Volume 82, number 5,6
Photoluminescence H. Lundt
1 May 1991
properties of Cr : Er : GGG
and H. Weidner
Wacker - Chemitronic Research Center, P.O. Box 1140, W-8263 Burghausen. Germany
Received 1 I December 1990; revised manuscript received 25 January I99 I
An analysis of the photoluminescence properties of Cr: Er: GGG indicates that the erbium emission at 1.6 pm and 2.8 pm can be efficiently sensitized by a chromium co-doping. Increasing the erbium concentration enhances the 2.8 pm emission, whiIe the 1.6 pm emission decreases. This effect is due to a cooperative upconversion of two erbium ions in the ‘I,s,r state. The lifetime of the 2.8 pm emission in Cr: Er: GGG is 1.3 ms which is about an order of magnitude longer than in Er:YAG.
1. Introduction Medical applications stimulate the development of efficient solid state lasers with a wavelength near 3 lrn. In laser systems of this kind Er:YAG and Er : YA103 laser crystals are often used as an active medium. These materials have the disadvantage of a high threshold energy and a high thermal load [ 11. The available pulse repetition rate and average output power are limited by these features. Thus erbium doped laser crystals with a higher efficiency are necessary for an improved laser operation. For neodymium lasers it has recently been demonstrated [2,4] that a co-doping with chromium significantly increased the laser efficiency due to an improved absorption of the flashlamp light. Such a sensitization with chromium was successful in gadolinium scandium gallium garnet (GSGG) [ 2,3 ] and gadolinium gallium garnet (GGG) [ 3,4] but unfortunately not in yttrium aluminum garnet (YAG). In YAG a sensitization failed because the transfer time from Cr3+ to Nd3+ is longer than the neodymium fluorescence decay time [ 5 1. In this paper we report on spectroscopic studies of Cr : Er : GGG in order to clarify how efficient the Cr3+ to Er3+ energy transfer is in this material. While a co-doping with Cr and Er has been successfully demonstrated in YSGG [ 6,7 ] GGG offers an economic advantage because the production of this laser crystal does not require the rare and expensive scandium. Another reason for chasing GGG as host material 484
for erbium instead of YAG is the higher quantum efficiency of the emission near 3 urn and the longer lifetime in the upper laser level. In Er : YAG this lifetime is only 0.1 ms [ 8 1, which can be explained by losses due to multiphonon nonradiative relaxations. Losses of this kind are expected to be lower in gallium garnets where the probability of such relaxations is lower [ 81. These features promise a lower threshold energy for a laser operation at this wavelength.
2. Crystal growth Erbium doped GGG crystals can be grown with a high erbium concentration up to a total substitution of gadolinium [ 91. While the distribution coefficient for erbium in GGG is 1.0 it is 3.0 for chromium in Cr:GGG. If the crystal is doped with erbium the chromium distribution coefficient increases and reaches a value of 3.5 for an erbium concentration of 50 at.%. A near-unity distribution coefficient of chromium was found in Cr: Er: YSGG [ 71. Cr: Er: GGG single crystals were grown by the Czochralski method using an inductively heated iridium crucible. The growth conditions were 20-40 rpm rotation rate, 1.2 mm/h pulling rate and a growth direction along the ( 111) crystal axis. The crystals had a diameter of 45 mm and a length of 80 mm. All examined samples had a chromium concentration of 0.5 at.%. The erbium concentration has
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Volume 82, number 5,6
1 May 1991
been varied from 2 at.% up to 50 at.%.
