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Abstract
Here we report the fabrication of a magneto-optic waveguide based on TGG crystal via 15 MeV C3+ ion irradiation. The ion irradiation process leads to the optical anisotropy in the as-irradiated TGG waveguide, which hinders the magneto-optical rotation in the waveguide. To remove the irradiation-induced optical anisotropy, we annealed the as-irradiated TGG waveguide under different conditions. After annealing at 400 °C for one hour, the magneto-optical rotation of 14° per centimeter is observed in the waveguide at the wavelength of 632.8 nm, under the magnetic field of 0.24 T, which is comparable to that observed in the TGG crystal under the same magnetic field. This work paves the way for applications of TGG waveguides as integrated optical rotators and isolators.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical waveguide isolators have attracted increasing attention for decades [1–8], due to the insistent demands arising from fiber-optical networks and optical chips. Compared with the bulk optical isolators, waveguide isolators have the advantages of small size and easy integration [9]. Until now, various kinds of waveguide isolators have been reported, such as rib GGG waveguide isolator [10], As2S3 and Ge33As12Se55 rib waveguides [11], and the magneto-optical isolator with Si waveguides [12]. Yttrium iron garnet (YIG) is generally applicable in the wavelength range of 1200-5000 nm, which cannot meet the research requirements of visible light and near-infrared magneto-optical devices [13]. There is continued interest in exploring novel ways to fabricate waveguide isolators with low optical loss, high specific rotation, and compact structure.
Terbium gallium garnet (Tb3Ga5O12, TGG) is an excellent magneto-optical material with high Verdet constant and low absorption coefficient in the visible and infrared spectral ranges [14–16], which make it prominent candidate in the fabrication of optical rotators and isolators. The optical isolator based on TGG magneto-optical waveguide has potential application value in the field of integrated optics in terms of miniaturization, high sensitivity and integration characteristics. One of the reasons is that TGG waveguide fabricated by traditional deposition or epitaxial methods usually have distinct effective refractive index (EI) difference between TE and TM polarization, induced by large refractive index contrast between waveguide core and substrate materials, which will suppress the Faraday rotation in waveguides, as will be discussed later in this article.
Several techniques have been developed to fabricate waveguides within various crystals, including metal ion diffusion [17], proton exchange [18], thin film epitaxy/deposition (pulsed laser deposition (PLD) [19], liquid phase epitaxy (LPE) [20], ultrafast laser writing [21–23], and ion irradiation [24–27]. Among them, ion irradiation has been demonstrated to have wide applications in a variety of crystals [28–30]. During the ion irradiation process, the energetic ions penetrate into the crystal and transfer the energy from the incident beams to the target material. The refractive index of the crystal can be modified via the energy deposition, which constructs the waveguide structure [31], while the crystal properties could be preserved in the guiding regions [32–34]. It is of particular interest to fabricate waveguides in magneto-optical materials using ion beam technique, as ion irradiated waveguide offer better EI matching for different polarizations, by which Faraday rotation efficiency near to the level of bulk crystal could be expected. Also, as one of the planar process in semiconductor industry, ion irradiation shows intriguing potential in volume production of magneto-optical waveguide devices.
In this work, we fabricated magneto-optic waveguide based on TGG crystal by 15 MeV C3+ ion irradiation. In the as-irradiated TGG waveguide, optical anisotropy existed, which limited the magneto-optical rotation performance in the waveguide. To get better consistancy of EI between TE and TM polarizations, we annealed the as-irradiated TGG waveguide under different conditions. Under the magnetic field of 0.24 T, an optical rotation of 14° per centimeter was realized in the waveguide at the wavelength of 632.8 nm, which is comparable to the value observed in bulk TGG crystal under same magnetic field.
2. Experiments
The TGG crystal used in this work was cut into a wafer with dimensions of 10 (x) × 5 (y) × 1 (z) mm3. Facets of the TGG wafer were optically polished and cleaned. One of the largest facet (10 mm × 5 mm) was irradiated by C3+ ions at the energy of 15 MeV with the fluence of 2×1014 ion/cm2, with a tilting angle of 7° to avoid tunnel effect. After the ion irradiation, the as-irradiated crystal (labeled as S0) was annealed to optimize properties of the waveguide. During the annealing process, the sample was heated in the furnace with a heating rate of 4.44 °C/min to a holding temperature (T) for a certain time (t). Four annealing steps were carried out in sequence, and labeled as S1 (T = 200 °C, t = 60 min), S2 (T = 300 °C, t = 60 min), S3 (T = 400 °C, t = 60 min) and S4 (T = 400 °C, t = 60 min), respectively.
