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Up-conversion spectroscopy of Nd3: KLiYF5
JOURNAL OF
LUMINESCENCE ELSEWIER
Journal
of Luminescence
Up-conversion
72-74 (1997) 9277929
spectroscopy of Nd3 ’ : KLiYF5
Keith Holliday*, David L. Russell, Brian Henderson Department
of Phvsics and Applied Phwics.
lJniversi@
ofStrathclvde, Glasgow GI IXY UK
Abstract
In KLiYFs, Nd3+ ions shelved in the metastable 4F3,2 state decay through one of the three routes; radiative decay through the emission of an infrared photon directly to the 41Jstates; after absorption of a second photon via radiation of blue or violet photons from the 4D3,z or ‘PJjz states; through energy transfer to Nd 3+ ions in the ground state or to Nd3’ ions that are also shelved in the 4F3,2 state, the latter resulting in green and orange fluorescence from the t4G5,z> 4G7,z) and 4G7j2 multiplets. Site-selective up-conversion spectra can be obtained when energy transfer is not the populating mechanism for the fluorescing state. Keywords: Up-conversion;
Site selectivity; Laser spectroscopy; Nd3+: KLiYF,; Energy transfer
Two different up-conversion processes take place simultaneously in Nd 3+ : KLiYF,. The relative strength of the two processes depends on the transition that is excited but central to both are ions that have electrons shelved in the metastable 4F3,2 state (at 11510 cm-’ for both of the two crystallographically inequivalent sites). Three depopulating processes take place, one of which is radiative decay to the 41, levels. The second process that decreases the 4F3,2 state population is an interionic process involving energy transfer, either to a Nd3+ ion in the ground state or to a second ion in the 4F3,2 state [ 11. Both of these processes take place at a fast rate in Nd3+ : KLiYF, [2,3]. Fig. 1 shows an up-converted fluorescence spectrum between 300-700 nm obtained by resonantly pumping the 419,2+ 4F3,2 transition at 10 K. By fitting the precise energies of each feature to the known energy levels of *Correspondingauthor. Fax: 0141-5534162; e-mail: [email protected]. 0022-2313/97/$17.00 $; 1997 Elsevier Science B.V. All rights reserved PI1 SOO22-23 13(97)00085-9
Nd3’ :KLiYFS [4, 51 it was ascertained that the three transitions that dominate the spectrum at about 530, 600 and 660 nm correspond to transitions from the (4G5j2, 4G,j2) and 4G,,z multiplets and terminate on the 41J levels [3]. Assignments have been attempted for similar fluorescence spectra in many Nd 3+-doped solids but usually measurements have been conducted at room temperature where individual lines overlap due to thermal effects and the final conclusions have rarely been complete (see Refs. [6,7] for instance). The upper states are populated through energy transfer between two ions in the 4F3,2 state that causes one ion to be excited to a higher state whilst the other decays to a 41, state. The emitting states retain a significant population despite their very short, non-radiative decay limited lifetime [8] due to constant renewal of the population through energy transfer [2,3,8]. The dynamics of these processes leads to the appearance of additional features in the fluorescence bands, even at 10 K, due to nontherma1 population of the excited state Stark levels [3].
928
K. Hollidav et al. / Journal of Luminescence
72-74 (19Y7j 927-929
1
0.9
0.8 x50 0.7
I
0.3
+
4&/2-141w2
300
350
400
450
500
Wavelength Fig. 1. Up-converted
fluorescence
spectrum
obtained
550
650
700
(nm)
by exciting
The third depopulation mechanism for the 4F3,2 state of Nd3+ : KLiYF, is also via higher excited states. Here an intraionic process involves the absorption of a second photon followed by emission of violet and blue photons from the 4D3j2 and states as first reported in Nd3+ : LiYF, [9]. 2p3,2 The first stage involves either resonant or phonon assisted absorption to the (4G5,2, 4G7,2) multiplet at around 17 000 cm-’ (590 nm). Rapid non-radiative decay to the metastable 4F3,2 state follows. A second photon is then absorbed, populating energy levels close to 29 000 cm- ‘, and fluorescence is observed from the site-dependent energy levels at about 28 050 and 26 220 cm-’ after non-radiative decay [S]. The probability of absorption from the 4F3,z state to a particular Stark level of the 2111j2 state has the strongest transition probability and, as the strongly absorbing Stark level is different for each of the two sites, site-selective up-converted fluorescence is observed. Alternatively, the reson-
600
the 419,2 + 4F3,2 transition
in Nd3+
: KLiYF, at 10 K
ance condition can be satisfied for either site for the first excitation step and this results in up-converted fluorescence from both sites due to strong radiative energy transfer to Nd3+ ions initially in their 419,2 ground state [2,3]. In the case of samples doped with less than 1% Nd3+, energy transfer between sites is much weaker and site-selective upconversion spectra can be obtained when the resonance condition is satisfied for either excitation step [S]. Excited state absorption from the 4F3,2 state results in the fluorescence transitions between 300 and 520 nm being two orders of magnitude stronger than shown in Fig. 1 where they are due to excited state absorption of a photon from the more rapidly decaying (4G5,2, 4G,,z ) and 4G,,z multiplets. The details of the mechanisms leading to upconversion in Nd3+ : KLiYF, are different to those suggested, for instance, in Nd3’ :LiYF, [8] and Nd3+ : LaF, [lo] and the transitions involved are different to those suggested, for instance, in reports
K. Holliday et al. 1 Journal of Luminescence 72-74 (1997) 927-929
on Nd3+ :LiYF, [7] and Nd3+-doped fluoride glasses [6]. It is conceivable that different mechanisms dominate in different materials but as the 4F3,2 state is central to all suggested mechanisms and is also the upper level for most Nd3+ laser transitions it is important to find a definitive answer to this problem.
