Open Access
Add to BibSonomy
Abstract
In the traditional dry etching process for photonic device fabrication, the etching effect is influenced in many ways, usually resulting in relatively large sidewall roughness and high transmission loss. In this study, an effective method, namely the secondary coating method, is proposed to reduce the transmission loss of a Ge-Sb-Se chalcogenide waveguide and increase the quality factor (Q-factor) of a Ge-Sb-Se chalcogenide micro-ring resonator. The Ge-Sb-Se waveguide and micro-ring resonator are fabricated by ultraviolet exposure/electron beam lithography and inductively coupled plasma etching technology. Afterward, a 10 nm-thick Ge-Sb-Se thin film is deposited by thermal evaporation. The measurements show that after secondary coating, the sidewall roughness of the waveguide is reduced from 11.96 nm to 6.52 nm, with the transmission loss reduced from 2.63± 0.19 dB/cm to 1.86± 0.11 dB/cm at 1.55 µm wavelength. Keeping an equal coupling condition with equal radius and coupling distance, the Q-factor of the micro-ring resonator is improved by 47.5% after secondary coating. All results indicate that the secondary coating method is a feasible way to generate low-loss and high Q-factor integrated chalcogenide photonic devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Integrated optical path refers to a thin film assembly composed of various optical components. Waveguides, as a fundamental component of integrated optical path, have garnered significant attention given their small size, high integration, and excellent optical performance. They are widely used in various optical devices, including waveguide amplifiers [1–3], waveguide lasers [4,5], optical switches [6–8], photonic integrated circuits [9–12], optical quantum computers [13,14], and photonic crystals [15,16].
Various materials, including lithium niobate [17,18], silicon (Si) [19,20], and chalcogenide glasses (ChGs) [21–23], have been researched for waveguides. Lithium niobate has large second- and third-order nonlinear coefficients, a wide optical low-loss window (0.35-4.5 µm), and stable physical and chemical properties. However, the small refractive index difference between the core and cladding limits its light confinement capability. This limitation hinders its performance in applications requiring efficient light confinement and strong light–matter interactions. Si-based waveguides exhibit excellent optical properties in the near-infrared wavelength range and benefit from mature complementary metal–oxide–semiconductor fabrication technology, making them conducive to integration [24]. However, Si-based waveguides are susceptible to two-photon absorption and exhibit significant carrier absorption in the near-infrared range. ChGs are amorphous compounds consisting of elements such as sulfur (S), selenium (Se), tellurium (Te), and other chalcogen elements, which form covalent bonds with other elements. ChGs possess flexible composition, low two-photon absorption, tunable refractive index, transmission capability from visible light to mid-infrared range, and excellent photosensitivity [25–28]. These characteristics make them well suited for applications in infrared micro–nanophotonics [29–32].
One of the main challenges in microscale waveguides is the transmission loss, which is caused by the high electric field amplitude at the interface between the core and cladding (waveguide surface). Various methods, including inductively coupled plasma (ICP) etching, wet etching, direct laser writing, hot embossing, and lift–off, have been applied for ChG waveguide fabrication [33–35]. Our previous work decreased the transmission loss of Ge11.5As24Se64.5 waveguides by 37% through thermal reflow [36]. However, this method is not suitable for submicron-level optical devices, such as micro-ring resonators and Bragg gratings, due to the changes in waveguide morphology caused by thermal reflow.
In this study, we propose the fabrication of Ge-Sb-Se waveguides by ICP dry etching and secondary coating to minimize sidewall roughness and decrease transmission loss. A Ge–Sb–Se glass was chosen because of its wide infrared transparency and high index. The secondary coating process could effectively decrease the waveguide transmission losses, mostly due to the reduction in the sidewall roughness [37]. The experimental results indicated the effectiveness of the secondary coating method in improving the performance of Ge-Sb-Se waveguides. After secondary coating, the sidewall roughness of the waveguides was reduced from 11.96 nm to 6.52 nm, representing a reduction of 45.5%, which contributed to a decrease in transmission loss by 29.3%. Additionally, we applied this method to the fabrication of micro-ring resonators and found that the quality factor (Q-factor) increased by 47.5% under the same device parameters. These results demonstrate that the secondary coating method significantly improves the performance of Ge-Sb-Se waveguides and holds great significance for applications in fields such as optical communications, sensing, and integrated photonics.
