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Holeburning and photon echo measurements in KEu(WO4)2
Journal of Luminescence 45 (1990) 429—430 North-Holland
429
HOLEBURNING AND PHOTON ECHO MEASUREMENTS IN KEu(WO4)2 Neil B. MANSON and Peter T.H. FISK Laser Physics Centre, Research School of Physical Sciences, A ustralian National University, P.O. Box 4, Canberra, Australia 2601
The variation of holeburning, echoesKEu(W0 and free induction decay rates across the 7F Raman heterodyne detected NQR, 3+ in photon concentrated inhomogeneously broadened 0 —~D0 zero-phonon line of Eu 4) 2 is reported and attributed to variations in the optical and hyperfine relaxation rates. Upconversion is also reported.
The holeburning characteristics vary over the inho7F mogeneously broadened 3~in KEu(W0 0— D0 (582 nm) zero-phonon line of Eu 4)2. In the wings — 15 GHz from line centre the holeburning is persistent, lasting for many minutes, and the details of the sideholes and antiholes associated with the ground state hyperfine splittings has been reported previously [1]. The hole lifetime is shorter towards the centre of the line andpumping at the line itselfa not even continuous optical can centre maintain measurable hole. As holeburning is associated with population storage in the Eu3 + nuclear quadrupole levels, these observations mdicate that a correlation exists between the relaxation rate in the hyperfine levels and the optical 7F 5D 0— 0 frequency. The coherent RF-optical technique of Raman heterodyne detection of NQR [2] has been used to obtain the hyperfine resonant frequencies. Signals can be obtamed with the laser frequency in the shoulders of the zero-phonon but no signal can be obtained in the central 3pearance GHz region the line. reason for the disapof theofsignal will The be discussed below. Away from the centre of the line the associated hyperfine resonances are inhomogeneously broadened as has been shown in a series of rf holeburning and rf echo measurements [3]. However, the distribution of hyperfine resonance frequencies shows little change with different resonant optical frequencies. Thus, although there is a correlation of optical frequency with ground state relaxation rate, there is not an equivalent correlation with ground state energies.
A third optically measured quantity which showed a variation over the inhomogeneously broadened zerophonon line is photon echo decay rates. The pulse sequence which consisted of a 7T/2 (= /3t where f3 is the Rabi frequency and t is the length of the pulse) and a ii pulse separated by a delay ~ is achieved by acoustooptically gating the output of a CW dye laser. The sample emits an echo after a period ~ which was detected using a heterodyne technique [4]. The variation 0022-2313/90/$03.5() © Elsevier Science Publishers B.V. (North-Holland)
of the echo amplitude as the laser is scanned through the optical line is shown in fig. 1 for various delays L~. The decay rate in the wings is — 20 ~s and shows little variation except near the line centre where an echo cannot be detected. In the central 3 GHz the decay is presumably too fast to be measured. There is also an indication that the measured rate depends on the laser intensity. Similar observations have and been 2~[5] and Y 3~[6] thismade effectfor is LiYF4 : Th to the second203 : Eu exciting ions which are attributed pulse not involved in the initial pulse but which causes a shift in the frequencies and hence an increase in the apparent dephasing rate. In the present study because of the limited range of laser intensity over which a signal could be obtained it has not been possible to determine whether this is the only process responsible for the large variation in dephasing rates across the optical line. Free induction decay measurements have likewise been studied as a function of the optical frequency. In the wings decay rates of several ~ssare obtained, limited 3~dephasing rate. The free by the laser jittermeasurements and the Eu are capable of detecting induction decay
I
I
I
30 ~
<2
uS
0 -10
-5 0 5 10 15 Frequency Shift (GHz) Fig. 1. Variation of photon echo amplitude as laser is scanned through the absorption line for various delays ~1from 30 ~s to 60 p.s in steps of 5 p.s. The laser intensity was 100 mW focused by a 10 cm focal length lens and t = 5 p.s.
430
N.B. Manson, P. T.H. Fisk
3
I
I
I
I
/ Holeburning and photon echoes in KEu(W04)2 I
Monitoring 526nm 61 3nm
2
.
