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Anomalous optical dephasing in crystalline Y2O3: Eu3+
JOURNAL OF
LUMINESCENCE Journal of Luminescence 58(1994)374 379
ELSEVIER
Anomalous optical dephasing in crystalline Y203 : Eu3 + G.P. Flinna,* K.W. Janga, Joseph Ganema 1, M.L. Jones~’,R.S. Meltzera, R.M. Macfarlaneb Dept. of Phvsi~s and A,stronom~,Unwersitv ot Georgia, Athens, GA 30602-2451. USA hJBM
Research Lahoratorkv, A1n,aden, San Jose, CA 95193, USA
Abstract Sample dependent optical dephasing is observed for the 7F
5D 3 in Y 0 0 transition of Eu 203 crystals prepared by flame fusion, arc imaging and laser heated pedestal growth techniques. Two pulse photon echo measurements at 1.4 K show that the homogeneous line widths vary nearly two orders of magnitude in the different ([‘H linear 0.8 to 3 +over concentration. The temperature dependence of [‘Hsamples is nearly in 42 kHz), and(1are12K), uncorrelated the Eu for disordered systems. The measured line widths may be associated with temperature similar to with that observed limited disorder in the crystalline state. We also report on the line width of optical holes burned in the 7F 5D 0 ~. larger0 3~.We find [‘1~ 12MHz at 1.4 K, about l0~times transition than that measured of a MOCVD in theY203 bulk film crystals, containing and it is also 2at.°o linearly Eu dependent on temperature.
1.
Introduction
In general, the optical dephasing of impurity ions in ordered crystalline environments is very different from that of amorphous or disordered systems. In disordered systems controlled by the interaction of the the dephasing ions with islow frequency modes of the host lattice (two-level systems, or TLS) [1], resulting in a broad homogeneous line width (Ta) possessing a low power law dependence on temperature (i.e. between about T’ and T2). Without disorder, the low temperature dephasing is largely dominated by interactions between the electronic or nuclear moment of the impurity ions both with each other and with those of the host constituents [2], resulting in a homogeneous line width Corresponding author. New address: Laser Physics Branch, Naval Research Laborat ory, Washington, DC 20375, USA.
which is many orders of magnitude less than in disordered materials. The temperature dependence is also quite different. We report on the homogeneous line width measurement of the 7F 5D 0~ 0 transition a num3 + in prepared her four of crystalline Y203 techniques, Eu by different samples crystal of growth and present evidence for dynamical properties in some of these samples which are characteristic of disordered systems. Two pulse photon echo measurements are made in samples grown by flame fusion, arc imaging, and laser heated pedestal growth techniques. The line widths are seen to be sample dependent (0.8 to 42 kHz at 1.4 K). In addition, optical hole burning is used to measure the line width in a 3 ~im film of the same material deposited on sapphire by MOCVD [3]. At 1.4 K, [‘H is 12MHz. This line width is similar to that measured in Eu3 + doped silicate glass at the same temperature [4], and is three orders of
0022 2313 94 S07.00 ~ 1994 Elsevier Science By. All rights reserved SSDJ 0022-2313(93)E0l49 R
G.P. F/inn et al.
magnitude larger than that observed in the bulk samples. 2. Experimental The crystal grown by the Verneuil process, or flame fusion, forms the standard for these measurements. Previous measurements of r’H for this crystal show a value of 0.76 kHz [5], one of the narrowest line widths observed in any material. This sample is compared to seven other bulk crystals and an MOCVD film. One of the bulk samples was grown in an arc imaging furnace, while the others were grown as crystalline fibers by a laser heated pedestal growth technique (diameter 0.4 mm). In both techniques a melt pool is generated on the upper end of a sintered preparation ceramic, wherein a seed crystal is dipped and then slowly drawn upward 0.5 mm/mm). These are slightly faster growth techniques than the Verneuil process. The ends of the crystals were then polished and a focused laser beam was observed to pass through unhindered. The MOCVD sample comprises a 3pm film of Eu3~(—~2at.%)doped Y 203 grown on sapphire. Previous examination of identical films grown by the same apparatus has included Auger and X-ray photoelectron spectroscopy [3]. The stoichiometry of the film is confirmed within the resolution of the instrument, and the presence of contaminants introduced during the MOCVD process is observed to be below the detection limits of the apparatus <1 O/~).X-ray diffraction patterns and scanning electron microscopy were used to examine the crystallinity of the films after annealing in air at 1200 C for one hour; the films have cracked into 3 jim islands, and are thus polycrystalline. The six fibers Fl F6, and the flame fusion and arc imaging crystals were all mounted on the same sample holder, and placed in a helium immersion cryostat equipped with optical windows. Fluorescence from the film was weak enough to require that this sample be examined in isolation from the other samples. The temperature was varied from 2 to 14K, with an accuracy ranging from + 0.1 K at 4.5 K, to + 1 K at 14K. Optical excitation was accomplished using a Coherent 599-2 1 single frequency dye laser (jitter (—j
375
Journal of Luminescence 58 (1994) 374 379
