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Transient and photon-gated persistent spectral holeburning in CaSO4:Sm
Journal of Luminescence 42 (1988) 83—88 North-Holland, Amsterdam
83
TRANSIENT AND PHOTON-GATED PERSISTENT SPECTRAL HOLEBURNING IN CaSO4:Sm Robert J. DANBY, Keith HOLLIDAY and Neil B. MANSON Laser Physics Centre, Research School of Physical Sciences, The Au.stralian National University. GPO Box 4, Canberra, A CT 2601, Australia Received 24 September 1987 Revised 22 March 1988 Accepted 26 April 1988 3 + centres in CaSO The selective emission and excitation of three Sm 6H 4G 4 are reported. For one of these, the substitutional centre, short lived optical holes have been burned in the2+ centre 5/2 — formed 5/2 transition at 562.7 nm. X-irradiation has been analysed. For the 7F 5D converts this centre to the divalent state, and the emission from the Sm 0 —~ 0 transition at 689.2 nm deep short-lived holes with a width of 4 MHz have been attained with single laser irradiation, but persistent holes with a width of 200 MHz were observed when a second laser source in the green or ultraviolet was introduced.
1. Introduction While the proposal that persistent holeburning (PHB) might be used as a means of optically storing digital information was suggested some time ago [1], a number of physical problems remains to be solved before a practical device can be produced [2]. When conventional single photon holeburning techniques are used, the stored information is degraded during the reading process. This has recently stimulated considerable interest in materials which show photon-gated holeburning [3]. Holeburrnng in such systems is achieved
2~in various hosts [4,6]. While SrTiO3materials [4] and Sm these show characteristics desirable for optical storage applications [2], a practical material has yet to be found. One likely area to search for a suitable material is among the many ultraviolet sensitive phosphors for thermoluminescent dosimetry of X-ray and ultraviolet radiation. Such dosimeters function by using photochemically induced valence changes of a rare earth of transition metal dopant ion to store information about the radiation or ultraviolet dose received. The material CaSO 4: Sm shows a considerable sensitivity to ultraviolet light [7] arising from a mechanism such as that outlined above. In this material the trivalent dopant ions are easily converted to the divalent state by X-irradiation and, to a more limited degree, by exposure to ultra2+ violet light. It has also been found the Sm ions generated by X-irradiation canthat be efficiently converted back to Sm3 ± ions by ultraviolet light in the wavelength range 250—300 nm. Either of these processes might be suitable for photon gated holeburning. In the first case this would be done by exciting the 6H transition the Sm3~ion at 562 nm 5/2 and gating with light at of 500
using two input beams of different frequencies. The first beam provides an initial selective excitation while the second, which may be broadband, initiates holeburning of the previously excited centres. The first beam alone may then read the material. burningor mechanism be gated on This or offallows by thethepresence absence oftothe second “gating beam”, effectively removing any interference between the reading and writing processes. Impurity ion systems exhibiting gated holeburning have been reviewed by Macfarlane [4]. 2~ions in LiGa 3~in These include Co 5O8 [5], Cr 0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
—~
—
84
R.J. Danby et al.
/ Persistent spectral holeburning in CaSO4 : Sm
5D nm while the +second the would 0 —‘vbe F0 excited transition of theinSm2 ion at case 689 nm and light at 400 nm used as the gating source, We have therefore studied the suitability of CaSO 4: Sm for photon gated holeburning. As the basic spectroscopy CaSO4: Sm has and not emisbeen reported previously, of selective excitation sion studies were also performed in order to identify the various Sm3 ± and Sm2 ± centres in the material and to ascertain which of these are responsible for the ultraviolet response.
4G 6H transiand for at 600 to the 5/2model —‘ tion Sm3 nmA due Spectra Physics 171 Argon ion laser operating in multiline mode was used as the gating source for the photon-gated holeburn-
2. Experimental
3. Results and discussion
—
~.
