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We report on optical channel waveguides in zinc selenide (ZnSe) single crystal fabricated by using mask-assisted 500 keV protons implantation at a fluence of 6 × 1016 ions/cm2. The formed waveguides are with typical “enhanced well” + “negative barrier” refractive index profile. The numerical calculated modal profiles are in good agreement with the measured near-field intensity distribution of the guided light. The propagation loss of the channel waveguide is determined to be ~4 dB/cm after thermal annealing treatment in air.
©2012 Optical Society of America
1. Introduction
Zinc selenide (ZnSe) crystal is an attractive material for the blue light emitting devices because it is a direct transition type semiconductor and it has a wide band gap of 2.7 eV at room temperature [1]. The electronic properties of ZnSe material lead to the possibility of using ZnSe as an alternative material to more widely-used InGaN-based [2–7] material for visible light-emitting devices, specifically with potential application in solid state lighting [1–7]. In addition to this, ZnSe is transparent in wide spectral range from visible to mid-infrared (MIR) regimes, which makes it widely used for high power carbon dioxide laser optics at 10.6 μm [8,9]. Integrated photonics deals with compact optical devices of micro- or nano- dimensions. The waveguide technology offers a unique platform to achieve multiple functions in small- scale circuits in many aspects of modern photonics and telecommunication systems [10]. Owing to the much compact geometry, optical waveguides with diverse configurations can confine the propagating light in very small volumes, in which some bulk-related performances may be considerably improved [11–14]. For practical applications, two dimensional (2D) waveguides (typically in channel or ridge configurations) are more attractive than one dimensional structures (such as planar and slab waveguides) because of the more compact configurations that further enhances the optical intensities [15]. Several techniques have been developed to fabricate optical waveguides in optical materials, such as proton/ion exchange [16], metal diffusion [17], ultrafast laser inscription [18–21] and ion implantation/irradiation [22–27]. As one of the most successful techniques for material-property modification, ion implantation has shown its unique capability for fabricating waveguide structures in most optical materials, exhibiting superior controllability of the refractive index profiles of the waveguides by selecting varying ion species, energies and fluences. In addition, it does not depend on the chemical properties of the target materials, which makes it unique and applicable for diverse materials [22–25]. Compared with medium-mass ions, proton implantation generates less damage in the waveguide region, and penetrates deeper inside the substrate [24,25].
ZnSe waveguides are good candidates for guided-wave nonlinear applications in the infrared wavelength regime. It has been confirmed that ZnSe waveguides are also promising for the astrophotonics. As of yet, ZnSe waveguides have been fabricated by metal organic vapor phase epitaxy (MOCVD) [28,29] and ultrafast laser inscription [30]. In this work, we report on the fabrication of optical channel waveguide in a ZnSe single crystal by 500 keV protons implantation with assistance of photomasking. The refractive index profiles and guiding properties of the waveguides are investigated at wavelength of 632.8 nm.
2. Experiments in details
The ZnSe single crystal sample we used in this work was cut to dimensions of 10 × 10 × 2 mm3, and optically polished. A specially designed photoresist stripe mask, which is composed of a series of open stripes of 5 μm (unshielded) with separation of 45μm (shielded) between the adjacent channels, is deposited on the polished sample surface (10 × 10 mm2) by applying the standard lithography technique. With this masking, the proton implantation at the energy of 500 keV and the fluence of 6 × 1016 ions/cm2 was performed on the sample surface (10 × 10 mm2) and the channel waveguides were formed in the open stripes regions. The proton beams were tilted by 7° off the normal direction of samples along the channels in order to minimize the channeling effect. The implantation was carried out by using the implanter at Institute of Semiconductors, Chinese Academy of Science, Beijing. Figures 1(a) and 1(b) depict the schematic plots of the channel waveguide formation process and Fig. 1(c) shows the microscope image of the transverse cross section of the channel waveguide. The trapezoidal configuration of the channel waveguide cross section is due to the wedged geometry of the photoresist masks at their edges. This technique has been recently successfully applied to produce channel waveguides in a couple of crystals by the implantation of diverse ions [31,32]. To improve the thermal stability and guiding properties, the sample was annealed at 200°C in air (an open oven), which include the following procedures: pre-heating stage (1h), annealing at target temperature (1h), and cooling process (1.5h). For comparison, another ZnSe crystal sample without any photoresist masks on the surface, allowing planar waveguide formation with same conditions to the channel one.
