Preparation and characterization of new magneto-optical crystals for optical communication Zhang Guangyu A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF ENGINEERING DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2005 Acknowledgements ii Acknowledgements I would like to express my sincere appreciation to my supervisors, Dr Xu Xuewu and Prof Chong Tow Chong for their supervision, constructive advice and help throughout this project Special thanks are given to Dr Xu Wei, Mr Tay Yong Soon, Dr Li Minghua, and Mr Yuan Shaoning I am also grateful to Mr Bill Freeman from Finisar for his help in setting up the Faraday rotation tester Lastly, I would like to acknowledge Data Storage Institute for supporting me to finish my study ii List of Publications iii List of publications Guangyu Zhang, Xuewu Xu and Tow-Chong Chong, “Faraday rotation spectra of bismuth substituted rare earth iron garnet crystals in optical communication band”, Journal of Applied Physics, Vol 95, No 10, pp 5276-5270, 2004 W Xu, X.W Xu, T.C Chong, Y.S Tay, G.Y Zhang, M.H Li, B Freeman, “Magneto-optical properties of Ce and Bi co-substituted iron garnets grown by the Bi2O3 self-flux technique”, Applied Physics A: Materials Science and Processing, (in press) Guangyu Zhang, Xuewu Xu, Wei Xu, and Tow Chong Chong, “ Magneto-optical properties of Bi Substituted rare-earth iron garnets in 1.55 µm wavelength band”, Japanese Journal of Applied Physics, Vol 43, No 10, , pp 7042-7044, 2004 Guangyu Zhang, Minghua Li, Tow Chong Chong, Xuewu Xu, and Bill Freeman, “Congruency and Morphology of Ca3(LiNbGa)5O12 Garnet Crystals Grown by the Czochralski Method”, Journal of Crystal Growth, Volume /Issue: 250/1-2, pp 90 – 93, 2002 Xuewu Xu, Tow-chong Chong, Guangyu Zhang, Minghua Li, Lay Hiok Soo, Wei Xu, and Bill Freeman., “Vertical Bridgman Growth of Calcium Lithium Niobium Gallium Garnet Crystals”, Journal of Crystal Growth, Volume /Issue: 250/1-2, pp 62-66, 2002 iii Contents iv Contents Acknowledgements ii List of publications iii Abstract viii INTRODUCTION 1.1 Optical isolator 1.2 Objectives of the thesis 1.3 Literature review EXPERIMENTS 44 2.1 Flux growth 44 2.2 Czochralski growth of CLNGG crystals .49 2.3 Lapping and polishing 52 2.4 Faraday rotation tester 52 RESULTS AND DISCUSSION: PREPARATION OF GARNET CRYSTALS .54 3.1 Bulk crystal growth of (TbBi)3Fe5O12 55 3.2 Bulk crystal growth of (HoBi)3Fe5O12 57 3.3 Bulk crystal growth of (YbBi)3Fe5O12 58 3.4 Bulk crystal growth of (HoYbBi)3Fe5O12 59 3.5 Bulk crystal growth of (TbHoYbBi)3Fe5O12 59 3.6 Bulk crystal growth of (HoYbCeBi)3Fe5O12 60 3.7 Bulk crystal growth of CLNGG 61 3.7.1 Morphology of CLNGG crystal grown by CZ .62 3.7.2 Lattice constant measurement of CLNGG crystal 65 3.7.3 Feasibility study of CLNGG crystal as substrate for LPE growth 67 RESULTS AND DISCUSSION: MO PROPERTIES OF NEW GARNET CRYSTALS 71 4.1 (TbBi)3Fe5O12 crystal 72 4.1.1 Saturation field of (TbBi)3Fe5O12 72 4.1.2 Wavelength dependence of the Faraday rotation in (TbBi)3Fe5O12 .78 iv Contents v 4.1.3 Temperature dependence of the Faraday rotation in (TbBi)3Fe5O12 79 4.1.4 Optical property of (TbBi)3Fe5O12 79 4.1.5 Summary of results on (TbBi)3Fe5O12 81 4.2 (HoBi)3Fe5O12 crystal 81 4.2.1 MO properties of (HoBi)3Fe5O12 81 4.2.2 Optical property of (HoBi)3Fe5O12 84 4.2.3 Summary of (HoBi)3Fe5O12 84 4.3 (YbBi)3Fe5O12 CRYSTAL 85 4.3.1 MO properties of (YbBi)3Fe5O12 85 4.3.2 Optical property of (YbBi)3Fe5O12 87 4.3.3 Summary of (YbBi)3Fe5O12 87 4.4 Comparison of MO properties of (REBi)3Fe5O12 crystal (Re=Tb, Ho, orYb) 88 4.5 MO properties of (Re1Re2Bi)3Fe5O12 crystal (Re1=Ho, Re2=Yb) 96 4.6 MO properties of (Re1Re2Re3Bi)3Fe5O12 crystal (Re1=Ho, Re2=Yb, Re3=Tb) 100 4.7 MO properties of (Re1Re2Re3Bi)3Fe5O12 crystal (Re1=Ho, Re2=Yb, Re3=Ce) 102 4.8 Comparison of Bi and Ce contributions to FR in garnet crystals 106 4.9 Feasibility of application in 1550 nm wideband optical isolators .108 4.10 Summary of the properties of new garnet crystals 110 RESULTS AND DISCUSSION: THEORETICAL CALCULATION OF FARADAY ROTATION 111 5.1 Helseth’s model on FR from electron dipole transitions 112 5.2 Calculation of FR of ReBiIG crystal in the 1550 nm band 115 CONCLUSION .124 REFERENCE .127 v viii Abstract Name: Zhang Guangyu Degree: Ph.D Thesis title: Preparation and characterization of new magneto-optical crystals for optical communication Abstract In optical fiber communication system, optical isolators are needed to eliminate the influence of reflected light Faraday rotators made of magneto-optical (MO) garnet crystals can be used for optical isolators The MO properties of the garnet crystals in the visible wavelength region have been heavily studied in the past decades However, the research on the MO mechanism in the 1.55 µ m band is not yet advanced enough to provide a material design for the MO crystal Furthermore, the MO properties of rare-earth substituted ion garnets are not yet thoroughly studied in details in the optical communication band This thesis was to explore the contributions from the Ho3+, Yb3+, Tb3+, Bi3+ and Ce3+ in the garnet crystals to the Faraday rotation (FR), and FR’s stability against wavelength and temperature in the communication band The theoretical calculation for FR in the 1.55 µ m band was also to be established for the first time The new materials relating to MO isolators like new substrate garnet ( Ca3(LiNbGa)5O12 ), new MO crystals ( (TbBi)3Fe5O12, (HoBi)3Fe5O12, (YbBi)3Fe5O12, (HoYb)3Fe5O12, (TbHoYbBi)3Fe5O12, (CeHoYbBi)3Fe5O12 ) were prepared and their specific FR and FR’s stability against wavelength and temperature were characterized Ca3(LINbGa)5O12 (CLNGG) crystal with inch diameter was grown by CZ method as a promising garnet substrate for high Bi-containing iron garnet film deposition The new MO crystals were grown by TSSG method Results indicated that viii ix Abstract all these garnet crystals showed a strong growth habits of {110} faces except CLNGG with {211} facets The crystal growth conditions affected the values of the saturation field, although the specific FR and the saturation field were independent of the crystallographic direction The specific FR in (TbBi)3Fe5O12 (TbBiIG) crystals was as low as 300o/cm, but its saturation field was only 0.2 kG Thick MO crystal is needed for the MO isolator if the specific FR is low, and magnet can be very small if MO element’s saturation field is small The specific FR and the saturation field in the REBiIG (RE=Yb, Ho, Tb) crystals decreased with the increase of the radius of rare-earth ions (rYb3+ < rHo3+ < rTb3+) As a Faraday rotator in the optical isolator, the MO crystal must have good stability of FR against both wavelength and temperature This target can be realized by doping more than one rare-earth element in the garnet crystals Experimental results showed that the values of FR wavelength coefficient (FWC) and FR temperature coefficient (FTC) in HoYbBiIG crystal were between the values in HoBiIG crystal and YbBiIG crystal Adding Tb3+ ions into HoYbBiIG crystal