ing experiments. This was focussed to2 provide in the optical (principally power densities of and20514 W nm) cm and visible 488 nm 2 W cm 2 in the ultraviolet (330—360 nm). For these measurements the samples were maintained at 1.8 K by immersion in superfluid liquid helium. —
—
—
The single crystal samples of CaSO 4 : Sm used for this work were grown by slow evaporation from concentrated H2SO4 using a technique which has beenSm3 fully described [7]. All studies of the + centres in elsewhere CaSO 4 were performed using the “as grown” material a nominal 3t Thewith studies of Sm2~ concentration of 0.05% Sm centres were performed by first reducing a quantity of the Sm3~dopant ions to Sm2~by Xirradiating the sample to an exposure level of io~C kg1 using a Co X-ray tube operating at 40 kV. The conventional excitation and emission measurements described below were carried out using a Molectron DL-II pulsed dye laser (bandwidth 0.01 nm) as the excitation source and a gated photomultiplier tube and single grating monochromator (bandwidth 0.1 mn) to analyse the fluorescent emission. For these studies single crystal samples were mounted in an optical quartz gas flow tube and cooled to 10 K by a flow of cold helium gas from a liquid helium storage dewar. The high-resolution holeburrnng measurements were performed using a Coherent 599-21 standing wave dye laser (nominal bandwidth 2 MHz). A dye solution of 10-2 M Exciton LD688 dye in a 4 to 1 mixture of butyl alcohol and glycerol [8] was used for the Sm2~measurements at 690 nm and a 10—2 M solution of Exciton Rhodamine 560 dye in ethylene glycol was used for the Sm3 ± measurement at 560 nm. A measure of the absorption was —
3.1. General spectroscopy
The excitation 3~are and emission spectra “asshown in figs. 1ofand 2 grown” CaSO4 Sm respectively. Figure 1 was recorded by monitoring all from the while ofscanmngthe thefluorescence excitation source oversample the region the 6H 3t Selective exci5/2 spectra were transitions of Sm while monitoring tation also recorded the individual emission lines. This revealed the presence of three crystallographically distinct Sm3 ± centres, the principal excitation lines of which have been labelled as either A, B or C in fig. 1. The emission of each centre is shown in fig. 2. The lifetimes of the centres were for A 3.4 ms, B 2.6 —‘
CaSO
3~
A
4~Sm
—
C 2 -
-
—
obtained by monitoring the 7F emission intensity 2at± 725 nm due to the SD0 2 transition for Sm —‘
1
o
L_________________________________ 561.5
562.0
562.5
563.0
563.5
WAVELENGTH (n m) . . Fig. 1. Excitation spectrum of CaSO
4 Sm at 10 K detecting all emissions. The groupsSm3 of lines weredenoted found toA,beBassociated three distinct + centres and C. with
/ Persistent spectral holeburning in CaSO4
R.J. Danby et at. I
I
I
I
3+ CaSO4:Sm 4G~—~‘6H~
I
I
liii
Sm
85
11,1111
4G½..PH 7
~
o
560
I
562
564
566
568 594
596
,,
598
600
602
604
606
608
WAVELENGTH (n~ Fig. 2. Emission of the three centres A, B and C. The A trace is obtained by excitation at 562.7 mu which is resonant with the highest energy emission line. The B trace excitation is at 563.2 nm and the C trace at 561.8 nm.
ms, and C 2.5 ms. Some of the weak lines evident in fig. 1 did not appear to be correlated with the emission lines of these three centres, suggesting thatThe further centres may also be present. emission spectrum of the sample following X-irradiation revealed a series of new lines between 680 nm and 740 nm (fig. 3). The position,
number, and polarisation dependences of these lines 5D were 7F consistent 7F 7Fwith their assignment as the 0 2~centre 0, 1 as andindicated 2 transitions in fig. of 3. a single type of The Sm observation of only a single Sm2 centre in X-irradiated CaSO 3 despite the fact that 4 : Sm contains at least three the “as grown” material major types of Sm3 centre, suggests that one of these centres is preferentially reduced by its divalent state by X-irradiation. This proposition is supported by a comparison of the emission spectra of a given CaSO 3 + sample made before 4: Sm and after X-irradiation. A drop of approximately —‘
±
~,
+
~
5D
7F~
CaSO
0”I
2~ 4:Sm
2
5
1
5
‘b’
F 2
I I ______
890
___________
700 WAVELENGTH(nm) 710 ~0
_____
730
740
2±
Fig. 3. Emission of the Sm
centre in CaSO
4 at 10 K. The excitation wavelength is at 689.2 nm.