Fig. 1 (a) Schematic plot of the channel waveguides formation process in ZnSe crystal with photoresist masking, (b) magnification of a cross section of the channel waveguides sample and (c) the microscope image of the transverse cross section of the channel waveguide after removing the photoresist mask.
The m-line technique was used to measure the dark-mode spectroscopy of the ZnSe planar waveguides via a prism coupler at 632.8 nm. Based on the dark-line spectrum, the 1D refractive index profile of the planar waveguides was reconstructed by using the Reflectivity Calculation Method (RCM) [33]. The 2D refractive index distributions of the channel waveguides were constructed by considering the 1D planar waveguide index profile and the trapezoidal boundary of channel waveguide cross section. The modal profiles of the waveguides were characterized by using an end-face coupling system. The sample was mounted on a six-dimensional optical stage to for adjustment not only along the x, y, or z axes but also within the x-y, y-z, or z-x planes. The polarized light (at 632.8 nm) was coupled into the channel waveguides by a microscope objective ( × 25). Another microscope lens ( × 25) was used to collect the output light at the other facet of the sample. A CCD camera was used to image the waveguide modes.
The propagation loss of the channel waveguide was measured through the Fabry-Perot (F-P) resonance method through the end-face arrangement [34]. The F-P resonator was constructed due to the Fresnel reflections of light occurred in both input and output facet. As the length of the waveguide (resonator) changed the output light would have an intensity modulation because of the interference condition alternations, which leaded to periodically changed power oscillations. In this work, we measured the propagation loss of the channel waveguide by slightly heating the sample gradually (up to increment of 5°C), by which the propagation length of the waveguide (resonator) increased due to the thermal expansion. Based on this periodically power alternations of the output light the propagation loss (α) of the channel waveguide could be calculated in dB/cm by using the formula
where L is the waveguide length, R and K are the Fresnel reflection coefficient and the independent contrast factor, respectively, which are determined by following equations where neff is the effective refractive index of the ZnSe waveguide, and Imax and Imin are the maximum and minimum light powers measured by the powermeter, respectively.3. Results and discussion
Figure 2 shows reconstructed refractive index profile of the ZnSe planar waveguide produced by the 500 keV proton implantation. As one can see, the incident protons create an enhanced-index well with ∆nw = + 0.001 (a positive index change of ~0.04%) in the near surface region and an optical barrier with ∆nb = −0.004 (a negative index change of ~0.16%) at a depth of ~4.7 μm beneath the sample surface, constructing a distribution with typical “well + barrier” shape. In addition, the peak position of the barrier is in good agreement with the mean projected range of incident 500 keV protons in the ZnSe crystal calculated by the SRIM 2010 code [35].
Fig. 2 The obtained refractive index profile of the ZnSe planar waveguide fabricated by 500keV proton implantation at the fluence of 6 × 1016 /cm2 based on RCM.
Based on the 1D refractive index profiles we reconstructed the 2D refractive index distribution of the ZnSe channel waveguides at a wavelength of 632.8 nm (see Fig. 3(a) ). According to the 2D refractive index distribution, the modal distribution of the waveguide was obtained by utilizing the FD-BPM method (Rsoft© BeamProp 8.0) [36], see Fig. 3(b). By comparing the simulated and experimental near-field modal profiles in Figs. 3(b) and 3(c), one can conclude that there is a good agreement between the experiment and simulation results.