effectively lowered the saturation field CeHoYbBiIG crystal had lower saturation field (0.76 kG) and higher specific Faraday rotation (625 o/cm) than that in HoYbBiIG crystal (0.98 kG and 545 o/cm) However, the FWC and FTC in CeHoYbBiIG were a little bit worse than HoYbBiIG and the maximum doping amount of Ce3+ in CeHoYbBiIG crystal were moderate The theoretical calculation of FR in bismuth- substituted rare- earth iron garnet in the optical communication band was established for the first time The experimental results agreed with the theoretical calculation of FR in (HoBi)3Fe5O12, (YbBi)3Fe5O12, (YYbBi)3Fe5O12, and (TbBi)3Fe5O12 The theoretical models can be ix x Abstract used to calculate the FR at different wavelength for the bismuth- substituted iron garnets with different Bi3+ content For REBiIG crystals (RE=Yb, Ho, or Tb), they had almost the same cut-off edge at the short wavelength end, but the long wavelength end of the transparent region was shifted to longer wavelength side in the ascending atom radii sequence of Yb, Ho and Tb The features of the transmittance spectrum of REBiIG crystals (RE=Yb, Ho, or Tb) were kept in the two- or three- rare-earth ions co-doped iron garnets, like (HoYbBi)3Fe5O12 crystal, and (TbHoYbBi)3Fe5O12 crystal These basic MO characteristic results for the rare- earth iron garnets may provide the basis for the practical MO optical isolator design in the optical communication industry x xi List of Tables List of Tables Table Magneto-optical coefficients induced by the electric dipole transition for various garnet compositions 25 Table The Faraday rotation for rare –earth iron garnets, for λ =1.06 µ m and T=300K.[3] .38 Table Some properties of the substrates used for LPE film growth 68 Table Flux systems and their corrosion on CLNGG .69 Table Chemical composition of the grown crystals 89 Table Changes in anisotropy constants K1 and unit cell parameters in RE3Fe5O12 crystals,from those of Y3Fe5O12 (YIG) by replacing Y3+ with RE3+ For YIG, K1 (300 K)=-5.0 × 103 erg cm-3, K1(78 K)=-22.4 × 103 erg cm-3, K1(4 K)=24.8 × 103 erg cm-3 .89 Table Changes in magnetization π Ms , Hk, and (K1/a)1/2 in RE3Fe5O12, and the saturation field in their corresponding Bi-doping garnets 89 Table Changes in FWC of (REBi)3Fe5O12 (RE=Ho, Yb, or Tb) crystals 93 Table Changes in FTC of (REBi)3Fe5O12 (RE=Ho, Yb, or (Ho+Yb)) crystals 95 Table 10 Changes in FWC of (REBi)3Fe5O12 (RE=Ho, Yb, or (Ho+Yb)) crystals .97 Table 11 Changes in FTC of (REBi)3Fe5O12 (RE=Ho, Yb, or (Ho+Yb)) crystals 98 Table 12 Changes in FWC of (REBi)3Fe5O12 (RE=Ho, Yb, or (Ho+Yb)) crystals .101 Table 13 Changes in FWC of (REBi)3Fe5O12 (RE=(Ce+Ho+ Yb), or (Ho+Yb)) crystals .103 Table 14 Changes in FTC of (REBi)3Fe5O12 (RE==(Ce+Ho+ Yb), or (Ho+Yb)) crystals .105 Table 15 Measured compositions of (HoYb)3-x-yCexBiyFe5O12 and (HoYb)3-yBiyFe5O12 crystals .106 xi xii List of Tables Table 16 Optical and MO properties of (HoYb)2.171Ce0.164Bi0.665Fe5O12 and (HoYb)2.501Bi0.499Fe5O12 crystals at 1550 nm 108 Table 17 Chemical composition of the grown crystals 117 Table 18 Parameters used in Eq (3) Nfi/x is calculated in [cm-3], whereas ∆ i , ϖ i , and Γi are sample-independent and given in [eV] 117 xii Theoretical calculation of Faraday rotation 200 experimental data from reference [12] calculated data -200 -400 o Specific FR ( /cm) -600 -800 -1000 -1200 -1400 -1600 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Bi content, x Figure 86 Specific FR of (TbBi)3Fe5O12 crystal as a function of bismuth