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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 12 — Jun. 6, 2011
  • pp: 11786–11791
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CeF3 and PrF3 as UV-Visible Faraday rotators

P. Molina, V. Vasyliev, E. G. Víllora, and K. Shimamura  »View Author Affiliations


Optics Express, Vol. 19, Issue 12, pp. 11786-11791 (2011)
http://dx.doi.org/10.1364/OE.19.011786


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Abstract

CeF3 and PrF3 single crystals are investigated as Faraday rotators for the UV-visible region. Their properties are compared with those of the industrial standard reference terbium-gallium-garnet (TGG) single crystal. In contrast to TGG, CeF3 exhibits a higher transparency in the whole near UV-visible-IR, and a remarkable higher figure of merit which rapidly increases towards the cutoff. In the case of PrF3, the transparency extends to even shorter wavelengths, and analogously to CeF3 its figure of merit increases notably in the UV. These results indicate the potential of CeF3 and PrF3 as optical isolators in the UV-visible, where at present there are no alternative candidates.

© 2011 OSA

1. Introduction

The continuous development of high-power laser-diodes and high-power fiber-lasers operating at shorter wavelengths is leading in short time to increased requirements for optical isolators (OIs). These devices are used for the unidirectional propagation of laser light, avoiding optical feedback. This factor is very important in order to avoid parasitic oscillations in amplifier systems and frequency instabilities that decrease the beam quality as well as the lifetime of laser sources. The central part of an OI is a Faraday rotator (FR), normally a single crystal, which rotates the polarization plane of the laser beam when a magnetic field B is applied parallel to its propagation path. The rotation angle β is proportional to both B and the crystal length L according to the well know equation β = VBL, where V is the proportionality factor known as Verdet constant (V).

The most common FRs are garnets single crystals. Yttrium-iron-garnet Y3Fe5O12 (YIG) is used for long distance fiber communications in the near IR region, where it is transparent and exhibits an extremely high V value. Bellow 1100 nm terbium-gallium-garnet Tb3Ga5O12 (TGG) is utilized, mainly for fiber-laser machinery applications. TGG is characterized by a high V in comparison with similar compounds [1

1. C. B. Rubinstein, L. G. Van Uitert, and W. H. Grodkiewicz, “Magneto-optical properties of rare earth (III) aluminum garnets,” J. Appl. Phys. 35(10), 3069 (1964). [CrossRef]

], however, it presents two mayor disadvantages. Firstly, its transparency decreases continuously towards shorter wavelengths in the visible (VIS) wavelength region, leading to optical losses and other detrimental effects on the device performance. Secondly, although it melts congruently at 1825°C, its synthesis by the Czochralski (Cz) technique is not exempt of difficulties [2

2. R. C. Linares, “Growth of garnet laser crystals,” Solid State Commun. 2(8), 229–231 (1964). [CrossRef]

], as well as a high production cost [3

3. M. Geho, T. Sekijima, and T. Fujii, “Growth of terbium aluminum garnet (Tb3Al5O12; TAG) single crystals by the hybrid laser floating zone machine,” J. Cryst. Growth 267(1–2), 188–193 (2004). [CrossRef]

]. Recently a new garnet crystal, [Tb3][Sc2-xLux](Al3)O12 (TSLAG), has been proposed as an alternative VIS-IR FR. It exhibits not only a higher VIS transparency and a higher V than TGG, but also favorable growth characteristics for its industrial implementation [4

4. K. Shimamura, T. Kito, E. Castel, A. Latynina, P. Molina, E. G. Víllora, P. Mythili, P. Veber, J.-P. Chaminade, A. Funaki, T. Hatanaka, and K. Naoe, “Growth of {Tb3}[Sc2-xLux](Al3)O12 single crystals for visible-infrared optica isolators,” Cryst. Growth Des. 10(8), 3466–3470 (2010). [CrossRef]

]. However, in spite of the superior properties respect to TGG, TSLAG presents the same limiting features, namely the strong absorption bands in the near UV and blue wavelength regions. These are originated in the partially spin-allowed 4f-4f transitions of the magneto-optical (MO) active ion Tb3+, and therefore these crystals cannot be used at wavelengths neither bellow 400 nm nor at around 490 nm. Consequently, there is a need to find FRs which can be used in non-covered UV and VIS wavelength regions.

