold, etc.Because of the possible wide practical use of GSGG:Cr 3 +crystals as the active medium for tunable solid-state lasers,it is important to measure the emission cross section of theelectronic-vibronic transition 4T 2 -
4 A2 , which is responsi-ble for broadband luminescence in the region 700-900 nm.2
The effective emission cross section eff. of the transitionfrom the excited states, 4T2 and 2E, of Cr3+ in thermal equi-librium is
n1 (n2) is the population of the 2E(4T 2) level, g is the degen-eracy of the particular level, and AE is the energy gap be-tween the 4T2 and 2 E levels.
Similarly to Eq. (1) the effective decay rate of Cr3+ is givenby
Aeff
AE +-ATni
(3)
1+ +ni
where AE and AT are decay rates of the 2E and 4T2 levels,respectively. The well-known equation for the transitioncross section is the following:
a(V) = Ac g(v),81Inf2V2Apeff
(4)
where c is the velocity of light in vacuum, n is the mediumrefractive index, A veff. is the effective width of the emissionline (in inverse centimeters) defined as a ratio of the areaunder the luminescence-spectrum curve (in intensity perunit frequency interval) to the intensity at the maximum ofthe luminescence band, and g(v) is the normalized lineshape.
Note that Eq. (4) does not put any restrictions on the lineshape of the luminescence spectrum. Substituting Eq. (4)into Eq. (1), we obtain:
YGG 600 (Ref. 22) - 250GGG 370 1.1 X 10-38 160YSGG 320 1.4 X 10-38 145GSGG '-10 2.2 X 0-38 120
excited states 4T2 and 2E of Cr3+ ions. For GSGG:Cr3+ Aeff.= 8300 sec'1 at 300K.7 The effective width AVeff.2, evaluatedfrom the 4T2 -
4A2 emission spectrum, was found to be near2000 cm-'.
The effective emission cross section ffL versus wave-length X for the electronic-vibronic transition 2E, 4T2 - 4A2of Cr3 + in GSGG crystal is plotted in Fig. 1. The maximumvalue is near 0.85 X 10-20 cm2, which is close to the valueobtained from laser experiments."1
These results show that the value of oeff. in GSGG:Cr3+ ishigher than the corresponding value in alexandrite crystal at300 K (0.7 X 10-20 cm2).'4 It should be noted that, accordingto our estimations, based on data taken from Ref. 14, thevalue of Ueff. is really less than the value given in this paper.
It is noteworthy that, although the Cr3+ site symmetry inalexandrite is lower than in GSGG and the oscillatorstrength of the 4T2 - 4A2 transition is higher, the populationvalue of the 4T 2 excited level is lower because of the largerenergy gap AE, which leads to a smaller value of ef incomparison with GSGG:Cr3 + crystals. Thus the relativelysmall values of oscillator strengths arising from high sitesymmetry of Cr3+ ions in GSGG crystal are compensated forby a smaller energy gap AE, leading to a higher-population4T2 excited level. Note that increasing eff. by decreasingAE appears to be preferable to increasing 0eff. by use of alower site symmetry. The latter is usually achieved by useof anisotropic host crystals, which are less preferable thanisotropic crystals for laser operation.
The value of energy gap AE( 2E - 4T 2) in the gallium-
garnet crystals studied was determined from the tempera-ture dependence of TCr Aeff'-l. In the case of the gadolini-um gallium garnet (GGG) crystal doped by chromium and
neodymium, this value was also determined from the tem-perature dependence of CDA(Cr-Nd):
CDAE + CDAT X eAE/k
CDA(Cr-Nd) =
where CDA(Cr-Nd) is the microparameter of the Cr3+-Nd3+interaction, which denotes the probability of the interactionof these ions at the distance 1 cm, and CDAE and CDAT are themicroparameters of the energy transfer from the chromiumexcited levels 2E and 4T2 to the neodymium ions.
The values of AE, CDA(Cr-Nd), and TCr for gallium gar-nets at room temperature are given in Table 1.
The fluorescence spectra of Cr3+ in gallium garnet crystalsare shown in Fig. 2. The decreasing crystal field strengthcauses a fluorescence red shift as well as a decrease in the Rlines relative intensity and TCr.
I rel. units
I000 900 I00 700 , fl1
Fig. 2. The fluorescence spectra of Cr3+ in gallium garnet crystalsat room temperature.