Magneto-optic properties of the TGG waveguide were measured following the experimental setup in Fig. 1. A laser at the wavelength of 632.8 nm was used as the probe light. The power of the input light was modulated by a half-wave plate and a Glan-Taylor prism. Through a microscope objective (25 ×, N.A. = 0.4), the probe light was coupled into the TGG waveguide, traveling along the y direction (5 mm length), and collected by another microscope objective (25 ×, N.A. = 0.4). Via a Glan-Taylor prism, the magneto-optical rotation of the probe light was detected. The entire TGG crystal was enveloped by an external magnetic field same with beam propagation direction, which was produced by a permanent magnet with a magnetic induction of about 0.24 T.
Fig. 1. Schematic diagram of the experimental setup for magneto-optic measurement in the TGG optical waveguide.
3. Discussion
3.1. As-irradiated TGG waveguide
Figure 2(a) shows the cross-section of the as-irradiated TGG waveguide (S0) with layered structures, and the layer near the air is the irradiated region with a thickness of 8.1 μm. During the carbon ion irradiation, the incident carbon ions penetrate into the TGG crystal, and lose their energy via elastic (nuclear energy loss) and inelastic (electron energy loss) collisions with atoms in the crystal, resulting in damage of the crystal structure. We calculated the distribution of the energy loss in the TGG crystal by SRIM-2013 (Stopping and Range of Ions in Matter). As shown in Fig. 2(b), the electron energy loss is concentrated on the path of the incident ion, and the nuclear energy damage is located at the end of the ion motion. According to a previous report [31], the electron energy loss is the main reason for the change in refractive index of garnet crystal.
Fig. 2. (a) Optical microscope image of the cross-section of as-irradiated TGG waveguide (S0). (b) Electronic (dashed line) and nuclear (solid line) stopping powers as a function of depth of the TGG waveguide. Measured and simulated modal profiles of fundamental modes in the TGG waveguide with TE (c) and TM (e) polarizations at 632.8 nm. Reconstructed refractive index profiles of the waveguide (dashed line is S0 and solid line is S3) with TE (d) and TM (f) polarizations.
An M-line measurement is carried out by utilizing prism coupling technique. It is observed that the surface refractive index is unchanged, and no guiding mode could be excited through the prism. Combining with the near-field detection, it strongly suggests that the waveguide is formed by a buried waveguide core with enhanced refractive index. The detailed refractive index distribution in the irradiated region is reconstructed via multiple iteration fitting. First, the fundamental mode in the waveguide is experimentally measured. Second, we assume a refractive index distribution according to the calculated results in Fig. 2(b) [31]. Third, based on the assumed refractive index distribution, the fundamental mode is simulated by the finite-difference beam propagation method (FD-BPM). Then, we compare the simulated mode profile with the measured one. If results in good agreement with the simulated mode profile, the assumed refractive index distribution is reasonable. If not, the above steps are repeated. Although the waveguide is actually multi-mode, only the fundamental modes were studied here as higher coupling efficiency with input signal and for better practical applications.
Figures 2(c) and 2(e) show the simulated and the measured fundamental mode profile in the as-irradiated TGG waveguide (S0) with the detection light at the wavelength of 632.8 nm. The reconstructed refractive index distributions are shown in Figs. 2(d) and 2(f), which is a buried planar waveguide (with positive index change) with a depth of 8.1 μm. Please note, the maximum refractive index change is 0.00450 / 0.00495 with the TE / TM -polarization. We will show later that this difference cause distinct contrast between EI of the two polarization, leading to a decrease of Faraday rotation efficiency in waveguide.
The total insertion loss of the as-irradiated waveguide is measured to be more than 10 dB, which is not conducive for the stable propagation of light inside the waveguide. Besides, the propagation loss of the waveguide is polarization dependent with the TE (20 dB/cm) and TM (22 dB/cm) –polarization, which may further limit the performance of waveguide rotator. In the following sections, we attempt to overcome these two disadvantages via the annealing treatment.