2. Experimental

Transmittance and reflectance spectra in the UV-VIS range were obtained with a Jasco V570 spectrophotometer. The dispersion of the absorption coefficient α was calculated for CeF3, PrF3 and TGG taking into account both spectra. The FR angles were measured in the same region using a Xe-lamp and a spectrometer. 10 mm long crystals were placed in a magnet located between two Calcite Glan-Taylor polarizers mounted on rotation holders. The transmitted signal was recorded by the aid of an optical fiber coupled multichannel analyzer. In the particular case of CeF3 and PrF3, the crystals were oriented with the optical c-axis parallel to the magnetic field. All measurements were performed at room temperature.

3. Results and discussion

The absorbance of the PrF3 and CeF3 single crystals in the UV-VIS wavelength region is shown in Fig. 1
Fig. 1 Absorption coefficient dispersion of PrF3, CeF3 and TGG single crystals.
. in comparison with the reference crystal TGG. PrF3 presents the shortest cutoff among them at 210 nm, however, a narrow absorption band corresponding to the transition from the ground level 3H4 to the excited state 1S0 reduces the transparency below 220 nm. The typical green color of PrF3 is caused by two strong absorption bands in the blue (426-496 nm) and the red (564-618 nm) wavelength regions, attributed to the transitions 3H43PJ and 3H41D2, respectively. Instead, CeF3 is completely transparent in the near UV-VIS region, with the cutoff at 306 nm. In their transparent ranges both compounds show a small dispersion, the absorption coefficient of CeF3 being even lower than that of PrF3. In contrast with them, TGG absorption continuously increases in the VIS till the beginning of the 7F65D3, 5D2 Tb3+ absorption bands below 390 nm. This fact, together with the Tb3+ absorption peak at ~488 nm (7F65D4), are the reasons which critically limit the performance of TGG FRs in the VIS region, as mentioned in the introduction.

The FR characteristics of PrF3 and CeF3 are displayed in Fig. 2
Fig. 2 Verdet constant as a function of the wavelength for PrF3 and CeF3 in comparison with TGG.
. together with the ones of TGG. In the VIS region both compounds show a similar Verdet constant dispersion to the one of TGG, the values of TGG just being slightly larger. The Verdet constant increment of CeF3 and PrF3 in the UV wavelength region is very remarkable: V CeF3 varies from 414 to 1300 rad/Tm between 400 and 300 nm, while V PrF3 reaches even larger values, increasing from 817 rad/Tm at 300 nm to 3143 rad/Tm at 220 nm.

The standard theory of FR, considering a single transition frequency, predicts a linear dependence between the Verdet constant and the inverse of the wavelength square in the form [10

10. J. C. Suits, B. E. Argyle, and M. J. Freiser, “Magneto-optical properties of materials containing divalent europium,” J. Appl. Phys. 37(3), 1391–1397 (1966). [CrossRef]

]:
V=Eλ2λ02
(1)
where λο corresponds to the 4f-4f5d transition wavelength of the RE3+ ions, and the factor E is proportional to the concentration of magnetic ions per volume (i.e. N RE), the Lande splitting factor, and the transition probability [13

13. J. R. Qiu, K. Tanaka, N. Sugimoto, and K. Hirao, “Faraday effect in Tb3+-containing borate, fluoride and fluorophosphate glasses,” J. Non-Cryst. Solids 213-214, 193–198 (1997). [CrossRef]

]. The experimental data N RE/V RE are shown in Fig. 3
Fig. 3 Inverse of the ratio (V/N) as a function of the wavelength square for PrF3 and CeF3 in comparison with TGG. The experimental points are fitted by a linear regression.
. as a function of λ 2. The fitting curves according to Eq. (1). indicate a very good agreement between the theory and the experimental results. The fitting parameters are given in Table 1

Table 1. Coefficients for the Verdet Constant Dispersion of PrF3, CeF3 and TGG According to the Eq. (1), and the Respective RE3+ ion Concentrations NRE in the Crystals.

table-icon
View This Table
.