Denisov et al.
Vol. 3, No. 1/January 1986/J. Opt. Soc. Am. B 97
I rel. units
750 720 690z 900 800 700 Zn
Fig. 3. Dependence of the relative intensity of R-line and broad-band 4T 2 -
4A2 emission for different chromium-ion concentrationsin GGG (a, 2 X 1019 cm-3 ; b, 2.4 X 1020 cm-3 ; c, 8 X 1020 cm-3 ) andGSGG (d, 6 X 1018 cm-3 ; e, 6 X 1019 cm-3 ; f, 3 X 1020 cm- 3 ) crystals at77K.
It should be noted that, in both GGG:Cr3 + andGSGG:Cr3 + crystals, the relative intensities of the R linesand the broadband 4 T 2 -
4A2 emission depend on the con-centration of the chromium ions, as can be seen in Fig. 3.This phenomenon can be explained, for example, by theformation of exchange-coupled Cr3+ pairs in these crystals.The optical spectrum resulting from the exchange-coupledCr3 + pairs together with their phonon side bands is super-posed upon the spectrum of the single Cr3 + centers, whichleads to a decrease in the relative intensity of the R lines ofthe single centers.
The absorption spectra of the GGG:Nd3 + andGSGG:Nd3 + crystals are shown in Fig. 4. The neodymium-ion concentration was 2 X 1020 cm- 3 for GGG crystal and 3 X1020 cm- 3 for GSGG. For the curves marked by numbers,the absorption-coefficient linear dependence on the activa-tor concentration was checked (see Fig. 5).
The fluorescence spectra of Nd3 + in GGG and GSGGcrystals at room temperature are shown in Fig. 6. It shouldbe noted that incorporating chromium ions in these crystalsup to concentrations of 6 X 1020 cm- 3 does not change theneodymium fluorescence line shape.
For the practical use of the GSGG:Cr,Nd crystal, e.g., in amaster-oscillator-power-amplifier scheme, it is sometimesnecessary to know the temperature dependence of the fluo-rescence properties of this crystal. Figure 7 represents thetemperature dependence of line position and the FWHM ofthe fluorescence line on which lasing was achieved. Usingthese data, as was done in Ref. 15, one can get the tempera-ture dependence of the emission cross section of the lasingtransition, resonant losses, gain coefficient, and gain coeffi-cient of the amplifier versus the temperature difference be-
Absorption %I00
T=3001(
80.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~2
20 'I
0. . ._-OuW
100 -
80-
60w.
--40-
20-
0;-0-500
U 600 700 750 800.AI. -.. 1 .. .-._.. ... .......
550 600 700 750
850 Ain
800 850 Annm.
Fig. 4. Nd3 + absorption spectra in GGG and GSGG crystals atroom temperature. Neodymium-ion concentration and samplelength are, respectively, 2 X 1020 cm-3 and 1.27 mm for GGG crystaland 3 X 1020 cm-3 and 0.73 mm for GSGG crystal.
~b ( cm't) t .abs . ( )
GSGG Nd3+ 1 0 GOG 5d+
10 ~~~~~~~~~10
0 2 4 6 nd(lo0crn)o 2 4 nlNd(1020cm-3)
Fig. 5. Dependence of the absorption coefficient Kab.. from theNd3+ concentration for the lines marked in Fig. 4.
T=300K
GSGG: Nd3+
, B 5 (, ~~~~~~~~.. _ .,4
H [ItiV rug S s 0 .
Denisov et al.
7
98 J. Opt. Soc. Am. B/Vol. 3, No. 1/January 1986
Irel. units
I 2 0 T IN i cn ce s a n d G cr t a r te m p era .m
Fig. 6. The fluorescence spectra of Nd3+ in GGG and GSGG crystals at room temperature.
tween the active medium of the oscillator and the amplifier,based on the GSGG:Cr, Nd crystal.
The energy-level scheme for Nd3+ in GSGG crystals withdifferent contents of scandium and gallium ions in octahe-dral sites is given in Refs. 16 and 17. The absorption and
fluorescence spectra of the crystals studied in this paper areshown in Fig. 8; the energy scheme of the 4I9/2, 4I11/2, 4F3/2
levels of the neodymium ions in the same crystal is shown inFig. 9. Comparison of these results with the data of Refs. 16and 17 shows that varying the content of scandium and
U,
U,4
0d
Denisov et al.
