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Infrared absorption and Zeeman effect in Nd3+ doped YVO4
Optical Materials 19 (2002) 449–454 www.elsevier.com/locate/optmat
Infrared absorption and Zeeman effect in Nd3þ doped YVO4 S. Jandl a
b
a,*
, O. Guillot-No€el b, D. Gourier
b
D epartement de Physique, Centre de Recherche sur les Propri et es Electroniques des Mat eriaux Avanc es, Universit e de Sherbrooke, Que., Canada J1K2R1 Laboratoire de Chimie Appliqu ee del Etat Solide, Ecole Nationale Sup erieure de Chimie de Paris (ENSCP), UMR CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris C edex 05, France Received 2 September 2001; received in revised form 21 November 2001; accepted 7 January 2002
Abstract Low temperature infrared transmission studies of Nd3þ doped YVO4 were performed, under a magnetic field B ? c, in the 1800–8000 cm1 range of the 4 I9=2 ! 4 I11=2 , 4 I13=2 , and 4 I15=2 Nd3þ crystal-field transitions. Good agreement is obtained between the experimental and calculated g-factors. Frequencies of the satellites in the 4 I9=2 ! 4 F3=2 transitions of the Nd3þ isolated ion confirm the presence of ferromagnetic interactions between pairs of coupled Nd3þ ions that lift the Kramers doublet degeneracies of their ground state and excited multiplets. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction There has been in recent years, a renewed interest in the zircon-type structure Nd3þ :YVO4 diode-pumped microchip laser material due to its strong broad absorption band around 808 nm and its very intense emission in the 1 lm range [1–3]. The YVO4 space group is I41 /amd and the Y3þ ions, which are eightfold coordinated to oxygen ions, are located in a D2d site symmetry. In Nd3þ : YVO4 , 99.9% of the Nd3þ ions substitute for Y3þ ions in their regular sites, and only 0.1% are located in an orthorhombic D2 or C2v site symmetry [4]. Low temperature absorption spectra of
*
Corresponding author. Tel.: +1-819-821-8000; fax: +1-819821-8046. E-mail address: [email protected] (S. Jandl).
Nd3þ :YVO4 have allowed the determination of the five non-zero B02 , B04 , B44 , B06 , and B46 crystal-field parameters that describe the crystal-field Hamiltonian. The g-factors of the 4 I9=2 ground state multiplet lowest Kramers doublet have been calculated and compared to EPR measurements [5]. Recently, Zeeman effect measurements of the 4 F3=2 ! 4 I9=2 Nd3þ ion luminescence in the 11 350– 11 390 cm1 range were analyzed by adding to the free-ion and crystal-field Hamiltonian a Zeeman term, and the g-factors of the 4 F3=2 Kramers doublets have been determined [6]. It is worth noting that the agreement between the observed and calculated values of the g-factor for B ? c compared to the g-factor for B // c is less satisfactory in the Zeeman effect optical studies of Nd3þ doped LuPO4 and YPO4 Zircon-type materials [7], YLiF4 scheelite-type material [8], as well as NdOCl [9]. This discrepancy has been partly attributed to the
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S. Jandl et al. / Optical Materials 19 (2002) 449–454
crystal level displacements affecting the ket wavefunction composition associated with each quantum state [8]. Satellite lines due to Nd3þ –Nd3þ pair interactions have been observed in the electron paramagnetic resonance and optical spectra of Nd3þ : YVO4 and Nd3þ :YLiF4 [10–13] even at low concentrations such as 7:2 0:2 1019 Nd3þ ions/cm3 (0.58%) [4]. Luminescence studies of the Nd3þ ion 4 F3=2 ! 4 I9=2 transitions evidenced the existence of a ground state ferromagnetic interaction between the ions of the Nd3þ –Nd3þ coupled pairs. Positions of the satellite lines in the vicinity of the isolated Nd3þ ion line have been assigned to four different ferromagnetically coupled ion pairs characterized by the exchange energy coupling values: J1 ¼ 0:8 cm1 , J2 ¼ 1:6 cm1 , J3 ¼ 2:7 cm1 and J4 ¼ 4:9 cm1 [6]. In this model, for the ground state exchange coupled pair, a doublet of lines is expected and observed at energies D þ J =2 and D 3J =2, asymmetrically located around the isolated ion transition at the energy D. Under an external magnetic field, each line is expected to split into two components. As a continuation of previous investigations on the crystal-field of the Nd3þ ions in the YVO4 matrix, we present in this paper infrared transmission measurements of Nd3þ :YVO4 under a magnetic field. The objective, on the one hand, is to evaluate experimentally the 4 I11=2 , 4 I13=2 , 4 I15=2 g-factors for B ? c and to compare them to the general Hamiltonian of an ion in the presence of crystal and magnetic field predictions. On the other hand, we aim to determine if the satellites that are attributed to the Nd3þ –Nd3þ pair interactions are observed in the transmission spectra and if, contrary to the pair description model that restricts the splitting to the ground state of the interacting pairs [6], the excited multiplets are also split.