3.2. Annealing
Figure 3 shows the evolution of the maximum refractive index change (R) and the propagation loss (α) of the TGG waveguide under different annealing conditions. Before annealing, both R and α show anisotropy, which are 0.00495 and 22 dB/cm for the TM -polarization and 0.00450, 20 dB/cm for TE -polarization. After annealing stages of S1-S4, R and α gradually reduced to 0.0032 and 2.6 dB/cm with unpolarized dependence. It indicates that optical anisotropy between TE -polarization and TM -polarization was alleviated via the annealing treatment.
Fig. 3. (a) Variation of the maximum refractive index contrast of the TGG planar waveguide under different annealing conditions. (b) Polarization images of the propagation loss of waveguide at 632.8 nm.
To further confirm the alleviation of optical anisotropy, we measured Raman spectra of the TGG waveguide before and after the annealing. Figure 4(a) shows the Raman spectrum at a depth of 4 μm in the as-irradiated TGG waveguide (S0). At this point, the intensity of Raman signals with TE -polarization and TM -polarization are different, which demonstrate the anisotropy of the crystal structure. More evidence is shown in Figs. 4(b), 4(c) and 4(d), which show the intensity distribution of the Raman signal along with the depth. As one can see, the intensity of the Raman signals with TM -polarization is higher than the one with TE -polarization. It indicates that there is structure anisotropy in the as-irradiated TGG waveguide (S0). Under the same conditions, we measured the Raman spectra of the sample after annealing (S3). Figure 4(e) presents the Raman spectrum at a depth of 4 μm in the TGG waveguide, in which the intensity distributions are identical with TE -polarization and TM -polarization. Similar results are also observed in Figs. 4(f), 4(g) and 4(h). The intensity of the Raman signal is almost identical. It demonstrates that the annealing treatment eliminates the structural anisotropy of the TGG crystal inside the waveguide.
Fig. 4. Raman spectra at a depth of 4 μm in S0 (a) and S3 (e) excited by 532 nm laser with TE and TM -polarization. The intensity distribution of the Raman signal of S0 (d) and S3 (h) at 356.3 cm−1 along with the depth. Raman mapping of S0 excited by the light with TE -polarization (b) and TM -polarization (c). Raman mapping of S3 excited by the light with TE -polarization (f) and TM -polarization (g).
3.3. Magneto-optical rotation in the annealed TGG waveguide
The magneto-optical property of the TGG waveguide was measured following the experimental setup in Fig. 1. The TGG waveguide was placed in a permanent magnet port with a magnetic field strength of about 0.24 T. Figure 5 shows the magneto-optical rotation angle (θF) in the TGG waveguide, after S1-S4 annealing processes. Under S1, S2 and S3, values of θF are only 2°, which is much lower than the one (9°) observed in the TGG crystal under the same magnetic field. While, under the annealing condition of S3 and S4, θF reaches 7°, which is close to the optical rotation angle in the crystal (9°). The increase of rotating angle could be attributed to the change of effective refractive index contrast between TE and TM mode, as described below.
Fig. 5. The measured internal optical rotation angles of the TGG planar optical waveguide under different annealing conditions.
When there is no magnetic field, the permittivity of magnetic-optic materials became anisotropic, which causes the coupling of TE and TM mode in waveguide. In such case, the power of TE (TM) mode converted into TM (TE) mode after propagating for certain distance, which is similar to faraday rotation in bulk materials. The different propagation constant of TE, TM mode leads to a decrease of conversion efficiency, results in a decrease of practical rotation angle for a certain propagation distance compared to the bulk material. S. Tien et al. derived the analytic formulas of such energy conversion process for planar waveguide by solving the mode coupling equations [35]. The complex amplitude of electric field of TE and TM mode of same mode order could be expressed by:
By solving the beam power of both polarized modes, the practical rotation angle of the waveguide could be solved by the following expression, in which ${\theta _p}$ corresponds to the practical rotation angle:
In this work, the propagation distance $x$ is 5 mm, and Faraday rotation constant $\theta$ equals to 18°/cm under magnetic field of 0.24 T. Consider the relation $\Delta \beta = \Delta {n_{eff}} \cdot {k_0}$, where $\Delta {n_{eff}}$ is the difference between EI of TE and TM modes, the relationship between the practical rotation angle ${\theta _p}$ and $\Delta {n_{eff}}$ could be obtained by combining (1),(2) and (3). Here we define R to be the ratio of rotation angle in waveguide dividing the rotation angle of bulk crystal. In such case, the relationship between R and $\Delta {n_{eff}}$ is plotted in Fig. 6.Fig. 6. The relationship between rotation angle ratio R of the TGG waveguide and the EI contrast between TE and TM modes. Here the propagation distance is 5 mm with Faraday rotation constant θ of 18°/cm.
