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Ge20As20Se15Te45 chalcogenide glass was fabricated and systematically studied. This glass exhibits broad transmission range, high linear and non-linear refractive index, and good thermal stability. The low glass transition temperature allowed for the thermal nanoimprint to be accomplished directly on the bulk Ge20As20Se15Te45 glass to produce photonic crystals with a hybrid soft stamp. By optimizing the imprint conditions, uniform gratings with 500 nm depth grooves were fabricated by direct resist-free thermal nanoimprint lithography.
© 2016 Optical Society of America
1. Introduction
Photonic crystals (PCs) are periodic dielectric materials exhibiting photonic band gaps (PBG), for which light of certain energies cannot propagate. Since their discovery in 1987 and especially in recent years [1, 2], photonic crystals have been the objects of an intensive research [3–5] partly because of the advancements of extremely high solution fabrication techniques and partly because of the discovery of new materials that present photonic band gaps with technological feasibility [6–8]. A high refractive index, preferably higher than 2.7, is the most important requirement for a material’s use for the fabrication of PCs with a complete photonic band gap [9]. Furthermore, the material should afford fabrication as a large bulk of arbitrary shape; therefore, the material must be of glassy origin. Chalcogenide glasses (ChG) have recently attracted substantial interest because they have the highest linear (2.0 to 3.5) and nonlinear refractive index (100 to 1000 times higher than silica) among glasses, and they also allow fabrication of rib and fiber waveguides [10–12]. Furthermore, ChG possess extremely wide transmission window, which can be transparent from the visible region (sulfur-based glass) up to the mid-infrared region (~25 µm for telluride glasses) [13]. These properties allowed the application of ChG to fabricate efficient photonic band-gap crystals for visible to infrared applications [6–11].
Several methods have been proposed for fabricating chalcogenide PCs; these methods include laser interference lithography [14], femtosecond laser direct-writing [6], photolithography combined with dry plasma etching [7, 15]. However, for photolithography and laser direct-writing, the refractive index contrasts in the PCs are generally too small to produce a complete band-gap. Although a high refractive index contrast can be achieved using the etching method, it remained a challenge for high quality PCs because chalcogenides are aggressively attacked by the alkaline chemicals used in photolithography as well as most gases in the plasma form. Thus, very careful attention are necessary to process design, which introduces complexities, such as utilizing protective layers [16]; optimized annealing conditions [17]; and tailored process chemistries [18]. This process increases the number of processing steps, and leads to less than ideal process control.
Conversely, thermal nanoimprint lithography (NIL) provides a fast and low -cost technology, which requires only a single step to fabricate photonic devices [19–26]. This method is further popular for its ability to produce even nanometer sized features without utilizing any chemicals [20, 25]. Furthermore, they can be employed without modification with any glass composition, which has suitable softening characteristics. Awareness of these qualities suggests that changing the processing steps is no longer necessary because the material composition changes.
The nanoimprint implementing directly on glasses usually requires the material to have a low glass transition temperature. Many PC devices have been successfully imprinted in As2S3 glass [22, 23], whose glass transition temperature is lower than 200 °C. However, the IR transmission window of As2S3 is narrow, and the refractive index is also relative small, with about 2.4 in the mid-IR. Te-enriched Ge–As–Se–Te (GAST) chalcogenide system is a good candidate to prepare PCs for IR applications [13]. Refractive indices for the GAST glasses are very large (3.0–3.5), due to the heavier and more polarizable chalcogen Te. The heavier Te atom means that Ge–Te and As–Te bonds exhibit fundamental lattice vibrations, overtone and combination vibrations, at a smaller frequency than the analogous As–S and As–Se bonds. Hence, the intrinsic multiphonon infrared absorption edge of the Te-enriched GAST glasses is at a longer wavelength. But, the metallic character of Te leads to a greater tendency for crystallite formation, which prevents the production of low-loss photonic devices due to scattering effects. However, this drawback can be alleviated by the substitution of a small amount of Te by Se, which both lowers the conductivity and dramatically increases the resistance to crystallization, while retaining a wide optical window to long wavelengths [27]. So, Ge20As20Se15Te45 chalcogenide glass was selected in our work, which was measured to have high nonlinearity and wide optical window in IR regions.