content x at 1.55 µ m The solid line represents the calculation results The scattered points represent the experimental results in reference 172 In order to further confirm Helseth’s model in the 1.55µm band, the FR spectra in Y2.289Yb0.246Bi0.465Fe5O12 crystal was also calculated Two points, (θF=2521o/cm, at 1.0052 µ m) and (θF=583.59o/cm, at 1.4081 µ m), were taken from reference 171 to calculate the (Nfi/x) The values of FR at other wavelengths in Y2.289Yb0.246Bi0.465Fe5O12 crystal were calculated using Eq (5.2.3) The calculated FR at different wavelengths also agreed with the experimental data in reference [171] as shown in Figure 85 According to Eq (5.2.3), there is a linear relationship between FR and x In order to prove the linear relationship, garnet crystal with different Bi content (x) are needed In reference 172 , Tb3-xBixFe5O12 crystals (x=0, 0.15, 0.29, 0.35, 0.51, 0.84, 1.3) were reported The sample with x=0.84 was selected and two points, (θF= -1239.1o/cm, at 122 Theoretical calculation of Faraday rotation 1.4083µm) and (θF= -1043.5o/cm, at 1.5132µm), were obtained to calculate the values of (Nfi/x) The linear relationship between θF and x in Tb3-xBixFe5O12 crystals was also calculated using Eq (5.2.3) As shown in Figure 86, the calculated results agree with the experimental data These results suggest that Eq (5.2.3) can be used to predict the FR in a crystal at different wavelengths or with different Bi content only if two values of FR at two wavelengths are given In summary, in order to interpret the FR spectra of (HoBi)3Fe5O12, (YbBi)3Fe5O12, (YYbBi)3Fe5O12, and (TbBi)3Fe5O12 crystals, Helseth’s model was used that was based on molecular-orbital levels of Fe3+ in tetrahedral and octahedral sites It was assumed that the Faraday rotation (FR) in the wavelength range of 1.3-1.6 µ m was still dominated by the electric dipole transitions To some extent, the contributions from rare-earth and bismuth ions were included in spin orbit splittings in Fe3+ The experimnetal results agreed with the theoretical calculation results in the abovementioned wavelength range The FR in (HoBi)3Fe5O12, (YbBi)3Fe5O12, and (YYbBi)3Fe5O12 crystals decreased with the wavelength The linear relationship between the specific FR(θF) and Bi content (x) was calculated and confirmed by the experimental data in Tb3-xBixFe5O12 crystals 123 Conclusion CONCLUSION With the development of optical communication technique, much interest is attracted onto the new magneto-optical (MO) crystals as Faraday rotators in isolators working in the 1550 nm wavelength band In this study, the specific FR, saturation field, the stability of FR against wavelength and temperature, optical transmittance spectrum were investigated in REBiIG crystals (RE=Yb, Ho, Tb, (Ho+Yb), (Ce+Ho+Yb) ) REBiIG crystals were grown by TSSG method The new garnet substrate CLNGG crystal for MO film deposition by LPE method with inch diameter was grown by CZ method The morphology of these garnet crystals were studied CLNGG crystal followed the morphology BFDH theory and the most important growth form of {211} appeared as growth facets on the bottom of the CLNGG boule In contrast, the large flat facets of the grown REBiIG crystals were the second important growth form of {220} according to the XRD analysis The growth parameters had influence on the MO properties of the grown crystals The specific FR and the saturation field in the REBiIG crystals decreased with the increases of the radius of rare-earth ions The bigger