2. Fabrication and characterization
2.1 Waveguide device design
In this research, the structural parameters of a Ge-Sb-Se waveguide were simulated and designed to enable single-mode transmission at a wavelength of 1550 nm, ensuring that the secondary coating does not affect its mode field. On the basis of the simulation results, the structural parameters of the waveguide were designed with a width of 5 µm and a thickness of 800 nm, while the thickness of the secondary coating was set to 10 nm. Then, a Ge-Sb-Se micro-ring resonator was created to evaluate the impact of secondary coating on other optical characterization. From our previous simulation results [38,39], only the fundamental mode of TE/TM is effectively supported when the width of the waveguide is less than 750 nm, and the bending loss is negligible when the radius of the micro-ring is greater than 5 µm. Therefore, the width of the waveguide was set to 600 nm, and the radius of the ring was 24.6 µm.
2.2 Fabrication process
The fabrication process for Ge-Sb-Se waveguides and micro-ring resonators using a secondary coating method is illustrated in Fig. 1. It involves thermal evaporation, ultraviolet lithography/electron beam lithography, ICP etching, and secondary coating.
First, piranha cleaning was conducted to remove any organic residue from the substrate surface before proceeding with lithography. Next, a positive AZ5214 photoresist was spin-coated onto the substrate at a speed of 3000 rpm for 1 min, resulting in a uniform photoresist layer. After 10 s of exposure and 2 min of development, the mask pattern was transferred onto the photoresist. Then, mixed gases of CF4 and CHF3 were used for ICP etching, with an ICP power of 300 W and an etching rate of 12 nm/s. The fabricated straight waveguides and micro-ring resonator are shown in Fig. 2(a) and Fig. 2(b). Finally, a 10 nm-thick film was deposited on the device to complete the secondary coating process.
2.3 Surface roughness
The surface profiler measurements showed nearly no changes in roughness on the top surface of the waveguide following the application of secondary coating. An image analysis method was employed to evaluate the sidewall roughness of the waveguide [40,41]. The waveguide sidewall images were first captured using a scanning electron microscope (SEM). After the images were adjusted, including contrast, brightness, and magnification, the sidewall was outlined along the waveguide contour, obtaining the sidewall contour feature curve, as shown in Fig. 3. The starting point of the curve was defined as the origin (0 point) of the coordinate axis. Data points were recorded every 1 nm along the y-axis, resulting in a total of 50 sets. The waveguide sidewall roughness was calculated using the square root formula.
Fig. 3. (a) SEM image and sidewall curve of the initial waveguide; (b) SEM image and sidewall curve of waveguide after secondary coating.
2.4 Optical loss of waveguide
The magnitude of waveguide losses directly and significantly affects the performance of optical devices. The performance of the fabricated devices near a wavelength of 1550 nm was evaluated using the cut-back method, as shown in Fig. 4. In order to ensure the uniformity in the lengths of the waveguide, the waveguide was cut into three sections, with lengths of 4 mm, 8 mm, and 10 mm, respectively. The transmission loss of each waveguide segment was measured and calculated. Then, a thin film with a thickness of 10 nm was deposited on these three waveguide sections, before the transmission losses were measured again. In all transmission loss measurements, the input power (Pin) is kept constant, and the loss is expressed in dB/cm. The insertion loss can be derived as follows:
The transmission loss of the waveguide can be obtained as:
where αc represents the coupling loss; αi represents the insertion loss; α represents the transmission loss; Pin and Pout represent the input and output power, respectively; L1 and L2 represent the lengths of the waveguide segments after truncation; αi1 and αi2 represent the insertion loss at different lengths. From Eq. (1) and Eq. (2), αi can be expressed as a linear function of the waveguide length L, with the slope of the line representing the transmission loss of the waveguide.2.5 Q-factor of the micro-ring resonator
In micro-ring devices, a high Q-factor indicates low loss in the micro-ring, leading to long photon lifetimes and strong sensing capabilities of the device. Considering that a 10 nm-thick film deposition will change the structural parameters of the resonator, including radius, width, and coupling gap. In this study, after secondary coating, the coupling gap and the radius of the micro-ring were controlled to be consistent with those of the micro-ring before secondary coating to keep the same coupling condition for the influence evaluation of secondary coating on the Q-factor of the micro-ring resonator, as shown in Fig. 5. A tunable laser emitted laser beams within a wavelength range of 1500-1600 nm was employed, with a tuning increment of 0.01 nm. To ensure TE mode light, a polarization controller was utilized to adjust the light. Two single-mode fibers were vertically coupled into and out of the micro-ring resonator using grating couplers, and the output transmission spectrum was displayed on a computer through a power meter.
