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Abstract
The dispersion of the Verdet constant of LiY1.0-xErxF4 crystals was evaluated from 190 nm to 500 nm for different doping concentrations of Er ions. A 15% doping concentration yielded a high Verdet constant of 54.5 rad/(T·m) at 193 nm. This value can be explained by the contribution of the diamagnetic term associated with LiYF4 and the paramagnetic term of the Er ions. Although the LiYF4 crystal yielded a lower value of −36.6 rad/(T·m) at 193 nm from Er-doped LiYF4, it can be used in the vacuum–ultraviolet region because of its high transmittance at wavelengths longer than 120 nm.
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1. Introduction
The Faraday effect, a magneto-optic effect, is widely used in magnetic and electric current sensors and Faraday rotators (FRs). [1–3] The most important application for optical systems such as FRs is the optical isolator (OI), which is a polarization-dependent isolator for protecting light sources and preventing feedback to laser oscillators. The Faraday effect results in a rotation of the polarization plane of light traveling through a material when an external magnetic field is applied, and the rotation angle ${\theta}$ is expressed as
where V is the Verdet constant and H is the strength of the magnetic field over the medium length L. The Verdet constant is material-specific and wavelength and temperature-dependent. A high V facilitates a reduction in the thickness of the media and strength of the external magnetic field. Ideal candidate materials for FRs should have both a high transmittance and a high Verdet constant V at the wavelength of the light source. For example, terbium gallium garnet (TGG) crystals are commonly used as FR materials at a wavelength of approximately 1000 nm (VTGG = 40 rad/(T·m) at λ = 1064 nm) [4,5] and yttrium iron garnet (YIG) crystals are used at approximately 1500 nm (VYIG = 304 rad/(T·m) at λ = 1550 nm). [6,7] These materials exhibit high Verdet constants in their respective wavelength regions, but cannot be used at wavelengths shorter than 390 nm due to their large absorptions. In the deep-ultraviolet (λ = 193–300 nm, DUV) and ultraviolet (λ = 300–400 nm, UV) regions, some materials have been reported as candidates for DUV-UV FRs. [8–12] For example, the Verdet constant is 70.1 rad/(T·m) at λ = 193 nm in synthetic quartz glass, 180 rad/(T·m) at λ = 193 nm in ADA crystal, and 74.5 rad/(T·m) at λ = 193 nm in DKDP crystal. In particular, LiREF4 (RE = Tb, Dy, Ho, Er, and Yb) has been identified as a promising material because of its high Verdet constant, owing to the contribution of rare earth element doping and short-wavelength absorption edge. For example, the Verdet constant of LiErF4 with a short-wavelength absorption edge of 163 nm is 516 rad/(T·m) at λ = 193 nm and 279 rad/(T·m) at λ = 248 nm, which is suitable for constructing DUV FRs.In this paper, we report the Er doping concentration dependence of the dispersion of the Verdet constant in LiY1.0-xErxF4. Non-doped LiYF4 (YLF) is a well-known laser host material with a short-wavelength absorption edge of 120 nm. [13–15] This suggests that FRs can be constructed not only in the DUV region, but also in the vacuum ultraviolet (VUV) region below λ = 200 nm. Partially doped crystals (Er:YLF), which are known as laser active media at 551 nm and 2.8 µm [16–19] were also evaluated, because an enhancement in their magneto-optic properties was expected from the contribution of Er ions. In addition, the contribution of Er ions to the Verdet constant in the DUV region was evaluated by comparison with previous results for LiErF4.