2 MHz), operating with Rhodamine R6G at the 580~A Eu3~absorption. For the echo measurements, the laser beam was optically gated using two acousto-optic modulators (AOMs) operating in tandem, ensuring 10 or 10 6 rejection of the laser radiation during the off period. Optical pulse widths for the experiments were typically 600 ns (“ic/2” pulse) and 850 ns (“it” pulse), and were directed into the cryostat windows through a 30cm lens (spot size at the crystals was 50 jim). Laser power a~the cryostat windows was 10 mW. A good and reproducible launch into all the samples, without incurring any internal reflections, could be made by looking for a clear transmitted beam spot on the detection side of the cryostat. The echoes were detected using a photomultiplier (EMI 9558) and commercial transient digitizer. A double Pockel’s cell arrangement was used to temporally protect the photomultiplier from the transmitted preparation pulses (rejection 10~)by gating the cells on after the second preparation pulse. The photon echo at each pulse separation was averaged for 100 to 2000 pulse pairs (repetition rate 10 Hz), and the data recorded on a computer. ‘—j
Hole-burning measurements on the MOCVD film were performed using a variable burn/delay/sweep/erase cycle which was repeated for signal averaging. As the absorption of the film is less than 1%, the hole was detected in excitation. Provision was made for attenuation of the laser beam during positioning of the laser frequency before the sweep, and during the delay period before the sweep was initialized, but the sweep had to be performed at 5D full power due to the weakness of the 0 fluorescence. Following each individual recording of the hole spectrum, the laser was repetitively scanned about 10 times so as to erase the hole completely. This prevented problems associated with artificial widening of the hole line width due to excessive burning. Burn times were of the order of 3 s, and the laser sweep rate was 1.7 MHz/ms. —~
3. Results Table 1 summarizes the results obtained for all samples; except where indicated, all values are for 1.4K. The fluorescence lifetimes of all the crystals
376
G.P. F/inn et al.
Journal of Luminescence 58 (1994) 374 379
Table 1 5D TH and associated non exponential The lifetime of the fitting parameter X, and 0 excited the concentration state, the inhomogeneaus of Eu3~for line eachwidth, of the thesamples homogeneous discussed. line All width experiments made to determine these parameters were undertaken at 1.4K. See Ref. [8] regarding the value of T’H given for the flame fusion sample Sample
Fiber F6 Fiber Fl Fiber F5 Fiber F2 Fiber F3 Fiber F4 Flame Arc Film a This
Eu content
[‘H
5D
[at.°o]
[kHz]
0 lifetime [Rs]
Inhomogeneous absorption line width [GHz]
Non-exponential parameter (X)
0.55 0.3 0.13 0.1 0.05 0.004 0.3 — 0.1 —2
4.0 18
950 ±30 900 ±20 940 ±30 940 + 20 950 + 20 880 + 20 915 + 10 900 —.900
23.4 + 2.1 11.3 + 0.3 6.4 + 0.2 7.1 + 0.5 5.4 + 03 5 5 + 08 7.1 + 05 10 + 2 —90+20
1.5 1.5 2.5
42 26 38 9.4, 2.8a 2.5 27 l210~
1.5 2.5 1.5 2.5
1
value for fiber F4 indicates a value measured at 5 K, and is discussed in the text.
are very similar, i.e. in the region of 900 jis. With the exception of 7F the thin 5Dfilm, the inhomogeneous line width of the 0 ~ 0 transition in all the samples vary monotonically with concentration. The only difference between the bulk samples lies in the measured [‘H• While the flame fusion sample reproduced the results of previous authors [5,6], [‘H recorded in the other samples were significantly larger, sometimes by more than a factor 10. [‘H 3 +ofconcenappears to be uncorrelated with the Eu tration. Many of the echo decay curves exhibited a strong non-exponential nature. Here, the echo decay was fitted to a non-exponential line [7], and this is denoted by the non-exponential fitting parameter X given in the table. A value for X greater than 1 indicates that the dynamical processes are responsible for a spectral diffusion which occurs during the time scale of the echo measurements. For fiber F6, [‘H at 1.4 K (4kHz) compares favorably with the value of 2.5 kHz measured in the flame fusion crystal [8]. The fibers exhibited an inhomogeneity in [‘H as illustrated for fiber F4 in the table Tnitial measurements in this fiber showed = 9.4 kHz, whereas a subsequent measurement over a different beam path showed that we could obtain a value of 2.8 kHz. Fig. 1 shows the temperature dependence of [‘H for a number of the bulk samples. The flame fusion
sample displays a behavior characteristic of the two-phonon Raman process [9], exhibiting little temperature dependence below 8 K. Each of the other samples shows an additional temperature dependent contribution to F~which has a nearly linear behavior. For the fibers Fl and F2 we determine that [‘H is indeed linear in temperature. The behavior for fibers F4 and F6 lies close to that of the flame fusion crystal, except for acontribution small and nearly linear temperature dependent to [‘H at lower temperatures. For the F4 data shown, and that of fiber F6, the trend at higher temperatures is for the line width to converge with the ~‘7 Raman behavior of the flame fusion crystal. Fig. 2 illustrates the temperature behavior of the hole line width in the thin film. Also shown in this figure is a line representing [‘~ measured in a Eu3 + doped silicate glass sample [4]. The line width in the glass was also measured by hole burning, and follows 9 T’°MHz up to 4K, the maximum ternperature at which holes could be burned. The value measured in the film at 1.4 K is substantially larger than that measured in the bulk samples (by about three or four orders of magnitude), but is similar to that measured in the silicate glass at the same temperature. Inset into this figure is a hole spectrum for the thin film. The hole and anti-hole features are similar to those obtained by other authors
G.P. Flinn ci al.