It is clear that when $\Delta \beta = 0$, the rotation angle in waveguide is equal to that of bulk crystal. As $\Delta \beta$ increase, the rotation angle in waveguide decreases and oscillating of the curve could be observed. No matter how, the rotation angle in waveguide was limited in a low level when EI mismatch between TE and TM mode exceed a certain value.
It should be noticed that the exact value of EI difference for TE and TM mode is hard to be quantified because of small refractive index contrast in ion irradiated waveguides. According to reconstructed refractive index profile shown in Fig. 2(d) and 2(f), $\Delta {n_{eff}}$ is evaluated to be ∼2.0×10−4 for as-irradiated waveguide. It is clear that this value falls into the low rotation angle range in Fig. 6, and the corresponding R value is near to the measured one (∼0.22). By the same relationship shown in Fig. 6 and assuming the same Verdet constant of the waveguide compared to un-implanted material, $\Delta {n_{eff}}$ after S3 annealing is estimated to be 2.4×10−5 from the experimental results, which is reasonable as the detected waveguide core index change of different polarizations decreased after annealing. Here we come to the conclusion that proper annealing process after ion irradiation plays a crucial role to lessen the EI mismatch between TE and TM mode, leading to the recovery of Faraday rotation performance inside the TGG waveguide.
4. Conclusions
In summary, we fabricated a planar waveguide based on the TGG crystal via the carbon ion irradiation. The effective refractive index mismatch between TE and TM mode in the as-irradiated waveguide hinders the magneto-optical rotation in the waveguide. According to Raman spectra of the irradiated TGG crystal, we suspect that the optical anisotropy is resulted from the structure anisotropy of the TGG crystal induced by the ion irradiation. To alleviate the optical anisotropy induced by the irradiation and to reduce the propagation loss, the TGG waveguide was annealed under different conditions. After annealing at 400 °C for 60 min, the optical rotation of 14° per centimeter was realized in the annealed TGG waveguide at the wavelength of 632.8 nm, which is comparable to that observed in the TGG crystal under the same conditions.
Funding
National Natural Science Foundation of China (NSFC) (11535008, 11775136); Natural Science Foundation of Shandong Province (ZR2016AB03); Shandong Science Research Program for Universities (J16LJ08).