and electronic microprobe analysis were used to determine the exact neodymium concentration. Room temperature micro-Raman back-scattering measurements were performed, to insure absence of twinning, with a LABRAM-800 confocal system equipped with a charge-coupled-device (CCD) He–Ne laser line fodetector. A 5 mW, 6328 A cused to a 1 lm spot diameter on the sample, was used as the exciting source. For the infrared transmission study of the 4 I11=2 , 4 I13=2 , 4 I15=2 Nd3þ ion crystal-field excitations under a magnetic field, the samples, with their ac-plane perpendicular to the incident beam and to the magnetic field, were mounted in a continuous-flow temperature-regulated helium cryostat inserted in the core of an 8 T superconducting magnet. Spectra at a 0.5 cm1 resolution, in the 1800– 8000 cm1 range, were obtained using a fastFourier-transform interferometer (BOMEM DA 3.002) equipped with a quartz halogen lamp, a CaF2 beam splitter and an InSb detector. A quartz beamsplitter and a Si detector were also used for the study of the absorption bands around 11 300 cm1 at a 0.25 cm1 resolution.
3. Results and discussion Typical Raman spectra of Nd3þ :YVO4 , obtained on a 1 lm scale with the polarized incident light in the ac plane, is shown in Fig. 1. For 0.58%
2. Experimental YVO4 single crystals with 0.58% and 3% neodymium concentrations and typically 1:5 1:5 1 mm3 were grown by the Czochralski method. Inductively coupled plasma atomic spectroscopy
Fig. 1. Raman spectrum of 0.58% Nd:YVO4 at T ¼ 300 K.
S. Jandl et al. / Optical Materials 19 (2002) 449–454
451
Nd3þ ion substitution, no local distortions that would induce local or symmetry forbidden modes are observed in the spectra. Only the allowed A1g (380 and 892 cm1 ) and Eg (163, 264 and 841 cm1 ) modes, for the adopted experimental configuration, are observed. The absence of B1g and B2g modes confirms the orientation of the sample ac plane. The phonon frequencies and symmetries correspond to the ones observed in undoped YVO4 single crystals [14] strongly indicating that Nd3þ ions substitute for Y3þ ions in regular sites with no special defects induced. Infrared absorption spectra, under an applied magnetic field B ? c, from the 4 I9=2 multiplet ground state to the excited 4 I11=2 multiplet levels are shown in Fig. 2. The 1966, 1987, 2046, 2061, and 2153 cm1 absorption bands correspond to Kramers doublets whose degeneracies are lifted under the applied magnetic field. Since the levels
are not pure, absorption between the ground state Kramers doublet and an excited Kramers doublet would exhibit four split bands, if the corresponding dipole moments are strong enough, as observed in the case of the three lowest energy excitations. Lifting of the Kramers doublet degeneracies is also observed in both cases of the 4 I13=2 multiplet levels, 3908, 3930, 3977, 4040, 4087, 4156 and 4159 cm1 , and of the 4 I15=2 multiplet levels 5831, 5868, 5914, 6064, and 6258 cm1 (Figs. 3 and 4). Evolution of the level splittings under an applied magnetic field bf B ? c is presented, for the three excited multiplets 4 I11=2 , 4 I13=2 , and 4 I15=2 , in Figs. 5–7 respectively. In Table 1, the calculated g-factors that correspond to the splittings are compared to the theoretical predictions of the crystal-field Hamiltonian expressed in terms of the crystal-field parameters, B02 ¼ 200, B04 ¼ 628, B44 ¼ 1136, B06 ¼ 1233, and B46 ¼ 149 cm1 [5], with the magnetic interaction introduced through the L þ gS
Fig. 2. 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I11=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
Fig. 3. 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I13=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
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S. Jandl et al. / Optical Materials 19 (2002) 449–454
Fig. 6. Zeeman splitting of 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I13=2 absorption bands t T ¼ 10 K under applied magnetic field B ? c.