20 to 30% in relative magnitude of the A centre lines with respect to those of the other centres can 3~ be seen. Such a preferential reduction has been observed previously samples ofcentres CaSO4in : Euthe which contain four in major Eu3~ “as-grown” material but show only one Eu2~ centre following treatment with X-rays [9]. By
86
R.J. Danby et aL
/ Persistent spectral holeburning in CaSO4
analogy with this system, it appears likely that the A centre in 3fig. 1 corresponds + centre in CaSO to 3the ± inhighest which symmetry Sm replaces a Ca2 + ion 4: substitutionally Sm the Sm3 ± ion at a centre of C 2~symmetry with no local charge compensation. The photochemical behaviour of the Sm centres under ultraviolet illumination was studied using the focussed beam from a 150 W Hg arc lamp. Exposure of “as grown” samples to ultraviolet light at temperatures of 10 K revealed no detectable changes in the relative3~centres, or absoluteeven emission after levels from the various Sm exposure times in excess of 1 h. Very weak Sm2~ emission lines could be observed following this ultraviolet treatment; however, these were estimated to be two to three orders of magnitude less intense than those observed following X-irradiation. This lack of a significant ultraviolet response at low temperatures contrasts with the pronounced ultraviolet induced valence changes observed for this material at room temperature [7]. Similar behaviour has been observed previously for the related material CaSO 3 + [10]. The ultraviolet response of CaSO 4 : Eu 3 + is strongly temperature dependent as it 4: hasEubeen found to rely on the thermally induced population transfer between the upper states of the europium—oxygen complex. Within this model the pathway for photochemical conversion is essentially closed for temperatures below 50 K. A five minute ultraviolet illumination of previously X-irradiated CaSO 3 ± at 10 K was Sm found to produce a 95%4:decrease in the overall Sm2 ± emission level. This was accompanied by a corresponding increase in the Sm3 + A centre emission. This response was sufficient to suggest that photochemical and perhaps gated holeburning via the conversion of Sm2~to Sm3~may be possible in CaSO 4: Sm. 3.2. Holeburning: Sm3 + —
—
—
A high resolution study of the dominant A centre line at 562.7 nm was performed using the unfocussed beam from the standing wave dye laser described above (power density 2.5 W cm 2), This revealed that the line had an inhomo—
Sm
geneous width of approximately 8 GHz and exhibited nowas apparent while laser frequency scanned burning across the line the at sweep rates of 8 GHz s~or greater. When the laser —
scan was stopped within the line, however, the emission intensity dropped rapidly by approximately 30% of its unburned value. Decreasing the laser power density using neutral density filters decreased the proportional hole depth; however, increasing the power density by focussing the beam produced no increase in hole depth beyond the 30% seen the unfocussed This with suggests that using the hole burning is beam. associated 147Sm (I ~) and 149Sm (I ~) isotopes, which are 30% naturally abundant and the only naturally occurring Sm isotopes with non-zero nuclear spin. Being an odd electron system there will be a Kramers degeneracy and hence an angular momentum associated with the ground electronic state. For the isotopes with a nuclear spin of I 7/2, this will give eight hyperfine levels, and holeburning can arise through a laser induced redistribution of the population among these levels. There is no equivalent holeburning mechanism for the naturally abundant isotopes with zero nuclear spin. No long-lived holes were detected when sweeping the laser 100 ms after the burning process nor could any gated permanent holes be produced by introducing a second photon frequency in the ultraviolet. Considering that there is evidence of ultraviolet induced photochemical changes at room temperature [7], it is surprising that at low temperatures there is no response at all with the high ionisation energies associated with a green photon plus an ultraviolet photon. =
=
=
3.3. Holeburning: Sm2 ± 5D 7F 2~ The lies 0at—‘ 689.2 0 transition the single Sman centre nm and isof found to have inhomogeneous width of 5 GHz. For a laser power of 1 W cm -2 and stopping the laser scan when the frequency is within the inhomogeneous line, a hole was burned to a depth of 90% of the original absorption strength. However, the hole was short lived, and the hole spectrum could not be measured with a single laser beam. The spectrum was
R.J. Danby et aL
60
Sm
I
2+
~ ~
7F
I
/ Persistent spectral holeburning in
I
-
P 80 - - - -
.