3. Results and discussions
The measurement results show that the sidewall roughness of the waveguides decreased from 11.96 nm to 6.52 nm, with a reduction of 45.5%, after secondary coating, which means that the native surface smoothing capabilities of secondary coating make it well suited for enhancing the surface quality of waveguides. Figure 6 illustrates the loss curve of a waveguide before (black line) and after the application of secondary coating (red line). The slope of the curve indicates the transmission loss of the waveguide. At a wavelength of 1550 nm, the waveguide’s transmission loss is calculated as 2.63 ± 0.19 dB/cm. After applying the secondary coating, the transmission loss decreases to 1.86 ± 0.11 dB/cm, representing a significant 29.3% decrease.
Based on the results presented above, the impact of secondary coating on the optical performance of micro-ring resonators is investigated. Figure 7 shows the output transmission spectrum of the Ge-Sb-Se micro-ring. The FSR measurement is approximately 5 nm, which is consistent with the simulation values. The Q-factor can be obtained from the following equation:
where λFWHM is the full width at half maximum of the resonant peak. The load Q-factor of the micro-ring is 3.03 × 104 from the transmission spectrum. After secondary coating, the Q-factor of the micro-ring is increased by 47.5%, which is 4.47 × 104.Fig. 7. (a) Transmission spectrum of the micro-ring resonator before secondary coating. (b) Single resonance peak with a Lorentzian fit to Q-factor before secondary coating. Experimental transmission (black points) and Lorentzian fitting (red line) of transmission spectra. (c) Transmission spectrum of the micro-ring resonator after secondary coating. (d) Single resonance peak with a Lorentzian fit to Q-factor after secondary coating. Experimental transmission (black points) and Lorentzian fitting (red line) of transmission spectra.
4. Conclusion
In this study, a secondary coating method was utilized to create a low-loss Ge-Sb-Se chalcogenide photonic device. The experimental results revealed a significant improvement in the performance of the device. Specifically, the sidewall roughness of the waveguides decreased by 45.5% from 11.96 nm to 6.52 nm after the application of the secondary coating. Additionally, the transmission loss was reduced from 2.63 ± 0.19 dB/cm to 1.86 ± 0.11 dB/cm at a wavelength of 1.55 µm. Furthermore, the secondary coating method was also applied to a Ge-Sb-Se micro-ring resonator. Remarkably, the Q-factor of the micro-ring resonator increased by 47.5% following the secondary coating process, using the same structural parameters. This study demonstrated that the secondary coating method is a simple and practical way for realizing a low-loss and high-Q-factor integrated chalcogenide photonic device.
Funding
Natural Science Foundation of Zhejiang Province (No. LD22F040002, No. LY23F050008); National Natural Science Foundation of China (No. 62105172); Natural Science Foundation of Ningbo Municipality (No. 2022J105); K. C. Wong Magna Fund in Ningbo University.
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. J. D. Bradley and M. Pollnau, “Erbium-doped integrated waveguide amplifiers and lasers,” Laser Photonics Rev. 5(3), 368–403 (2011). [CrossRef]
2. Z. Chen, Q. Xu, K. Zhang, et al., “Efficient erbium-doped thin-film lithium niobate waveguide amplifiers,” Opt. Lett. 46(5), 1161–1164 (2021). [CrossRef]
3. L. Slooff, A. Van Blaaderen, A. Polman, et al., “Rare-earth doped polymers for planar optical amplifiers,” J. Appl. Phys. 91(7), 3955–3980 (2002). [CrossRef]
4. C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quant. Electron. 35(6), 159–239 (2011). [CrossRef]
5. Y. Ren, G. Brown, A. Ródenas, et al., “Mid-infrared waveguide lasers in rare-earth-doped YAG,” Opt. Lett. 37(16), 3339–3341 (2012). [CrossRef]
6. E. M. Vogel, M. Weber, and D. Krol, “Nonlinear optical phenomena in glass,” Phys. Chem. Glasses 32(6), 231–254 (1991).