Fig. 3 (a) 2D refractive index profile, (b) calculated modal profiles and (c) measured near-field intensity distributions of the ZnSe channel waveguides at the wavelength of 632.8 nm.
Figure 4 depicts the output light intensity spectrum (transmitted light intensity versus heating time) at a wavelength of 632.8 nm obtained for the ZnSe channel waveguide fabricated by 500 keV proton implantation after thermal annealing treatment at 200°C in air. As we can see, the output power showed a nearly periodical oscillation because of the thermal expansion. The propagation loss of the ZnSe channel waveguide was determined to be ~4 dB/cm at 632.8 nm based on the F-P method. For comparison, the waveguide loss was measured as larger than 10 dB/cm prior to the annealing process. This finding shows that the annealing process of the ZnSe sample leads to a reduction of the attenuation obtained from the waveguide. The detailed demonstration of this method can be obtained in [34].
Fig. 4 Relative intensity of output light as a function of heating time obtained for ZnSe channel waveguide fabricated by 500keV protons implantation after thermal annealing treatment at 200 °C in air. The propagation loss of the waveguide was determined to be ~4 dB/cm.
Figure 5 shows the relative atom displacement of the original lattice versus penetration depth of the protons in the ZnSe crystal. As we can see, the values of relative atom displacement are obtained within the range of 0-5μm, peaking at about 15% at a depth of ~4.7 μm, which is in good agreement with the optical barrier location. In addition, the distribution of relative atom displacement coincides with the 1D refractive index profiles. Therefore, it is reasonable to consider the lattice damage induced by nuclear collisions to be the main reason for the waveguides formation.
Fig. 5 Relative atom displacement of the original lattice versus penetration depth of ions in ZnSe crystal induced by 500keV proton implantation at the fluence of 6 × 1016 ions/cm2.
The energetic ion beams create waveguide structures through induced lattice disorder in the irradiated regions of materials, which exhibits wide applicability to a range of materials [12,24,25]. This is similar to the femtosecond laser inscription, which is also applicable to many materials [18–21]. Nevertheless, the performance of the formed waveguides strongly depends on the material related properties. As for ZnSe, the proton implanted waveguides show acceptable guiding properties. In addition, one could improve the quality further by optimizing the implantation parameters and the post-implantation annealing treatment.
4. Summary
In conclusion, we have reported on the fabrication of optical channel waveguides in ZnSe crystal by 500 keV proton implantation at the fluence of 6 × 1016/cm2. The propagation loss of the channel waveguides was determined to be ~4 dB/cm at 632.8 nm after annealing at 200°C in air. The present results show the potential applications of proton implanted ZnSe channel waveguide as nonlinear integrated devices.
Acknowledgments
The work is supported by the National Natural Science Foundation of China (No. 10925524).
References and links
1. H. Wenisch, K. Schüll, D. Hommel, G. Landwehr, D. Siche, and H. Hartmann, “Molecular beam epitaxial growth and characterization of ZnSe on (001) ZnSe substrates and its application in light-emitting diodes,” Semicond. Sci. Technol. 11(1), 107–115 (1996). [CrossRef]
2. H. Zhao, G. Liu, J. Zhang, J. D. Poplawsky, V. Dierolf, and N. Tansu, “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Opt. Express 19(S4Suppl 4), A991–A1007 (2011). [CrossRef] [PubMed]
3. R. M. Farrell, D. A. Haeger, P. S. Hsu, K. Fujito, D. F. Feezell, S. P. Denbaars, J. S. Speck, and S. Nakamura, “Determination of internal parameters for AlGaN-cladding-free m-plane InGaN/GaN laser diodes,” Appl. Phys. Lett. 99(17), 171115 (2011). [CrossRef]