- -
100
I I
LASER FREQUENCY SHIFT(GHz~ Fig. 4. Gated optical hole burned m the 689.2 nm zero phonon line of CaSO 2~using Ar~ion irradiated at 488 and 514 mis for the gating 4: Smlight. The strength of the signal, detected ~ excitation, is expressed as a percentage of the absorption in the centre of the zero-phonon line,
CaSO
4 Sm
87
producing the very deep holes therefore remains to be determined. Permanent holeburning was achieved when a second illumination source was introduced. With a 20 W cm2 Ar~ion beam at 488 and 514 nm, a 7% deep long-lived hole was obtained after 15 m exposure. Marginally faster burning was achieved with ultraviolet irradiation. For a 2 W cm2 beam at 351—363 nm a 7% permanent hole was attained after 300 s irradiation although at these wavelengths the ultraviolet itself causes some nonselective photochemical change. Even though the gating wavelengths may not be optimum, the rates are clearly still going to be very slow and totally unrealistic for any device application. The slow rate is accentuated by the short-lived holeburning effect which causes a large reduction in the number of optically active centres during irradiation. The widths of the permanent holes are 200 MHz, much larger than that of the short-lived holes 4 MHz). These holes, which are the result of burning for several mm, could acquire a width from the long-term laser drift. However, this effect can only account for a very small increase in width (<10 MHz). Rather, the increased width is indicative of a different mechanism such as there being a valence change associated with the permanent hole, whereas there is no such change for the short-lived hole. The redistribution of charge in the crystal can then change the effective fields at the crystal sites and account for the much broader holes. Similar, effects were observed in the gated holeburning of LiGaO 2 [6]. A Sm3 Sm2 8 : Cogave hole widths which valence change in BaFC1 were observed to be concentration-dependent [5]. For a concentration of 0.05% Sm2~the widths were found to be 200 MHz, similar to those observed here for CaSO 4: Sm. —
measured, however, using two laser beams. The two beams were generated by driving an acoustooptic modulator at two adjacent rf frequencies. One beam was fixed in frequency and gave rise to deep holes and hence only to weak emission. The second beam was swept in frequency and used to read the hole generated by the first. This second beam was weaker than the burn beam, but because there was no associated burning the emission level was high and the spectrum readily detectable using the excitation technique. A single sharp hole with no antihole was observed. The hole width, 4 MHz, is just that set by laser jitter. With focusing, the hole depth increased marginally, whereas a reduction in intensity reduced the hole depth. The hole depth is a measure of the fraction of centres held in levels other than the ground state. One of these levels will be the excited state, but since inversion cannot be achieved, the observed hole depths of 90% cannot be explained by population in the excited state. Other reservoirs of population should be long lived to give deep holes, but in that case the holes themselves should be long lived. This is not observed. Measurement of fast recovery of but the excitation hole in absorption has give not been successful, measurements a value limited by the excited lifetime, implying a recovery rate faster than 10 ms. The mechanism
(—
+
+
—
+
—
4. Conclusions It has been argued that CaSO4: Sm has promising characteristics forresonant gated holeburning 3 Sm2 using transition inboth the for Smand for Sm2 + green Sm3 + for a transition in the red. The latter has been demonstrated, and thus CaSO 2~ 4: Sm is a further example where a Sm ± —‘
±
—‘
88
RJ. Danby et aL
/ Persistent spectral holeburning in CaSO4
transition exhibits gated holeburing [4]. There is no technological promise for the material as burning rates were exceedingly slow requiring several minutes of two-laser irradiation to produce measurable permanent holes. Interesting short-lived holeburning 4H effects were observed for bothat ions. For the the holeburning 5/2 60 3~transition 562 nm is Sm apparently due to redistribution of population in the nuclear levels in each of the 15% natural abundant 147Sm and ‘49Sm isotopes. A similar effect could not account for the very deep holes produced in the 5D 7D 2 at 0 0 transition of Sm 689.2 nm and the holeburning process in that case remains to be explained. --‘
—
Sm
References [1] G. Castro, D. Haarer, R.M. Macfarlane and H.P. Trommsdorff, US Patent No. 4, 101, 976 (July 18, 1978). [2] W.E. Moerner and M.D. Levenson, J. Opt. Soc. Am. B 2 (1985) 915. [3] W. Lenth and W.E. Moerner, Opt. Commun. 58 (1986) [4] R.M. 249. Macfarlane, J. Lumin. 38 (1987) 20. [5] A. Winnacker, R.M. Shelby and R.M. Macfarlane, Opt. Lett. 10 (1985) 350. [6] R.M. Macfarlane, R.M. Shelby and A. Winnacker, Phys. Rev. B 33 (1986) 4207. [7] R.L. Calvert and R.J. Danby, Phys. Stat. Sol. (a) 83 (1984)
±
—
[8] J. Beber and A. Szabo, IEEE J. Quant. Electron. 20 (1984) 9. [9] R.J. Danby, J. Phys. C. Sol. St. Phys. 16 (1983) 3573. [10] R.J. Danby, J. Phys. C. Sal. St. Phys. 21(1988) 485.