Journal of Luminescence 58 (1994) 374 379
150
377
50.0
120
F2
90
ARC •
LU
z
/
•
Id
I
.1
fr
•....v........
/
F4
0
100
.:
.‘
300
500
A
.‘~
AL
A A A
A..~
~
0J
A
___
0 4
.100
10.0
AA~ 2
-300
I
0
0
-500
.
/I
.....
oe
I
/
~
~...“
~
30
I I /
.‘ ~
Fl ,~
20.0
.
..17•
V
.~
,.....1
...
..
U....
300
“ui
..~
I
S 60
~
V
F~M/
I 8
6
10
I 12
I 14
5.0 0.5
16
TEMPERATURE (K)
1.0
I 2.0
~ 5.0
I 10.0
TEMPERATURE (K)
Fig. 1. Temperature dependence of the homogeneous line width for fibers Fl through F4 and fiber F6, for the arc imaging and flame fusion samples. Fibers Fl and F2 clearly follow a T’° dependence; the dotted lines are linear fits. The solid curve represents the T7 two-phonon Raman behavior observed in the flame fusion crystal [9]; line widths measured as a function of temperature lie close to this line,
Fig. 2. Temperature dependence ofF~measured in the thin film (triangles). The extrapolated value of 11.5MHz for the OK line width prevents illustrating the linear fit to the data. The dotted line represents hole-burning measurements made of TH for Eu3 + in a silicate glass sample ([‘H 9T ‘ ~MHz) [4]. Inset into the figure is a hole spectrum for the thin film taken at 1.4 K, showing relative fluorescence intensity versus laser detuning in MHz.
in the flame fusion sample [9]; these result from population storage in the quadrupole split levels of the ground and excited states of each of the 151Eu and iS3Eu isotopes. The line width is seen to possess a weak linear dependence on temperature, indicating a value of [‘H 11.5 MHz at very low temperatures, and widening to a value of about 20 MHz at 11 K.
the impurity ions with the host 89Y nuclear moments is expected to contribute 0.1 kHz to the line width [2,5]. No temperature dependence from this process is predicted. Phonon induced contributions to the line width may be neglected on the grounds of sufficient energetic isolation of the ground and excited state levels. The lifetime contribution to the line width is 0.2 kHz. A component from excitation induced instantaneous diffusion 0.1 kHz) is also anticipated [6], and the remaining 0.3 kHz is expected to arise, at least in part, from weak Eu3’~’ Eu3+ interactions, and is thus concentration dependent [2,5]. All of these processes can be dismissed as sources of the additional line width; the broadened line widths are linearly dependent on temperature, are
=
(‘-..~
4. Discussion
The expected contributions to the homogeneous line width (0.76 kHz) in ordered Y 3 + have 2O3: Eu a magbeen discussed elsewhere [2,4—6].In brief, netic interaction between the nuclear moments of
378
G.P. F/inn
Cf
al.
Journal of Luminescence 58 (1994) 374 379
not susceptible to instantaneous diffusion, are unaffected by magnetic fields, and are uncorrelated with the Eu3 + concentration. We refer to this contribution as anomalous. It is well documented that glasses and disordered crystalline systems exhibit a greatly enhanced optical dephasing, and that this dephasing exhibits a low power law dependence on temperature [1]. The dynamics of these materials is governed by the interaction of the excited ions with two-level systems. The data presented here for these crystalline samples of Y 203 shows behavior quite similar to that of disordered materials. Although it is apparent that the magnitude of the anomalous line widths lie somewhat midway between that of good crystalline material and that of disordered systems, the linear temperature dependence of the dephasing implies a broad distribution of excitation frequencies for the dynamical modes. in addition, the non-exponential nature of the echo decay in most of the samples indicates the presence of spectral a characteristic in glasses whose dynamicsdiffusion, are controlled by TLS [I]. What sort of modes are responsible for the dynamics of Y 3 ~? We considered the possib203 might : Eu be a low density of small, ility that there highly distorted regions in the crystals where TLS might result in glass-like dynamics. We thus obtamed an excitation spectrum of the Eu3 + ions in a number of the samples up to 100 cm below the main line by monitoring fluorescence in the region of the 5D 7F~transition, but we could not in 0 —* any broad background to the crygeneral detect stalline absorption in any of the samples. Similarly, X-ray scattering measurements were made on some of the samples; the diffraction pattern was clearly that of a well ordered Y 2O3 crystal. In addition, several of the samples were annealed at 2000 C for half an hour. The effect on the photon echo decays was not dramatic, and it should be noted that within the context of the observed spatial inhomogeneity of the dephasing, little change was actually observed in any of the samples. Lastly, some of the samples were examined by confocal optical microscopy, imaging different planes within the crystals. Each of the samples examined contains different featuresprocess. that all appear to be consequences of the growth In particular, the growth of individual fibers is both non-reproducible and