References
1. H. Takeda and S. John, “Compact optical one-way waveguide isolators for photonic-band-gap microchips,” Phys. Rev. A 78(2), 023804 (2008). [CrossRef]
2. W. Zaets and K. Ando, “Optical waveguide isolator based on nonreciprocal loss/gain of amplifier covered by ferromagnetic layer,” IEEE Photonics Technol. Lett. 11(8), 1012–1014 (1999). [CrossRef]
3. B. J. H. Stadler and T. Mizumoto, “Integrated magneto-optical materials and isolators: a review,” IEEE Photonics J. 6(1), 1–15 (2014). [CrossRef]
4. Y. Wang, X. Shen, R. Zheng, P. Lv, C. Liu, and H. Guo, “Optical Planar Waveguides Fabricated by Using Carbon Ion Implantation in Terbium Gallium Garnet,” J. Korean Phys. Soc. 72(7), 765–769 (2018). [CrossRef]
5. C. Liu, X. Shen, R. Zheng, H. Guo, W. Li, and W. Wei, “Visible and near-infrared waveguides formed by double-energy proton implantation in magneto-optical glasses,” Appl. Phys. B 123(2), 56 (2017). [CrossRef]
6. Q. Zhu, Y. Wang, X. Shen, H. Guo, and C. Liu, “Optical Ridge Waveguides in Magneto-Optical Glasses Fabricated by Combination of Silicon Ion Implantation and Femtosecond Laser Ablation,” IEEE Photonics J. 10(5), 1–7 (2018). [CrossRef]
7. X. Long, B. Jing, and L. Xin, “Inscription of waveguides in terbium gallium garnet using femtosecond laser,” Acta Opt. Sin. 34(4), 0432002 (2014). [CrossRef]
8. Y. Wang, X. Shen, Q. Zhu, and C. liu, “Optical planar and ridge waveguides in terbium gallium garnet crystals produced by ion implantation and precise diamond blade dicing,” Opt. Mater. Express 8(11), 3288–3294 (2018). [CrossRef]
9. H. Shimizu and Y. Nakano, “Fabrication and characterization of an InGaAsp/InP active waveguide optical isolator with 14.7 dB/mm TE mode nonreciprocal attenuation,” J. Lightwave Technol. 24(1), 38–43 (2006). [CrossRef]
10. H. Dötsch, N. Bahlmann, O. Zhuromskyy, M. Hammer, L. Wilkens, R. Gerhardt, P. Hertel, and A. F. Popkov, “Applications of magneto-optical waveguides in integrated optics: review,” J. Opt. Soc. Am. B 22(1), 240–253 (2005). [CrossRef]
11. Y. L. Ruan, A. Jarvis, A. V. Rode, S. Madden, L. Davies, and Berry, “Wavelength dispersion of Verdet constants in chalcogenide glasses for magneto-optical waveguide devices,” Opt. Commun. 252(1-3), 39–45 (2005). [CrossRef]
12. Y. Shoji, T. Mizumoto, H. Yokoi, I.-W. Hsieh, and R. M. Osgood Jr, “Magneto-optical isolator with silicon waveguides fabricated by direct bonding,” Appl. Phys. Lett. 92(7), 071117 (2008). [CrossRef]
13. K. M. Samirand and J. H. S. Bethanie, “Novel designs for integrating YIG/air photonic crystal slab polarizers with waveguide faraday rotators,” IEEE Photonics Technol. Lett. 17(1), 127–129 (2005). [CrossRef]
14. U. Schlarb and B. Sugg, “Refractive index of terbium gallium garnet,” Phys. Status Solidi B 182(2), K91–K93 (1994). [CrossRef]
15. R. Yasuhara, H. Nozawa, T. Yanagitani, S. Motokoshi, and J. Kawanaka, “Temperature dependence of thermo-optic effects of single-crystal and ceramic TGG,” Opt. Express 21(25), 31443–31452 (2013). [CrossRef]
16. I. L. Snetkov, R. Yasuhara, A. V. Starobor, and O. V. Palashov, “TGG ceramics based Faraday isolator with external compensation of thermally induced depolarization,” Opt. Express 22(4), 4144–4151 (2014). [CrossRef]
17. D. Kip, “Photorefractive waveguides in oxide crystals: fabrication, properties, and applications,” Appl. Phys. B 67(2), 131–150 (1998). [CrossRef]
18. Y. N. Korkishko, V. A. Fedorov, T. M. Morozova, F. Caccavale, F. Gonella, and F. Segato, “Reverse proton exchange for buried waveguides in LiNbO3,” J. Opt. Soc. Am. A 15(7), 1838–1842 (1998). [CrossRef]
19. H. Uetsuhara, S. Goto, Y. Nakata, N. Vasa, T. Okada, and M. Maeda, “Fabrication of a Ti:sapphire planar waveguide by pulsed laser deposition,” Appl. Phys. A 69(7), S719–S722 (1999). [CrossRef]