The transition wavelengths of Tb3+ and Ce3+ in these compounds are close to each other at 239 and 258 nm, respectively, while the one of Pr3+ lies at a shorter wavelength, 184 nm. As the crystal field interaction with the RE3+ ions is smaller in ionic fluoride crystals, the transition wavelength is shorter than in the case of oxides. This relation found between the transition wavelengths, λο−Tbο−Ceο-Pr, is the expected one for the case of fluorides since both compounds are very similar from the chemical point of view and the predicted 4f5d levels (Eg) of the free RE3+ ions present the same trend (i.e. Eg-Ce<Eg-Pr~Eg-Tb) [7

7. P. Dorenbos, “The 5d level positions of the trivalent lanthanides in inorganic compounds,” J. Lumin. 91(3–4), 155–176 (2000). [CrossRef]

]. Furthermore, in agreement with this relationship, it has been observed that the transition wavelength of Tb3+-based fluoride FRs lies at ~200 nm [13

13. J. R. Qiu, K. Tanaka, N. Sugimoto, and K. Hirao, “Faraday effect in Tb3+-containing borate, fluoride and fluorophosphate glasses,” J. Non-Cryst. Solids 213-214, 193–198 (1997). [CrossRef]

,14

14. V. Vasyliev, P. Molina, E. G. Villora, and K. Shimamura, “Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals,” Opt. Mater. (to be published).

], also very close to the one of PrF3 and clearly different from the one of CeF3 and TGG.

In order to evaluate the rotation efficiency of the three RE3+ ions, it is necessary to consider their respective concentrations in the crystal (see Table 1). It is seen that although the concentration of both Pr3+ and Ce3+ is much higher than that of Tb3+, N Tb<<N Ce<N Pr, from the slopes in Fig. 3 it can be deduced that the rotation efficiency of Tb3+ in TGG clearly overcomes that of Pr3+ and of Ce3+, i.e. (E/N)TGG>>(E/N)PrF3>(E/N)CeF3 . It should be noticed that the rotation efficiency of Tb3+ in fluorides has been reported to be only slight larger than that of Pr3+ and Ce3+ in the presented trifluorides [14

14. V. Vasyliev, P. Molina, E. G. Villora, and K. Shimamura, “Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals,” Opt. Mater. (to be published).

].

The two most relevant magnitudes to consider at the time to select a FR have been shown so far, namely the absorbance α and the Verdet constant. The figure of merit (FM) of a FR, defined as the ratio V/α, is helpful to compare different FRs performances because it aims at a compromise between both parameters. The FM dispersions of PrF3, CeF3 and TGG have been calculated using the experimental results of Figs. 1 and 2 and are depicted in Fig. 4
Fig. 4 Figure of merit as a function of the wavelength for PrF3 and CeF3 in comparison with TGG.
. In the VIS wavelength region CeF3 presents a FM much higher than that of PrF3 or TGG, mainly due to the higher transparency of this crystal. It should be mentioned, that this is the only compound which can be used in the 490 nm region, since the other compounds are opaque. In the near UV region, between 300 and 400 nm, due to the quadratic increase of V Ce, the FM of CeF3 rises rapidly, reaching a value as high as 104 rad/Tm at about 300 nm. Below this wavelength, no FRs are available except PrF3. In this particular UV region, V Pr achieves exceptionally high values, which are comparable to those of YIG in the IR region. These values largely compensate for the absorption losses, yielding with it to an increase of the FM in the 224-300 nm range comparable to that of CeF3 in the near UV.

