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A spectral hole burning study of BaFCl0.5Br0.5:Sm2+
Journal of Luminescence 50 (1991) 89—100 North-Holland
89
A spectral hole burning study of BaFCl05Br05:Sm2~ Changjiang Wei
~, Keith Holliday, Alfred J. Meixner 2 Mauro Croci and Urs P. Wild Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zen:rwn. CH-8092 Zurich, Switzerland
Received 6 November 1990 Revised 18 March 1991 Accepted 19 March 1991
Investigations of parameters for two-photon hole burning (single colour and photon-gated) and hole erasing in the three 4f6 7F 5D 2 + are presented. This material is of particular interest as it has unusually large zero-phonon line widths allowing holes to be burnt at relatively high temperatures. For the transition to 0 — inhomogeneous 3 electronic transitions in BaFCI05 Br05: Sm the 5D 5D 0 excited state, only photon-gated holes could be burnt whereas gating light-enhanced single-colour hole burning 5D in the 1 transition by a factor of about five. Gating light did not significantly increase the hole burning rate for the 2 transition but did increase the resistance of the holes to erasing by irradiation from the gating beam alone. Such erasing during the burning process decreases the effective gating ratios for all transitions. The hole width burning power 5D dependence was 3 investigated, the minimum hole width at 1.8 K being 14 MHz. The temperature dependence of the 2 hole width was investigated 1.8 and 90 K, suggesting regions. Below about 20 K the width found to beinT’ whilst above between 20 K it was found to be T2-9. A two similar temperature dependence washole found fordependence the widths was of holes burnt the other two transitions in the high-temperature regime but the holes were generally broader below 20 K. This was attributed to the requirement of higher burn and read power densities. Holes could be burnt at temperatures up to 133 K and could be cycled from 1.8 K to room temperature, becoming much shallower in the process.
1. Introduction Persistent spectral hole burning has now been observed in many materials, providing an enormous increase in the resolution of data available from optical experiments and consequently revealing much about physical processes occurring in solids. Spectral hole burning also shows great promise as a technique for the implementation of technological goals such as high-density image storage [1] and molecular computing [2] but the search for a material possessing the required properties to produce an efficient digital memory device [3] continues. Such a hole burning memory
2
Present address: Laser Physics Centre. Research School of Physical Sciences. Australian National University. Canberra, ACT 2601. Australia Present address: IBM Almaden Research Center. 650 Harry Road. San Jose, California 95120-6099, USA
0022-2313/91/$03.50 © 1991
—
represents ‘l’s or ‘0’s as the presence or absence of spectral holes produced across the range of an inhomogeneously broadened band. Two fundamental problems rule out most materials; the degradation of data caused by repeated reading and the rapid increase in hole instability and width with increasing temperature. Photon-gated hole burning circumvents the first difficulty. Here a second beam, at a different wavelength, greatly enhances the hole burning rate. Only light at the resonant frequency is required to probe for the existence of a hole and therefore the information may be accessed repeatedly with little degradation. Photon-gated spectral holes were first burnt in the 4f6 7F 5D 2~ 0— as 0 transition in BaFCl:Sm [4] with a gating ratio high as 100, that is, the hole burning rate is enhanced by this factor upon .
addttton of the gating beam. The mechantsm for hole burning was attributed to excited state absorption of a gating photon leading to oxidation
Elsevier Science Publishers B.V. (North-Holland)
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C. Wei et al.
/ Spectral hole burning in BaFC10 5Br0
of the Sm2~ion to Sm3t In samples which have not been fully reduced (type I), Sm3 + ions may act as traps for the released electrons. In such samples holes could be erased through irradiation of the sample with light corresponding to the 4f 55d absorption band of Sm2~,the interpretation being that this non-selectively releases electrons into the conduction band where they may be captured by Sm3~ions. In samples containing no Sm3~ions (type II), other traps act as electron acceptors and
E
2+ 5:Sm gating beam (514 nm)
-t
(cm 30000
4f55d b~ds 20000 ____________
hole erasure was not observed. Additionally, type II samples showed strong single-colour transient hole burning. It was speculated that transient holes with a lifetime of less than 1 ms and depths of almost 100% may be attributable to electron ejection to energetically nearby traps from which re3~is rapid. combination with burnt Sm in the same transition in Spectral holes certain samples of BaFCl:Sm2~were found to be
4f05D 5D 2(560nm):t=0.72ms 41’s65D1 (630 nm): t=t.0 ms 4f 0 (688 nm): r= 1.5
burning beams (thin, up) principal emission detected (down) gating/erasing beam (thick, up) _____________
4f67F 0
resistant to cycling to room temperature [4] showing the photoproduct to detected be stable at at high 300 K. However, holes could not be temperatures because the transition is homogeneously broadened at 77 K and above, A study of anthracene in 2,3-dimethylnaphthalene mixed crystals [5] demonstrated the effectiveness of introducing statistical disorder to the host lattice to increase inhomogeneous line widths. Wei et al. [6] succeeded in broadening the inhomogeneous line widths of the 4f6 7F 0—~D~ transitions in BaFCl:Sm2~through the addition of bromine to the melt. Holes with a width of 1.1 cm’ were burnt in the 7F 5D 2 transition in 2 ± at 77 K 0— suggesting that the BaFC1 05Br05: Sm homogeneous line width of the transition is not greatly affected by the addition of bromine. However, the inhomogeneous line widths of the transitions are broadened to approximately 50 cm’, due to substitutional ligand disorder [7], without appreciably splitting or shifting the transition energies. It is consistent with these observations to expect holes to be observable at room temperature in brominated samples since the homogeneous line widths of transitions may be expected to be close to the 3.2 cmt observed in BaFC1: Sm2 + at 300 K. Here we report results of hole burning experiments [8] in all three 4f6 7F 5D 0— 1 transitions (fig.
2+ 6 excited Fig. 1. Schematic energy level diagram for BaFCI05Br05:Sm showing hole burning/erasing scheme for all three 4f electronic states and indicating principal emission detected during read-out for each. Excited-state radiative lifetimes i~ are also given [91. .
S
1). The temperature dependence of the hole width has been studied between 1.8 and 133 K. Extrapolation of this data to room temperature gives a hole width of 17 cm’, contrary to expectations 2~ data mentioned derived from thesupposition BaFCl:Sm was substantiated above. Neither however, as photochemistry at room temperature resulted in non-selective spectrally resolved holes werebleaching observed.and Thenopowder used for this investigation had characteristics associated with type II samples in that, at 2 K, 50% deep holes could be burnt and deep transient holes were observed but also of type I samples in that the holes could be erased by irradiation corresponding to the 4f 55d absorption band of Sm2 + and holes burnt at 2 K could be cycled to room temperature without complete erasure.