In this work, Ge20As20Se15Te45 chalcogenide glass was fabricated using melt-quenching method and characterized for photonic crystal. The prepared glasses were further measured to have the following advantages: a) broad transmission window from approximately 1.5 to 23 μm, b) extremely high nonlinearity of 6.1727 × 10−18 m2/W @3.5 μm in infrared, c) excellent glass-stability, which enables applications of the surface molding and hot pressing techniques to produce photonic devices. Photonic crystals of 1D gratings were fabricated using direct resist-free thermal nanoimprint on the surface of a 2 mm thick glass instead of the film. Remarkably, due to the low softening temperature of GAST, the nanoimprint was accomplished with a hybrid soft Polydimethylsiloxane (PDMS) stamp in contrast to hard stamps. By optimizing the imprint temperature, a simple, low cost PDMS soft stamp is shown to produce grating with excellent surface morphology.
2. Glass synthesis and properties study
2.1 Glass synthesis
Bulk chalcogenide glass with the composition of 20Ge-20As-15Se-45Te (in mol%) were prepared by using melt-quenching method on silica ampoules under vacuum [13, 28]. High-purity raw materials, i.e., germanium, arsenic, selenium, and tellurium 5N, were used for glass preparation. The starting materials were distilled in different tubes by performing multiple distillations to eliminate water, carbon, and other impurities that hindered the transmission of glasses [29]. After purification, the different elements were sealed in the same silica ampoules under vacuum (10−3 bar). The ampoules were then heated in a rocking furnace at 850 °C for 10 hours, and then cooled to 650 °C for 1 hour, allowing the fusion of the elements and the homogenization of the melt. Then, the ampoules were quenched in ice water to allow glass formation and to avoid crystallization. The advantage of the GAST system is the relatively low vapor pressure of the components at temperatures used during melting. Melting and quenching of the melt are thus more straightforward than that for sulphur-containing systems. In fact, researchers have shown that telluride melts can be cast in air [13]; whereas normally chalcogenide glass melts are quenched in situ inside a sealed silica ampoule. Finally, the vitreous sample in situ inside the sealed silica ampoule was annealed at the temperature (190 °C) slightly below the glass transition temperature (Tg) before slowly cooling to room temperature. The silica glass ampoule was carefully cut away, and the chalcogenide glass came cleanly away from the inner surface of the ampoule. The glasses were then cut into 2mm-thick disks, and their two parallel sides were optically polished for the measurement of their thermal and optical properties.
2.2 Properties of glass
The infrared transparency spectrum was measured using a PerkinElmer-Lambda 950 UV/VIS/NIR spectrophotometer over a spectral range from 400 to 2,500 nm, and Fourier transform infrared spectroscopy (Thermo Nicolet, Nexus 380, USA) over the range of 2.5–25 μm at room temperature. The transmission curve of the fabricated Ge20As20Se15Te45 chalcogenide glass with 2 mm thickness is presented in Fig. 1(a). The glass is shown to have extremely wide transmission window, which can be transparent from 1.5 to 23 μm. Furthermore, no clear absorption band exists because of the multiple distillations of raw materials, indicating excellent optical quality for photonic devices. The refractive indices of the prepared the GAST glass were measured with an Infrared-Variable Angle Spectroscopic Ellipsometer (IR-VASE Mark II, J.A.Woollam, USA), which was shown in Fig. 1(b). It was shown that the refractive indices of the Ge20As20Se15Te45 chalcogenide glass are around 3.1 in IR region, which are high enough to obtain photonic band gaps for PC applications.
The thermal stability of the glass is an important parameter of the glass for nanoimprint lithography. The linear thermal expansion property of Ge20As20Se15Te45 chalcogenide glass was measured using a Netzsch DIL 402 C dilatometer from room temperature to 550 K, with a heating rate of 5 K/min, as shown in Fig. 2. The glass transition temperature Tg and the softening temperature were measured to be 203.8 °C and 244.2 °C, respectively.