2. Experimental methods
The dispersion of the Verdet constant was measured using the polarization-stepping method. [20] The experimental setup is illustrated in Fig. 1. An optical discharge plasma light source (Energetiq Technology, Inc. EQ-99X LDLS) was used as the seed white light source (λ = 170–2100 nm). Two types of polarizers and spectrometers were used for accurate measurement in a wide wavelength region (λ = 190–500 nm). In the short-wavelength region (λ = 193–300 nm), a Wollaston-GlanTaylor polarizer (Kogakugiken Corp. WoG-193-E) and DUV spectrometer (Ocean insight Maya200) were used, whereas a GlanTaylor polarizer (Thorlabs, Inc. GL10) and a second spectrometer (Ocean Insight USB2000) were used in the long-wavelength region (λ = 300–500 nm). The sample was positioned, and an external magnetic field (B = 1.18 T at L = 6 mm, B = 1.16 T at L = 10 mm) was applied between the two polarizers. Two types of samples were measured: LiYF4 (non-doped YLF, L = 10 mm) and LiY0.85Er1.5F4 (15% doped Er:YLF, L = 6 mm). One of the polarizers is rotated using a stepping motor with a sampling pitch of 1°. The intensity of the transmitted light without an applied external magnetic field can be expressed as follows:
where I0 is the maximum intensity, θ is the rotation angle of the polarizers, θ0 is the angular difference between the two polarizers, and Imin is the minimum intensity. By applying a magnetic field to the sample, polarization rotation ${\theta _F}$ due to the Faraday effect is induced, and thus, the above equation becomes${\theta _F}$ is determined by evaluating the phase difference between the two measurement data. From these data, the Verdet constant is derived using Eq. (1) for all wavelengths.
3. Result and discussion
Figure 2 shows the wavelength dependence of the Verdet constant and the transmittance in a non-doped YLF crystal. Figure 3 shows the results in a 15%-doped Er:YLF crystal. The gray dotted lines indicate the wavelength of a typical DUV laser source. The Verdet constant exhibited opposite signs because Er ions are paramagnetic, whereas YLF crystals are diamagnetic. The Verdet constant dispersion of a diamagnetic material can be determined using the following equation [21]:
Similarly, the properties of paramagnetic materials are given by the following equation [21]:
Fig. 4. Doping concentration dependence of Verdet constant of LiY1.0-xErxF4 for a typical DUV-excimer laser.
Table 1. Comparison of Verdet constants for several DUV-VUV excimer lasers
4. Conclusion
The Verdet constant of LiY1.0-xErxF4 crystals with different doping concentration was measured over the range of 190–500 nm. To the best of our knowledge, this paper presents the first measurement of the Verdet constant of non-doped YLF. The measurements of partially doped Er:YLF crystals and previously reported data on LiErF4 crystals revealed a relationship between the Er dopant concentration and the Verdet constant in the DUV region for the first time. In this region, crystals with a high Er doping concentration, which exhibited a high Verdet constant, were found to be suitable. However, non-doped YLF crystals can be used in the VUV region, which is shorter than the absorption edge (169 nm) of LiErF4 crystals. For example, at 157 nm—the wavelength of the F2 excimer laser—the Verdet constant is predicted to be −63.7 rad/(T·m). LiY1.0-xErxF4 is a good candidate material for an FR in the DUV-VUV region.
Funding
Amada Foundation (AF-2019221-B3); Japan Society for the Promotion of Science (18H01204).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. A. D. Buckingham and P. J. Stephens, “Magnetic optical activity,” Annu. Rev. Phys. Chem. 17(1), 399–432 (1966). [CrossRef]
2. M. N. Deeter, A. H. Rose, and G. W. Day, “Fast, sensitive magnetic-field sensors based on the Faraday effect in YIG,” J. Lightwave Technol. 8(12), 1838–1842 (1990). [CrossRef]
3. K. Bohnert, P. Gabus, J. Nehring, and H. Brändle, “Temperature and vibration insensitive fiber-optic current sensor,” J. Lightwave Technol. 20(2), 267–276 (2002). [CrossRef]
4. 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), 3069–3070 (1964). [CrossRef]