Fig. 4. 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I15=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
Fig. 7. Zeeman splitting of 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I15=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
Fig. 5. Zeeman splitting of 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I11=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
tensor operator as a perturbation. The agreement between the observed and predicted g-factors is
rather good, indicating that in contrast to Nd3þ :LuPO 4 , YPO4; YLiF4 and NdOCl [7–9], the crystal-field parameters determined for Nd3þ : YVO4 , predict adequately the g-factors for B ? c. Infrared transmission spectra of Nd3þ :YVO4 , corresponding to the 4 I9=2 ! 4 F3=2 transitions, are shown in Fig. 8 for the two Nd concentrations (0.58% and 3%). In addition to the isolated Nd3þ ion absorption bands (DÞ at 11 364.6 cm1 (*) and 11 384.2 cm1 (*), satellites marked with arrows and shifted 5.5, 2.5, 1.2 and 1.8, 2.4 cm1 from 11 364.6 cm1 and 8.1, 5.5, 2.6, 1.3,
S. Jandl et al. / Optical Materials 19 (2002) 449–454 Table 1 B ? c g-factors of Nd3þ crystal field levels in YVO4 Multiplet
Energy level (cm1 )
Experimental g-factor (0.2)
Calculated g-factor
4
I11=2
1966 1987 2046 2061
5.1 4.8 0.8 0.2
5.23 4.96 0.96 0.23
4
I13=2
3908 3930 3977 4040 4087 4156 4159
7.0 6.1 0.5 0.1 2.3 6.9 2.7
7.30 6.16 0.50 0.16 2.44 6.89 2.66
4
I15=2
5831 5868 5914 6064 6258
9.1 0.2 7.5 0.9 5.0
9.36 0.42 6.69 0.80 5.23
1.8 and 2.7 cm1 from 11 384.2 cm1 are observed. These satellites correspond to the D + J =2 and D 3J =2 excitations assigned to four different
453
ferromagnetically interacting ion pairs that are characterized by the exchange energy J coupling values (0:1 cm1 ):J1 ¼ 0:8 cm1 , J2 ¼ 1:7 cm1 , J3 ¼ 2:7 cm1 and J4 ¼ 4:9 cm1 , in agreement with the J values as determined by the photoluminescence measurements [6]. An additional pair, not resolved in the photoluminescence spectra, is observed as indicated by the dashed arrows and corresponds to an interacting exchange value J ¼ 3:6 cm1 . The relative intensities and bandwidths of the satellites, compared to the isolated ion absorption bands, increase as expected with the Nd concentration. Most of their absorption bands are strong, even for the low concentration of 0.58%, indicating an important dipole oscillator strength associated with the 4 I9=2 ! 4 F3=2 pair transitions. A few absorption bands, not indicated with arrows, could not be unambiguously assigned to pair interactions. Satellites are also observed in the absorption spectra corresponding to the 4 I9=2 ! 