/ 0
1’—’
.I ~ I
I
~
______________________________________ -10 0 +10
Laser 7FFrequency Shift (0Hz) Fig. 2. Excitation of 0 —~D0 zero-phonon line in KEu(W04 ) 2 at 4.2 K: monitoring the red (solid line) and the green (dashed line) emission respectively. faster responses than the echo measurements but again no signal could be obtained in the centre of the optical line. 3~ions is given indication interacting Eu by Athedirect observation of ofupconversion (fig. 2). Pumping into the 5D 0 state results in green emission 5D associated with excitation of the higher energy 1 state. The upconversion 5Dwill arise when two neighbouring ions in the excited 0 state relax with one decaying to a lower energy state while the other ion is raised in energy. This only occurs with high densities of excited ions which can be more readily achieved by pumping in the centre of the line. The upconverted light is at the expense of the red emission and hence the process accounts for a
dip in the centre of the excitation line when measured by monitoring the red emission. The involvement of a common physical phenomenon might be expected to account for the variation of signals across the zero-phonon line. However, this does not appear to be solely the case. The holeburning only involves the ground state whereas the other measure-
ments are more related to theincrease excited in state. Also, upconversion leads a small radiative lifetime notecho oftosufficient magnitude tothe account for the loss but of the signals. The loss of photon echoes, Raman signals and free induction decay measurements at line centre could, however, all be attributed to the frequency shifts caused by the optical pumping although the detailed relationships have still to be established.
References [11 A.J. Silversmith and N.B. Manson, J. Phys. C 17 (1984) L97. [2] J. Mlynek, N.C. Wong, R.G. DeVoe, E.S. Kintzer and R.G. Brewer, Phys. Rev. Lett. 50 (1983) 993. [3] N.B. Manson and A.J. Silversmith, J. Phys. C 20 (1987) 1507. [41 R.M. Macfarlane, R.M. Shelby and R.L. Shoemaker, Phys. Rev. Lett. 43 (1979) 1726. [5] G.K. Liu, M.F. Joubert, R.L. Cone and B. Jacquier, J. Lumin. 38 (1987) 34. [6] J. Huang, J.M. Zhang, A. Lezama and T.W. Mossberg, Phys. Rev. Lett. 63 (1989) 78.
429
HOLEBURNING AND PHOTON ECHO MEASUREMENTS IN KEu(WO4)2 Neil B. MANSON and Peter T.H. FISK Laser Physics Centre, Research School of Physical Sciences, A ustralian National University, P.O. Box 4, Canberra, Australia 2601
The variation of holeburning, echoesKEu(W0 and free induction decay rates across the 7F Raman heterodyne detected NQR, 3+ in photon concentrated inhomogeneously broadened 0 —~D0 zero-phonon line of Eu 4) 2 is reported and attributed to variations in the optical and hyperfine relaxation rates. Upconversion is also reported.
The holeburning characteristics vary over the inho7F mogeneously broadened 3~in KEu(W0 0— D0 (582 nm) zero-phonon line of Eu 4)2. In the wings — 15 GHz from line centre the holeburning is persistent, lasting for many minutes, and the details of the sideholes and antiholes associated with the ground state hyperfine splittings has been reported previously [1]. The hole lifetime is shorter towards the centre of the line andpumping at the line itselfa not even continuous optical can centre maintain measurable hole. As holeburning is associated with population storage in the Eu3 + nuclear quadrupole levels, these observations mdicate that a correlation exists between the relaxation rate in the hyperfine levels and the optical 7F 5D 0— 0 frequency. The coherent RF-optical technique of Raman heterodyne detection of NQR [2] has been used to obtain the hyperfine resonant frequencies. Signals can be obtamed with the laser frequency in the shoulders of the zero-phonon but no signal can be obtained in the central 3pearance GHz region the line. reason for the disapof theofsignal will The be discussed below. Away from the centre of the line the associated hyperfine resonances are inhomogeneously broadened as has been shown in a series of rf holeburning and rf echo measurements [3]. However, the distribution of hyperfine resonance frequencies shows little change with different resonant optical frequencies. Thus, although there is a correlation of optical frequency with ground state relaxation rate, there is not an equivalent correlation with ground state energies.