83
TRANSIENT AND PHOTON-GATED PERSISTENT SPECTRAL HOLEBURNING IN CaSO4:Sm Robert J. DANBY, Keith HOLLIDAY and Neil B. MANSON Laser Physics Centre, Research School of Physical Sciences, The Au.stralian National University. GPO Box 4, Canberra, A CT 2601, Australia Received 24 September 1987 Revised 22 March 1988 Accepted 26 April 1988 3 + centres in CaSO The selective emission and excitation of three Sm 6H 4G 4 are reported. For one of these, the substitutional centre, short lived optical holes have been burned in the2+ centre 5/2 — formed 5/2 transition at 562.7 nm. X-irradiation has been analysed. For the 7F 5D converts this centre to the divalent state, and the emission from the Sm 0 —~ 0 transition at 689.2 nm deep short-lived holes with a width of 4 MHz have been attained with single laser irradiation, but persistent holes with a width of 200 MHz were observed when a second laser source in the green or ultraviolet was introduced.
1. Introduction While the proposal that persistent holeburning (PHB) might be used as a means of optically storing digital information was suggested some time ago [1], a number of physical problems remains to be solved before a practical device can be produced [2]. When conventional single photon holeburning techniques are used, the stored information is degraded during the reading process. This has recently stimulated considerable interest in materials which show photon-gated holeburning [3]. Holeburrnng in such systems is achieved
2~in various hosts [4,6]. While SrTiO3materials [4] and Sm these show characteristics desirable for optical storage applications [2], a practical material has yet to be found. One likely area to search for a suitable material is among the many ultraviolet sensitive phosphors for thermoluminescent dosimetry of X-ray and ultraviolet radiation. Such dosimeters function by using photochemically induced valence changes of a rare earth of transition metal dopant ion to store information about the radiation or ultraviolet dose received. The material CaSO 4: Sm shows a considerable sensitivity to ultraviolet light [7] arising from a mechanism such as that outlined above. In this material the trivalent dopant ions are easily converted to the divalent state by X-irradiation and, to a more limited degree, by exposure to ultra2+ violet light. It has also been found the Sm ions generated by X-irradiation canthat be efficiently converted back to Sm3 ± ions by ultraviolet light in the wavelength range 250—300 nm. Either of these processes might be suitable for photon gated holeburning. In the first case this would be done by exciting the 6H transition the Sm3~ion at 562 nm 5/2 and gating with light at of 500
using two input beams of different frequencies. The first beam provides an initial selective excitation while the second, which may be broadband, initiates holeburning of the previously excited centres. The first beam alone may then read the material. burningor mechanism be gated on This or offallows by thethepresence absence oftothe second “gating beam”, effectively removing any interference between the reading and writing processes. Impurity ion systems exhibiting gated holeburning have been reviewed by Macfarlane [4]. 2~ions in LiGa 3~in These include Co 5O8 [5], Cr 0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
—~
—
84
R.J. Danby et al.
/ Persistent spectral holeburning in CaSO4 : Sm
5D nm while the +second the would 0 —‘vbe F0 excited transition of theinSm2 ion at case 689 nm and light at 400 nm used as the gating source, We have therefore studied the suitability of CaSO 4: Sm for photon gated holeburning. As the basic spectroscopy CaSO4: Sm has and not emisbeen reported previously, of selective excitation sion studies were also performed in order to identify the various Sm3 ± and Sm2 ± centres in the material and to ascertain which of these are responsible for the ultraviolet response.
4G 6H transiand for at 600 to the 5/2model —‘ tion Sm3 nmA due Spectra Physics 171 Argon ion laser operating in multiline mode was used as the gating source for the photon-gated holeburn-
2. Experimental
3. Results and discussion
—
~.