7. M. Frumar, B. Frumarova, P. Nemec, et al., “"Thin chalcogenide films prepared by pulsed laser deposition–new amorphous materials applicable in optoelectronics and chemical sensors,” J. Non-Cryst. Solids 352(6-7), 544–561 (2006). [CrossRef]
8. M. Daneshmand, R. R. Mansour, and N. Sarkar, “RF MEMS waveguide switch,” IEEE Trans. Microwave Theory Techn. 52(12), 2651–2657 (2004). [CrossRef]
9. J. F. Bauters, M. L. Davenport, M. J. Heck, et al., “"Silicon on ultra-low-loss waveguide photonic integration platform,” Opt. Express 21(1), 544–555 (2013). [CrossRef]
10. M. Law, D. J. Sirbuly, J. C. Johnson, et al., “Nanoribbon waveguides for subwavelength photonics integration,” Science 305(5688), 1269–1273 (2004). [CrossRef]
11. A. L. Pyayt, B. Wiley, Y. Xia, et al., “"Integration of photonic and silver nanowire plasmonic waveguides,” Nat. Nanotechnol. 3(11), 660–665 (2008). [CrossRef]
12. Y. Bian, Z. Zheng, X. Zhao, et al., “"Symmetric hybrid surface plasmon polariton waveguides for 3D photonic integration,” Opt. Express 17(23), 21320–21325 (2009). [CrossRef]
13. T. V. Galstyan, J.-F. Viens, A. Villeneuve, et al., “"Photoinduced self-developing relief gratings in thin film chalcogenide As2S3 glasses,” J. Lightwave Technol. 15(8), 1343–1347 (1997). [CrossRef]
14. D. Goyal and A. Maan, “"Far-infrared absorption in amorphous Sb15GexSe85-x glasses,” J. Non-Cryst. Solids 183(1-2), 182–185 (1995). [CrossRef]
15. J. Hu, L. Li, H. Lin, et al., “"Flexible integrated photonics: where materials, mechanics and optics meet,” Opt. Mater. Express 3(9), 1313–1331 (2013). [CrossRef]
16. T. Han, S. Madden, S. Debbarma, et al., “"Improved method for hot embossing As2S3 waveguides employing a thermally stable chalcogenide coating,” Opt. Express 19(25), 25447–25453 (2011). [CrossRef]
17. M. Bazzan and C. Sada, “Optical waveguides in lithium niobate: Recent developments and applications,” Appl. Phys. Rev. 2(4), 040603 (2015). [CrossRef]
18. C. Hu, A. Pan, T. Li, et al., “"High-efficient coupler for thin-film lithium niobate waveguide devices,” Opt. Express 29(4), 5397–5406 (2021). [CrossRef]
19. K. Okamoto, “"Progress and technical challenge for planar waveguide devices: silica and silicon waveguides,” Laser Photonics Rev. 6(1), 14–23 (2012). [CrossRef]
20. Y. Zhang, S. Yang, A. E.-J. Lim, et al., “"A compact and low loss Y-junction for submicron silicon waveguide,” Opt. Express 21(1), 1310–1316 (2013). [CrossRef]
21. C. Lin, C. Rüssel, and S. Dai, “Chalcogenide glass-ceramics: Functional design and crystallization mechanism,” Prog. Mater. Sci. 93, 1–44 (2018). [CrossRef]
22. J. Sanghera and I. Aggarwal, “Active and passive chalcogenide glass optical fibers for IR applications: a review,” J. Non-Cryst. Solids 256-257, 6–16 (1999). [CrossRef]
23. A. T. Vu, A. N. Vu, T. Grunwald, et al., “"Modeling of thermo-viscoelastic material behavior of glass over a wide temperature range in glass compression molding,” J. Am. Ceram. Soc. 103(4), 2791–2807 (2020). [CrossRef]
24. W. Wu, Q. Sun, L. Wang, et al., “"Influence of two-photon absorption and free-carrier effects on all-optical logic gates in silicon waveguides,” Appl. Phys. Express 12(4), 042005 (2019). [CrossRef]
