Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Harold Hopkins (physicist) wikipedia , lookup
Ultraviolet–visible spectroscopy wikipedia , lookup
Phase-contrast X-ray imaging wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Anti-reflective coating wikipedia , lookup
Silicon photonics wikipedia , lookup
Surface plasmon resonance microscopy wikipedia , lookup
Sol–gel process wikipedia , lookup
SCIENCE CHINA Physics, Mechanics & Astronomy • Research Paper • November 2011 Vol.54 No.11: 1967–1971 doi: 10.1007/s11433-011-4463-1 Novel sol-gel material for fabrication of a long period waveguide grating filter as a precise thermometer WANG Xin1, LI XiaoChan2, ZHANG Tao2, HU CanDong1, ZHU QiuXiang1, CHEN SiHai3, LI Yun1, XU Jia1, HE Miao1*, NIU QiaoLi1, ZHAO LingZhi1, LI ShuTi1 & ZHANG Yong1 1 Key Laboratory of Electroluminescent Devices of Guangdong Provincial Education Department, Institute of Optoelectronic Materials & Technology, South China Normal University, Guangzhou 510631, China; 2 College of Information Science & Engineering, Wuhan University of Science & Technology, Wuhan 430081, China; 3 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology, Wuhan 430074, China Received December 31, 2010; accepted January 25, 2011; published online August 15, 2011 A new kind of organic-inorganic hybrid HfO2/SiO2 sol-gel material with a large thermo-optic coefficient and a wide linear tunable temperature range has been developed for fabrication of a long period waveguide grating (LPWG) filter, whose parameters were optimized and designed by using finite difference time domain (FDTD) simulations. The LPWG filter, a periodic rectangle-corrugated grating structure, was easily fabricated with soft-lithography technique. At a temperature range from 19°C to 70°C, the fabricated LPWG filter element demonstrated a high temperature sensitivity of about 6.5 nm/°C and a wide linear tunable temperature range of 51°C, so that it can be used as a precise thermometer. Our results are useful for the designs of LPWG filters for the implementation of a wide range of thermo-optic functions. LPWG filter, soft-lithography, sol-gel material, temperature sensitivity PACS: 42.79.Gn, 81.20.Fw, 42.79.Dj, 42.82.Cr 1 Introduction Long-period grating (LPG) devices are useful in both optical communication and sensor fields. Much research efforts have been devoted to the study of long-period fiber gratings (LPFGs), which can transfer light energy from the guide mode to the selected cladding modes of the fiber at specific resonance wavelength. One of the most important applications of LPGs is temperature sensors [1]. But it is worth nothing that the resonance wavelength of an ordinary LPFG, written in a conventional communication fiber, shifts by only 3–10 nm for a temperature change of 100°C , which is not sufficient for many applications. Because of the geometry and material constraints of optical fibers, it is difficult to improve the tunability of LPFGs [2,3]. To relax the con*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2011 straints of fibers, LPGs formed in planar waveguides, referred to as long period waveguide gratings (LPWGs), have been proposed [4,5]. Compared with the LPFGs, LPWGs fabricated in planar waveguides have more advantages in the structure fabrication and material choice, and the design of the transmission spectrum is more flexible. There have been a number of experimental demonstrations on fabrications of the LPWGs in different materials, such as glass [6], lithium niobate [7], semiconductor [8], and polymer [4,9]. In addition, we have noticed that the sol-gel materials became more and more popular in waveguide material preparation, due to their low cost and simple processing steps. Especially, the thermo-optic coefficients of the sol-gel materials are in general much larger than that of the other optical materials, so LPWGs in sol-gel materials may show higher temperature sensitivity and a larger temperature tunable range. At present, the fabrication of LPWGs has been phys.scichina.com www.springerlink.com 1968 Wang X, et al. Sci China Phys Mech Astron mostly limited to the costly semiconductor fabrication techniques such as reactive ion etching (RIE) and ultraviolet (UV) writing before an alternative fabrication technique utilizing the UV-curable sol-gel material, called soft-stamp replica-molding lithography and shortened as soft-lithography, appeared [10]. In the soft-lithography technique, a mechanically flexible and elastomeric Poly (di-methylsiloxane) (PDMS) mold stamp was used to transfer the desired pattern because its relatively low surface energy results in weak adhesion to other materials [11]. Because of its convenience, effectiveness and low cost for the formation and manufacture of micro- and nano-structures, the soft-lithography technique gradually plays an important role in surface chemistry, materials science, optics, Micro-Electro-Mechanical Systems (MEMS) and microelectronics. In fact, soft-lithography techniques have been used to fabricate optical devices, such as distributed feedback structures [12], waveguides [13], micro lens arrays [14], and optical resonators [15]. In this paper, we have reported on the development of a new kind of organic-inorganic hybrid HfO2/SiO2 sol-gel material with a large thermo-optic coefficient and a wide linear tunable temperature range, with which a long-period grating has been fabricated on the surface of a planar waveguide core by using soft-lithography. 