20. W. Bolaños, J. J. Carvajal, X. Mateos, E. Cantelar, G. Lifante, U. Griebner, V. Petrov, V. L. Panyutin, G. S. Murugan, J. S. Wilkinson, M. Aguiló, and F. Díaz, “Continuous-wave and Q-switched Tm-doped KY(WO4)2 planar waveguide laser at 1.84 μm,” Opt. Express 19(2), 1449–1454 (2011). [CrossRef]
21. M. Ams, G. D. Marshall, P. Dekker, J. A. Piper, and M. J. Withford, “Ultrafast laser written active devices,” Laser Photonics Rev. 3(6), 535–544 (2009). [CrossRef]
22. J. Siebenmorgen, T. Calmano, K. Petermann, and G. Huber, “Highly efficient Yb:YAG channel waveguide laser written with a femtosecond-laser,” Opt. Express 18(15), 16035–16041 (2010). [CrossRef]
23. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. M. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18(24), 24994–24999 (2010). [CrossRef]
24. T. Ruiz, A. Mendez, M. Carrascosa, J. Carnicero, A. Garcia-Cabañes, J. Olivares, F. Agulló-López, A. García-Navarro, and G. García, “Tailoring of refractive index profiles in LiNbO3 optical waveguides by low-fluence swift-ion irradiation,” J. Phys. D 40(15), 4454–4459 (2007). [CrossRef]
25. N. Dong, F. Chen, D. Jaque, A. Benayas, F. Qiu, and T. Narusawa, “Characterization of active waveguides fabricated by ultralow-fluence swift heavy ion irradiation in lithium niobate crystals,” J. Phys. D 44(10), 105103 (2011). [CrossRef]
26. Y. Ren, N. Dong, F. Chen, A. Benayas, D. Jaque, F. Qiu, and T. Narusawa, “Swift heavy-ion irradiated active waveguides in Nd:YAG crystals: fabrication and laser generation,” Opt. Lett. 35(19), 3276–3278 (2010). [CrossRef]
27. J. Olivares, M. L. Crespillo, O. Caballero-Calero, M. D. Ynsa, A. García-Cabañes, M. Toulemonde, C. Trautmann, and F. Agulló-López, “Thick optical waveguides in lithium niobate induced by swift heavy ions (∼10 MeV/amu) at ultralow fluences,” Opt. Express 17(26), 24175–24182 (2009). [CrossRef]
28. J. Lv, Z. Shang, Y. Tan, J. R. V. D. Aldana, and F. Chen, “Cladding-like waveguide fabricated by cooperation of ultrafast laser writing and ion irradiation: characterization and laser generation,” Opt. Express 25(16), 19603–19608 (2017). [CrossRef]
29. Z. Shang, Y. Tan, S. Akhmadaliev, S. Zhou, and F. Chen, “Cladding-like waveguide structure in Nd:YAG crystal fabricated by multiple ion irradiation for enhanced waveguide lasing,” Opt. Express 23(21), 27612–27617 (2015). [CrossRef]
30. F. Chen, “Micro- and submicrometric waveguiding structures in optical crystals produced by ion beams for photonic applications,” Laser Photonics Rev. 6(5), 622–640 (2012). [CrossRef]
31. Y. Ren, N. Dong, F. Chen, A. Benayas, D. Jaque, F. Qiu, and T. Narusawa, “Swift heavy-ion irradiated active waveguides in Nd:YAG crystals: fabrication and laser generation,” Opt. Lett. 35(19), 3276–3278 (2010). [CrossRef]
32. Y. Jia, N. Dong, F. Chen, J. R. V. D. Aldana, S. Akhmadaliev, and S. Zhou, “Ridge waveguide lasers in Nd:GGG crystals produced by swift carbon ion irradiation and femtosecond laser ablation,” Opt. Express 20(9), 9763–9768 (2012). [CrossRef]
33. Y. Tan, S. Akhmadaliev, S. Zhou, S. Sun, and F. Chen, “Guided continuous-wave and graphene-based Q-switched lasers in carbon ion irradiated Nd:YAG ceramic channel waveguide,” Opt. Express 22(3), 3572–3577 (2014). [CrossRef]
34. Y. Tan, Q. Luan, F. Liu, S. Akhmadaliev, S. Zhou, and F. Chen, “Swift carbon ion irradiated Nd:YAG ceramic optical waveguide amplifier,” Opt. Express 21(12), 13992–13997 (2013). [CrossRef]
35. P. K. Tien, D. P. Schinke, and S. L. Blank, “Magneto-optics and motion of the magnetization in a film-waveguide optical switch,” J. Appl. Phys. 45(7), 3059–3068 (1974). [CrossRef]