4. Conclusions

Summarizing, the MO properties of CeF3 and PrF3 single crystals have been determined in the UV-VIS wavelength region from the point of view of Faraday rotators. The characteristic features of both crystals have been compared with those of the reference crystal TGG, which is the only FR available in the market in the considered wavelength region. The presented compounds show superior properties in the whole UV-VIS region. CeF3 is noteworthy for its higher transparency and FM in the whole VIS, as well as a superior FM in the regions 300 to 400 nm and around 490 nm, where TGG is already opaque. Below the cutoff of CeF3 only PrF3 can be used. PrF3 possesses a superior FM in the UV region from 220 to 300 nm. These results indicate that CeF3 has a high potential to be implemented in OIs operating in the near UV-VIS wavelength region, while PrF3 is the unique candidate for devices that should work in the UV, below 300 nm.

Acknowledgments

This work has been partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 22560316, 2010.

References and links

1.

C. B. Rubinstein, L. G. Van Uitert, and W. H. Grodkiewicz, “Magneto-optical properties of rare earth (III) aluminum garnets,” J. Appl. Phys. 35(10), 3069 (1964). [CrossRef]

2.

R. C. Linares, “Growth of garnet laser crystals,” Solid State Commun. 2(8), 229–231 (1964). [CrossRef]

3.

M. Geho, T. Sekijima, and T. Fujii, “Growth of terbium aluminum garnet (Tb3Al5O12; TAG) single crystals by the hybrid laser floating zone machine,” J. Cryst. Growth 267(1–2), 188–193 (2004). [CrossRef]

4.

K. Shimamura, T. Kito, E. Castel, A. Latynina, P. Molina, E. G. Víllora, P. Mythili, P. Veber, J.-P. Chaminade, A. Funaki, T. Hatanaka, and K. Naoe, “Growth of {Tb3}[Sc2-xLux](Al3)O12 single crystals for visible-infrared optica isolators,” Cryst. Growth Des. 10(8), 3466–3470 (2010). [CrossRef]

5.

S. B. Berger, C. B. Rubinstein, C. R. Kurkjian, and A. W. Treptow, “Faraday rotation of rare-earth (III) phosphate glass,” Phys. Rev. 133(3A), A723–A727 (1964). [CrossRef]

6.

D. R. MacFarlane, C. R. Bradbury, P. J. Newman, and J. Javorniczky, “Faraday rotation in rare earth fluorozirconate glasses,” J. Non-Cryst. Solids 213-214, 199–204 (1997). [CrossRef]

7.

P. Dorenbos, “The 5d level positions of the trivalent lanthanides in inorganic compounds,” J. Lumin. 91(3–4), 155–176 (2000). [CrossRef]

8.

Y. Xu and M. Q. Duan, “Theory of Faraday rotation and susceptibility of rare-earth trifluorides,” Phys. Rev. B Condens. Matter 46(18), 11636–11641 (1992). [CrossRef] [PubMed]

9.

C. Leycuras, H. Legall, M. Guillot, and A. Marchand, “Magnetic-susceptibility and Verdet constant in Rare-Earth trifluorides,” J. Appl. Phys. 55(6), 2161–2163 (1984). [CrossRef]

10.

J. C. Suits, B. E. Argyle, and M. J. Freiser, “Magneto-optical properties of materials containing divalent europium,” J. Appl. Phys. 37(3), 1391–1397 (1966). [CrossRef]

11.

M. J. Weber, R. Morgret, S. Y. Leung, J. A. Griffin, D. Gabbe, and A. Linz, “Magneto-optical properties of KTb3F10 and LiTbF4 crystals,” J. Appl. Phys. 49(6), 3464–3469 (1978). [CrossRef]

12.

K. Shimamura, E. G. Villora, S. Nakakita, M. Nikl, and N. Ichinose, “Growth and scintillation characteristics of CeF3, PrF3 and NdF3 single crystals,” J. Cryst. Growth 264(1–3), 208–215 (2004). [CrossRef]

13.

J. R. Qiu, K. Tanaka, N. Sugimoto, and K. Hirao, “Faraday effect in Tb3+-containing borate, fluoride and fluorophosphate glasses,” J. Non-Cryst. Solids 213-214, 193–198 (1997). [CrossRef]

14.

V. Vasyliev, P. Molina, E. G. Villora, and K. Shimamura, “Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals,” Opt. Mater. (to be published).