4 I11=2 , 4 I9=2 ! 4 I13=2 , and 4 I9=2 ! 4 I15=2 transitions as indicated by arrows in Figs. 2–4. Compared to the 4 I9=2 ! 4 F3=2 pair transitions, only a few satellites are observed, and their oscillator strengths are weaker. Up to B ¼ 2 T they do not split, and for higher magnetic fields, they are masked by the isolated ion transitions. Frequency shifts of the satellites, given in brackets, and associated with the 1966 ð2:6Þ, 1987 (2.7) and 2046 (8.3 and þ2.6) cm1 absorption bands, indicate that the 4 I11=2 pair excited levels are split by the pair interactions since they do not correspond to the splittings of the pair ground states. Same remarks apply to the satellites accompanying 4 I13=2 and 4 I15=2 excited multiplet levels: 3908 ð3:8Þ, 3977 (3.4, 6.5, 3.8), 4040 (10.5), 5831 (3.4, 4.9), 5868 (þ5.0) cm1 . The overall small number of satellites hinders the exact determination of the excited state splittings needed to determine the additional terms of the Hamiltonian that describe more precisely the pair interactions.
4. Conclusion Fig. 8. 0.58% and 3% Nd:YVO4 intermultiplet 4 I9=2 ! 4 F3=2 absorption bands at T ¼ 10 K.
A study under a B ? c applied magnetic field configuration of the Nd3þ :YVO4 absorption bands
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S. Jandl et al. / Optical Materials 19 (2002) 449–454
that correspond to the 4 I9=2 ! 4 I11=2 , 4 I9=2 ! 4 I13=2 , and 4 I9=2 ! 4 I15=2 transitions has allowed the determination of their crystal-field level g-factors. The g-factor values agree with the predictions of a crystal-field Hamiltonian that includes a Zeeman term [5] in contrast to the case of Nd3þ :LuPO4 , YPO4; YLiF4 and NdOCl studies that could not adequately predict the g-factors for B ? c. The 4 I9=2 ! 4 F3=2 transition absorption bands have confirmed the Nd–Nd pair ferromagnetic exchange interaction model predictions, and an additional pair, compared to the photoluminescence measurements, has been detected. The pair crystal-field excited multiplets are split indicating that the pair exchange interaction not only influences the ground state levels but also affects the excited multiplets.
Acknowledgements The authors are grateful to Dr. B. Ferrand from the LETI-CEA for providing the Nd3þ :YVO4 single crystals used in this work and to J. Rousseau for technical assistance. S.J. acknowledges support from the National Science and Engineering Research Council of Canada and le Fonds de Formation de Chercheurs et l’aide a la Recherche du Quebec.