5. Northrop Grumman Corporation, Fairview Park Drive, Falls Church, VA; 2980 (United States).
6. R. W. Cooper, W. A. Crossley, J. L. Page, and R. F. Pearson, “Faraday rotation in YIG and TbIG,” J. Appl. Phys. 39(2), 565–567 (1968). [CrossRef]
7. A. Ikesue and Y. L. Aung, “Development of optical grade polycrystalline YIG ceramics for Faraday rotator,” J. Am. Ceram. Soc. 101(11), 5120–5126 (2018). [CrossRef]
8. P. Molina, V. Vasyliev, E. G. Víllora, and K. Shimamura, “CeF3 and PrF3 as UV-visible Faraday rotators,” Opt. Express 19(12), 11786–11791 (2011). [CrossRef]
9. V. Vasyliev, E. G. Villora, M. Nakamura, Y. Sugahara, and K. Shimamura, “UV-visible Faraday rotators based on rare-earth fluoride single crystals: LiREF4 (RE = Tb, Dy, Ho, Er and Yb), PrF3 and CeF3,” Opt. Express 20(13), 14460–14470 (2012). [CrossRef]
10. J. L. Dexter, J. Landry, D. G. Cooper, and J. Reintjes, “Ultraviolet optical isolators utilizing KDP-isomorphs,” Opt. Commun. 80(2), 115–118 (1990). [CrossRef]
11. Y. Tamaru, H. Chen, A. Fuchimukai, H. Uehara, T. Miura, and R. Yasuhara, “Wavelength dependence of the Verdet constant in synthetic quartz glass for deep-ultraviolet light source,” Opt. Mater. Express 11(3), 814–882 (2021). [CrossRef]
12. H. Nishioka, H. Hisano, T. Kaminaga, K. Ueda, and H. Takuma, “Development of the UV Faraday rotator,” rle 12(11), 660–662 (1984). [CrossRef]
13. E. Garcia and R. R. Ryan, “Structure of the laser host material LiYF4,” Acta Crystallogr. C Cryst. Struct. Commun. 49(12), 2053–2054 (1993). [CrossRef]
14. Qishu Fang, Hongbing Chen, Fang Xu, Sujing Wang, Zhe Liang, and Chengyong Jiang, “Bridgman growth of LiYF4 single crystal in nonvacuum atmosphere,” Chin. Opt. Lett. 8(11), 1071–1074 (2010). [CrossRef]
15. T. M. Pollak, R. C. Folweiler, E. P. Chicklis, J. W. Baer, A. Linz, and D. Gabbe, “Properties and fabrication of crystalline fluoride materials for high power laser applications,” ASTM Special Technical Publications, 1980, pp. 127–135.
16. R. Brede, T. Danger, E. Heumann, G. Huber, and B. Chai, “Room-temperature green laser emission of Er:LiYF4,” Appl. Phys. Lett. 63(6), 729–730 (1993). [CrossRef]
17. F. Heine, E. Heumann, T. Danger, T. Schweizer, G. Huber, and B. Chai, “Green upconversion continuous wave Er3+:LiYF4 laser at room temperature,” Appl. Phys. Lett. 65(4), 383–384 (1994). [CrossRef]
18. G. J. Kintz, R. Allen, and L. Esterowitz, “Cw and pulsed 2.8-µm laser emission from diode-pumped Er3+:LiYF4 at room temperature,” Appl. Phys. Lett. 50(22), 1553–1555 (1987). [CrossRef]
19. F. Auzel, S. Hubert, and D. Meichenin, “Multifrequency room-temperature continuous diode and Ar* laser-pumped Er3+ laser emission between 2.66 and 2.85 µm,” Appl. Phys. Lett. 54(8), 681–683 (1989). [CrossRef]
20. J. L. Flores and J. A. Ferrari, “Verdet constant dispersion measurement using polarization-stepping techniques,” Appl. Opt. 47(24), 4396–4399 (2008). [CrossRef]
21. M. J. Weber, “Faraday rotator materials for laser systems,” Proc. SPIE 0681, 75–90 (1987). [CrossRef]
22. N. P. Barnes and D. J. Gettemy, “Temperature variation of the refractive indices of yttrium lithium fluoride,” J. Opt. Soc. Am. 70(10), 1244–1247 (1980). [CrossRef]