2 Analysis and design In order to realize a resonance-wavelength-tunable long period waveguide grating (LPWG) filter which can offer higher tuning temperature sensitivity and much wider tunable temperature range, we have designed a multi-layer structure as shown in Figure 1. Figure 1 shows the cross-section of a designed LPWG filter, which is in fact a periodic rectangle-corrugated planar waveguide. The grating structure of the filter is fabricated on the interface between the waveguide core and the upper cladding of the planar waveguide, and the waveguide core is made from a kind of organic-inorganic hybrid HfO2/SiO2 sol-gel material with a large thermo-optic coefficient. When a light beam is incident into and propagating along the z-direction in the core, the propagation of the light wave will include core modes and cladding modes according to the coupling mode theory. The LPWG can couple the core modes and the cladding modes, and the couplings among the cladding modes are so weak that they can be ignored. We can adjust the geometry, sizes and refractive indexes of the core and the claddings to allow only the fundamental core modes to exist in the core so that only the coupling between the fundamental core modes and the first cladding modes needs to be taken into account. Owing to the LPWG, the light energy coupled from the fundamental core modes to the first cladding modes can be greatly intensified and reach maximum when the phase-matching condition co 1cl 2 / is November (2011) Vol. 54 No. 11 Figure 1 The cross-section of a LPWG filter. fulfilled [16], where co and 1cl are respectively the propagation constant of the fundamental core modes and the first cladding modes, and is the grating period. On the phase matching condition, the transmission spectrum of a LPWG filter exhibits several resonance loss peaks, corresponding to different couplings. Thus the phase matching condition can be also called as resonance condition and more simply expressed as 0 ( N co N1cl ), where 0 is the response wavelength, N co and N1cl are respectively effective refractive indexes of the fundamental core modes and the first cladding modes, which vary with the temperature. Provided that the cladding is thick enough and the small thermal expansion of the grating is ignored [17], the temperament sensitivity of the resonance wavelength d 0 / dT can be derived and expressed as: d0 C co C cl 0co 1cl , dT 1 dN co dN1cl 1 d d , (1) where T is the temperature, C co and C cl are respectively the thermal-optic coefficients of the core and the cladding, and 0co and 1cl are the fraction powers of the guided mode and the first cladding mode in the core. According to eq. (1), the temperature sensitivity of the LPWG filter depends on the grating geometry, the modal dispersion parameter , and the thermal-optic coefficients of the core and cladding materials. In order to realize minimum transmission of a light wave propagating through a LPWG filter, the parameters of the waveguide and the LPWG need to be optimized and designed by using the FDTD simulations. The dependence of resonance wavelength on the grating period can be calculated and illustrated in Figure 2, where the five lines represent five cladding modes, and obviously the resonance wavelength is linear with the grating period. In addition, the parameters should be chosen so that the core mode was coupled to only the first cladding mode. With the FDTD Wang X, et al. Sci China Phys Mech Astron Figure 2 The resonance wavelength vs. the grating period. simulations, the optimized parameters were as follows. The thicknesses of the core, the upper cladding and the lower cladding are 1 m, 2 m and 2 m, respectively. The refractive indexes of the core, the upper cladding and the lower cladding were 1.502, 1.496 and 1.496, respectively. The depth, period, length and duty cycle of the LPWG were respectively 86 nm, 450 m, 20 mm and 0.5. 