The nonlinearity of prepared Ge20As20Se15Te45 glass at the mid-infrared wavelengths was also studied with the Z-scan technique. An optical parametric amplifier system (Coherent, Legend Elite and OperA Solo) with pulse width of 150 fs and repetition frequency of 1 kHz was employed as the pump source. The femtosecond laser was focused on the glass samples using a CaF2 lens. The average laser power incident on the surface of glass samples was set at 3 ( ± 0.1) mW, corresponding to laser density of 8.1 ( ± 0.3) and 4.4 ( ± 0.2) GW/cm2 at the lens focus. The laser power was recorded using a highly sensitive pyroelectric power probe (Laser Probe, RkP-575). Figure 3 shows the closed-aperture (CA) Z-scan test of the Ge20As20Se15Te45 glass at 3.5 μm. The CA curve exhibits a valley-and-peak configuration, indicating self-focusing, namely the nonlinear refractive indexes (n2) of glasses with a positive sign at the mid-infrared wavelengths. Using the well-established fitting procedure [30], the nonlinear refraction of Ge20As20Se15Te45 glass at 3.5 μm is estimated to be n2 = 6.1727 × 10−18 m2/W. The high nonlinearity of the prepared Ge20As20Se15Te45 glass shows significant potential for nonlinear optical devices in mid-infrared.
3. Nanoimprint and measurements
Nanoimprint lithography has been demonstrated as a high-throughput and low-cost lithographic technique with sub-10 nm resolution [20]. Instead of following the established hard stamp thermal imprint route, we chose to carry out direct resist-free thermal nanoimprint with a hybrid soft stamp, while taking advantage of the low glass softening temperature of Ge20As20Se15Te45 at ~244.2 °C. Figure 4(a) shows the schematic of the hybrid mold, which consisted of a rigid cross-linked patterning layer on flexible polymer support (dimethylsiloxane) (PDMS). This PDMS mold itself is thermally stable up to 250°C. A ~10 nm thick anti-stick layer was deposited using plasma processing with CHF3 for easy peeling after imprint [25]. This mold combined the advantages of both a rigid nanoimprint mold to achieve a high-resolution pattern transfer and a flexible soft lithography stamp to enable conformal contact without an externally applied pressure.
The simple schematic of the nanoimprint lithography with hybrid soft stamp is shown in Fig. 4(b). Here, the chalcogenide glass disk with the stamp in contact sat on a hotplate with an elastic membrane suspended above it, this being vacuum sealed back to the hotplate surface. A second sealed chamber lay above the membrane. By evacuating both chambers, but keeping the upper one at slightly lower pressure, the membrane bows up and all the air can be removed from the stamp. The hotplate was then heated to the softening temperature of the glass, and 1Mpa (10 atmospheres) of pressure was applied to the upper chamber. The membrane elastically deformed, applying the pressure isobarically to the stamp to create the imprint, as shown in Fig. 4(c). After ~30 minutes, the hotplate was flash cooled at ~40°C/min rate by forcing compressed air through a cooling chamber incorporated at the bottom surface of the hotplate. Upon cooling below the glass transition temperature, both chambers were vented and the sample removed, as shown in Fig. 4(d). The stamp was released by peeling it off by hand, the radically different compositions of the stamp and the ChG glass, plus the anti-stick layer, ensuring no adhesion occurred at the molded glass surface.