non-symmetrical, resulting in an internal structure which is seen to include microbubbles, regiolis of varying refractive index, and polycrystalline regions that are all distributed inhomogeneously over the fiber cross section. The arc crystal was found to principally contain microbubbles, whose dimensions vary from less than 1 up to about 10 jim, and that are randomly distributed throughout the crystal. In general, both the laser heated pedestal and the arc imaging furnace growth procedures are both susceptible to varying growth parameters. in the latter procedure for example, the incorporation of defects during the growth is known to be critically dependent on both the temperature gradient and on the growth rate [10]. This may at least qualitatively account for the disorder responsible for the larger line widths seen in some of the bulk samples. For the thin film, the large value for [‘H suggests a material which is highly disordered. However, its behavior is significantly different from that whichthe is 3 + glass observed in a glass [4]. Whereas in a Eu inhomogeneous line widths are 100 cm 1, those of the thin film are 3 cm more like the bulk crystals (0.1 1 cm 1)• While [‘H in a glass extrapolates to about zero at low temperature, the film seems to have a much larger low temperature contribution. Of course, there might still be a large temperature dependent contribution for T < 1 K. In addition, spectral diffusion may be active over the relatively long time scale between the burn and probe phase of the experiment (—~0.5 s), although [‘H is not observed to broaden for times greater than one second after the burn. The hole lifetime in the film at 1.4 K of 2 mm is longer than that of the glass (— 20 s) [4], but is much less than that ‘—
~,
‘~
of the bulk crystals ( > 10 h). The relative behavior of the hole lifetime in these systems suggests that disorder induced inhomogeneous broadening of the quadrupole levels is responsible for determining the hole lifetimes.
5. Summary We have measured the homogeneous line widths 3~,produced in a number of crystals of Y2O3 : Eu by four different growth processes. For the bulk samples produced by alternate methods to the
G.P. F/inn ci al.
Journal of Luminescence 58 (1994) 374 379
Verneuil process (i.e. the arc imaging crystal and the fiber samples), an enhancement of the dynamic environment is observed for the impurity ions. Instead of a narrow homogeneous line width and a temperature dependence associated with a twophonon Raman process, these anomalous crystals show a behavior akin to that observed in disordered systems; [‘H is wider and shows a nearly linear temperature dependence. This behavior is attributed to some disorder present within the samples which introduces a broad frequency distribution of modes. The homogeneous line width of the same transition is also examined in a thin film prepared by MOCYD. The line width at 1.4K shows a value very similar to that measured in a silicate glass sample, with a nearly linear temperature behavior up to 12K. However, a large contribution remains at very low temperatures, in contrast to the glass. This may indicate a large density of states for the system at low frequencies. Acknowledgements The authors wish to thank Lizhu Li for growing the fiber crystals, Charles Greskovich of GE for annealing some of the samples, and Mark Farmer and Andrew Maselli at UGA are acknowledged for their assistance with the microscopy work. Gary West of Allied Signal Inc. is acknowledged for supplying the thin film. This work was supported by the National Science Foundation, Grant 4IDMR-9015468.
379
References [I] R.M. Macfarlane and R.M. Shelby, J. Lumin. 36 (1987) 179; DL. Huber, in: Dynamical Processes In Disordered Sys tems, ed. W.M. Yen (Materials Science Forum, Trans Tech Publications, 1989) [2] R.M. Macfarlane and R.M. Shelby, in: Spectroscopy Of Solids Containing Rare Earth Ions, eds. A.A. Kaplyanskii and R.M Macfarlane (North Holland, Amsterdam, 1987). [3] GA. West and K.W. Beeson, J. Mater. Res. 5 (1990) 1573.
[4] Th. Schmidt, J. Baak, D.A. van de Straat, H B. Brow and S. VOlker, to be published; see also: P.M. Seizer, DL. Huber, D.S. Hamilton, W.M. Yen and M.J. Weber, Phys. Rev. Lett. 36(1976)813; R.M. Macfarlane and R.M. Shelby, Opt. Commun. 45 (1983) 46; P.J. Van der Zaag, B.C. Schokker, Th. Schmidt, R.M. Macfarlane and S. Völker, J. Lumin. 45 (1990) 80. [5] R.M. Macfarlane and R.M. Shelby, Opt. Commun. 39 (1981) 169. [6] Jin Huang, J.M. Zhang, A. Lezama and T.W. Mossberg, Phys. Rev. Lett. 63 (1989) 78;
[7]
Jin Huang. J.M. Zhang and T W. Mossberg. Opt. Commun. 75 (1990) 29. J. Ganem, Y.P. Wang, D. Boye, R.S. Meltzer and W.M. Yen, Phys. Rev Lett. 66 (1991) 695;
R.S. Meltzer, J. Ganem, Y.P. Wang, D. Boye, W.M. Yen, D.P. Landau, R. Wannemacher and R.M. Macfarlane, J. Lumin. 53 (1992) 1. TH in the flame crystal (2.5 kHz) [8] The value quoted here for is that measured by us and other authors [5,6] at line center, where instantaneous diffusion effects make a contribution to the line width. [9] W.R. Babbitt, A. Lezama and T.W. Mossberg, Phys. Rev. B 39 (1989) 1987 [10] H. Tsuiki, K. Kitazawa, T. Masamoto, K Shiroki and K. Fueki, J. Cryst. Growth 49(1980) 7l.