References [l] W.D. Partlow, Phys. Rev. Lett. 21 (1968) 90. [2] H. Weidner, P.L. Summers, R.E. Peale and B.H.T. Chai, OSA Proc. Adv. Solid State Lasers, 20 (1994) 61.
929
131D.L. Russell and K. Holliday, in preparation. r41 P.L. Summers, H. Weidner, R.E. Peale and B.H.T. Chai, I. Appl. Phys. 75 (1994) 2184. c51D.L. Russell, B. Henderson, B.T.H. Chai, J.F.H. Nicholls and K. Holliday, Opt. Commun. 134 (1997) 398. [61 A.T. Stanley, E.A. Harris, T.M. Searle and J.M. Parker, J. Non-Cryst. Solids 161 (1993) 235. c71T. Chuang and H.R. Verdun, OSA Proc. Adv. Solid State Lasers 20 (1994) 77. t-81A.D. Novo-Gradac, W.M. Dennis, A.J. Silversmith, SM. Jacobsen and W.M. Yen, J. Lumin. 60 & 61 (19941 695. c91T.Y. Fan and R.L. Byer, J. Opt. Sot. Am. B 3 (1986) 1519. [lOI V.V. Ter-Mikirtychev and T. Tsuboi, Phys. Rev. B 52 (1995) 15027.
LUMINESCENCE ELSEWIER
Journal
of Luminescence
Up-conversion
72-74 (1997) 9277929
spectroscopy of Nd3 ’ : KLiYF5
Keith Holliday*, David L. Russell, Brian Henderson Department
of Phvsics and Applied Phwics.
lJniversi@
ofStrathclvde, Glasgow GI IXY UK
Abstract
In KLiYFs, Nd3+ ions shelved in the metastable 4F3,2 state decay through one of the three routes; radiative decay through the emission of an infrared photon directly to the 41Jstates; after absorption of a second photon via radiation of blue or violet photons from the 4D3,z or ‘PJjz states; through energy transfer to Nd 3+ ions in the ground state or to Nd3’ ions that are also shelved in the 4F3,2 state, the latter resulting in green and orange fluorescence from the t4G5,z> 4G7,z) and 4G7j2 multiplets. Site-selective up-conversion spectra can be obtained when energy transfer is not the populating mechanism for the fluorescing state. Keywords: Up-conversion;
Site selectivity; Laser spectroscopy; Nd3+: KLiYF,; Energy transfer
Two different up-conversion processes take place simultaneously in Nd 3+ : KLiYF,. The relative strength of the two processes depends on the transition that is excited but central to both are ions that have electrons shelved in the metastable 4F3,2 state (at 11510 cm-’ for both of the two crystallographically inequivalent sites). Three depopulating processes take place, one of which is radiative decay to the 41, levels. The second process that decreases the 4F3,2 state population is an interionic process involving energy transfer, either to a Nd3+ ion in the ground state or to a second ion in the 4F3,2 state [ 11. Both of these processes take place at a fast rate in Nd3+ : KLiYF, [2,3]. Fig. 1 shows an up-converted fluorescence spectrum between 300-700 nm obtained by resonantly pumping the 419,2+ 4F3,2 transition at 10 K. By fitting the precise energies of each feature to the known energy levels of *Correspondingauthor. Fax: 0141-5534162; e-mail: [email protected]. 0022-2313/97/$17.00 $; 1997 Elsevier Science B.V. All rights reserved PI1 SOO22-23 13(97)00085-9
Nd3’ :KLiYFS [4, 51 it was ascertained that the three transitions that dominate the spectrum at about 530, 600 and 660 nm correspond to transitions from the (4G5j2, 4G,j2) and 4G,,z multiplets and terminate on the 41J levels [3]. Assignments have been attempted for similar fluorescence spectra in many Nd 3+-doped solids but usually measurements have been conducted at room temperature where individual lines overlap due to thermal effects and the final conclusions have rarely been complete (see Refs. [6,7] for instance). The upper states are populated through energy transfer between two ions in the 4F3,2 state that causes one ion to be excited to a higher state whilst the other decays to a 41, state. The emitting states retain a significant population despite their very short, non-radiative decay limited lifetime [8] due to constant renewal of the population through energy transfer [2,3,8]. The dynamics of these processes leads to the appearance of additional features in the fluorescence bands, even at 10 K, due to nontherma1 population of the excited state Stark levels [3].