References [1] D.G. Matthews, J.R. Boon, R.S. Conroy, B.D. Sinclair, J. Modern Opt. 43 (1996) 1079. [2] G. Feugnet, C. Bussac, C. Larat, M. Schwarz, J.P. Pocholle, Opt. Lett. 20 (1995) 157. [3] B.H.T. Chai, G. Loutts, J. Lefaucheur, X.X. Zhang, P. Hong, M. Bass, I.A. Shesherbakov, A.I. Zagumennyi, OSA Proceedings on Advanced Solid State Lasers 20 (1994) 41. [4] O. Guillot-No€el, B. Viana, B. Bellamy, D. Gourier, G.B. Zogo-Mboulou, S. Jandl, Opt. Mat. 13 (2000) 427. [5] O. Guillot-No€el, A. Khan-Harari, B. Viana, D. Vivien, E. Antic-Fidancev, P. Porcher, J. Phys.: Condens. Matter 10 (1998) 6491. [6] V. Mehta, O. Guillot-No€el, D. Gourier, Z. Ichalalene, M. Castonguay, S. Jandl, J. Phys.: Condens. Matter 12 (2000) 7149. [7] T. Hayhurst, G. Shalimoff, J.G. Conway, N. Edelstein, L.A. Boatner, M.M. Abraham, J. Chem. Phys. 76 (1982) 3961. [8] M.A. Couto dos Santos, P. Porcher, J.C. Krupa, J.Y. Gesland, J. Phys.: Condens. Matter 8 (1996) 4643. [9] L. Beaury, J. Derouet, P. Porcher, P. Caro, P.G. Feldman, J. Less-Common Met. 126 (1986) 263. [10] O. Guillot-No€el, V. Mehta, B. Viana, D. Gourier, M. Boukhris, S. Jandl, Phys. Rev. B 61 (2000) 15338. [11] F.S. Ermeneux, C. Goutaudier, R. Moncorge, M.T. Cohen-Adad, M. Bettinelli, E. Cavalli, Opt. Mater. 13 (1999) 193. [12] R.B. Barthem, R. Buisson, J.C. Vial, H. Harmand, J. Lumin. 34 (1986) 295. [13] M. Boukhris, S. Jandl, O. Guillot-No€el, D. Gourier, J. Chem. Phys. of Solids 63 (2002) 527. [14] B.M. Jin, S. Erdei, A.S. Bhalla, F.W. Ainger, Mat. Res. Bull. 30 (1995) 1293.
Infrared absorption and Zeeman effect in Nd3þ doped YVO4 S. Jandl a
b
a,*
, O. Guillot-No€el b, D. Gourier
b
D epartement de Physique, Centre de Recherche sur les Propri et es Electroniques des Mat eriaux Avanc es, Universit e de Sherbrooke, Que., Canada J1K2R1 Laboratoire de Chimie Appliqu ee del Etat Solide, Ecole Nationale Sup erieure de Chimie de Paris (ENSCP), UMR CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris C edex 05, France Received 2 September 2001; received in revised form 21 November 2001; accepted 7 January 2002
Abstract Low temperature infrared transmission studies of Nd3þ doped YVO4 were performed, under a magnetic field B ? c, in the 1800–8000 cm1 range of the 4 I9=2 ! 4 I11=2 , 4 I13=2 , and 4 I15=2 Nd3þ crystal-field transitions. Good agreement is obtained between the experimental and calculated g-factors. Frequencies of the satellites in the 4 I9=2 ! 4 F3=2 transitions of the Nd3þ isolated ion confirm the presence of ferromagnetic interactions between pairs of coupled Nd3þ ions that lift the Kramers doublet degeneracies of their ground state and excited multiplets. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction There has been in recent years, a renewed interest in the zircon-type structure Nd3þ :YVO4 diode-pumped microchip laser material due to its strong broad absorption band around 808 nm and its very intense emission in the 1 lm range [1–3]. The YVO4 space group is I41 /amd and the Y3þ ions, which are eightfold coordinated to oxygen ions, are located in a D2d site symmetry. In Nd3þ : YVO4 , 99.9% of the Nd3þ ions substitute for Y3þ ions in their regular sites, and only 0.1% are located in an orthorhombic D2 or C2v site symmetry [4]. Low temperature absorption spectra of
*
Corresponding author. Tel.: +1-819-821-8000; fax: +1-819821-8046. E-mail address: [email protected] (S. Jandl).