LUMINESCENCE Journal of Luminescence 58(1994)374 379
ELSEVIER
Anomalous optical dephasing in crystalline Y203 : Eu3 + G.P. Flinna,* K.W. Janga, Joseph Ganema 1, M.L. Jones~’,R.S. Meltzera, R.M. Macfarlaneb Dept. of Phvsi~s and A,stronom~,Unwersitv ot Georgia, Athens, GA 30602-2451. USA hJBM
Research Lahoratorkv, A1n,aden, San Jose, CA 95193, USA
Abstract Sample dependent optical dephasing is observed for the 7F
5D 3 in Y 0 0 transition of Eu 203 crystals prepared by flame fusion, arc imaging and laser heated pedestal growth techniques. Two pulse photon echo measurements at 1.4 K show that the homogeneous line widths vary nearly two orders of magnitude in the different ([‘H linear 0.8 to 3 +over concentration. The temperature dependence of [‘Hsamples is nearly in 42 kHz), and(1are12K), uncorrelated the Eu for disordered systems. The measured line widths may be associated with temperature similar to with that observed limited disorder in the crystalline state. We also report on the line width of optical holes burned in the 7F 5D 0 ~. larger0 3~.We find [‘1~ 12MHz at 1.4 K, about l0~times transition than that measured of a MOCVD in theY203 bulk film crystals, containing and it is also 2at.°o linearly Eu dependent on temperature.
1.
Introduction
In general, the optical dephasing of impurity ions in ordered crystalline environments is very different from that of amorphous or disordered systems. In disordered systems controlled by the interaction of the the dephasing ions with islow frequency modes of the host lattice (two-level systems, or TLS) [1], resulting in a broad homogeneous line width (Ta) possessing a low power law dependence on temperature (i.e. between about T’ and T2). Without disorder, the low temperature dephasing is largely dominated by interactions between the electronic or nuclear moment of the impurity ions both with each other and with those of the host constituents [2], resulting in a homogeneous line width Corresponding author. New address: Laser Physics Branch, Naval Research Laborat ory, Washington, DC 20375, USA.
which is many orders of magnitude less than in disordered materials. The temperature dependence is also quite different. We report on the homogeneous line width measurement of the 7F 5D 0~ 0 transition a num3 + in prepared her four of crystalline Y203 techniques, Eu by different samples crystal of growth and present evidence for dynamical properties in some of these samples which are characteristic of disordered systems. Two pulse photon echo measurements are made in samples grown by flame fusion, arc imaging, and laser heated pedestal growth techniques. The line widths are seen to be sample dependent (0.8 to 42 kHz at 1.4 K). In addition, optical hole burning is used to measure the line width in a 3 ~im film of the same material deposited on sapphire by MOCVD [3]. At 1.4 K, [‘H is 12MHz. This line width is similar to that measured in Eu3 + doped silicate glass at the same temperature [4], and is three orders of
0022 2313 94 S07.00 ~ 1994 Elsevier Science By. All rights reserved SSDJ 0022-2313(93)E0l49 R
G.P. F/inn et al.
magnitude larger than that observed in the bulk samples. 2. Experimental The crystal grown by the Verneuil process, or flame fusion, forms the standard for these measurements. Previous measurements of r’H for this crystal show a value of 0.76 kHz [5], one of the narrowest line widths observed in any material. This sample is compared to seven other bulk crystals and an MOCVD film. One of the bulk samples was grown in an arc imaging furnace, while the others were grown as crystalline fibers by a laser heated pedestal growth technique (diameter 0.4 mm). In both techniques a melt pool is generated on the upper end of a sintered preparation ceramic, wherein a seed crystal is dipped and then slowly drawn upward 0.5 mm/mm). These are slightly faster growth techniques than the Verneuil process. The ends of the crystals were then polished and a focused laser beam was observed to pass through unhindered. The MOCVD sample comprises a 3pm film of Eu3~(—~2at.%)doped Y 203 grown on sapphire. Previous examination of identical films grown by the same apparatus has included Auger and X-ray photoelectron spectroscopy [3]. The stoichiometry of the film is confirmed within the resolution of the instrument, and the presence of contaminants introduced during the MOCVD process is observed to be below the detection limits of the apparatus <1 O/~).X-ray diffraction patterns and scanning electron microscopy were used to examine the crystallinity of the films after annealing in air at 1200 C for one hour; the films have cracked into 3 jim islands, and are thus polycrystalline. The six fibers Fl F6, and the flame fusion and arc imaging crystals were all mounted on the same sample holder, and placed in a helium immersion cryostat equipped with optical windows. Fluorescence from the film was weak enough to require that this sample be examined in isolation from the other samples. The temperature was varied from 2 to 14K, with an accuracy ranging from + 0.1 K at 4.5 K, to + 1 K at 14K. Optical excitation was accomplished using a Coherent 599-2 1 single frequency dye laser (jitter (—j
375
Journal of Luminescence 58 (1994) 374 379
