Waveguides and gratings fabrication in zirconium-based organic/inorganic hybrids
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- Vicente, C.M.S., Pecoraro, E., Ferreira, R.A.S. et al. J Sol-Gel Sci Technol (2008) 48: 80. doi:10.1007/s10971-008-1775-3
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Sol–gel derived poly(oxyethylene)/siloxane organic/inorganic di-ureasil hybrids containing different amounts of methacrylic acid (McOH, CH2=C(CH3)COOH)) modified zirconium oxo-clusters (Zr-OMc) were processed as thin films deposited in glassy substrates via spin coating and as transparent and shape controlled monoliths. Channel monomode waveguides and diffraction gratings were UV patterned using the Talbot interferometer and the Lloyd mirror interferometer experimental setups. The time dependence of the diffraction gratings efficiency was studied for hybrids containing different amounts of Zr-OMc. Finally, the number of propagating modes and the refractive index gradient within the waveguide region, determined as a Gaussian section located below the patterned channel, was evaluated and modeled, a maximum index contrast of 2.43 × 10−5 being estimated.
KeywordsOrganic/inorganic hybridsIntegrated opticsSol–gelWaveguidesZirconium oxo-clusters
During the last years, the fabrication of integrated optics devices using hybrid materials has received an increasing amount of attention [1–5]. The advantages of the sol–gel process, such as the low-temperature processing and shaping, high sample homogeneity and purity, the possibility of mixing the inorganic and organic precursor components at the nanometer scale, and the availability and low cost of numerous metallo-organic precursors makes this method very suitable for the development of organic/inorganic hybrid materials for the production of functional integrated optics devices [6, 7]. Among the various organic/inorganic hosts that have been developed in the last years, those containing amine functionalities, namely urea cross-linked hybrids classed as di-ureasils [8–11], present acceptable transparency, mechanical flexibility and thermal stability to be processed as thin films [12–14]. The combination of di-ureasils with metal oxide precursors, specifically methacrylic acid (McOH, CH2=C(CH3)COOH)) modified zirconium tetrapropoxide, Zr(OPrn)4, is recursively used for the fabrication of integrated optics devices. Recently, the local structure of these di-ureasil organic/inorganic hybrids was discussed in terms of the Zr-OMc content . For low Zr-OMc content the Si- and Zr-based networks are inter-constrained, since the Zr-based clusters are embedded in the polymeric phase between the siliceous domains, whereas segregation of the individual components at the 0.1 μm scale occurs for high contents . The presence of increasing size Zr-rich aggregates (∼200–450 nm) as the Zr-OMc relative content in the hybrids increases was detected . It was also demonstrated that the chemical process that takes place under UV illumination is the polymerization of the methacrylate groups of the Zr-OMc aggregates .
Di-ureasil hybrids have already been used as integrated optics substrates, namely in the production of patternable gratings, channels and monomode planar waveguides with low propagation losses (<0.3 dB/cm), presenting good thermal stability [12, 13, 15]. Distributed feedback lasers (DFB) by using dynamic gratings have been demonstrated for di-ureasil thin films incorporating rodhamine 6G . Furthermore, Fabry-Perot cavities were also obtained by UV writing without the need of photoinitiators revealing a reflection coeficient of 0.042 with a free spectral range (FSR) value of 35.6 GHz which makes it adequate to be used in optical clock recovery in optical signals with bit rate of 35.6 Gbit/s .
In the context of the development of innovative integrated optics devices, such as low-cost optical power splitters for the general use spreading of all optical access networks a window of opportunities is open for sol–gel derived organic/inorganic hybrids. Examples of applications encompass narrow band optical filters (used as demultiplexers to access the desirable wavelengths in a multi-wavelength system), low losses optical power splitters, and optical cavities (for the optical clock extraction function).
In this work, we use di-ureasil-zirconium oxo-clusters organic/inorganic hybrids processed as thin films and transparent monoliths as integrated optics substrates. UV patterned channel monomode waveguides and diffraction gratings are obtained without the need of photoinitiators. In particular, diffraction gratings have been written inside and outside the channel waveguides and the refractive index contrast will be compared. The diffraction gratings efficiency was studied as function of time and Zr-OMc concentration. The mode field distribution within the waveguide region, previously set as a Gaussian section located below the patterned channel , is addressed in greater detail and the refractive index contrast modeled.
2.1 Materials synthesis and processing
2.1.1 Di-ureasil precursor (d-UPTES(600))
2.1.2 McOH-modified Zr(OPrn)4 precursor
The precursor used in the preparation of the hybrid materials, named hereafter as Zr-OMc, together with d-UPTES(600), was obtained by mixing Zr(OPrn)4 (Fluka) and McOH at room temperature with a molar ratio Zr(OPrn)4:McOH=1:1 in butanol (CH3(CH2)3OH, BuOH) and stirred for 3 h .