Nd3þ :YVO4 have allowed the determination of the five non-zero B02 , B04 , B44 , B06 , and B46 crystal-field parameters that describe the crystal-field Hamiltonian. The g-factors of the 4 I9=2 ground state multiplet lowest Kramers doublet have been calculated and compared to EPR measurements [5]. Recently, Zeeman effect measurements of the 4 F3=2 ! 4 I9=2 Nd3þ ion luminescence in the 11 350– 11 390 cm1 range were analyzed by adding to the free-ion and crystal-field Hamiltonian a Zeeman term, and the g-factors of the 4 F3=2 Kramers doublets have been determined [6]. It is worth noting that the agreement between the observed and calculated values of the g-factor for B ? c compared to the g-factor for B // c is less satisfactory in the Zeeman effect optical studies of Nd3þ doped LuPO4 and YPO4 Zircon-type materials [7], YLiF4 scheelite-type material [8], as well as NdOCl [9]. This discrepancy has been partly attributed to the
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 0 2 6 - 5
450
S. Jandl et al. / Optical Materials 19 (2002) 449–454
crystal level displacements affecting the ket wavefunction composition associated with each quantum state [8]. Satellite lines due to Nd3þ –Nd3þ pair interactions have been observed in the electron paramagnetic resonance and optical spectra of Nd3þ : YVO4 and Nd3þ :YLiF4 [10–13] even at low concentrations such as 7:2 0:2 1019 Nd3þ ions/cm3 (0.58%) [4]. Luminescence studies of the Nd3þ ion 4 F3=2 ! 4 I9=2 transitions evidenced the existence of a ground state ferromagnetic interaction between the ions of the Nd3þ –Nd3þ coupled pairs. Positions of the satellite lines in the vicinity of the isolated Nd3þ ion line have been assigned to four different ferromagnetically coupled ion pairs characterized by the exchange energy coupling values: J1 ¼ 0:8 cm1 , J2 ¼ 1:6 cm1 , J3 ¼ 2:7 cm1 and J4 ¼ 4:9 cm1 [6]. In this model, for the ground state exchange coupled pair, a doublet of lines is expected and observed at energies D þ J =2 and D 3J =2, asymmetrically located around the isolated ion transition at the energy D. Under an external magnetic field, each line is expected to split into two components. As a continuation of previous investigations on the crystal-field of the Nd3þ ions in the YVO4 matrix, we present in this paper infrared transmission measurements of Nd3þ :YVO4 under a magnetic field. The objective, on the one hand, is to evaluate experimentally the 4 I11=2 , 4 I13=2 , 4 I15=2 g-factors for B ? c and to compare them to the general Hamiltonian of an ion in the presence of crystal and magnetic field predictions. On the other hand, we aim to determine if the satellites that are attributed to the Nd3þ –Nd3þ pair interactions are observed in the transmission spectra and if, contrary to the pair description model that restricts the splitting to the ground state of the interacting pairs [6], the excited multiplets are also split.
and electronic microprobe analysis were used to determine the exact neodymium concentration. Room temperature micro-Raman back-scattering measurements were performed, to insure absence of twinning, with a LABRAM-800 confocal system equipped with a charge-coupled-device (CCD) He–Ne laser line fodetector. A 5 mW, 6328 A cused to a 1 lm spot diameter on the sample, was used as the exciting source. For the infrared transmission study of the 4 I11=2 , 4 I13=2 , 4 I15=2 Nd3þ ion crystal-field excitations under a magnetic field, the samples, with their ac-plane perpendicular to the incident beam and to the magnetic field, were mounted in a continuous-flow temperature-regulated helium cryostat inserted in the core of an 8 T superconducting magnet. Spectra at a 0.5 cm1 resolution, in the 1800– 8000 cm1 range, were obtained using a fastFourier-transform interferometer (BOMEM DA 3.002) equipped with a quartz halogen lamp, a CaF2 beam splitter and an InSb detector. A quartz beamsplitter and a Si detector were also used for the study of the absorption bands around 11 300 cm1 at a 0.25 cm1 resolution.
3. Results and discussion Typical Raman spectra of Nd3þ :YVO4 , obtained on a 1 lm scale with the polarized incident light in the ac plane, is shown in Fig. 1. For 0.58%
2. Experimental YVO4 single crystals with 0.58% and 3% neodymium concentrations and typically 1:5 1:5 1 mm3 were grown by the Czochralski method. Inductively coupled plasma atomic spectroscopy
Fig. 1. Raman spectrum of 0.58% Nd:YVO4 at T ¼ 300 K.