rare-earth ions in the garnet structure cause fewer Bi3+ incorporated and reduce the domain wall’s energy and thickness in the crystal Bismuth addition enhances strongly the FR in REBiIG crystal The difference between the experimental saturation field and the anisotropy field depends on the domain wall’s energy and thickness Therefore, TbBiIG crystal had the lowest specific FR (300 o/cm) and saturation field (0.2 kG) due to largest radius of Tb3+ among TbBiIG, HoBiIG, and YbBiIG crystals 124 Conclusion The specific FR, its stability against both wavelength and temperature, and the saturation field can be improved by doping more than one rare-earth element The specific FR in HoYbBiIG crystal was higher than that in HoBiIG crystal but lower than that in YbBiIG crystal The FR wavelength coefficient (FWC) of HoYbBiIG crystals was better than that of YbBiIG crystal but worse than that of HoBiIG crystals whereas the FR temperature coefficient of HoYbBiIG crystal was better than HoBiIG crystal but worse than that of YbBiIG crystal HoYbBiIG crystal is a trade-off between HoBiIG crystal and YbBiIG crystal in MO properties The possible explanation is that HoYbBiIG crystal is the solid solution of HoBiIG crystal and YbBiIG crystal, and that the MO properties of HoYbBiIG crystals is the combination of MO activities in both HoBiIG and YbBiIG crystals This explanation is also supported by the experimental results from TbHoYbBiIG crystal TbBiIG crystal had as low saturation field as 0.2kG HoYbBiIG crystal had saturation field of 0.98 kG After Tb was added, the TbHoYbBiIG crystal had lower saturation field than HoYbBiIG but higher than TbBiIG crystal The assumption that solid solution demonstrates the combination of MO properties of two individual garnet crystals can also explain: FWC (HoYbBiIG) < FWC (TbHoYbBiIG) < FWC (TbBiIG) Besides Bi3+ ions that enhance the MO properties in garnet crystals, Ce3+ ions in CeHoYbBiIG crystal brought in lower saturation field (0.76 kG) and higher specific FR (625o/cm) than HoYbBiIG (0.98 kG and 545o/cm) However, the stability of FR against both wavelength and temperature in CeHoYbBiIG was a little bit worse than HoYbBiIG The mechanism behind this is unknown yet For Bi-substituted iron garnet crystal, Helseth’s model in the visible wavelength band was applied to interpret the FR spectra in the 1.55 µ m band It was assumed that FR in the wavelength range of 1.3-1.6 µ m was still dominated by the electric dipole 125 Conclusion transitions To some extent, the contributions from rare-earth and bismuth ions were included in spin orbit splitting in Fe3+ The variation of FR with wavelength for (HoBi)3Fe5O12, (YbBi)3Fe5O12 and (YYbBi)3Fe5O12 crystals agreed with the theoretical calculation results The linear relationship between the specific FR and Bi content was also calculated and 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Comparison of Bi and Ce contributions to FR in garnet crystals 106 4.9 Feasibility of application in 1550 nm wideband optical isolators .108 4.10 Summary of the properties of new garnet crystals. .. magneto- optical crystals for optical communication Abstract In optical fiber communication system, optical isolators are needed to eliminate the influence of reflected light Faraday rotators made of magneto- optical. .. fiber-optic connector and other functional optical devices Therefore, there is a demand for optical isolators working at the 1550nm wavelength band in the optical communication industry 1.1 Optical isolator