OCIS Codes
(230.0230) Optical devices : Optical devices
(230.2240) Optical devices : Faraday effect
(230.3810) Optical devices : Magneto-optic systems
(260.7190) Physical optics : Ultraviolet

ToC Category:
Optical Devices

History
Original Manuscript: March 1, 2011
Manuscript Accepted: March 28, 2011
Published: June 2, 2011

Citation
P. Molina, V. Vasyliev, E. G. Víllora, and K. Shimamura, "CeF3 and PrF3 as UV-Visible Faraday rotators," Opt. Express 19, 11786-11791 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-12-11786


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References

  1. C. B. Rubinstein, L. G. Van Uitert, and W. H. Grodkiewicz, “Magneto-optical properties of rare earth (III) aluminum garnets,” J. Appl. Phys. 35(10), 3069 (1964). [CrossRef]
  2. R. C. Linares, “Growth of garnet laser crystals,” Solid State Commun. 2(8), 229–231 (1964). [CrossRef]
  3. M. Geho, T. Sekijima, and T. Fujii, “Growth of terbium aluminum garnet (Tb3Al5O12; TAG) single crystals by the hybrid laser floating zone machine,” J. Cryst. Growth 267(1–2), 188–193 (2004). [CrossRef]
  4. K. Shimamura, T. Kito, E. Castel, A. Latynina, P. Molina, E. G. Víllora, P. Mythili, P. Veber, J.-P. Chaminade, A. Funaki, T. Hatanaka, and K. Naoe, “Growth of {Tb3}[Sc2-xLux](Al3)O12 single crystals for visible-infrared optica isolators,” Cryst. Growth Des. 10(8), 3466–3470 (2010). [CrossRef]
  5. S. B. Berger, C. B. Rubinstein, C. R. Kurkjian, and A. W. Treptow, “Faraday rotation of rare-earth (III) phosphate glass,” Phys. Rev. 133(3A), A723–A727 (1964). [CrossRef]
  6. D. R. MacFarlane, C. R. Bradbury, P. J. Newman, and J. Javorniczky, “Faraday rotation in rare earth fluorozirconate glasses,” J. Non-Cryst. Solids 213-214, 199–204 (1997). [CrossRef]
  7. P. Dorenbos, “The 5d level positions of the trivalent lanthanides in inorganic compounds,” J. Lumin. 91(3–4), 155–176 (2000). [CrossRef]
  8. Y. Xu and M. Q. Duan, “Theory of Faraday rotation and susceptibility of rare-earth trifluorides,” Phys. Rev. B Condens. Matter 46(18), 11636–11641 (1992). [CrossRef] [PubMed]
  9. C. Leycuras, H. Legall, M. Guillot, and A. Marchand, “Magnetic-susceptibility and Verdet constant in Rare-Earth trifluorides,” J. Appl. Phys. 55(6), 2161–2163 (1984). [CrossRef]
  10. J. C. Suits, B. E. Argyle, and M. J. Freiser, “Magneto-optical properties of materials containing divalent europium,” J. Appl. Phys. 37(3), 1391–1397 (1966). [CrossRef]
  11. M. J. Weber, R. Morgret, S. Y. Leung, J. A. Griffin, D. Gabbe, and A. Linz, “Magneto-optical properties of KTb3F10 and LiTbF4 crystals,” J. Appl. Phys. 49(6), 3464–3469 (1978). [CrossRef]
  12. K. Shimamura, E. G. Villora, S. Nakakita, M. Nikl, and N. Ichinose, “Growth and scintillation characteristics of CeF3, PrF3 and NdF3 single crystals,” J. Cryst. Growth 264(1–3), 208–215 (2004). [CrossRef]
  13. J. R. Qiu, K. Tanaka, N. Sugimoto, and K. Hirao, “Faraday effect in Tb3+-containing borate, fluoride and fluorophosphate glasses,” J. Non-Cryst. Solids 213-214, 193–198 (1997). [CrossRef]
  14. V. Vasyliev, P. Molina, E. G. Villora, and K. Shimamura, “Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals,” Opt. Mater. (to be published).

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