S. Jandl et al. / Optical Materials 19 (2002) 449–454
451
Nd3þ ion substitution, no local distortions that would induce local or symmetry forbidden modes are observed in the spectra. Only the allowed A1g (380 and 892 cm1 ) and Eg (163, 264 and 841 cm1 ) modes, for the adopted experimental configuration, are observed. The absence of B1g and B2g modes confirms the orientation of the sample ac plane. The phonon frequencies and symmetries correspond to the ones observed in undoped YVO4 single crystals [14] strongly indicating that Nd3þ ions substitute for Y3þ ions in regular sites with no special defects induced. Infrared absorption spectra, under an applied magnetic field B ? c, from the 4 I9=2 multiplet ground state to the excited 4 I11=2 multiplet levels are shown in Fig. 2. The 1966, 1987, 2046, 2061, and 2153 cm1 absorption bands correspond to Kramers doublets whose degeneracies are lifted under the applied magnetic field. Since the levels
are not pure, absorption between the ground state Kramers doublet and an excited Kramers doublet would exhibit four split bands, if the corresponding dipole moments are strong enough, as observed in the case of the three lowest energy excitations. Lifting of the Kramers doublet degeneracies is also observed in both cases of the 4 I13=2 multiplet levels, 3908, 3930, 3977, 4040, 4087, 4156 and 4159 cm1 , and of the 4 I15=2 multiplet levels 5831, 5868, 5914, 6064, and 6258 cm1 (Figs. 3 and 4). Evolution of the level splittings under an applied magnetic field bf B ? c is presented, for the three excited multiplets 4 I11=2 , 4 I13=2 , and 4 I15=2 , in Figs. 5–7 respectively. In Table 1, the calculated g-factors that correspond to the splittings are compared to the theoretical predictions of the crystal-field Hamiltonian expressed in terms of the crystal-field parameters, B02 ¼ 200, B04 ¼ 628, B44 ¼ 1136, B06 ¼ 1233, and B46 ¼ 149 cm1 [5], with the magnetic interaction introduced through the L þ gS
Fig. 2. 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I11=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
Fig. 3. 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I13=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
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Fig. 6. Zeeman splitting of 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I13=2 absorption bands t T ¼ 10 K under applied magnetic field B ? c.
Fig. 4. 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I15=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
Fig. 7. Zeeman splitting of 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I15=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
Fig. 5. Zeeman splitting of 0.58% Nd:YVO4 intermultiplet 4 I9=2 ! 4 I11=2 absorption bands at T ¼ 10 K under applied magnetic field B ? c.
tensor operator as a perturbation. The agreement between the observed and predicted g-factors is
rather good, indicating that in contrast to Nd3þ :LuPO 4 , YPO4; YLiF4 and NdOCl [7–9], the crystal-field parameters determined for Nd3þ : YVO4 , predict adequately the g-factors for B ? c. Infrared transmission spectra of Nd3þ :YVO4 , corresponding to the 4 I9=2 ! 4 F3=2 transitions, are shown in Fig. 8 for the two Nd concentrations (0.58% and 3%). In addition to the isolated Nd3þ ion absorption bands (DÞ at 11 364.6 cm1 (*) and 11 384.2 cm1 (*), satellites marked with arrows and shifted 5.5, 2.5, 1.2 and 1.8, 2.4 cm1 from 11 364.6 cm1 and 8.1, 5.5, 2.6, 1.3,
S. Jandl et al. / Optical Materials 19 (2002) 449–454 Table 1 B ? c g-factors of Nd3þ crystal field levels in YVO4 Multiplet
Energy level (cm1 )
Experimental g-factor (0.2)
Calculated g-factor
4
I11=2
1966 1987 2046 2061
5.1 4.8 0.8 0.2
5.23 4.96 0.96 0.23
4
I13=2
3908 3930 3977 4040 4087 4156 4159