2. Experimental details A powdered sample of BaFCI 0.5 Br
2 + was 05:Sm prepared as described previously [61. No lumines-
C. Wei eta!.
/
91
05Br05:Sm
cence corresponding to Sm3 ± impurities could be detected suggesting that the sample is likely to be type II as defined for BaFC1: Sm2 + [4]. The sample was placed in a modified Oxford Instruments MD-4 cryostat and could be cooled to 1.8 K through thermal contact to a pumped liquid helium
The full experimental arrangement is shown in fig. 2. A single-mode dye laser having a nominal linewidth of 1 MHz excited the 7F 5 D~transi0—of up to 30 tions and could be swept over ranges GHz to record hole profiles. The laser dyes used were DCM for excitation to the 5D 5D 0 and 5D1 levels and rhodamine 60 for excitation to the 2 level. The power reaching the sample, controlled by a Cambridge Research LS200 laser power stabiliser and neutral density filters, was typically 20 ~.tW—l0mW, focused to a 12)mm anddiameter at leastspot, an for burning (2—100 mW/cm order of magnitude less for reading. Burning times were typically of the order of 30 s. Increasing burn fluences were required at higher temperatures as the hole width increased though it was not necessary to increase reading fluences. Fluorescence, from the 5D~—~F~ transitions, was
reservoir. By removing the exchange gas from the sample space the temperature could be stabilised at 6 K whilst temperatures between 10 and 300 K could be selected using an Oxford Instruments ITC4 temperature controller coupled to a thermal element exchanger. Returning the temperature intothe 1.8heat K could be rapidly achieved by refilling the reservoir from the cryostat’s main helium tank. The temperature could be monitored to an accuracy of 0.1 K by measuring the resistance of a Lake Shore Cryotronics calibrated germanium resistor mounted next to the sample.
± F
laser__I~j~øu gating/erasing beam
2+
Spectra! hole burning in BaFC!
specuum
1
I:sj~.~~ ~1
dye laser
analyser
s~~~er
~ I
resonant beam
wavemeter
Ii
~
p
‘4
(
jstat
j::mmt~mmamrI
e
computer
single photon counting
~~~ean1 splitter \mirror
~
______
neutral density filter
()lens
~
shutter
Fig. 2. Schematic illustration of experimental arrangement. The photomultiplier could be placed directly behind the cut-off filters for measurements on the two transitions to higher energy.
92
C. Wei eta!.
/ Spectral hole burning in BaFCI0 5Br0 5.Sm2 +
directed to a photomultiplier shielded from parasitic laser scatter5Dby various cut-off filters and, in the case of the 0 level, with the addition of a Perkin—Elmer model 98 prism monochrornator. Gating light was provided by an argon ion laser operating at 514 nm. The gating power was usually chosen to be about five times that of the resonant beam as a compromise between long burning times necessitated by a low ratio and local heating of the sample caused by high incident power. To equalise conditions for gated hole burning in density all threeratio transitions the gating to resonant power was chosen to be equal for all experiments. Single-photon detecting electronics acted as an interface between the photomultiplier and a Digital Equipment Corporation PDP-11 computer. The computer was also used to control the laser scan and shutter action, 7F Data could be most accurately recorded for the 0—~D2transition as fluorescence from the two electronic transitions at lower energy could be easily distinguished from parasitic laser scatter with the use of cut-off filters. In the case of the 7F 5D 7F 0— 0 transition, weak fluorescence to the 1 level distinguish from backgroundwas dueharder to thetoclose proximity of the excitation and emission. Furthermore, the 7F 5D 0— 0 transient hole depth was much greater than the other transitions necessitating higher burnforfluences to compensate for the period of time in which the centres are not resonant with the laser irradiation. Consequently, studies of hole burning dynamics were largely confined to the 7F 5D 0— 2 transition. Higher read fluences are also required to cornpensate for the reduction in emission 7F 5D caused by transient hole burning in the 0— 0 transition, though the much higher gating efficiency allows this to be undertaken without degradation of the data. 3. Results and discussion A summary of hole burning and erasing mechanisms is given in fig. 3. Narrow 7Fspectral holes could be burnt in all three of the 0—~D1transitions (fig. 1) at 1.8 K. The gating efficiency 7F depended strongly on the transition. For the 0—
5D 0 transition, holes could only be observed with the presence of the gating beam, a gating ratio, determined by signal to noise considerations, of at 7F 5D least iO~.The gating ratio for the 0— 1 transition was of the order of 5 whereas the presence of gating light only slightly 7F 5D increased the hole burning rate for the 0— 2 transition. These data imply a threshold to produce persistent oxidation close7f~—5D to the energy of two photons resonant with the 1transition, that is 32000 cm’ (fig. 3). This corresponds well 2to oxidation thresholds + by varying the gating measured for BaFC1 light wavelength for :Sm hole burning in the 7F 0—~D0 transition [4]. Consequently, excited-state absorp5D tion of a second resonant photon from the 0 level does not provide the centre with enough energy for oxidation. 5D Excited state absorption of gating light from the 2 level may increase the probability of ionisation but any enhancement of the hole burning rate is hidden by hole refilling caused by single-photon absorption of the gating beam 7F (see 5D below). The measured gating ratio for the 0— 1 transition is similarly reduced.7F Gated and ungated holes burnt in the 0—~D2 transition to a depth of approximately 210% of were light subsequently irradiated with 50 mW/cm at 514 nm (fig. 4). Gated holes were shown to be consistently more stable. Electrons removedoffrom 2 + ions through excited-state absorption gatSm ing photons are excited higher into the conduction band than those which are removed through absorption of a second resonant photon. Consequently, it may be expected that more energetically remote traps may be reached through gated hole burning and that these holes will then be more resistant to refilling. As highly excited electrons are likely to have a short lifetime before relaxing to the bottom of the conduction band it is also possible that two different hole burning mechanisms exist, ungated hole burning relying on photon assisted tunnelling to a local trap and gated hole burning progressing through excitation of the electron to the conduction band from where more remote traps may be reached. Furthermore, in either case, holes created due to electrons being removed to shallow traps may be refilled by single-photon absorption of gating light during burning leaving only those which may not easily
C. Wei et aL
/ Spectral hole burning in BaFC105Br05:Sm2 + gating am increases hole-burning rate
~ Sc~ stron hole-burnin nhotons ca U g g
Energy
(cmi)
gating beam increases holeburning rate sufficiently to cancel effect ofrefilling due to gating beam and increases
final hole stability
gating beam essential
for hole-burning
93
two resonant photons cause weak hole-burning
32000
oxidation threshold
4f~5Sd bands
two resonant photons cause no hole-burning
4
4f65D
____
2 17900
—
4t’~~l)~ 15900 14500
—~
~
S —
/
/
/
/
/
______
____ —
.0
\ unknown traps ~ \ hole-burning \
hole refilling caused by single photon absorption of gating beam during photongated hole-burning in ALL transitions
reduces efficiency hole-burning of persistent
6 ~i’ 0
—
4f’
Fig. 3. Schematic summary of single-colour hole burning, photon-gated hole burning and hole-erasing processes which occur in BaFCI 2 ~. Line thicknesses qualitatively represent relative strengths. 0 5Br05 : Sm
be refilled. This is not possible for holes burnt with only resonant light. Figure 5 illustrates the dependence of the un~ated hole width on burning power density for the F 5D 2 transition with a typical fitted to0— a Lorentzian line shape inset.hole Theprofile broadening follows the usual form [10] but a theoretical fit has not been made as the exact hole burning scheme or schemes are not known. It would appear that the holes, when extrapolated to zero burning power, are still not laser line width limited. The fluorescence lifetime T 5D 1 of the 2 state has been measured as 0.72 ms [9] and so, invoking the usual relationship, 1 1 Fhomogeneous
=
+
~7,
where “homogeneous is the homogeneous line width, measured as being half the hole width for zero burning power and time (neglecting spectral diffu-
sion), it would seem that, even at 1.8 K, the pure dephasing time T is several orders of magnitude shorter. Hole burning studies of BaFCl:Sm2~ showed a strong dependence of homogeneous line2 + concentration indicating that dewidth onisSm phasing due to interactions between impurity molecules [41.Time-resolved studies of hole burning in the microsecond and millisecond regime in other rare-earth-doped crystals containing fluorine ions [11,12]have revealed strong optical dephasing due to nuclear interactions. In these cases, the large nuclear magnetic moment of the rare-earth ion causes a local magnetic-field perturbation, iso19F ions from the bulk lating the neighbouring and slowing spin exchange. The impurity ion is thus partially shielded from optical dephasing during the rapid-burning period but the hole is observed to broaden on a longer time scale, a spectral diffusion process. The Sm2 + ion in BaFC1 has no such large nuclear magnetic moment [13] and