4. X.-H. Li, R. Song, Y.-K. Re, P. Kumnorkaew, J. F. Gilchrist, and N. Tansu, “Light extraction efficiency and radiation patterns of III-nitride light-emitting diodes with colloidal microlens arrays with various aspect ratios,” IEEE Photon. J. 3(3), 489–499 (2011). [CrossRef]
5. H. Zhao, J. Zhang, G. Liu, and N. Tansu, “Surface plasmon dispersion engineering via double-metallic Au/Ag layers for III-nitride based light-emitting diodes,” Appl. Phys. Lett. 98(15), 151115 (2011). [CrossRef]
6. G. Liu, H. Zhao, J. Zhang, J. H. Park, L. J. Mawst, and N. Tansu, “Selective area epitaxy of ultra-high density InGaN quantum dots by diblock copolymer lithography,” Nanoscale Res. Lett. 6(1), 342 (2011). [CrossRef] [PubMed]
7. J. Zhang and N. Tansu, “Improvement in spontaneous emission rates for InGaN quantum wells on ternary InGaN substrate for light-emitting diodes,” J. Appl. Phys. 110(11), 113110 (2011). [CrossRef]
8. B. S. Patel, “Optical suitability of window materials for CO2 lasers,” Appl. Opt. 16(5), 1232–1235 (1977). [CrossRef] [PubMed]
9. E. M. Gavrishchuk and É. V. Yashina, “Zinc sulfide and zinc selenide optical elements for IR engineering,” J. Opt. Technol. 71(12), 822–827 (2004). [CrossRef]
10. G. Lifante, Integrated Photonics: Fundamentals (Wiley, West Sussex, UK, 2003).
11. E. J. Murphy, Integrated Optical Circuits and Components: Design and Applications (Marcel Dekker, New York, 1999).
12. D. Kip, “Photorefractive waveguides in oxide crystals: fabrication, properties, and applications,” Appl. Phys. B 67(2), 131–150 (1998). [CrossRef]
13. C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quantum Electron. 35(6), 159–239 (2011). [CrossRef]
14. G. I. Stegeman and C. T. Seaton, “Nonlinear integrated optics,” J. Appl. Phys. 58(12), R57–R77 (1985). [CrossRef]
15. F. Chen, “Construction of two-dimensional waveguides in insulating optical materials by means of ion beam implantation for photonic applications: fabrication methods and research progress,” Crit. Rev. Solid State Mater. Sci. 33(3-4), 165–182 (2008). [CrossRef]
16. I. Savatinova, I. Savova, E. Liarokapis, C. C. Ziling, V. V. Atuchin, M. N. Armenise, and V. M. N. Passaro, “A comparative analysis of Rb:KTP and Cs:KTP optical waveguides,” J. Phys. D 31(14), 1667–1672 (1998). [CrossRef]
17. E. Zolotoyabko, Y. Avrahami, W. Sauer, T. H. Metzger, and J. Peisl, “High-temperature phase transformation in Ti-diffused waveguide layers of LiNbO3,” Appl. Phys. Lett. 73(10), 1352–1354 (1998). [CrossRef]
18. J. Siebenmorgen, K. Petermann, G. Huber, K. Rademaker, S. Nolte, and A. Tünnermann, “Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser,” Appl. Phys. B 97(2), 251–255 (2009). [CrossRef]
19. F. M. Bain, A. A. Lagatsky, R. R. Thomson, N. D. Psaila, N. V. Kuleshov, A. K. Kar, W. Sibbett, and C. T. Brown, “Ultrafast laser inscribed Yb:KGd(WO4)2 and Yb:KY(WO4)2 channel waveguide lasers,” Opt. Express 17(25), 22417–22422 (2009). [CrossRef] [PubMed]
20. G. A. Torchia, P. F. Meilán, A. Rodenas, D. Jaque, C. Mendez, and L. Roso, “Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics,” Opt. Express 15(20), 13266–13271 (2007). [CrossRef] [PubMed]
21. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18(24), 24994–24999 (2010). [CrossRef] [PubMed]
22. F. Chen, “Micro-and submicrometric waveguiding structures in optical crystals produced by ion beams for photonic applications,” Laser Photon. Rev. DOI . [CrossRef]
23. A. García-Navarro, J. Olivares, G. García, F. Agulló-López, S. García-Blanco, C. Merchant, and J. S. Aitchison, “Fabrication of optical waveguides in KGW by swift heavy ion beam irradiation,” Nucl. Instrum. Methods Phys. Res. B 249(1-2), 177–180 (2006). [CrossRef]
24. P. D. Townsend, P. J. Chandler, and L. Zhang, Optical Effects of Ion Implantation (Cambridge Univ. Press, Cambridge, UK, 1994).