2 MHz), operating with Rhodamine R6G at the 580~A Eu3~absorption. For the echo measurements, the laser beam was optically gated using two acousto-optic modulators (AOMs) operating in tandem, ensuring 10 or 10 6 rejection of the laser radiation during the off period. Optical pulse widths for the experiments were typically 600 ns (“ic/2” pulse) and 850 ns (“it” pulse), and were directed into the cryostat windows through a 30cm lens (spot size at the crystals was 50 jim). Laser power a~the cryostat windows was 10 mW. A good and reproducible launch into all the samples, without incurring any internal reflections, could be made by looking for a clear transmitted beam spot on the detection side of the cryostat. The echoes were detected using a photomultiplier (EMI 9558) and commercial transient digitizer. A double Pockel’s cell arrangement was used to temporally protect the photomultiplier from the transmitted preparation pulses (rejection 10~)by gating the cells on after the second preparation pulse. The photon echo at each pulse separation was averaged for 100 to 2000 pulse pairs (repetition rate 10 Hz), and the data recorded on a computer. ‘—j
Hole-burning measurements on the MOCVD film were performed using a variable burn/delay/sweep/erase cycle which was repeated for signal averaging. As the absorption of the film is less than 1%, the hole was detected in excitation. Provision was made for attenuation of the laser beam during positioning of the laser frequency before the sweep, and during the delay period before the sweep was initialized, but the sweep had to be performed at 5D full power due to the weakness of the 0 fluorescence. Following each individual recording of the hole spectrum, the laser was repetitively scanned about 10 times so as to erase the hole completely. This prevented problems associated with artificial widening of the hole line width due to excessive burning. Burn times were of the order of 3 s, and the laser sweep rate was 1.7 MHz/ms. —~
3. Results Table 1 summarizes the results obtained for all samples; except where indicated, all values are for 1.4K. The fluorescence lifetimes of all the crystals
376
G.P. F/inn et al.
Journal of Luminescence 58 (1994) 374 379
Table 1 5D TH and associated non exponential The lifetime of the fitting parameter X, and 0 excited the concentration state, the inhomogeneaus of Eu3~for line eachwidth, of the thesamples homogeneous discussed. line All width experiments made to determine these parameters were undertaken at 1.4K. See Ref. [8] regarding the value of T’H given for the flame fusion sample Sample
Fiber F6 Fiber Fl Fiber F5 Fiber F2 Fiber F3 Fiber F4 Flame Arc Film a This
Eu content
[‘H
5D
[at.°o]
[kHz]
0 lifetime [Rs]
Inhomogeneous absorption line width [GHz]
Non-exponential parameter (X)
0.55 0.3 0.13 0.1 0.05 0.004 0.3 — 0.1 —2
4.0 18
950 ±30 900 ±20 940 ±30 940 + 20 950 + 20 880 + 20 915 + 10 900 —.900
23.4 + 2.1 11.3 + 0.3 6.4 + 0.2 7.1 + 0.5 5.4 + 03 5 5 + 08 7.1 + 05 10 + 2 —90+20
1.5 1.5 2.5
42 26 38 9.4, 2.8a 2.5 27 l210~
1.5 2.5 1.5 2.5
1
value for fiber F4 indicates a value measured at 5 K, and is discussed in the text.
are very similar, i.e. in the region of 900 jis. With the exception of 7F the thin 5Dfilm, the inhomogeneous line width of the 0 ~ 0 transition in all the samples vary monotonically with concentration. The only difference between the bulk samples lies in the measured [‘H• While the flame fusion sample reproduced the results of previous authors [5,6], [‘H recorded in the other samples were significantly larger, sometimes by more than a factor 10. [‘H 3 +ofconcenappears to be uncorrelated with the Eu tration. Many of the echo decay curves exhibited a strong non-exponential nature. Here, the echo decay was fitted to a non-exponential line [7], and this is denoted by the non-exponential fitting parameter X given in the table. A value for X greater than 1 indicates that the dynamical processes are responsible for a spectral diffusion which occurs during the time scale of the echo measurements. For fiber F6, [‘H at 1.4 K (4kHz) compares favorably with the value of 2.5 kHz measured in the flame fusion crystal [8]. The fibers exhibited an inhomogeneity in [‘H as illustrated for fiber F4 in the table Tnitial measurements in this fiber showed = 9.4 kHz, whereas a subsequent measurement over a different beam path showed that we could obtain a value of 2.8 kHz. Fig. 1 shows the temperature dependence of [‘H for a number of the bulk samples. The flame fusion
sample displays a behavior characteristic of the two-phonon Raman process [9], exhibiting little temperature dependence below 8 K. Each of the other samples shows an additional temperature dependent contribution to F~which has a nearly linear behavior. For the fibers Fl and F2 we determine that [‘H is indeed linear in temperature. The behavior for fibers F4 and F6 lies close to that of the flame fusion crystal, except for acontribution small and nearly linear temperature dependent to [‘H at lower temperatures. For the F4 data shown, and that of fiber F6, the trend at higher temperatures is for the line width to converge with the ~‘7 Raman behavior of the flame fusion crystal. Fig. 2 illustrates the temperature behavior of the hole line width in the thin film. Also shown in this figure is a line representing [‘~ measured in a Eu3 + doped silicate glass sample [4]. The line width in the glass was also measured by hole burning, and follows 9 T’°MHz up to 4K, the maximum ternperature at which holes could be burned. The value measured in the film at 1.4 K is substantially larger than that measured in the bulk samples (by about three or four orders of magnitude), but is similar to that measured in the silicate glass at the same temperature. Inset into this figure is a hole spectrum for the thin film. The hole and anti-hole features are similar to those obtained by other authors
G.P. Flinn ci al.