94
C. Wei et a.
/
2+
Spectral hole burning in BaFCI
05 Br0 5:Sm
Hole-refilling (photon-gated hole)
1000 0.00
-
0.10
I
I
I
I
I
I
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
I
I
I
0.70
0.80
0.90
1.00
Frequency (GHz)
Hole-refilling (ungated hole) I
I
1100
0.00
0.10
0.20
0.30
0.40
0.50
0.60
1.00
Frequency (GHz) Fig. 4. Hole refilling for holes burnt in the 7F 5D 2 of resonant light. The upper trace was burnt with an additional 50 mW/cm2 of gating light. 0—Refilling 2 transition was caused in 50 sby using subsequent 10 mW/cm irradiation of the sample by the gating beam for 80 s. Despite background erasure due to intermediate read-out of the hole profiles, the photon-gated hole is seen to be more stable than that burnt with resonant light only. Hole shifts are due to laser drift.
so such spectral diffusion would not be expected. Interaction with bulk ‘9F ions, though weaker than in the other cases studied [11,12], may still cause optical dephasing and thus hole broadening.
It should be noted that persistent hole widths much greater than homogeneous widths, measured from transient hole spectra, have been reported previously in a Sm2 + system [14] but a full ex-
C. Wei et aL
/
05Br05:Sm
Power Dependence of Hole Width
100
95
2+
Spectral hole burning in BaFCI
I
I
80-
-
*
~60-
*
~t~II.
40-
laser limited hole width 0
.
0
Fig.
5.
•
I
I
100 150 Resonant Light Power Density (mW/cm”2)
50
200
Ungated hole widths for the 1F
0—~D2transition plotted as a function of burning power for a burn time of 10 s. Inset is a typical best fit to a Lorentzian line shape with residuals shown below.
planation of this phenomenon has yet to be given, It is thought that the effect of charge redistribution may play a role however, Hole widths burnt with constant fluence were found to follow a similar dependence on burn power to those burnt for constant time, indicating that the burn power is much more significant in the determination of hole width than the burn time, for unsaturated holes. Holes burnt for different times using constant power in the regime of linear hole growth showed little broadening, confirming this. Non-Gaussian emission inhomogeneous lineshapes, similar to those observed in this study for excitation (fig. 6), have been attributed to shifts in transition energy of the order of tens of wave numbers for samarium ions having different combinations of bromine and chlorine nearest neighbours [7]. No dependence of hole width on position within the inhomogeneously broadened excitation line was found however, showing the hole width to be independent of the Sm2 + nearest neighbours. This is consistent with dephasing due to interactions with both 59F ions and with other impurity ions. The temperature dependence of the gated hole
width for holes burnt and read at the same temperature between 1.8 and 90 K was investigated. It was found that power broadening at higher temperatures is much reduced and consequently data was not extrapolated to zero burning power in most cases. Instead, widths of holes with a depth of approximately 5—10% were measured and these are plotted in fig. 7. There are two distinct regions of temperature dependence and the boundary temperature is taken to be about 20 K. For temperatures hole a 29 higher relationship whereas the below 20 width K the obeys proporT tionality is to T13. The data below 20 K must be regarded as less reliablç than that above due to the scarcity of points and difficulty in extrapolating to zero burning power. Holes burnt above 20 K in all three transitions had very similar widths (fig. 8), the best-fit temperature dependencies being T29, T26 and T27 respectively. Holes burnt at temperatures lower than 20 K in different transitions had differing hole widths, becoming broader with decreasing transition energy. This effect is interpreted as being due, at least in part, to decreased persistent hole burning efficiency because of an increased transient hole depth resulting in the requirement of
96
C. Wei et aL
/ Spectral hole burning in BaFC105Br0
2+ 5.Sm
Zero Phonon Line Excitation Profile 1200
I
630 Wavelength (nm)
629
631
Fig. 6. Inhomogeneous zero-phonon line profile of the 7F
5D 0— 1 transition recorded in excitation. The non-Gaussian shape has been previously observed in emission and attributed to varying impurity ligand configurations [7].
27 higher burn tofluences. Extrapolation thewidth T dependence 300 K gives a potentialofhole of 500 GHz (17 wave numbers), somewhat higher than the BaFCl:Sm2~ room-temperature homogeneous line width but suggesting that a few holes could be burnt in the inhomogeneously broadened
zero-phonon lines of BaFCl 2~at room 05Br05:Sm temperature if the temperature dependence remains constant.
Hole burning in the 7F
5D 0— 1 transition was attempted at higher temperatures. A hole burnt at 133 K, burnt and scanned with a home-made
Temperature Dependence of Hole Width 10.00
~i
-
T 2.91
1.00-
‘0 SI
0
0.10
-
-
1.27
T 1
-
10 Temperature (K)
100
5D T’3, respectively. Fig. 7. 7F Temperature dependence of gated hole widths for holes of depth 5—10% burnt and read 29 at and the same temperature in the 0— 2 transition. The best-fit temperature dependences above and below about 20 K are T
C. Wei et aL I
*
15
/ Spectral hole burning in BaFCI05Br05:Sm2 +
Temperature Dependence of Hole Width 1
I
I
i
I
35
40
50
97
I
I
70
80
I
5D2/T42.9t
20
25
30
60
90 toO
Temperature (K) Fig. 8. Temperature dependence of gated hole widths for holes of depth 5—10% burnt and read at the same temperature in all three 4f6 7F 27, T2’6 and T29. 1~—~DJ transitions. The best-fit temperature dependences are, from low to high energy, T
multi-mode dye laser with a total line width of, nominally, 30 GHz, is shown in fig. 9. The hole width lies close to that obtained by extrapolation from the dependence between 20 and 90 K. At room temperature persistent bleaching was observed but no discernible hole could be dis-
tinguished. Holes could be cycled to room temperature though significant hole refilling occurs. Figure 10 shows a holeburnt at 1.8 K before and after cycling to room temperature over a period of approximately one hour. The data presented here suggests that BaFC1 05
Hole at 133 K
.200
I
I
I
.100
0 Frequency (GHz)
100
200
7F Fig. 9. Persistent spectral hole burnt at 133 K in the 0 —~D5 transition near the centre of the inhomogeneous line at 630.1 nm using 30 mW of resonant and gating light for 30 mm. Dashed line approximately indicates original profile.
98
C. Wei et aL
/ Spectral hole burning in
2+
BaFCI
0 5Br0 5:Sm
Hole before and after cycling to 300 K 1500
I
~I400T~T
1000 0
I
I
I
I
I
1
2
3
4
Frequency (GHz) 7F 5D Fig. 10. Deep, power-broadened photon-gated spectral hole burnt in the 0— 1 transition at 1.8 K before (shifted 100 units lower) cycling the temperature to 300 K during a period of approximately one hour. Frequency shift is attributed to laser drift.