Figure 5 shows the optical images of the imprinted chalcogenide grating, from which uniform grating has clearly been obtained. The thickness of the glass disk is 2 mm, and the heating temperature for imprint is 250 °C. However, some holes and defects are present on the surface of the prepared grating as well. These may be caused by the residual air bubbles and dust particles between the stamp and the sample. To minimize the amount of air trapped between the stamp and the sample, the stamp was manually and carefully placed on the substrate from one side to the other side to push air out. Furthermore, the bottom chamber in Fig. 4(b) was pumped into a vacuum (10−3 bar). However, observing air bubbles trapped between the mold and the substrate was still common. When imprinting the substrate coating with nanoimprint resist, air bubbles are found to possibly dissolve in the nanoimprint resist given a sufficient processing time [31]. However, this effect is difficult and unexpected to happen when directly imprinted on the ChG glass sample; thus, higher vacuum should be used to improve the imprinted structure. Meanwhile, the imprint pressure of 10 atmospheres was also too high, which led to some structure defects on the surface of the sample.
Fig. 5 Optical images of imprinted grating in Ge20As20Se15Te45 Chalcogenide glass: (a) magnified 2000 times and (b) magnified 4000 times.
Figure 6 shows the improved imprinting results, where we increased the vacuum of the bottom chamber to 10−4 bar, and reduced the imprint pressure to 5 atmospheres. We further cleared the glass surface by cleaning compressed nitrogen before vacuum pumping the chamber. The quality of the grating has been significantly improved, except for extremely few defects in the sample. Figure 7 shows the detailed cross section profiles of the prepared gratings, by using a Dektak 150 stylus surface profiler. It was found that the surface of the imprinted sample was very smooth, with a roughness of the patterned surface of Ra = 5~10 nm. Such a smooth surface, with roughness in the nanometer range, is very important for the design and fabrication of optical and photonic devices with minimal scattering losses. The period of the prepared grating is 2.0 μm, which is consistent with the original mold. However, the observed depths of the grooves in the imprinted grating are about 500 nm, which are lower than in the original mold (1.0μm). This should be result from the limited flow of chalcogenide at the low pressure. Also, the viscosity of the GAST glass is still high at current imprinting temperature of 250 °C, which was only 46 °C above glass transition temperature. It was also found that the top faces of the gratings are in arc shapes instead of complete rectangles. A higher heating temperature might improve the imprinted structure in GAST glasses when higher temperature resistance PDMS molds are available.
Fig. 6 Optical images of imprinted grating in Ge20As20Se15Te45 Chalcogenide glass: (a) magnified 2000 times and (b) magnified 4000 times.
We further conducted the XRD study of the glasses before and after imprinting. Figure 8 shows the XRD results of the original glass and the glass after thermal imprinting. XRD analysis showed that prepared grating at both conditions are still in the amorphous state, which confirms the thermal stability of the prepared Ge20As20Se15Te45 glass.
Fig. 8 XRD result of the original glass (black line) and the glass after thermal imprinting (black and red line).
The transmission spectrum of the obtained 1D photonic crystal at normal incidence was calculated by a plane-wave-based transfer-matrix method (TMM) [32], which was shown in Fig. 9. It was found that there are several photonic band gaps (PBG) in the IR region, including two main band gaps of 3.6–5.0 µm and 7.0–11.7 µm, which are consistent with the very important atmospheric infrared windows and the fundamental absorption bands of variety of gases and chemicals in the mid-IR. So, the imprinted PC structure has great potentials for IR applications.
4. Conclusion
We have demonstrated the chalcogenide glass grating fabricated by using thermal nano-imprint lithography in Ge20As20Se15Te45 glass. The Ge20As20Se15Te45 chalcogenide glass was prepared using melt-quenching method, and was measured to have broad transmission range, high linear and non-linear refractive index, and good thermal stability. The low glass transition temperature allowed for the use of a hybrid soft PDMS stamp in contrast to previous works wherein hard stamps were used. The hybrid soft PDMS stamp was shown capable of providing excellent performance. By optimizing the imprint condition, such as atmospheric pressure and imprint temperature, uniform gratings with 500 nm depth grooves were successfully obtained.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 61307060, 61435009 and 61377061), the Zhejiang Open Foundation of the Most Important Subjects (No. xkx11408), and the K. C. Wong Magna Fund in Ningbo University.