Journal of Luminescence 58 (1994) 374 379
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Fig. 1. Temperature dependence of the homogeneous line width for fibers Fl through F4 and fiber F6, for the arc imaging and flame fusion samples. Fibers Fl and F2 clearly follow a T’° dependence; the dotted lines are linear fits. The solid curve represents the T7 two-phonon Raman behavior observed in the flame fusion crystal [9]; line widths measured as a function of temperature lie close to this line,
Fig. 2. Temperature dependence ofF~measured in the thin film (triangles). The extrapolated value of 11.5MHz for the OK line width prevents illustrating the linear fit to the data. The dotted line represents hole-burning measurements made of TH for Eu3 + in a silicate glass sample ([‘H 9T ‘ ~MHz) [4]. Inset into the figure is a hole spectrum for the thin film taken at 1.4 K, showing relative fluorescence intensity versus laser detuning in MHz.
in the flame fusion sample [9]; these result from population storage in the quadrupole split levels of the ground and excited states of each of the 151Eu and iS3Eu isotopes. The line width is seen to possess a weak linear dependence on temperature, indicating a value of [‘H 11.5 MHz at very low temperatures, and widening to a value of about 20 MHz at 11 K.
the impurity ions with the host 89Y nuclear moments is expected to contribute 0.1 kHz to the line width [2,5]. No temperature dependence from this process is predicted. Phonon induced contributions to the line width may be neglected on the grounds of sufficient energetic isolation of the ground and excited state levels. The lifetime contribution to the line width is 0.2 kHz. A component from excitation induced instantaneous diffusion 0.1 kHz) is also anticipated [6], and the remaining 0.3 kHz is expected to arise, at least in part, from weak Eu3’~’ Eu3+ interactions, and is thus concentration dependent [2,5]. All of these processes can be dismissed as sources of the additional line width; the broadened line widths are linearly dependent on temperature, are
=
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4. Discussion
The expected contributions to the homogeneous line width (0.76 kHz) in ordered Y 3 + have 2O3: Eu a magbeen discussed elsewhere [2,4—6].In brief, netic interaction between the nuclear moments of
378
G.P. F/inn
Cf
al.
Journal of Luminescence 58 (1994) 374 379
not susceptible to instantaneous diffusion, are unaffected by magnetic fields, and are uncorrelated with the Eu3 + concentration. We refer to this contribution as anomalous. It is well documented that glasses and disordered crystalline systems exhibit a greatly enhanced optical dephasing, and that this dephasing exhibits a low power law dependence on temperature [1]. The dynamics of these materials is governed by the interaction of the excited ions with two-level systems. The data presented here for these crystalline samples of Y 203 shows behavior quite similar to that of disordered materials. Although it is apparent that the magnitude of the anomalous line widths lie somewhat midway between that of good crystalline material and that of disordered systems, the linear temperature dependence of the dephasing implies a broad distribution of excitation frequencies for the dynamical modes. in addition, the non-exponential nature of the echo decay in most of the samples indicates the presence of spectral a characteristic in glasses whose dynamicsdiffusion, are controlled by TLS [I]. What sort of modes are responsible for the dynamics of Y 3 ~? We considered the possib203 might : Eu be a low density of small, ility that there highly distorted regions in the crystals where TLS might result in glass-like dynamics. We thus obtamed an excitation spectrum of the Eu3 + ions in a number of the samples up to 100 cm below the main line by monitoring fluorescence in the region of the 5D 7F~transition, but we could not in 0 —* any broad background to the crygeneral detect stalline absorption in any of the samples. Similarly, X-ray scattering measurements were made on some of the samples; the diffraction pattern was clearly that of a well ordered Y 2O3 crystal. In addition, several of the samples were annealed at 2000 C for half an hour. The effect on the photon echo decays was not dramatic, and it should be noted that within the context of the observed spatial inhomogeneity of the dephasing, little change was actually observed in any of the samples. Lastly, some of the samples were examined by confocal optical microscopy, imaging different planes within the crystals. Each of the samples examined contains different featuresprocess. that all appear to be consequences of the growth In particular, the growth of individual fibers is both non-reproducible and
non-symmetrical, resulting in an internal structure which is seen to include microbubbles, regiolis of varying refractive index, and polycrystalline regions that are all distributed inhomogeneously over the fiber cross section. The arc crystal was found to principally contain microbubbles, whose dimensions vary from less than 1 up to about 10 jim, and that are randomly distributed throughout the crystal. In general, both the laser heated pedestal and the arc imaging furnace growth procedures are both susceptible to varying growth parameters. in the latter procedure for example, the incorporation of defects during the growth is known to be critically dependent on both the temperature gradient and on the growth rate [10]. This may at least qualitatively account for the disorder responsible for the larger line widths seen in some of the bulk samples. For the thin film, the large value for [‘H suggests a material which is highly disordered. However, its behavior is significantly different from that whichthe is 3 + glass observed in a glass [4]. Whereas in a Eu inhomogeneous line widths are 100 cm 1, those of the thin film are 3 cm more like the bulk crystals (0.1 1 cm 1)• While [‘H in a glass extrapolates to about zero at low temperature, the film seems to have a much larger low temperature contribution. Of course, there might still be a large temperature dependent contribution for T < 1 K. In addition, spectral diffusion may be active over the relatively long time scale between the burn and probe phase of the experiment (—~0.5 s), although [‘H is not observed to broaden for times greater than one second after the burn. The hole lifetime in the film at 1.4 K of 2 mm is longer than that of the glass (— 20 s) [4], but is much less than that ‘—
~,
‘~
of the bulk crystals ( > 10 h). The relative behavior of the hole lifetime in these systems suggests that disorder induced inhomogeneous broadening of the quadrupole levels is responsible for determining the hole lifetimes.