2~ is an anomalous material. X-ray difBr0 5:Sm studies have shown the structure to be fraction polycrystalline [6] but the temperature dependence of the hole width does not show behaviour typical for ordered materials. A T2 dependence due to two-phonon scattering processes would be expected above the Debye temperature but a value of 400 K has been suggested for BaFC1: Sm2 + [15] and a similar value would be expected for BaFCI 2 A significant lowering of the Sm Debye 05Br05 temperature has been observed in glasses ~.
and ascribed thefrequencies large density of materials two-level system states attolow in such [16]. The temperature dependence is, nonetheless, even stronger than T2 in this case and no present theory seems to predict the observed behaviour. The temperature dependence of the hole width below 20 K is also characteristic of an amorphous material. Many studies have shown dependencies 2 for glasses [17], T’3 being of between Tcommon and T and explained in terms of particularly off-diagonal modulation plus exchange coupling via low-frequency impurity-induced phonons [18]. Other theories predict similar results [19] and a more complete study of the temperature dependence at low temperature may shed more light on the unique nature of BaFCl 2~.Further 05Br05:Sm
to this, the temperature dependence of the hole shift has been measured to be T4 [20], as expected for crystalline materials, but with a coupling constant more typical of amorphous materials. A previous study of an inorganic crystalline material showing hole burning in three electronic transitions, SrWO 6 + [21], showed an enormous 4: U dependence of hole width on transition, as would be expected for ions relaxing to the lowest excited electronic state non-radiatively. The non-radiative decay rates of the 5D 5D 1 and different 2 excited states in 2 ± show radically temperature BaFC1 Sm dependencies between 50—100 K [22]. The hole widths in BaFCl 2~ are similar for all 05Br05:Sm three transitions however, indicating that this process plays no role in the determination of hole width. The 4f6 7F 0—~D1 transitions involve inner electrons which are shielded from phonon coupling, as structure evidencedinbythetheabsorption very smalland amount of vibronic fluorescence spectra of rare-earth ions in crystals at low temperatures. Such weak phonon coupling would suggest that this process cannot account for hole widths several orders of magnitude larger than the lifetime-limited value at 1.8 K. If nuclear magnetic interactions are too weak to cause such broadening, it is possible that spectral hole widths mea-
C. Wei et al.
/
2+
Spectral hole burning in BaFCI
sured on the present time scale may not provide a good measure of homogeneous line width and that a process such as spectral diffusion due to coupling to two-level systems, also associated more frequently with amorphous materials [23], may be responsible. The precise nature of the broadening mechanism in the material studied here is not yet understood but occurs on a fast time scale as hole profiles reread up to an hour after an initial scan showed no increase in hole width. High-speed time-resolved hole burning measurements or fluorescence-detected photon echo experiments [24] may prove useful in answering this question.
99
05Br05:Sm
a closely related material with technological applicability.
Acknowledgements The authors would like to thank Prof. J. Yu of the Changchun Institute of Physics for the kind gift of the sample of BaFCl 2~powder 05Br05:Sm used in this study. This work was supported by the Swiss National Science Foundation.
References
4. Conclusion Persistent spectral holes have been burnt in the three zero-phonon lines corresponding to the 4f6 7F 0—~D~electronic transitions in BaFC105 2 + and parameters such as power broadBr05: Smhole stability and gating ratios have been ening, investigated. The temperature dependence of the hole width at temperatures above 20 K has been found to be similar for all three transitions, at T28 ±02, Persistent holes have been burnt at temperatures up to 133 K and it is predicted that stable holes could be burnt at higher temperatures. It is thought spectral diffusion may play a role in the determination of the hole width. This is unusual for inorganic crystalline materials as is the hole width temperature dependence below 20 K, both being characteristic of amorphous materials. These results contribute towards the search for a photon-gated hole burning material with a high inhomogeneous to homogeneous line width ratio at room temperature. The introduction of disorder into materials already known to exhibit photongated hole burning, and those known to be thermally cyclable to room temperature, appears to be a promising direction in which to pursue this goal. Whilst the sample studied here is still unsuitable for device implementation, the variability of such materials [4] suggests that further attention to sample preparation and composition could reveal
[1] A. Renn, C. De Caro and UP. Wild, Jpn. J. AppI. Phys. 28 (1989) 257. [2] U.P. Wild, C. Dc Caro, S. Bernet, M. Traber and A. Renn, J. Lumin. 48 & 49 (1991) 335. [3] W.E. Moerner and M.D. Levenson, J. Opt. Soc. Am. B 2 (1985) 915.
141
A. Winnacker, R.M. Shelby and R.M. Macfarlane, Opt. Lett. 10 (1985) 350. [5] N. Karl, H. Heym and J.J. Stezowski, Mol. Cryst. Liq. Cryst. 131 (1985) 163. [6] C. Wei, S. Huang and J. Yu, J. Lumin. 43 (1989) 161. [7] L. Zhang, J. Yu and S. Huang, J. Lumin. 45 (1990) 301. [8] First reported at tnt. Conf. on Luminescence, Lisbon, July 1990.. [9] J.C. Gâcon, State Doctorate Thesis, Lyon University (1978). [10] H. de Vries and D.A. Wiersma, J. Chem. Phys. 72 (1980) 1851. [11] R.M. Shelby and R.M. Macfarlane, J. Lumin. 31 &32 (1984) 839. [12] R. Wannemacher, R.S. Meltzer and R.M. Macfarlane, J. Lumin. 45 (1990) 307. [13] R.M. Macfarlane, R.M. Shelby and A. Winnacker, Phys. Rev. B 33 (1986) 4207. [14] R.J. Danby, K. Holliday and NB. Manson, J. Lumin. 42 (1988) 83. [15] B. Birang, A.S.M. Mahbub’ul Alam and B. Di Bartolo, J. Chem. Phys. 50 (1969) 2750. [16] D.L. Huber, J. Non-Cryst. Solids 51(1982) 241. [17] 5. Vdlker, in: Relaxation Processes in Molecular Excited States, ed. J. Funfschilling (Kluwer, Dordrecht, 1989) p. 113. [18] B. 331.Jackson and R. Silbey, Chem. Phys. Lett. 99 (1983) [19] J. M. Hayes, R. Jankowiak and G.J. Small, in: Persistent Spectral Hole-Burning: Science and Applications, ed. W.E. Moerner (Springer. Berlin, 1988) p. 167.
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2+ 05Br05:Sm [23] M. Berg, C.A. Walsh, L.R. Narasimhan, K.A. Littau and M.D. Fayer, J. Chem. Phys. 88 (1988) 1564. [24] K. Uchikawa, H. Ohsawa, T. Suga and S. Saikan, Opt. Lett. 16 (1991) 13.
/ Spectral hole burning in
[20] A. Oppenlander, F. Madeore, J.-C. Vial and J.-P. Chaminade, J. Lumin. 50 (1991) 1. [21] K. Holliday and N.B. Manson, J. Phys. Condens. Mat. 1 (1989) 1339. [22] G.C. Gâcon, J.C. Souillat, J. Seriot, F. Gaume-Mahn and B. Di Bartolo, J. Lumin. 18 & 19 (1979) 244.