ing experiments. This was focussed to2 provide in the optical (principally power densities of and20514 W nm) cm and visible 488 nm 2 W cm 2 in the ultraviolet (330—360 nm). For these measurements the samples were maintained at 1.8 K by immersion in superfluid liquid helium. —
—
—
The single crystal samples of CaSO 4 : Sm used for this work were grown by slow evaporation from concentrated H2SO4 using a technique which has beenSm3 fully described [7]. All studies of the + centres in elsewhere CaSO 4 were performed using the “as grown” material a nominal 3t Thewith studies of Sm2~ concentration of 0.05% Sm centres were performed by first reducing a quantity of the Sm3~dopant ions to Sm2~by Xirradiating the sample to an exposure level of io~C kg1 using a Co X-ray tube operating at 40 kV. The conventional excitation and emission measurements described below were carried out using a Molectron DL-II pulsed dye laser (bandwidth 0.01 nm) as the excitation source and a gated photomultiplier tube and single grating monochromator (bandwidth 0.1 mn) to analyse the fluorescent emission. For these studies single crystal samples were mounted in an optical quartz gas flow tube and cooled to 10 K by a flow of cold helium gas from a liquid helium storage dewar. The high-resolution holeburrnng measurements were performed using a Coherent 599-21 standing wave dye laser (nominal bandwidth 2 MHz). A dye solution of 10-2 M Exciton LD688 dye in a 4 to 1 mixture of butyl alcohol and glycerol [8] was used for the Sm2~measurements at 690 nm and a 10—2 M solution of Exciton Rhodamine 560 dye in ethylene glycol was used for the Sm3 ± measurement at 560 nm. A measure of the absorption was —
3.1. General spectroscopy
The excitation 3~are and emission spectra “asshown in figs. 1ofand 2 grown” CaSO4 Sm respectively. Figure 1 was recorded by monitoring all from the while ofscanmngthe thefluorescence excitation source oversample the region the 6H 3t Selective exci5/2 spectra were transitions of Sm while monitoring tation also recorded the individual emission lines. This revealed the presence of three crystallographically distinct Sm3 ± centres, the principal excitation lines of which have been labelled as either A, B or C in fig. 1. The emission of each centre is shown in fig. 2. The lifetimes of the centres were for A 3.4 ms, B 2.6 —‘
CaSO
3~
A
4~Sm
—
C 2 -
-
—
obtained by monitoring the 7F emission intensity 2at± 725 nm due to the SD0 2 transition for Sm —‘
1
o
L_________________________________ 561.5
562.0
562.5
563.0
563.5
WAVELENGTH (n m) . . Fig. 1. Excitation spectrum of CaSO
4 Sm at 10 K detecting all emissions. The groupsSm3 of lines weredenoted found toA,beBassociated three distinct + centres and C. with
/ Persistent spectral holeburning in CaSO4
R.J. Danby et at. I
I
I
I
3+ CaSO4:Sm 4G~—~‘6H~
I
I
liii
Sm
85
11,1111
4G½..PH 7
~
o
560
I
562
564
566
568 594
596
,,
598
600
602
604
606
608
WAVELENGTH (n~ Fig. 2. Emission of the three centres A, B and C. The A trace is obtained by excitation at 562.7 mu which is resonant with the highest energy emission line. The B trace excitation is at 563.2 nm and the C trace at 561.8 nm.
ms, and C 2.5 ms. Some of the weak lines evident in fig. 1 did not appear to be correlated with the emission lines of these three centres, suggesting thatThe further centres may also be present. emission spectrum of the sample following X-irradiation revealed a series of new lines between 680 nm and 740 nm (fig. 3). The position,
number, and polarisation dependences of these lines 5D were 7F consistent 7F 7Fwith their assignment as the 0 2~centre 0, 1 as andindicated 2 transitions in fig. of 3. a single type of The Sm observation of only a single Sm2 centre in X-irradiated CaSO 3 despite the fact that 4 : Sm contains at least three the “as grown” material major types of Sm3 centre, suggests that one of these centres is preferentially reduced by its divalent state by X-irradiation. This proposition is supported by a comparison of the emission spectra of a given CaSO 3 + sample made before 4: Sm and after X-irradiation. A drop of approximately —‘
±
~,
+
~
5D
7F~
CaSO
0”I
2~ 4:Sm
2
5
1
5
‘b’
F 2
I I ______
890
___________
700 WAVELENGTH(nm) 710 ~0
_____
730
740
2±
Fig. 3. Emission of the Sm
centre in CaSO
4 at 10 K. The excitation wavelength is at 689.2 nm.