3. Spectroscopy The absorption spectrum of Cr : Er : GGG has been obtained using a Perkin Elmer lambda 9 spectrophotometer. It consists of the narrow lines due to the erbium transitions from the 4I,5,2 ground state to excited levels and the broad bands of Cr3+ (4T, and 4T2), centered at 460 nm and 630 nm (fig. 1). A change of the absorption spectrum did not occur even after UV irradiation (Hg lamp). This indicates that the crystals were free of color centers. The 633 nm line of a HeNe laser was used as an excitation source for photoluminescence (PL). The incident light power of 10 mW was focused on to a spot with a diameter of 0.3 mm. All PL spectra were spectrally dispersed with I /4 m monochromator and recorded with a liquid-nitrogen-cooled InAs detector. The erbium transitions 41,3,2+41,5,2 and 411,,2-+ 4I,3,2 (fig. 2) cause emissions near 1.6 urn and 2.8 urn, respectively (figs. 3 and 4). The resolution of the narrow lines in the recorded spectra due to the Stark splitting of the energy levels was limited by the spectrometer. The concentration dependance of the 1.6 urn emission intensity has a maximum at about 15 at.W, while the 2.8 urn emission intensity shows a. continuous increase at higher erbium concentrations (fig. 5). The growing emission intensities at both wavelengths for an increase of the erbium concentration up to 15 at.% can be explained by an improvement of the Cr3+ to Er3+ energy transfer ef-
&I9/2 “I 11/2
Fig. 2. Pump scheme of the erbium photoluminescence Cr:Er:GGG.
Fig. 3. Photoluminescence spectrum (Er3+ emission) Cr: Er: GGG in the 1.6 Km range at 300 K.
Cr:Er:GGG Cr:Er:GGG T=300K
d k E 2
P 9=: 0.50
Fig. 1.Absorption spectrum of Cr: Er: GGG at 300 K.
Fig. 4. Photoluminescence spectrum (E+ Cr: Er: GGG in the 2.8 pm range at 300 K.
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Fig. 5. Normated photoluminescence intensities of Cr: Er: GGG at 1608 nm and 2808 nm versus erbium concentration at 300 K.
ficiency. The decrease of the 1.6 pm emission at higher erbium concentrations, while the 2.8 ym emission further increases, is due to a cooperative unconversion process [ 10 1. This upconversion due to an Er-Er interaction occurs if the energy for a 3113,2+419,2 transition is gained by a transition from the 4113,2 level to the ground state in a neighboring erbium ion (fig. 2). A rapid relaxation from the 4I9,2 level populates the 4111,1 level. High erbium concentrations are a necessary condition for this process. The upconversion should also depend on the excitation density [ 111. Due tb the fact that only a low excitation power was available such a dependence could not be [email protected]
Although it can not totally be ruled out that energy migration to impurities contributes to the decrease of the 1.6 pm emission at high erbium concentrations it is unlikely, becorresponds the decrease cause observed quantitatively with the increase of the 2.8 pm emission. It has to be noted that other upconversion processes are possible. An excitation of the Cr : Er : GGG with a 647 nm line of a krypton ion laser resulted in intense green emission [ 121. Fig. 6 shows the chromium emission of Cr : GGG and Cr: Er: GGG for differ&t erbium concentrations. The broad chromium emission (~T*--P~A~) has its maximum near 730 nm. A shoulder near 720 nm is due to a weak contribution of the 2E-+pA2 transition. In Cr: Er: GGG the chromium’-emission decreases strongly with increasing erbium concentration. The spectra reveal a series of sharp absorption dips near 800 nm, corresponding to overlapping Ef+ 486
Wavelength/nm Fig. 6. Photoluminescence Cr : Er : GGG at 300 K.
xT2808nm \ 1
Erbium Concedration/% Fig. 7. Lifetime of the photoluminescence of Cr : Er: GGG at 1608 nm and 2808 nm versus erbium concentration at 300 K.