BaFC1
89
A spectral hole burning study of BaFCl05Br05:Sm2~ Changjiang Wei
~, Keith Holliday, Alfred J. Meixner 2 Mauro Croci and Urs P. Wild Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zen:rwn. CH-8092 Zurich, Switzerland
Received 6 November 1990 Revised 18 March 1991 Accepted 19 March 1991
Investigations of parameters for two-photon hole burning (single colour and photon-gated) and hole erasing in the three 4f6 7F 5D 2 + are presented. This material is of particular interest as it has unusually large zero-phonon line widths allowing holes to be burnt at relatively high temperatures. For the transition to 0 — inhomogeneous 3 electronic transitions in BaFCI05 Br05: Sm the 5D 5D 0 excited state, only photon-gated holes could be burnt whereas gating light-enhanced single-colour hole burning 5D in the 1 transition by a factor of about five. Gating light did not significantly increase the hole burning rate for the 2 transition but did increase the resistance of the holes to erasing by irradiation from the gating beam alone. Such erasing during the burning process decreases the effective gating ratios for all transitions. The hole width burning power 5D dependence was 3 investigated, the minimum hole width at 1.8 K being 14 MHz. The temperature dependence of the 2 hole width was investigated 1.8 and 90 K, suggesting regions. Below about 20 K the width found to beinT’ whilst above between 20 K it was found to be T2-9. A two similar temperature dependence washole found fordependence the widths was of holes burnt the other two transitions in the high-temperature regime but the holes were generally broader below 20 K. This was attributed to the requirement of higher burn and read power densities. Holes could be burnt at temperatures up to 133 K and could be cycled from 1.8 K to room temperature, becoming much shallower in the process.
1. Introduction Persistent spectral hole burning has now been observed in many materials, providing an enormous increase in the resolution of data available from optical experiments and consequently revealing much about physical processes occurring in solids. Spectral hole burning also shows great promise as a technique for the implementation of technological goals such as high-density image storage [1] and molecular computing [2] but the search for a material possessing the required properties to produce an efficient digital memory device [3] continues. Such a hole burning memory
2
Present address: Laser Physics Centre. Research School of Physical Sciences. Australian National University. Canberra, ACT 2601. Australia Present address: IBM Almaden Research Center. 650 Harry Road. San Jose, California 95120-6099, USA
0022-2313/91/$03.50 © 1991
—
represents ‘l’s or ‘0’s as the presence or absence of spectral holes produced across the range of an inhomogeneously broadened band. Two fundamental problems rule out most materials; the degradation of data caused by repeated reading and the rapid increase in hole instability and width with increasing temperature. Photon-gated hole burning circumvents the first difficulty. Here a second beam, at a different wavelength, greatly enhances the hole burning rate. Only light at the resonant frequency is required to probe for the existence of a hole and therefore the information may be accessed repeatedly with little degradation. Photon-gated spectral holes were first burnt in the 4f6 7F 5D 2~ 0— as 0 transition in BaFCl:Sm [4] with a gating ratio high as 100, that is, the hole burning rate is enhanced by this factor upon .
addttton of the gating beam. The mechantsm for hole burning was attributed to excited state absorption of a gating photon leading to oxidation
Elsevier Science Publishers B.V. (North-Holland)
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C. Wei et al.
/ Spectral hole burning in BaFC10 5Br0
of the Sm2~ion to Sm3t In samples which have not been fully reduced (type I), Sm3 + ions may act as traps for the released electrons. In such samples holes could be erased through irradiation of the sample with light corresponding to the 4f 55d absorption band of Sm2~,the interpretation being that this non-selectively releases electrons into the conduction band where they may be captured by Sm3~ions. In samples containing no Sm3~ions (type II), other traps act as electron acceptors and
E
2+ 5:Sm gating beam (514 nm)
-t
(cm 30000
4f55d b~ds 20000 ____________
hole erasure was not observed. Additionally, type II samples showed strong single-colour transient hole burning. It was speculated that transient holes with a lifetime of less than 1 ms and depths of almost 100% may be attributable to electron ejection to energetically nearby traps from which re3~is rapid. combination with burnt Sm in the same transition in Spectral holes certain samples of BaFCl:Sm2~were found to be
4f05D 5D 2(560nm):t=0.72ms 41’s65D1 (630 nm): t=t.0 ms 4f 0 (688 nm): r= 1.5
burning beams (thin, up) principal emission detected (down) gating/erasing beam (thick, up) _____________
4f67F 0
resistant to cycling to room temperature [4] showing the photoproduct to detected be stable at at high 300 K. However, holes could not be temperatures because the transition is homogeneously broadened at 77 K and above, A study of anthracene in 2,3-dimethylnaphthalene mixed crystals [5] demonstrated the effectiveness of introducing statistical disorder to the host lattice to increase inhomogeneous line widths. Wei et al. [6] succeeded in broadening the inhomogeneous line widths of the 4f6 7F 0—~D~ transitions in BaFCl:Sm2~through the addition of bromine to the melt. Holes with a width of 1.1 cm’ were burnt in the 7F 5D 2 transition in 2 ± at 77 K 0— suggesting that the BaFC1 05Br05: Sm homogeneous line width of the transition is not greatly affected by the addition of bromine. However, the inhomogeneous line widths of the transitions are broadened to approximately 50 cm’, due to substitutional ligand disorder [7], without appreciably splitting or shifting the transition energies. It is consistent with these observations to expect holes to be observable at room temperature in brominated samples since the homogeneous line widths of transitions may be expected to be close to the 3.2 cmt observed in BaFC1: Sm2 + at 300 K. Here we report results of hole burning experiments [8] in all three 4f6 7F 5D 0— 1 transitions (fig.
2+ 6 excited Fig. 1. Schematic energy level diagram for BaFCI05Br05:Sm showing hole burning/erasing scheme for all three 4f electronic states and indicating principal emission detected during read-out for each. Excited-state radiative lifetimes i~ are also given [91. .
S
1). The temperature dependence of the hole width has been studied between 1.8 and 133 K. Extrapolation of this data to room temperature gives a hole width of 17 cm’, contrary to expectations 2~ data mentioned derived from thesupposition BaFCl:Sm was substantiated above. Neither however, as photochemistry at room temperature resulted in non-selective spectrally resolved holes werebleaching observed.and Thenopowder used for this investigation had characteristics associated with type II samples in that, at 2 K, 50% deep holes could be burnt and deep transient holes were observed but also of type I samples in that the holes could be erased by irradiation corresponding to the 4f 55d absorption band of Sm2 + and holes burnt at 2 K could be cycled to room temperature without complete erasure.