References and links
1. S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58(23), 2486–2489 (1987). [CrossRef] [PubMed]
2. E. Yablonovitch, “Inhibited Spontaneous Emission in Solid-State Physics and Electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). [CrossRef] [PubMed]
3. J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, “Photonic crystals: putting a new twist on light,” Nature 386(6621), 143–149 (1997). [CrossRef]
4. S. Noda, K. Tomoda, N. Yamamoto, and A. Chutinan, “Full three-dimensional photonic bandgap crystals at near-infrared wavelengths,” Science 289(5479), 604–606 (2000). [CrossRef] [PubMed]
5. K. Ishizaki, M. Koumura, K. Suzuki, K. Gondaira, and S. Noda, “Realization of three-dimensional guiding of photons in photonic crystals,” Nat. Photonics 7(2), 133–137 (2013). [CrossRef]
6. S. Wong, M. Deubel, F. Pérez-Willard, S. John, G. A. Ozin, M. Wegener, and G. van Freymann, “Direct laser writing of three-dimensional photonic crystals with a complete photonic bandgap in chalcogenide glasses,” Adv. Mater. 18(3), 265–269 (2006). [CrossRef]
7. D. Freeman, S. Madden, and B. Luther-Davies, “Fabrication of planar photonic crystals in a chalcogenide glass using a focused ion beam,” Opt. Express 13(8), 3079–3086 (2005). [CrossRef] [PubMed]
8. V. N. Astratov, A. M. Adawi, M. S. Skolnik, V. K. Tikhomirov, V. M. Lyubin, D. G. Lidzey, M. Ariu, and A. L. Reynolds, “Opal photonic crystals infiltrated with chalcogenide glasses,” Appl. Phys. Lett. 78(26), 4094–4096 (2001). [CrossRef]
9. K. Paivasaari, V. K. Tikhomirov, and J. Turunen, “High refractive index chalcogenide glass for photonic crystal applications,” Opt. Express 15(5), 2336–2340 (2007). [CrossRef] [PubMed]
10. C. Quémard, F. Smektala, V. Couderc, A. Barthelemy, and J. Lucas, “Chalcogenide glasses with high non linear optical properties for telecommunications,” J. Phys. Chem. Solids 62(8), 1435–1440 (2001). [CrossRef]
11. J. M. Harbold, F. O. Ilday, F. W. Wise, J. S. Sanghera, V. Q. Nguyen, L. B. Shaw, and I. D. Aggarwal, “Highly nonlinear As-S-Se glasses for all-optical switching,” Opt. Lett. 27(2), 119–121 (2002). [CrossRef] [PubMed]
12. A. Zakery and S. R. Elliott, “Optical properties and applications of chalcogenide glasses: a review,” J. Non-Cryst. Solids 330(1-3), 1–12 (2003). [CrossRef]
13. V. K. Tikhomirov, D. Furniss, A. B. Seddon, J. A. Savage, P. D. Mason, D. A. Orchard, and K. L. Lewis, “Glass formation in the Te-enriched part of the quaternary Ge-As-Se-Te system and its implication for mid-infrared optical fibres,” Infrared Phys. Technol. 45(2), 115–123 (2004). [CrossRef]
14. A. Arsh, M. Klebanov, V. Lyubin, L. Shapiro, A. Feigel, M. Veinger, and B. Sfez, “Glassy mAs2S3·nAs2Se3 photoresist films for interference laser lithography,” Opt. Mater. 26(3), 301–304 (2004). [CrossRef]
15. S. J. Madden, D. Y. Choi, D. A. Bulla, A. V. Rode, B. Luther-Davies, V. G. Ta’eed, M. D. Pelusi, and B. J. Eggleton, “Long, low loss etched As2S3 chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 15(22), 14414–14421 (2007). [CrossRef] [PubMed]