absorption transitions. For an erbium concentration of 15 at.Oh the total integrated intensity of the chromium emission was less than 1% of the intensity in Cr : GGG. At an erbium concentration of 50 at.% no chromium emission was detectable. The decrease of this emission intensity indicates an Cr3+ to Er3+ energy transfer efficiency close to unity for high erbium concentrations. Such a high efficiency has also been reported for Cr : Er : YSGG [ 6 1. The energy transfer predominantly occurs as a radiationless dipole-dipole interaction. It proceeds either to the 411I,z upper laser level or the 4I9,2 level [ 13 1, followed by a rapid relaxation to the 41LI,2 level (see fig. 2). The fluorescence decay of the Er3+ emission was measured after excitation with pulses of a HeNe laser (633 nm), which were generated with an acoustooptic modulator. The time dependence of the fluo-
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rescence was detected with an InAs detector and a storage oscilloscope. For the emission at 2.8 urn the lifetime was found to be 1.3 ms, which did not depend on the erbium concentration. The lifetime of the 1.6 pm emission decreased with increasing erbium concentration from 11.4 ms for 2 at.% Er down to 6.1 ms for 50 at.% Er. This kind of quenching of the 4I13,2 lifetime also occurs in Cr:Er:YSGG [ 141. It is due to the cooperative upconversion described above. Thus it should also depend on the excitation density [ 111. In Cr : Er : YSGG with an erbium concentration of 30 at.% the 41, ,,* and 4I,3,2 lifetimes were found to be 5.1 ms and 1.4 ms, respectively [ 7 1. For a low erbium concentration in this crystal Huber and coworkers [ 141 report these lifetimes to be 10.68 ms and 2.25 ms, respectively.
4. Conclusions Our spectroscopic data indicate that the Cr3+ to Er3+ energy transfer in Cr: Er: GGG is highly efficient. The lifetime of the 2.8 urn emission ( 1.3 ms) is an order of magnitude longer than in Er : YAG ( 0.1 ms [ 8 ] ) . High erbium concentrations are favorable to enhance the 2.8 urn emission and reduce the 41i3,* lifetime. Both effects are due to cooperative upconversion. Further investigations have to be made to find out if an co-doping with holmium can cause an additional depopulation of the 4I,3,2 state of Er3+ as suggested by Huber and coworkers [ 141. Such a
1 May 1991
depopulation channel is essential for cw laser operation. Cr : Er : GGG is a promising candidate in the search for efficient laser crystals for the wavelength region near 3 urn.
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1J.A. Caird, M.D. Shinn, T.A. Kirchhoff, L.K. Smith and R.E. Wilder, Appl. Optics 25 ( 1986) 4294. H. Lundt and H. Weidner, Optics Comm. 75 ( 1990) 430. H. Lund& H. Weidner, H. FuBstetter, R.A. Huber and R. IfBander, SPIE Proc. 1021 (1988) 55. Z.J. Kissand R.C. Duncan, Appl. Phys. Lett. 5 (1964) 200. E.V. Zharikov, N.N. Ilichev, S.P. Kalitin, V.V. Laptev, A.A. Malyutin, V.V. Osiko, P.P. Pashinin, A.M. Prokorov, Z.S. Saidov, V.A. Smimov, A.F. Umyskov and I.A. Shcherbakov, Sov. J. Quantum Electron. 16 (1986) 635. [ 71 P.F. Moulton, J.G. Manni and GA. Rines, IEEE J. Quantum Electron. 24 (1988) 960.  M.K. Ashurov, T.T. Basiev, Y.K. Voronko, E.V. Zharikov, V.I. Zhekov, T.M. Murina, V.V. Osiko, M.I. Timishechkin and LA. Sherbakov, Sov. J. Quantum Electron. 8 (1978) 588.  CD. Brandle and A.J. Valentino, J. Crystal Growth 12 (1972) 3. [ IO] P. Xie and S.C. Rand, Conf. on Advanced Solid State Lasers, Technical Digest, paper WC6, Salt Lake City 1990. [ 111 P. Xie and S.C. Rand, Optics Lett. 15 ( 1990) 848. [ 121 W. Gellermann and H. Lundt, unpublished results. [ 131 V.A. Smimov and I.A. Shcherbakov, IEEE J. Quantum Electron. 24 (1988) 949. [ 141 G. Huber, E.W. Ducynski and K. Petermann, IEEE J. Quantum Electron. 24 ( 1988) 920.