25. T. Inoue and S. Namiki, “"Pulse compression techniques using highly nonlinear fibers,” Laser Photonics Rev. 2(1-2), 83–99 (2008). [CrossRef]
26. J. Xu, X. Zhang, and J. Mørk, “"Investigation of patterning effects in ultrafast SOA-based optical switches,” IEEE J. Quantum Elect. 46(1), 87–94 (2010). [CrossRef]
27. L. K. Oxenløwe, F. Gómez-Agis, C. Ware, et al., “"640-Gbit/s data transmission and clock recovery using an ultrafast periodically poled lithium niobate device,” J. Lightwave Technol. 27(3), 205–213 (2009). [CrossRef]
28. P. Steglich, C. Villringer, B. Dietzel, et al., “"Electric field-induced linear electro-optic effect observed in silicon-organic hybrid ring resonator,” IEEE Photon. Technol. Lett. 32(9), 526–529 (2020). [CrossRef]
29. S. Madden, D.-Y. Choi, D. Bulla, et al., “"Long, low loss etched As2S3 chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 15(22), 14414–14421 (2007). [CrossRef]
30. C. Zhang, M. Zhang, Y. Xie, et al., “"Wavelength-selective 2×2 optical switch based on a Ge2Sb2Te5-assisted microring,” Photonics Res. 8(7), 1171–1176 (2020). [CrossRef]
31. G. Hodes, J. Manassen, and D. Cahen, “"Photoelectrochemical energy conversion and storage using polycrystalline chalcogenide electrodes,” Nature 261(5559), 403–404 (1976). [CrossRef]
32. J. Hu, V. Tarasov, A. Agarwal, et al., “"Fabrication and testing of planar chalcogenide waveguide integrated microfluidic sensor,” Opt. Express 15(5), 2307–2314 (2007). [CrossRef]
33. J. Hu, V. Tarasov, N. Carlie, et al., “"Exploration of waveguide fabrication from thermally evaporated Ge–Sb–S glass films,” Opt. Mater. 30(10), 1560–1566 (2008). [CrossRef]
34. A. Zoubir, M. Richardson, C. Rivero, et al., “"Direct femtosecond laser writing of waveguides in As2S3 thin films,” Opt. Lett. 29(7), 748–750 (2004). [CrossRef]
35. J. Hu, V. Tarasov, N. Carlie, et al., “"Si-CMOS-compatible lift-off fabrication of low-loss planar chalcogenide waveguides,” Opt. Express 15(19), 11798–11807 (2007). [CrossRef]
36. Y. Zhao, C. Li, P. Guo, et al., “"Exploration of lift-off Ge–As–Se chalcogenide waveguides with thermal reflow process,” Opt. Mater. 92, 206–211 (2019). [CrossRef]
37. M. Häyrinen, M. Roussey, V. Gandhi, et al., “"Low-loss titanium dioxide strip waveguides fabricated by atomic layer deposition,” J. Lightwave Technol. 32(2), 208–212 (2014). [CrossRef]
38. X. Zhang, C. Zhou, Y. Luo, et al., “"High Q-factor, ultrasensitivity slot microring resonator sensor based on chalcogenide glasses,” Opt. Express 30(3), 3866–3875 (2022). [CrossRef]
39. W. Huang, Y. Luo, W. Zhang, et al., “"High-sensitivity refractive index sensor based on Ge–Sb–Se chalcogenide microring resonator,” Infrared Phys. Technol. 116, 103792 (2021). [CrossRef]
40. K. A. Buzaverov, A. S. Baburin, E. V. Sergeev, et al., “"Low-loss silicon nitride photonic ICs for near-infrared wavelength bandwidth,” Opt. Express 31(10), 16227–16242 (2023). [CrossRef]
41. Q. Du, Y. Huang, J. Li, et al., “Low-loss photonic device in Ge-Sb-S chalcogenide glass,” Opt. Lett. 41(13), 3090–3093 (2016). [CrossRef]