16. D. Choi, S. Madden, D. Bulla, R. Wang, A. Rode, and B. Luther-Davies, “Submicrometer-Thick Low-Loss As2S3 Planar Waveguides for Nonlinear Optical Devices,” IEEE Photonics Technol. Lett. 22(7), 495–497 (2010). [CrossRef]
17. D. Choi, S. Madden, D. Bulla, R. Wang, A. Rode, and B. Luther-Davies, “Thermal annealing of arsenic trisulphide thin film and its influence on device performance,” J. Appl. Phys. 107(5), 053106 (2010). [CrossRef]
18. D. Y. Choi, S. Madden, A. Rode, R. P. Wang, A. Ankiewicz, and B. Luther-Davies, “Surface roughness in plasma-etched As2S3 films: Its origin and improvement,” IEEE Trans. NanoTechnol. 7(3), 285–290 (2008). [CrossRef]
19. H. Schift, “Nanoimprint lithography: An old story in modern times? A review,” J. Vac. Sci. Technol. B 26(2), 458–480 (2008). [CrossRef]
20. M. Austin, H. Ge, W. Wu, M. Li, Z. Yu, D. Wasserman, S. Lyon, and S. Chou, “Fabrication of 5 nm linewidth and 14 nm pitch features by nanoimprint lithography,” Appl. Phys. Lett. 84(26), 5299–5301 (2004). [CrossRef]
21. T. Han, S. Madden, M. Zhang, R. Charters, and B. Luther-Davies, “Low loss high index contrast nanoimprinted polysiloxane waveguides,” Opt. Express 17(4), 2623–2630 (2009). [CrossRef] [PubMed]
22. T. Han, S. Madden, D. Bulla, and B. Luther-Davies, “Low loss Chalcogenide glass waveguides by thermal nano-imprint lithography,” Opt. Express 18(18), 19286–19291 (2010). [CrossRef] [PubMed]
23. J. Orava, T. Kohoutek, A. L. Greer, and H. Fudouzi, “Soft imprint lithography of a bulk chalcogenide glass,” Opt. Mater. Express 1(5), 796–802 (2011). [CrossRef]
24. M. Solmaz, H. Park, C. K. Madsen, and X. Cheng, “Patterning chalcogenide glass by direct resist-free thermal nanoimprint,” J. Vac. Sci. Technol. B 26(2), 606–610 (2008). [CrossRef]
25. Z. Li, Y. Gu, L. Wang, H. Ge, W. Wu, Q. Xia, C. Yuan, Y. Chen, B. Cui, and R. S. Williams, “Hybrid nanoimprint-soft lithography with sub-15 nm resolution,” Nano Lett. 9(6), 2306–2310 (2009). [CrossRef] [PubMed]
26. Y. Xia and G. M. Whitesides, “Soft lithography,” Annu. Rev. Mater. Sci. 28(1), 153–184 (1998). [CrossRef]
27. Z. Yang, T. Luo, S. Jiang, J. Geng, and P. Lucas, “Single-mode low-loss optical fibers for long-wave infrared transmission,” Opt. Lett. 35(20), 3360–3362 (2010). [CrossRef] [PubMed]
28. C. Yi, P. Zhang, F. Chen, S. Dai, X. Wang, T. Xu, and Q. Nie, “Fabrication and characterization of Ge20Sb15S65 chalcogenide glass for photonic crystal fibers,” Appl. Phys. B 116(3), 653–658 (2014). [CrossRef]
29. X. Zhang, H. Ma, and J. Lucas, “Applications of chalcogenide glass bulks and fibres,” J. Optoelectron. Adv. Mater. 5(5), 1327–1333 (2003).
30. M. Yin, H. P. Li, S. H. Tang, and W. Ji, “Determination of nonlinear absorption and refraction by single Z-scanmethod,” Appl. Phys. B 70(4), 587–591 (2000). [CrossRef]
31. X. Liang, H. Tan, Z. Fu, and S. Y. Chou, “Air bubbleformation and dissolution in dispensing nanoimprint lithography,” Nanotechnology 18(2), 025303 (2007). [CrossRef]
32. Z. Y. Li and L. L. Lin, “Photonic band structures solved by a plane-wave-based transfer-matrix method,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 67(4), 046607 (2003). [CrossRef] [PubMed]