25. F. Chen, X. L. Wang, and K. M. Wang, “Development of ion-implanted optical waveguides in optical materials: A review,” Opt. Mater. 29(11), 1523–1542 (2007). [CrossRef]
26. G. G. Bentini, M. Bianconi, M. Chiarini, L. Correra, C. Sada, P. Mazzoldi, N. Argiolas, M. Bazzan, and R. Guzzi, “Effect of low dose high energy O3+ implantation on refractive index and linear electro-optic properties in X-cut LiNbO3: Planar optical waveguide formation and characterization,” J. Appl. Phys. 92(11), 6477–6483 (2002). [CrossRef]
27. G. B. Montanari, P. De Nicola, S. Sugliani, A. Menin, A. Parini, A. Nubile, G. Bellanca, M. Chiarini, M. Bianconi, and G. G. Bentini, “Step-index optical waveguide produced by multi-step ion implantation in LiNbO3,” Opt. Express 20(4), 4444–4453 (2012). [CrossRef] [PubMed]
28. B. G. Kim, E. Garmire, N. Shibata, and S. Zembutsu, “Optical bistability and nonlinear switching due to increasing absorption in single-crystal ZnSe waveguides,” Appl. Phys. Lett. 51(7), 475–477 (1987). [CrossRef]
29. M. Kühnelt, T. Leichtner, S. Kaiser, B. Hahn, H. P. Wagner, D. Eisert, G. Bacher, and A. Forchel, “Quasiphase matched second harmonic generation in ZnSe waveguide structures modulated by focused ion beam implantation,” Appl. Phys. Lett. 73(5), 584–586 (1998). [CrossRef]
30. J. R. Macdonald, R. R. Thomson, S. J. Beecher, N. D. Psaila, H. T. Bookey, and A. K. Kar, “Ultrafast laser inscription of near-infrared waveguides in polycrystalline ZnSe,” Opt. Lett. 35(23), 4036–4038 (2010). [CrossRef] [PubMed]
31. J. H. Zhao, X. H. Liu, Q. Huang, P. Liu, L. Wang, and X. L. Wang, “The array waveguides formed in LiNbO3 crystal by oxygen-ion implantation,” Nucl. Instrum. Methods Phys. Res. B 268(19), 2923–2925 (2010). [CrossRef]
32. S. Berneschi, G. Nunzi Conti, I. Banyasz, A. Watterich, N. Q. Khanh, M. Fried, F. Paszti, M. Brenci, S. Pelli, and G. C. Righini, “Ion beam irradiated channel waveguides in Er3+-doped tellurite glass,” Appl. Phys. Lett. 90(12), 121136 (2007). [CrossRef]
33. P. J. Chandler and F. L. Lama, “A new approach to the determination of planar waveguide profiles by means of a non-stationary mode index calculation,” Opt. Acta (Lond.) 33(2), 127–143 (1986). [CrossRef]
34. R. Regener and W. Sohler, “Loss in low-finesse Ti:LiNbO3 optical waveguide resonators,” Appl. Phys. B 36(3), 143–147 (1985). [CrossRef]
35. J. F. Ziegler, computer code, SRIM, http://www.srim.org.
36. D. Yevick and W. Bardyszewski, “Correspondence of variational finite-difference (relaxation) and imaginary-distance propagation methods for modal analysis,” Opt. Lett. 17(5), 329–330 (1992). [CrossRef] [PubMed]