2. Experimental details A powdered sample of BaFCI 0.5 Br
2 + was 05:Sm prepared as described previously [61. No lumines-
C. Wei eta!.
/
91
05Br05:Sm
cence corresponding to Sm3 ± impurities could be detected suggesting that the sample is likely to be type II as defined for BaFC1: Sm2 + [4]. The sample was placed in a modified Oxford Instruments MD-4 cryostat and could be cooled to 1.8 K through thermal contact to a pumped liquid helium
The full experimental arrangement is shown in fig. 2. A single-mode dye laser having a nominal linewidth of 1 MHz excited the 7F 5 D~transi0—of up to 30 tions and could be swept over ranges GHz to record hole profiles. The laser dyes used were DCM for excitation to the 5D 5D 0 and 5D1 levels and rhodamine 60 for excitation to the 2 level. The power reaching the sample, controlled by a Cambridge Research LS200 laser power stabiliser and neutral density filters, was typically 20 ~.tW—l0mW, focused to a 12)mm anddiameter at leastspot, an for burning (2—100 mW/cm order of magnitude less for reading. Burning times were typically of the order of 30 s. Increasing burn fluences were required at higher temperatures as the hole width increased though it was not necessary to increase reading fluences. Fluorescence, from the 5D~—~F~ transitions, was
reservoir. By removing the exchange gas from the sample space the temperature could be stabilised at 6 K whilst temperatures between 10 and 300 K could be selected using an Oxford Instruments ITC4 temperature controller coupled to a thermal element exchanger. Returning the temperature intothe 1.8heat K could be rapidly achieved by refilling the reservoir from the cryostat’s main helium tank. The temperature could be monitored to an accuracy of 0.1 K by measuring the resistance of a Lake Shore Cryotronics calibrated germanium resistor mounted next to the sample.
± F
laser__I~j~øu gating/erasing beam
2+
Spectra! hole burning in BaFC!
specuum
1
I:sj~.~~ ~1
dye laser
analyser
s~~~er
~ I
resonant beam
wavemeter
Ii
~
p
‘4
(
jstat
j::mmt~mmamrI
e
computer
single photon counting
~~~ean1 splitter \mirror
~
______
neutral density filter
()lens
~
shutter
Fig. 2. Schematic illustration of experimental arrangement. The photomultiplier could be placed directly behind the cut-off filters for measurements on the two transitions to higher energy.
92
C. Wei eta!.
/ Spectral hole burning in BaFCI0 5Br0 5.Sm2 +
directed to a photomultiplier shielded from parasitic laser scatter5Dby various cut-off filters and, in the case of the 0 level, with the addition of a Perkin—Elmer model 98 prism monochrornator. Gating light was provided by an argon ion laser operating at 514 nm. The gating power was usually chosen to be about five times that of the resonant beam as a compromise between long burning times necessitated by a low ratio and local heating of the sample caused by high incident power. To equalise conditions for gated hole burning in density all threeratio transitions the gating to resonant power was chosen to be equal for all experiments. Single-photon detecting electronics acted as an interface between the photomultiplier and a Digital Equipment Corporation PDP-11 computer. The computer was also used to control the laser scan and shutter action, 7F Data could be most accurately recorded for the 0—~D2transition as fluorescence from the two electronic transitions at lower energy could be easily distinguished from parasitic laser scatter with the use of cut-off filters. In the case of the 7F 5D 7F 0— 0 transition, weak fluorescence to the 1 level distinguish from backgroundwas dueharder to thetoclose proximity of the excitation and emission. Furthermore, the 7F 5D 0— 0 transient hole depth was much greater than the other transitions necessitating higher burnforfluences to compensate for the period of time in which the centres are not resonant with the laser irradiation. Consequently, studies of hole burning dynamics were largely confined to the 7F 5D 0— 2 transition. Higher read fluences are also required to cornpensate for the reduction in emission 7F 5D caused by transient hole burning in the 0— 0 transition, though the much higher gating efficiency allows this to be undertaken without degradation of the data. 3. Results and discussion A summary of hole burning and erasing mechanisms is given in fig. 3. Narrow 7Fspectral holes could be burnt in all three of the 0—~D1transitions (fig. 1) at 1.8 K. The gating efficiency 7F depended strongly on the transition. For the 0—
5D 0 transition, holes could only be observed with the presence of the gating beam, a gating ratio, determined by signal to noise considerations, of at 7F 5D least iO~.The gating ratio for the 0— 1 transition was of the order of 5 whereas the presence of gating light only slightly 7F 5D increased the hole burning rate for the 0— 2 transition. These data imply a threshold to produce persistent oxidation close7f~—5D to the energy of two photons resonant with the 1transition, that is 32000 cm’ (fig. 3). This corresponds well 2to oxidation thresholds + by varying the gating measured for BaFC1 light wavelength for :Sm hole burning in the 7F 0—~D0 transition [4]. Consequently, excited-state absorp5D tion of a second resonant photon from the 0 level does not provide the centre with enough energy for oxidation. 5D Excited state absorption of gating light from the 2 level may increase the probability of ionisation but any enhancement of the hole burning rate is hidden by hole refilling caused by single-photon absorption of the gating beam 7F (see 5D below). The measured gating ratio for the 0— 1 transition is similarly reduced.7F Gated and ungated holes burnt in the 0—~D2 transition to a depth of approximately 210% of were light subsequently irradiated with 50 mW/cm at 514 nm (fig. 4). Gated holes were shown to be consistently more stable. Electrons removedoffrom 2 + ions through excited-state absorption gatSm ing photons are excited higher into the conduction band than those which are removed through absorption of a second resonant photon. Consequently, it may be expected that more energetically remote traps may be reached through gated hole burning and that these holes will then be more resistant to refilling. As highly excited electrons are likely to have a short lifetime before relaxing to the bottom of the conduction band it is also possible that two different hole burning mechanisms exist, ungated hole burning relying on photon assisted tunnelling to a local trap and gated hole burning progressing through excitation of the electron to the conduction band from where more remote traps may be reached. Furthermore, in either case, holes created due to electrons being removed to shallow traps may be refilled by single-photon absorption of gating light during burning leaving only those which may not easily
C. Wei et aL
/ Spectral hole burning in BaFC105Br05:Sm2 + gating am increases hole-burning rate
~ Sc~ stron hole-burnin nhotons ca U g g
Energy
(cmi)
gating beam increases holeburning rate sufficiently to cancel effect ofrefilling due to gating beam and increases
final hole stability
gating beam essential
for hole-burning
93
two resonant photons cause weak hole-burning
32000
oxidation threshold
4f~5Sd bands
two resonant photons cause no hole-burning
4
4f65D
____
2 17900
—
4t’~~l)~ 15900 14500
—~
~
S —
/
/
/
/
/
______
____ —
.0
\ unknown traps ~ \ hole-burning \
hole refilling caused by single photon absorption of gating beam during photongated hole-burning in ALL transitions
reduces efficiency hole-burning of persistent
6 ~i’ 0
—
4f’
Fig. 3. Schematic summary of single-colour hole burning, photon-gated hole burning and hole-erasing processes which occur in BaFCI 2 ~. Line thicknesses qualitatively represent relative strengths. 0 5Br05 : Sm
be refilled. This is not possible for holes burnt with only resonant light. Figure 5 illustrates the dependence of the un~ated hole width on burning power density for the F 5D 2 transition with a typical fitted to0— a Lorentzian line shape inset.hole Theprofile broadening follows the usual form [10] but a theoretical fit has not been made as the exact hole burning scheme or schemes are not known. It would appear that the holes, when extrapolated to zero burning power, are still not laser line width limited. The fluorescence lifetime T 5D 1 of the 2 state has been measured as 0.72 ms [9] and so, invoking the usual relationship, 1 1 Fhomogeneous
=
+
~7,
where “homogeneous is the homogeneous line width, measured as being half the hole width for zero burning power and time (neglecting spectral diffu-
sion), it would seem that, even at 1.8 K, the pure dephasing time T is several orders of magnitude shorter. Hole burning studies of BaFCl:Sm2~ showed a strong dependence of homogeneous line2 + concentration indicating that dewidth onisSm phasing due to interactions between impurity molecules [41.Time-resolved studies of hole burning in the microsecond and millisecond regime in other rare-earth-doped crystals containing fluorine ions [11,12]have revealed strong optical dephasing due to nuclear interactions. In these cases, the large nuclear magnetic moment of the rare-earth ion causes a local magnetic-field perturbation, iso19F ions from the bulk lating the neighbouring and slowing spin exchange. The impurity ion is thus partially shielded from optical dephasing during the rapid-burning period but the hole is observed to broaden on a longer time scale, a spectral diffusion process. The Sm2 + ion in BaFC1 has no such large nuclear magnetic moment [13] and