3 Fabrication The hybrid HfO2/SiO2 sol-gel material developed in this study was prepared by uniform hydrolysis and subsequent copolymerization of [3-(methacryloxy) propyl] trimethoxysilane (MAPTMS) and Hafnium (IV) in nitrogen atmosphere. Firstly, MAPTMS was hydrolyzed in isopropanol and deionized water in a molar ratio of 1:2:2 with 0.03 M hydrochloric acid as the catalyst, and stirred at 70°C for 30 min. Next, Hafnium (IV) (70 wt% solution in n-butanol) was hydrolyzed in methacrylic acid in a molar ratio of 1:2 with 0.02 M acetic acid as the catalyst, and stirred at 40°C for 40 min. Finally, these two solutions were mixed with a Si/Hf molar ratio of 3:2, and the polycondensation of the mixture was allowed to occur at room temperature with vigorous stirring for 12 h. At last, 4 wt% of photo initiator, a mixture of bis (2,4,6-trimethylbenzoyl)-phenylphosphine oxide and 1-hydroxycyclohexyl phenylketone in a molar ratio of 3:1, was added into the mixed sol solution to make the final HfO2/SiO2 sol UV-sensitive. Refractive index of the prepared sol-gel material can be adjusted by changing the Si/Hf molar ratio. The LPWG filter was fabricated by using soft-lithography technique including micro-contact printing and subsequent pattern transfer. Firstly, master structure with the desired grating pattern was patterned by conventional photolithography, and then the polydimethylsiloxane (PDMS) liquid was poured onto the top of the master to cover it and cured for 3 h at 50°C. After the thermal curing process, the PDMS material, transformed into a silicone elastomer, was November (2011) Vol. 54 No. 11 1969 easily peeled off the master as a negative mold against the master. Afterwards, a mechanically-flexible PDMS stamp was formed and peeled from the master substrate, and the elastomeric stamp having grating structure on its surface was then used to transfer the pattern onto the UV-curable hybrid HfO2/SiO2 sol-gel layer to form waveguide core, and the replication process is illustrated in Figure 3. Initially, a film in the prepared solgel material I was spincoated on the surface of a 1.5-m-thick SiO2 buffer layer of a silicon substrate to be the lower cladding of the waveguide. To improve the adhesion of the sol-gel film to silicon substrate, the sample was soft-baked on a hotplate at 80°C for 5 min to remove the excess solvent in the film, and then subjected to UV exposure for 5 min and heated on a hotplate at 150°C for 60 min. With a prism coupler (Metricon Corporation), the thickness and refractive index of the prepared solgel lower cladding layer were measured as 2 m and 1.496 at a wavelength of 1320 nm at room temperature. In a subsequent replication process, a film in the prepared sol-gel material II was spin-coated onto the lower cladding to be the waveguide core, and the thickness and refractive index of the prepared core layer were measured as 2 m and 1.502 at 1320 nm at room temperature. Then the PDMS stamp was placed and pressed on the sol-gel film so that the stamp was kept in contact with the surface of the core layer film. After the core layer was exposed upon UV radiation and solidified, the PDMS stamp was mechanically removed off so that the corrugated grating structure was printed on the surface of the core layer. Afterwards, the sample was developed in acetone for 15 s, and the unilluminated zones were washed away. Finally, a 2-m-thick film in the prepared sol-gel material I was spin-coated onto the grating surface as the upper cladding. After the sample was subjected UV radiation and baked at 170°C for 3 h, the desired LPWG filter was fabricated successfully. 4 Measurement results Figure 4 shows the profile image of the cross-section of the Figure 3 structure. Schematic diagram of the replication process of the LPWG 1970 Wang X, et al. Sci China Phys Mech Astron November (2011) Vol. 54 No. 11 5 Figure 4 100× optical image of the cross-section of the fabricated rectangle-corrugated LPWG. fabricated rectangle-corrugated LPWG taken by a chargecoupled device (CCD) microscopic camera (DP25, Olympus). It is noted that the fabricated element profile has strict periodicity and steep wall, and the period is about 450 m, in agreement with the design. In order to measure the transmission characteristics of the LPWG filter, a broadband infrared source was fed through a polarizer and input into the LPWG filter. The output light was collected through a 40× microscope objective, and the transmission spectra of LPWG filter for TE polarization at different temperatures were measured and recorded by using an optical spectrum analyzer (OSA). As a result, a series of transmission loss peaks with a wavelength range of 1260–1630 nm and a temperature range from 19°C to 70°C were observed from the transmission spectra. The temperature dependence of the resonance wavelength of LPWG filter for the TE polarization is shown in Figure 5. As shown in Figure 5, the resonance wavelength of LPWG filter shifted toward longer wavelength as the temperature increased. Because of the negative thermo-optic coefficient of the material of the core and cladding, the effective refractive index of the core and cladding