A third optically measured quantity which showed a variation over the inhomogeneously broadened zerophonon line is photon echo decay rates. The pulse sequence which consisted of a 7T/2 (= /3t where f3 is the Rabi frequency and t is the length of the pulse) and a ii pulse separated by a delay ~ is achieved by acoustooptically gating the output of a CW dye laser. The sample emits an echo after a period ~ which was detected using a heterodyne technique [4]. The variation 0022-2313/90/$03.5() © Elsevier Science Publishers B.V. (North-Holland)
of the echo amplitude as the laser is scanned through the optical line is shown in fig. 1 for various delays L~. The decay rate in the wings is — 20 ~s and shows little variation except near the line centre where an echo cannot be detected. In the central 3 GHz the decay is presumably too fast to be measured. There is also an indication that the measured rate depends on the laser intensity. Similar observations have and been 2~[5] and Y 3~[6] thismade effectfor is LiYF4 : Th to the second203 : Eu exciting ions which are attributed pulse not involved in the initial pulse but which causes a shift in the frequencies and hence an increase in the apparent dephasing rate. In the present study because of the limited range of laser intensity over which a signal could be obtained it has not been possible to determine whether this is the only process responsible for the large variation in dephasing rates across the optical line. Free induction decay measurements have likewise been studied as a function of the optical frequency. In the wings decay rates of several ~ssare obtained, limited 3~dephasing rate. The free by the laser jittermeasurements and the Eu are capable of detecting induction decay
I
I
I
30 ~
<2
uS
0 -10
-5 0 5 10 15 Frequency Shift (GHz) Fig. 1. Variation of photon echo amplitude as laser is scanned through the absorption line for various delays ~1from 30 ~s to 60 p.s in steps of 5 p.s. The laser intensity was 100 mW focused by a 10 cm focal length lens and t = 5 p.s.
430
N.B. Manson, P. T.H. Fisk
3
I
I
I
I
/ Holeburning and photon echoes in KEu(W04)2 I
Monitoring 526nm 61 3nm
2
.
/ 0
1’—’
.I ~ I
I
~
______________________________________ -10 0 +10
Laser 7FFrequency Shift (0Hz) Fig. 2. Excitation of 0 —~D0 zero-phonon line in KEu(W04 ) 2 at 4.2 K: monitoring the red (solid line) and the green (dashed line) emission respectively. faster responses than the echo measurements but again no signal could be obtained in the centre of the optical line. 3~ions is given indication interacting Eu by Athedirect observation of ofupconversion (fig. 2). Pumping into the 5D 0 state results in green emission 5D associated with excitation of the higher energy 1 state. The upconversion 5Dwill arise when two neighbouring ions in the excited 0 state relax with one decaying to a lower energy state while the other ion is raised in energy. This only occurs with high densities of excited ions which can be more readily achieved by pumping in the centre of the line. The upconverted light is at the expense of the red emission and hence the process accounts for a
dip in the centre of the excitation line when measured by monitoring the red emission. The involvement of a common physical phenomenon might be expected to account for the variation of signals across the zero-phonon line. However, this does not appear to be solely the case. The holeburning only involves the ground state whereas the other measure-
ments are more related to theincrease excited in state. Also, upconversion leads a small radiative lifetime notecho oftosufficient magnitude tothe account for the loss but of the signals. The loss of photon echoes, Raman signals and free induction decay measurements at line centre could, however, all be attributed to the frequency shifts caused by the optical pumping although the detailed relationships have still to be established.
References [11 A.J. Silversmith and N.B. Manson, J. Phys. C 17 (1984) L97. [2] J. Mlynek, N.C. Wong, R.G. DeVoe, E.S. Kintzer and R.G. Brewer, Phys. Rev. Lett. 50 (1983) 993. [3] N.B. Manson and A.J. Silversmith, J. Phys. C 20 (1987) 1507. [41 R.M. Macfarlane, R.M. Shelby and R.L. Shoemaker, Phys. Rev. Lett. 43 (1979) 1726. [5] G.K. Liu, M.F. Joubert, R.L. Cone and B. Jacquier, J. Lumin. 38 (1987) 34. [6] J. Huang, J.M. Zhang, A. Lezama and T.W. Mossberg, Phys. Rev. Lett. 63 (1989) 78.