2.1.3 Di-ureasil-Zr-OMc hybrids
The Zr-OMc and d-UPTES(600) precursors were pre-hydrolyzed with a hydrochloric acid solution (HCl 0.01 mol L−1), with a water-to-metal molar ratio of 0.5:4. The pre-hydrolyzed solutions were mixed with different Zr relative molar percentage ranging from 5% to 75%. The synthesis details were given elsewhere [12, 15].
The di-ureasil-Zr-OMc hybrids is henceforth identified as d-UZ(X mol% Zr) and were processed both as transparent and with shape control monoliths and as thin films on glass substrates using the spin-coating technique.
2.1.4 Fabrication of monomode waveguides and gratings
The channel waveguide writing was performed in the monoliths through the exposure to a c.w. Ar-ion laser, frequency doubled (244 nm) by a BBO crystal, operating at 40 mW. The laser beam was shaped by an iris in order to remove spatial noise and by a plano-cylindrical lens to create a narrow laser line with 2 mm length. Then, the laser line was translated by a motorized positioning system in order to create the channel waveguide. The speed of the translation and the exposure time were varied between 0.02–0.08 mm/s and 2–18 min, respectively.
Diffraction gratings were recorded under different methods and laser radiations. One method is based on a modified Talbot interferometer with a phase mask acting as beam splitter, using the laser source described above for the waveguide patterning . This method was used for the d-UZ(23) monolithic hybrids.
2.2.1 Atomic force microscopy (AFM)
The images were obtained using a AFM Nanoscope Instruments equipment, in tapping mode, with a super sharp silicon probe having a radius of 10 nm, resonance frequency 330 kHz and spring constant 42 N/m. The images were deconvoluted considering the probe’s shape using the software WSXM® . In order to improve the images quality, flattening and elimination of line noise tools and a Gaussian filter were used. The same tip was employed in all the images recording to avoid the influence of the tip radius variations in the square roughness values.
2.2.2 Mode field distribution
The modal field distribution was acquired using a positioning system and a Laser Beam Profiler from Newport LPB-1 with microscope objective. The measurements were made with two lasers operating at 632 nm and 980 nm. The light was coupled into the waveguide region previously patterned, using an optical fiber and a xyz positioning system (Thorlabs Namomax) in order to excite a propagation mode. The propagating region limits were set when the output light tends to zero. The output mode profile was measured with a beam profile analyzer using an integration time of 10 s and the xy position experimental error is 0.15 μm.
2.2.3 Refractive spectrum
The refractive grating spectrum of the monolithic sample was obtained with an Amonics ALS-CL-17optical broadband source covering the spectral region between 1,530 nm and 1,620 nm. The signal is coupled to the waveguide using a standard monomode optical fiber. In order to minimize de optical losses in the coupling, an index matching gel (Thorlabs) was used. An optical circulator monitored the reflected spectrum, measured with an optical spectrum analyzer (Advantest Q8384).
3 Results and discussion
Channel monomode waveguides were UV written in homogeneous transparent poly(oxyethylene)/siloxane di-ureasils containing McOH modified Zr(OMc) processed as monoliths and thin films. The Talbot interferometer and the Lloyd mirror interferometer experimental setups were successfully used to write difraction gratings, either in monolithic samples surfaces or thin films deposited on glass substrates. The maximum diffraction efficiency was observed to be independently of the Zr-OMc relative content. On the other hand, the higher is the Zr-OMc content the faster is the patterning. The patterned samples were also characterized by AFM. Very low roughness values below 1 nm were evaluated. From mode field distribution experiments the guidance section was determined as a Gaussian region placed below the patterned channel, with a typical width of 280 μm and a maximum refractive index contrast of 2.43 × 10−5 in close agreement with previously reported experimental data. The dimensions of the central waveguide region with higher refractive index contrast and monomode propagation were estimated as 38.8 μm × 63.8 μm, through the analysis of the variation of the refractive index values along the vertical and horizontal directions.
The support of NoE “Functionalised Advanced Materials Engineering of Hybrids and Ceramics” (FAME) is gratefully acknowledged. This work was also supported by Fundação para a Ciência e Tecnologia (Portuguese agency), FEDER and POCI programs under contract POCI/CTM/59075/2004, PTDC/CTM/72093/2006, FAPESP and CNPq (Brazilian agencies) and CAPES-GRICES Brazil–Portugal cooperation program, contract BEX 2866/05-6. The authors thank the help of Daniela C. Oliveira (LNLS), Máximo S. Li (USP) and Andreia G. Macedo (University of Aveiro) for some synthesis, diffraction efficiency measurements and AFM analysis, respectively.