20 to 30% in relative magnitude of the A centre lines with respect to those of the other centres can 3~ be seen. Such a preferential reduction has been observed previously samples ofcentres CaSO4in : Euthe which contain four in major Eu3~ “as-grown” material but show only one Eu2~ centre following treatment with X-rays [9]. By
86
R.J. Danby et aL
/ Persistent spectral holeburning in CaSO4
analogy with this system, it appears likely that the A centre in 3fig. 1 corresponds + centre in CaSO to 3the ± inhighest which symmetry Sm replaces a Ca2 + ion 4: substitutionally Sm the Sm3 ± ion at a centre of C 2~symmetry with no local charge compensation. The photochemical behaviour of the Sm centres under ultraviolet illumination was studied using the focussed beam from a 150 W Hg arc lamp. Exposure of “as grown” samples to ultraviolet light at temperatures of 10 K revealed no detectable changes in the relative3~centres, or absoluteeven emission after levels from the various Sm exposure times in excess of 1 h. Very weak Sm2~ emission lines could be observed following this ultraviolet treatment; however, these were estimated to be two to three orders of magnitude less intense than those observed following X-irradiation. This lack of a significant ultraviolet response at low temperatures contrasts with the pronounced ultraviolet induced valence changes observed for this material at room temperature [7]. Similar behaviour has been observed previously for the related material CaSO 3 + [10]. The ultraviolet response of CaSO 4 : Eu 3 + is strongly temperature dependent as it 4: hasEubeen found to rely on the thermally induced population transfer between the upper states of the europium—oxygen complex. Within this model the pathway for photochemical conversion is essentially closed for temperatures below 50 K. A five minute ultraviolet illumination of previously X-irradiated CaSO 3 ± at 10 K was Sm found to produce a 95%4:decrease in the overall Sm2 ± emission level. This was accompanied by a corresponding increase in the Sm3 + A centre emission. This response was sufficient to suggest that photochemical and perhaps gated holeburning via the conversion of Sm2~to Sm3~may be possible in CaSO 4: Sm. 3.2. Holeburning: Sm3 + —
—
—
A high resolution study of the dominant A centre line at 562.7 nm was performed using the unfocussed beam from the standing wave dye laser described above (power density 2.5 W cm 2), This revealed that the line had an inhomo—
Sm
geneous width of approximately 8 GHz and exhibited nowas apparent while laser frequency scanned burning across the line the at sweep rates of 8 GHz s~or greater. When the laser —
scan was stopped within the line, however, the emission intensity dropped rapidly by approximately 30% of its unburned value. Decreasing the laser power density using neutral density filters decreased the proportional hole depth; however, increasing the power density by focussing the beam produced no increase in hole depth beyond the 30% seen the unfocussed This with suggests that using the hole burning is beam. associated 147Sm (I ~) and 149Sm (I ~) isotopes, which are 30% naturally abundant and the only naturally occurring Sm isotopes with non-zero nuclear spin. Being an odd electron system there will be a Kramers degeneracy and hence an angular momentum associated with the ground electronic state. For the isotopes with a nuclear spin of I 7/2, this will give eight hyperfine levels, and holeburning can arise through a laser induced redistribution of the population among these levels. There is no equivalent holeburning mechanism for the naturally abundant isotopes with zero nuclear spin. No long-lived holes were detected when sweeping the laser 100 ms after the burning process nor could any gated permanent holes be produced by introducing a second photon frequency in the ultraviolet. Considering that there is evidence of ultraviolet induced photochemical changes at room temperature [7], it is surprising that at low temperatures there is no response at all with the high ionisation energies associated with a green photon plus an ultraviolet photon. =
=
=
3.3. Holeburning: Sm2 ± 5D 7F 2~ The lies 0at—‘ 689.2 0 transition the single Sman centre nm and isof found to have inhomogeneous width of 5 GHz. For a laser power of 1 W cm -2 and stopping the laser scan when the frequency is within the inhomogeneous line, a hole was burned to a depth of 90% of the original absorption strength. However, the hole was short lived, and the hole spectrum could not be measured with a single laser beam. The spectrum was
R.J. Danby et aL
60
Sm
I
2+
~ ~
7F
I
/ Persistent spectral holeburning in
I
-
P 80 - - - -
.