94
C. Wei et a.
/
2+
Spectral hole burning in BaFCI
05 Br0 5:Sm
Hole-refilling (photon-gated hole)
1000 0.00
-
0.10
I
I
I
I
I
I
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
I
I
I
0.70
0.80
0.90
1.00
Frequency (GHz)
Hole-refilling (ungated hole) I
I
1100
0.00
0.10
0.20
0.30
0.40
0.50
0.60
1.00
Frequency (GHz) Fig. 4. Hole refilling for holes burnt in the 7F 5D 2 of resonant light. The upper trace was burnt with an additional 50 mW/cm2 of gating light. 0—Refilling 2 transition was caused in 50 sby using subsequent 10 mW/cm irradiation of the sample by the gating beam for 80 s. Despite background erasure due to intermediate read-out of the hole profiles, the photon-gated hole is seen to be more stable than that burnt with resonant light only. Hole shifts are due to laser drift.
so such spectral diffusion would not be expected. Interaction with bulk ‘9F ions, though weaker than in the other cases studied [11,12], may still cause optical dephasing and thus hole broadening.
It should be noted that persistent hole widths much greater than homogeneous widths, measured from transient hole spectra, have been reported previously in a Sm2 + system [14] but a full ex-
C. Wei et aL
/
05Br05:Sm
Power Dependence of Hole Width
100
95
2+
Spectral hole burning in BaFCI
I
I
80-
-
*
~60-
*
~t~II.
40-
laser limited hole width 0
.
0
Fig.
5.
•
I
I
100 150 Resonant Light Power Density (mW/cm”2)
50
200
Ungated hole widths for the 1F
0—~D2transition plotted as a function of burning power for a burn time of 10 s. Inset is a typical best fit to a Lorentzian line shape with residuals shown below.
planation of this phenomenon has yet to be given, It is thought that the effect of charge redistribution may play a role however, Hole widths burnt with constant fluence were found to follow a similar dependence on burn power to those burnt for constant time, indicating that the burn power is much more significant in the determination of hole width than the burn time, for unsaturated holes. Holes burnt for different times using constant power in the regime of linear hole growth showed little broadening, confirming this. Non-Gaussian emission inhomogeneous lineshapes, similar to those observed in this study for excitation (fig. 6), have been attributed to shifts in transition energy of the order of tens of wave numbers for samarium ions having different combinations of bromine and chlorine nearest neighbours [7]. No dependence of hole width on position within the inhomogeneously broadened excitation line was found however, showing the hole width to be independent of the Sm2 + nearest neighbours. This is consistent with dephasing due to interactions with both 59F ions and with other impurity ions. The temperature dependence of the gated hole
width for holes burnt and read at the same temperature between 1.8 and 90 K was investigated. It was found that power broadening at higher temperatures is much reduced and consequently data was not extrapolated to zero burning power in most cases. Instead, widths of holes with a depth of approximately 5—10% were measured and these are plotted in fig. 7. There are two distinct regions of temperature dependence and the boundary temperature is taken to be about 20 K. For temperatures hole a 29 higher relationship whereas the below 20 width K the obeys proporT tionality is to T13. The data below 20 K must be regarded as less reliablç than that above due to the scarcity of points and difficulty in extrapolating to zero burning power. Holes burnt above 20 K in all three transitions had very similar widths (fig. 8), the best-fit temperature dependencies being T29, T26 and T27 respectively. Holes burnt at temperatures lower than 20 K in different transitions had differing hole widths, becoming broader with decreasing transition energy. This effect is interpreted as being due, at least in part, to decreased persistent hole burning efficiency because of an increased transient hole depth resulting in the requirement of
96
C. Wei et aL
/ Spectral hole burning in BaFC105Br0
2+ 5.Sm
Zero Phonon Line Excitation Profile 1200
I
630 Wavelength (nm)
629
631
Fig. 6. Inhomogeneous zero-phonon line profile of the 7F
5D 0— 1 transition recorded in excitation. The non-Gaussian shape has been previously observed in emission and attributed to varying impurity ligand configurations [7].
27 higher burn tofluences. Extrapolation thewidth T dependence 300 K gives a potentialofhole of 500 GHz (17 wave numbers), somewhat higher than the BaFCl:Sm2~ room-temperature homogeneous line width but suggesting that a few holes could be burnt in the inhomogeneously broadened
zero-phonon lines of BaFCl 2~at room 05Br05:Sm temperature if the temperature dependence remains constant.
Hole burning in the 7F
5D 0— 1 transition was attempted at higher temperatures. A hole burnt at 133 K, burnt and scanned with a home-made
Temperature Dependence of Hole Width 10.00
~i
-
T 2.91
1.00-
‘0 SI
0
0.10
-
-
1.27
T 1
-
10 Temperature (K)
100
5D T’3, respectively. Fig. 7. 7F Temperature dependence of gated hole widths for holes of depth 5—10% burnt and read 29 at and the same temperature in the 0— 2 transition. The best-fit temperature dependences above and below about 20 K are T
C. Wei et aL I
*
15
/ Spectral hole burning in BaFCI05Br05:Sm2 +
Temperature Dependence of Hole Width 1
I
I
i
I
35
40
50
97
I
I
70
80
I
5D2/T42.9t
20
25
30
60
90 toO
Temperature (K) Fig. 8. Temperature dependence of gated hole widths for holes of depth 5—10% burnt and read at the same temperature in all three 4f6 7F 27, T2’6 and T29. 1~—~DJ transitions. The best-fit temperature dependences are, from low to high energy, T
multi-mode dye laser with a total line width of, nominally, 30 GHz, is shown in fig. 9. The hole width lies close to that obtained by extrapolation from the dependence between 20 and 90 K. At room temperature persistent bleaching was observed but no discernible hole could be dis-
tinguished. Holes could be cycled to room temperature though significant hole refilling occurs. Figure 10 shows a holeburnt at 1.8 K before and after cycling to room temperature over a period of approximately one hour. The data presented here suggests that BaFC1 05
Hole at 133 K
.200
I
I
I
.100
0 Frequency (GHz)
100
200
7F Fig. 9. Persistent spectral hole burnt at 133 K in the 0 —~D5 transition near the centre of the inhomogeneous line at 630.1 nm using 30 mW of resonant and gating light for 30 mm. Dashed line approximately indicates original profile.
98
C. Wei et aL
/ Spectral hole burning in
2+
BaFCI
0 5Br0 5:Sm
Hole before and after cycling to 300 K 1500
I
~I400T~T
1000 0
I
I
I
I
I
1
2
3
4
Frequency (GHz) 7F 5D Fig. 10. Deep, power-broadened photon-gated spectral hole burnt in the 0— 1 transition at 1.8 K before (shifted 100 units lower) cycling the temperature to 300 K during a period of approximately one hour. Frequency shift is attributed to laser drift.