928
K. Hollidav et al. / Journal of Luminescence
72-74 (19Y7j 927-929
1
0.9
0.8 x50 0.7
I
0.3
+
4&/2-141w2
300
350
400
450
500
Wavelength Fig. 1. Up-converted
fluorescence
spectrum
obtained
550
650
700
(nm)
by exciting
The third depopulation mechanism for the 4F3,2 state of Nd3+ : KLiYF, is also via higher excited states. Here an intraionic process involves the absorption of a second photon followed by emission of violet and blue photons from the 4D3j2 and states as first reported in Nd3+ : LiYF, [9]. 2p3,2 The first stage involves either resonant or phonon assisted absorption to the (4G5,2, 4G7,2) multiplet at around 17 000 cm-’ (590 nm). Rapid non-radiative decay to the metastable 4F3,2 state follows. A second photon is then absorbed, populating energy levels close to 29 000 cm- ‘, and fluorescence is observed from the site-dependent energy levels at about 28 050 and 26 220 cm-’ after non-radiative decay [S]. The probability of absorption from the 4F3,z state to a particular Stark level of the 2111j2 state has the strongest transition probability and, as the strongly absorbing Stark level is different for each of the two sites, site-selective up-converted fluorescence is observed. Alternatively, the reson-
600
the 419,2 + 4F3,2 transition
in Nd3+
: KLiYF, at 10 K
ance condition can be satisfied for either site for the first excitation step and this results in up-converted fluorescence from both sites due to strong radiative energy transfer to Nd3+ ions initially in their 419,2 ground state [2,3]. In the case of samples doped with less than 1% Nd3+, energy transfer between sites is much weaker and site-selective upconversion spectra can be obtained when the resonance condition is satisfied for either excitation step [S]. Excited state absorption from the 4F3,2 state results in the fluorescence transitions between 300 and 520 nm being two orders of magnitude stronger than shown in Fig. 1 where they are due to excited state absorption of a photon from the more rapidly decaying (4G5,2, 4G,,z ) and 4G,,z multiplets. The details of the mechanisms leading to upconversion in Nd3+ : KLiYF, are different to those suggested, for instance, in Nd3’ :LiYF, [8] and Nd3+ : LaF, [lo] and the transitions involved are different to those suggested, for instance, in reports
K. Holliday et al. 1 Journal of Luminescence 72-74 (1997) 927-929
on Nd3+ :LiYF, [7] and Nd3+-doped fluoride glasses [6]. It is conceivable that different mechanisms dominate in different materials but as the 4F3,2 state is central to all suggested mechanisms and is also the upper level for most Nd3+ laser transitions it is important to find a definitive answer to this problem.
References [l] W.D. Partlow, Phys. Rev. Lett. 21 (1968) 90. [2] H. Weidner, P.L. Summers, R.E. Peale and B.H.T. Chai, OSA Proc. Adv. Solid State Lasers, 20 (1994) 61.
929
131D.L. Russell and K. Holliday, in preparation. r41 P.L. Summers, H. Weidner, R.E. Peale and B.H.T. Chai, I. Appl. Phys. 75 (1994) 2184. c51D.L. Russell, B. Henderson, B.T.H. Chai, J.F.H. Nicholls and K. Holliday, Opt. Commun. 134 (1997) 398. [61 A.T. Stanley, E.A. Harris, T.M. Searle and J.M. Parker, J. Non-Cryst. Solids 161 (1993) 235. c71T. Chuang and H.R. Verdun, OSA Proc. Adv. Solid State Lasers 20 (1994) 77. t-81A.D. Novo-Gradac, W.M. Dennis, A.J. Silversmith, SM. Jacobsen and W.M. Yen, J. Lumin. 60 & 61 (19941 695. c91T.Y. Fan and R.L. Byer, J. Opt. Sot. Am. B 3 (1986) 1519. [lOI V.V. Ter-Mikirtychev and T. Tsuboi, Phys. Rev. B 52 (1995) 15027.