- -
100
I I
LASER FREQUENCY SHIFT(GHz~ Fig. 4. Gated optical hole burned m the 689.2 nm zero phonon line of CaSO 2~using Ar~ion irradiated at 488 and 514 mis for the gating 4: Smlight. The strength of the signal, detected ~ excitation, is expressed as a percentage of the absorption in the centre of the zero-phonon line,
CaSO
4 Sm
87
producing the very deep holes therefore remains to be determined. Permanent holeburning was achieved when a second illumination source was introduced. With a 20 W cm2 Ar~ion beam at 488 and 514 nm, a 7% deep long-lived hole was obtained after 15 m exposure. Marginally faster burning was achieved with ultraviolet irradiation. For a 2 W cm2 beam at 351—363 nm a 7% permanent hole was attained after 300 s irradiation although at these wavelengths the ultraviolet itself causes some nonselective photochemical change. Even though the gating wavelengths may not be optimum, the rates are clearly still going to be very slow and totally unrealistic for any device application. The slow rate is accentuated by the short-lived holeburning effect which causes a large reduction in the number of optically active centres during irradiation. The widths of the permanent holes are 200 MHz, much larger than that of the short-lived holes 4 MHz). These holes, which are the result of burning for several mm, could acquire a width from the long-term laser drift. However, this effect can only account for a very small increase in width (<10 MHz). Rather, the increased width is indicative of a different mechanism such as there being a valence change associated with the permanent hole, whereas there is no such change for the short-lived hole. The redistribution of charge in the crystal can then change the effective fields at the crystal sites and account for the much broader holes. Similar, effects were observed in the gated holeburning of LiGaO 2 [6]. A Sm3 Sm2 8 : Cogave hole widths which valence change in BaFC1 were observed to be concentration-dependent [5]. For a concentration of 0.05% Sm2~the widths were found to be 200 MHz, similar to those observed here for CaSO 4: Sm. —
measured, however, using two laser beams. The two beams were generated by driving an acoustooptic modulator at two adjacent rf frequencies. One beam was fixed in frequency and gave rise to deep holes and hence only to weak emission. The second beam was swept in frequency and used to read the hole generated by the first. This second beam was weaker than the burn beam, but because there was no associated burning the emission level was high and the spectrum readily detectable using the excitation technique. A single sharp hole with no antihole was observed. The hole width, 4 MHz, is just that set by laser jitter. With focusing, the hole depth increased marginally, whereas a reduction in intensity reduced the hole depth. The hole depth is a measure of the fraction of centres held in levels other than the ground state. One of these levels will be the excited state, but since inversion cannot be achieved, the observed hole depths of 90% cannot be explained by population in the excited state. Other reservoirs of population should be long lived to give deep holes, but in that case the holes themselves should be long lived. This is not observed. Measurement of fast recovery of but the excitation hole in absorption has give not been successful, measurements a value limited by the excited lifetime, implying a recovery rate faster than 10 ms. The mechanism
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+
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4. Conclusions It has been argued that CaSO4: Sm has promising characteristics forresonant gated holeburning 3 Sm2 using transition inboth the for Smand for Sm2 + green Sm3 + for a transition in the red. The latter has been demonstrated, and thus CaSO 2~ 4: Sm is a further example where a Sm ± —‘
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88
RJ. Danby et aL
/ Persistent spectral holeburning in CaSO4
transition exhibits gated holeburing [4]. There is no technological promise for the material as burning rates were exceedingly slow requiring several minutes of two-laser irradiation to produce measurable permanent holes. Interesting short-lived holeburning 4H effects were observed for bothat ions. For the the holeburning 5/2 60 3~transition 562 nm is Sm apparently due to redistribution of population in the nuclear levels in each of the 15% natural abundant 147Sm and ‘49Sm isotopes. A similar effect could not account for the very deep holes produced in the 5D 7D 2 at 0 0 transition of Sm 689.2 nm and the holeburning process in that case remains to be explained. --‘
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Sm
References [1] G. Castro, D. Haarer, R.M. Macfarlane and H.P. Trommsdorff, US Patent No. 4, 101, 976 (July 18, 1978). [2] W.E. Moerner and M.D. Levenson, J. Opt. Soc. Am. B 2 (1985) 915. [3] W. Lenth and W.E. Moerner, Opt. Commun. 58 (1986) [4] R.M. 249. Macfarlane, J. Lumin. 38 (1987) 20. [5] A. Winnacker, R.M. Shelby and R.M. Macfarlane, Opt. Lett. 10 (1985) 350. [6] R.M. Macfarlane, R.M. Shelby and A. Winnacker, Phys. Rev. B 33 (1986) 4207. [7] R.L. Calvert and R.J. Danby, Phys. Stat. Sol. (a) 83 (1984)
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[8] J. Beber and A. Szabo, IEEE J. Quant. Electron. 20 (1984) 9. [9] R.J. Danby, J. Phys. C. Sol. St. Phys. 16 (1983) 3573. [10] R.J. Danby, J. Phys. C. Sal. St. Phys. 21(1988) 485.