2~ is an anomalous material. X-ray difBr0 5:Sm studies have shown the structure to be fraction polycrystalline [6] but the temperature dependence of the hole width does not show behaviour typical for ordered materials. A T2 dependence due to two-phonon scattering processes would be expected above the Debye temperature but a value of 400 K has been suggested for BaFC1: Sm2 + [15] and a similar value would be expected for BaFCI 2 A significant lowering of the Sm Debye 05Br05 temperature has been observed in glasses ~.
and ascribed thefrequencies large density of materials two-level system states attolow in such [16]. The temperature dependence is, nonetheless, even stronger than T2 in this case and no present theory seems to predict the observed behaviour. The temperature dependence of the hole width below 20 K is also characteristic of an amorphous material. Many studies have shown dependencies 2 for glasses [17], T’3 being of between Tcommon and T and explained in terms of particularly off-diagonal modulation plus exchange coupling via low-frequency impurity-induced phonons [18]. Other theories predict similar results [19] and a more complete study of the temperature dependence at low temperature may shed more light on the unique nature of BaFCl 2~.Further 05Br05:Sm
to this, the temperature dependence of the hole shift has been measured to be T4 [20], as expected for crystalline materials, but with a coupling constant more typical of amorphous materials. A previous study of an inorganic crystalline material showing hole burning in three electronic transitions, SrWO 6 + [21], showed an enormous 4: U dependence of hole width on transition, as would be expected for ions relaxing to the lowest excited electronic state non-radiatively. The non-radiative decay rates of the 5D 5D 1 and different 2 excited states in 2 ± show radically temperature BaFC1 Sm dependencies between 50—100 K [22]. The hole widths in BaFCl 2~ are similar for all 05Br05:Sm three transitions however, indicating that this process plays no role in the determination of hole width. The 4f6 7F 0—~D1 transitions involve inner electrons which are shielded from phonon coupling, as structure evidencedinbythetheabsorption very smalland amount of vibronic fluorescence spectra of rare-earth ions in crystals at low temperatures. Such weak phonon coupling would suggest that this process cannot account for hole widths several orders of magnitude larger than the lifetime-limited value at 1.8 K. If nuclear magnetic interactions are too weak to cause such broadening, it is possible that spectral hole widths mea-
C. Wei et al.
/
2+
Spectral hole burning in BaFCI
sured on the present time scale may not provide a good measure of homogeneous line width and that a process such as spectral diffusion due to coupling to two-level systems, also associated more frequently with amorphous materials [23], may be responsible. The precise nature of the broadening mechanism in the material studied here is not yet understood but occurs on a fast time scale as hole profiles reread up to an hour after an initial scan showed no increase in hole width. High-speed time-resolved hole burning measurements or fluorescence-detected photon echo experiments [24] may prove useful in answering this question.
99
05Br05:Sm
a closely related material with technological applicability.
Acknowledgements The authors would like to thank Prof. J. Yu of the Changchun Institute of Physics for the kind gift of the sample of BaFCl 2~powder 05Br05:Sm used in this study. This work was supported by the Swiss National Science Foundation.
References
4. Conclusion Persistent spectral holes have been burnt in the three zero-phonon lines corresponding to the 4f6 7F 0—~D~electronic transitions in BaFC105 2 + and parameters such as power broadBr05: Smhole stability and gating ratios have been ening, investigated. The temperature dependence of the hole width at temperatures above 20 K has been found to be similar for all three transitions, at T28 ±02, Persistent holes have been burnt at temperatures up to 133 K and it is predicted that stable holes could be burnt at higher temperatures. It is thought spectral diffusion may play a role in the determination of the hole width. This is unusual for inorganic crystalline materials as is the hole width temperature dependence below 20 K, both being characteristic of amorphous materials. These results contribute towards the search for a photon-gated hole burning material with a high inhomogeneous to homogeneous line width ratio at room temperature. The introduction of disorder into materials already known to exhibit photongated hole burning, and those known to be thermally cyclable to room temperature, appears to be a promising direction in which to pursue this goal. Whilst the sample studied here is still unsuitable for device implementation, the variability of such materials [4] suggests that further attention to sample preparation and composition could reveal
[1] A. Renn, C. De Caro and UP. Wild, Jpn. J. AppI. Phys. 28 (1989) 257. [2] U.P. Wild, C. Dc Caro, S. Bernet, M. Traber and A. Renn, J. Lumin. 48 & 49 (1991) 335. [3] W.E. Moerner and M.D. Levenson, J. Opt. Soc. Am. B 2 (1985) 915.
141
A. Winnacker, R.M. Shelby and R.M. Macfarlane, Opt. Lett. 10 (1985) 350. [5] N. Karl, H. Heym and J.J. Stezowski, Mol. Cryst. Liq. Cryst. 131 (1985) 163. [6] C. Wei, S. Huang and J. Yu, J. Lumin. 43 (1989) 161. [7] L. Zhang, J. Yu and S. Huang, J. Lumin. 45 (1990) 301. [8] First reported at tnt. Conf. on Luminescence, Lisbon, July 1990.. [9] J.C. Gâcon, State Doctorate Thesis, Lyon University (1978). [10] H. de Vries and D.A. Wiersma, J. Chem. Phys. 72 (1980) 1851. [11] R.M. Shelby and R.M. Macfarlane, J. Lumin. 31 &32 (1984) 839. [12] R. Wannemacher, R.S. Meltzer and R.M. Macfarlane, J. Lumin. 45 (1990) 307. [13] R.M. Macfarlane, R.M. Shelby and A. Winnacker, Phys. Rev. B 33 (1986) 4207. [14] R.J. Danby, K. Holliday and NB. Manson, J. Lumin. 42 (1988) 83. [15] B. Birang, A.S.M. Mahbub’ul Alam and B. Di Bartolo, J. Chem. Phys. 50 (1969) 2750. [16] D.L. Huber, J. Non-Cryst. Solids 51(1982) 241. [17] 5. Vdlker, in: Relaxation Processes in Molecular Excited States, ed. J. Funfschilling (Kluwer, Dordrecht, 1989) p. 113. [18] B. 331.Jackson and R. Silbey, Chem. Phys. Lett. 99 (1983) [19] J. M. Hayes, R. Jankowiak and G.J. Small, in: Persistent Spectral Hole-Burning: Science and Applications, ed. W.E. Moerner (Springer. Berlin, 1988) p. 167.
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2+ 05Br05:Sm [23] M. Berg, C.A. Walsh, L.R. Narasimhan, K.A. Littau and M.D. Fayer, J. Chem. Phys. 88 (1988) 1564. [24] K. Uchikawa, H. Ohsawa, T. Suga and S. Saikan, Opt. Lett. 16 (1991) 13.
/ Spectral hole burning in
[20] A. Oppenlander, F. Madeore, J.-C. Vial and J.-P. Chaminade, J. Lumin. 50 (1991) 1. [21] K. Holliday and N.B. Manson, J. Phys. Condens. Mat. 1 (1989) 1339. [22] G.C. Gâcon, J.C. Souillat, J. Seriot, F. Gaume-Mahn and B. Di Bartolo, J. Lumin. 18 & 19 (1979) 244.
BaFC1