Vol. 3, No. 1/January 1986/J. Opt. Soc. Am. B 99
'A ,(A)10620 -
10610
10600 ,
10590
10580 1
100 200 300 400
Temperature (K)
Fig. 7. Temperature dependence of the position(A) of the line for which lasing was obtained.
I
2 ) after excitation of the sample by short (15-nsec) laserpulses. At low concentrations of Nd3+, its excited-state
22 decay was exponential with the time constant ro = 270 ,usec,which is in a good agreement with the value given in Ref. 16.
18 When the neodymium-ion concentration is increased, theluminescence lifetime is shortened, and the exponential de-
gallium ions in the octahedral sites does not affect the posi-tion of Stark sublevels of the examined states of neodymiumions.
In order to study the processes of the Nd3 +-Nd3 + interac-tion in GSGG crystal, the luminescence decay was studied
The energy-level diagram for Nd3+ ion in GSGG crystal at
Irel. units
0 2 4 6 8
Time, q secFig. 10. Nd3
+ luminescence-decay curves in GSGG crystals at 300K. Neodymium-ion concentration is (1) 4.7 X 1020 cm- 3 , (2) 8.8 X1020 cm-3 , and (3) 11 X 1020 cm-3 .
Nd3
: Gd3 (Sc.,Ga) 2 Ga 30 1 2
4P3/2 11493
11432
2430
2390
4 11/2 21072068
2009
1984
759
260
49/2 170
106
0
Denisov et al.
100 J. Opt. Soc. Am. B/Vol. 3, No. 1/January 1986
cay is distinctly displayed only at the final stages of theprocess:
[T(Nd)]-' = W + [TO(Nd)]-l.
In the quenching region of Nd3+ luminescence (up to theneodymium-ion concentration 5 X 1020 cm'3), the decaycurves of the 4F3/2 excited state may be approximated by thefollowing expression:
tI(t) - exp(- - Y- -t), (7)To
where 'y is the direct Nd3+-Nd3+ static dipole-dipole inter-action macroparameterl8"19 : y = 4/37r3/
2nNd CDA(Nd-
Nd)"1/2, nNd is the neodymium-ion concentration, andCDA(Nd-Nd) is the microparameter of the Nd3+-Nd 3 + di-pole-dipole interaction equal to 1.3 X 10-40 cm6 sec-. 20
The accuracy of the decay-curve approximation for theexcited state 4 F3/2 given by Eq. (7) is quite sufficient for thecalculation of the optimal concentrations required to obtainthe maximum population in the upper Nd3+ lasing level.Nevertheless, detailed analysis of the nonradiative energytransfer Nd3+-Nd 3 + has shown that, at the very beginning ofthe 4 F3/2 excited-state decay, a fast component of lumines-cence decay takes place. This phenomenon cannot be de-scribed by the dipole-dipole approximation and should beascribed to more-short-range mechanisms of the Nd3+-Nd3+quenching interaction. This phenomenon plays an increas-ingly important role with rising neodymium-ion concentra-tion and becomes quite significant at a Nd3+ concentrationgreater than 5 X 1020 cm3 (Fig. 10).
Thus, from the point of view of the luminescence concen-tration quenching, the behavior of the neodymium ions inthe crystals with garnet structure differs from that in thecrystalline and glassy phosphates and fluorides.21 It shouldbe noted that, taking into account the effects of the short-range energy transfer, the Nd-Nd luminescence decay is ahighly complicated problem and is a subject for future inves-tigations.
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1-
Cd
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Z. S. Saidov
A. L. Denisov was born in Moscow,USSR, on April 26, 1960. He receivedthe diploma degree in material engineer-ing from Moscow Chemical-TechnologyInstitute in 1983. Since 1983 he hasbeen staff engineer at Institute of Gener-al Physics, Academy of Sciences of theUSSR, Moscow. His main interest is inthe crystal growth of garnets doped byrare-earth ions.
Z. S. Saidov was born in Makhachkala,USSR, on January 21, 1960. He re-ceived the diploma degree in physicsfrom Moscow Physical-Technical Insti-tute in 1983. Currently he is at the In-stitute of General Physics, Moscow,working toward the candidate (Ph.D.)degree. He is engaged in the research ofenergy transfer from transition-metalions to rare-earth ions in garnet crystals.