5. Summary We have measured the homogeneous line widths 3~,produced in a number of crystals of Y2O3 : Eu by four different growth processes. For the bulk samples produced by alternate methods to the
G.P. F/inn ci al.
Journal of Luminescence 58 (1994) 374 379
Verneuil process (i.e. the arc imaging crystal and the fiber samples), an enhancement of the dynamic environment is observed for the impurity ions. Instead of a narrow homogeneous line width and a temperature dependence associated with a twophonon Raman process, these anomalous crystals show a behavior akin to that observed in disordered systems; [‘H is wider and shows a nearly linear temperature dependence. This behavior is attributed to some disorder present within the samples which introduces a broad frequency distribution of modes. The homogeneous line width of the same transition is also examined in a thin film prepared by MOCYD. The line width at 1.4K shows a value very similar to that measured in a silicate glass sample, with a nearly linear temperature behavior up to 12K. However, a large contribution remains at very low temperatures, in contrast to the glass. This may indicate a large density of states for the system at low frequencies. Acknowledgements The authors wish to thank Lizhu Li for growing the fiber crystals, Charles Greskovich of GE for annealing some of the samples, and Mark Farmer and Andrew Maselli at UGA are acknowledged for their assistance with the microscopy work. Gary West of Allied Signal Inc. is acknowledged for supplying the thin film. This work was supported by the National Science Foundation, Grant 4IDMR-9015468.
379
References [I] R.M. Macfarlane and R.M. Shelby, J. Lumin. 36 (1987) 179; DL. Huber, in: Dynamical Processes In Disordered Sys tems, ed. W.M. Yen (Materials Science Forum, Trans Tech Publications, 1989) [2] R.M. Macfarlane and R.M. Shelby, in: Spectroscopy Of Solids Containing Rare Earth Ions, eds. A.A. Kaplyanskii and R.M Macfarlane (North Holland, Amsterdam, 1987). [3] GA. West and K.W. Beeson, J. Mater. Res. 5 (1990) 1573.
[4] Th. Schmidt, J. Baak, D.A. van de Straat, H B. Brow and S. VOlker, to be published; see also: P.M. Seizer, DL. Huber, D.S. Hamilton, W.M. Yen and M.J. Weber, Phys. Rev. Lett. 36(1976)813; R.M. Macfarlane and R.M. Shelby, Opt. Commun. 45 (1983) 46; P.J. Van der Zaag, B.C. Schokker, Th. Schmidt, R.M. Macfarlane and S. Völker, J. Lumin. 45 (1990) 80. [5] R.M. Macfarlane and R.M. Shelby, Opt. Commun. 39 (1981) 169. [6] Jin Huang, J.M. Zhang, A. Lezama and T.W. Mossberg, Phys. Rev. Lett. 63 (1989) 78;
[7]
Jin Huang. J.M. Zhang and T W. Mossberg. Opt. Commun. 75 (1990) 29. J. Ganem, Y.P. Wang, D. Boye, R.S. Meltzer and W.M. Yen, Phys. Rev Lett. 66 (1991) 695;
R.S. Meltzer, J. Ganem, Y.P. Wang, D. Boye, W.M. Yen, D.P. Landau, R. Wannemacher and R.M. Macfarlane, J. Lumin. 53 (1992) 1. TH in the flame crystal (2.5 kHz) [8] The value quoted here for is that measured by us and other authors [5,6] at line center, where instantaneous diffusion effects make a contribution to the line width. [9] W.R. Babbitt, A. Lezama and T.W. Mossberg, Phys. Rev. B 39 (1989) 1987 [10] H. Tsuiki, K. Kitazawa, T. Masamoto, K Shiroki and K. Fueki, J. Cryst. Growth 49(1980) 7l.