7.0 6.1 0.5 0.1 2.3 6.9 2.7
7.30 6.16 0.50 0.16 2.44 6.89 2.66
4
I15=2
5831 5868 5914 6064 6258
9.1 0.2 7.5 0.9 5.0
9.36 0.42 6.69 0.80 5.23
1.8 and 2.7 cm1 from 11 384.2 cm1 are observed. These satellites correspond to the D + J =2 and D 3J =2 excitations assigned to four different
453
ferromagnetically interacting ion pairs that are characterized by the exchange energy J coupling values (0:1 cm1 ):J1 ¼ 0:8 cm1 , J2 ¼ 1:7 cm1 , J3 ¼ 2:7 cm1 and J4 ¼ 4:9 cm1 , in agreement with the J values as determined by the photoluminescence measurements [6]. An additional pair, not resolved in the photoluminescence spectra, is observed as indicated by the dashed arrows and corresponds to an interacting exchange value J ¼ 3:6 cm1 . The relative intensities and bandwidths of the satellites, compared to the isolated ion absorption bands, increase as expected with the Nd concentration. Most of their absorption bands are strong, even for the low concentration of 0.58%, indicating an important dipole oscillator strength associated with the 4 I9=2 ! 4 F3=2 pair transitions. A few absorption bands, not indicated with arrows, could not be unambiguously assigned to pair interactions. Satellites are also observed in the absorption spectra corresponding to the 4 I9=2 ! 4 I11=2 , 4 I9=2 ! 4 I13=2 , and 4 I9=2 ! 4 I15=2 transitions as indicated by arrows in Figs. 2–4. Compared to the 4 I9=2 ! 4 F3=2 pair transitions, only a few satellites are observed, and their oscillator strengths are weaker. Up to B ¼ 2 T they do not split, and for higher magnetic fields, they are masked by the isolated ion transitions. Frequency shifts of the satellites, given in brackets, and associated with the 1966 ð2:6Þ, 1987 (2.7) and 2046 (8.3 and þ2.6) cm1 absorption bands, indicate that the 4 I11=2 pair excited levels are split by the pair interactions since they do not correspond to the splittings of the pair ground states. Same remarks apply to the satellites accompanying 4 I13=2 and 4 I15=2 excited multiplet levels: 3908 ð3:8Þ, 3977 (3.4, 6.5, 3.8), 4040 (10.5), 5831 (3.4, 4.9), 5868 (þ5.0) cm1 . The overall small number of satellites hinders the exact determination of the excited state splittings needed to determine the additional terms of the Hamiltonian that describe more precisely the pair interactions.
4. Conclusion Fig. 8. 0.58% and 3% Nd:YVO4 intermultiplet 4 I9=2 ! 4 F3=2 absorption bands at T ¼ 10 K.
A study under a B ? c applied magnetic field configuration of the Nd3þ :YVO4 absorption bands
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that correspond to the 4 I9=2 ! 4 I11=2 , 4 I9=2 ! 4 I13=2 , and 4 I9=2 ! 4 I15=2 transitions has allowed the determination of their crystal-field level g-factors. The g-factor values agree with the predictions of a crystal-field Hamiltonian that includes a Zeeman term [5] in contrast to the case of Nd3þ :LuPO4 , YPO4; YLiF4 and NdOCl studies that could not adequately predict the g-factors for B ? c. The 4 I9=2 ! 4 F3=2 transition absorption bands have confirmed the Nd–Nd pair ferromagnetic exchange interaction model predictions, and an additional pair, compared to the photoluminescence measurements, has been detected. The pair crystal-field excited multiplets are split indicating that the pair exchange interaction not only influences the ground state levels but also affects the excited multiplets.
Acknowledgements The authors are grateful to Dr. B. Ferrand from the LETI-CEA for providing the Nd3þ :YVO4 single crystals used in this work and to J. Rousseau for technical assistance. S.J. acknowledges support from the National Science and Engineering Research Council of Canada and le Fonds de Formation de Chercheurs et l’aide a la Recherche du Quebec.
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