decreased while the temperature increased, resulting in the shifts towards longer wavelengths as per resonance condition. We can conclude from the data in Figure 5 that the fabricated LPWG filter had a temperature sensitivity of 6.5 nm/°C, much larger than 0.8 nm/°C of tunable filters and 0.89 nm/°C of polymer LPWG [18]. In addition, the fabricated LPWG filter had a linear tunable temperature range of 51°C, wider than that of the past work of others. Thus, the fabricated LPWG filter can be used as a precise thermometer. In summary, we have successfully demonstrated the fabrication of a LPWG filter based on a new kind of UV-tunable sol-gel material using soft-lithography technique, and it had a high temperature sensitivity and wide linear tunable temperature range so that it can be used as a precise thermometer. In addition, the sol-gel waveguide elements can be easily integrated with other optical components on the same substrate to form new functional planar light wave circuits. It should be mentioned that the fabrication procedure of the LPWG filter is very simple and cost-effective so that it is suitable for manufacture. Furthermore, the novel hybrid organic-inorganic sol-gel materials have great applicability for mass-production of other micro-optical elements. This work was supported by the Natural Science Foundation of Guangdong Province, China (Grant Nos. 8251063101000007, 10151063101000009 and 9451063101002082), the Scientific & Technological Plan of Guangdong Province (Grant Nos. 2008B010200004, 2010B010600030 and 2009B011100003), the National Natural Science Foundation of China (Grant Nos. 61078046 and 10904042), the Key Project of Chinese Ministry of Education (Grant No. 210157), the Scientific & Technological Project of Education Department of Hubei Province (Grant No. D20101104), the Fundamental Research Funds for the Central Universities (Grant No. HUST 2010MS069), and Program for New Century Excellent Talents in University, China (Grant No. 07-0319). 1 2 3 4 5 6 7 8 9 10 11 Figure 5 Dependence of the wavelengths of LPWG transmission loss peaks on temperatures. Conclusions 12 O’Flaherty F J, Ghassemlooy Z, Mangat P S, et al. Temperature characterisation of long-period gratings for sensor applications. Microw Opt Technol Lett, 2004, 42: 402–405 Chiang K S, Lor K P, Liu Q, et al. Long-period waveguide gratings. Jpn J Appl Phys, 2004, 43: 5690–5696 Lor K P, Liu Q, Chiang K S. UV-written long-period gratings on polymer waveguides. IEEE Photon Technol Lett, 2005, 17: 594–596 Tsoi H C, Wong W H, Pun E Y B. Polymeric long-period waveguide gratings. IEEE Photon Technol Lett, 2003, 15: 721–723 Rastogi V, Chiang K S. Long-period gratings in planar optical waveguides. Appl Opt, 2002, 41: 6351–6355 Christophe M, Bertrand H, Laurent C, et al. Advanced spectral filtering functionalities in ion-exchanged waveguides with artificial cladding gratings. Opt Commun, 2004, 233(1–3): 97–106 Jin W, Chiang K S, Liu Q. Electro-optic long-period waveguide gratings in lithium niobate. Opt Express, 2008, 16: 20409–20417 Tabuchi H, Abe T, Terada K, et al. Long-period-grating-loaded semiconductor separate-confinement heterostructure waveguide for polarization-independent gain-equalizing device. J Appl Phys, 2005, 44(49): L1488–L1490 Kwon M S, Cho Y B, Shin S Y. Experimental demonstration of a long-period grating based on the sampling theorem. Appl Phys Lett, 2006, 88(22): 211103 He M, Yuan X C, Bu J, et al. Hybrid sample-inverted reflow and soft-lithography technique for fabrication of conicoid microlens array. Appl Opt, 2005, 44 (19): 4130–4135 He M, Bu J, Ong B H, et al. Two-microlens coupling scheme with revolved-hyperboloid solgel microlens arrays for high-power-efficiency optical coupling. J Lightwave Technol, 2006, 24 (7): 2940– 2945 Pisignano D, Anni M, Gigli G, et al. Flexible organic distributed Wang X, et al. 13 14 15 Sci China Phys Mech Astron feedback structures by soft lithography. Synth Met, 2003, 137: 1057– 1058 Horvath R, Lindvold L R, Larsen N B. Fabrication of all-polymer freestanding waveguides. J Micromech Microeng, 2003, 13: 419–424 Kunnavakkam M V, Houlihan F M, Schlax M, et al. Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process. Appl Phys Lett, 2003, 82: 1152–1154 Huang Y Y, Paloczi G, Scheuer J, et al. Soft lithography replication of polymeric microring optical resonators. Opt Express, 2003, 11: November (2011) Vol. 54 No. 11 16 17 18 1971 2452–2458 Vengsarkar A M, Pedrazzani J R, Judkins J B, et al. Long-period fiber-grating-based gain equalizer. Opt Lett, 1996, 21: 336–338 Liu Q, Chiang K S, Lor K P, et al. Temperature sensitivity of a longperiod waveguide grating in a channel waveguide. Appl Phys Lett, 2005, 86: 241115 Tang H Y, Wong W H, Pun E Y B. Long period polymer waveguide grating device with positive temperature sensitivity. Appl Phys B, 2004, 79: 95–98