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NanoForum CANADAJune 16 – 18, 2004 The goal of integrated optics is to integrate multiple optical functions on a single chip. Optical integration is considered.

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Presentation on theme: "NanoForum CANADAJune 16 – 18, 2004 The goal of integrated optics is to integrate multiple optical functions on a single chip. Optical integration is considered."— Presentation transcript:

1 NanoForum CANADAJune 16 – 18, 2004 The goal of integrated optics is to integrate multiple optical functions on a single chip. Optical integration is considered to be a key future requirement for advances in communication networks and computing. Optical integration has been hindered by the relatively large size of optical devices (modulators, filters, etc.), mainly arising from the large bending radius in traditional optical waveguides (to avoid excessive radiation loss). In recent developments [1],[2], mainly based on silicon-on-insulator (SOI) material system, highly confining photonic wire optical waveguides (with sub-micron cross- sectional dimensions) have been realized, aided by the high refractive index of the core material (silicon). In addition to the small dimensions, the ability to bend them tightly (with micron scale radius) with minimum loss holds promise for chip-scale optical integration. Sub-micron photonic wire optical waveguides in chalcogenide glass N. Ponnampalam *, R. G. DeCorby, P. K. Dwivedi, H. T. Nguyen, M. M. Pai, C. J. Haugen, and J. N. McMullin ECE Department, University of Alberta and TRLabs, Edmonton AB, Canada T6G 2V4 *pnakeera@trlabs.ca Abstract: We have fabricated high index contrast, sub-micron photonic wire waveguides with a chalcogenide glass (arsenic tri-selenide – As 2 Se 3 ) core embedded in polymer claddings. Waveguides were patterned directly in the chalcogenide layer without the use of organic photoresist steps and the wires were formed by selective etching. The waveguides were tested for light guiding at different wavelengths. The modal volume at 980 nm wavelength is approximately 1 µm 2. Waveguide losses are on the order of 10 dB/cm, which is comparable to losses reported in other high-contrast, small core waveguides. Introduction: Fabrication: Benzocyclobutene (BCB – n~1.5) polymer was used as undercladding material and was deposited on silicon wafer by spin casting. Approximately 1 µm As 2 Se 3 was thermally evaporated on cleaned substrates at room temperature. Waveguides were patterned directly on the chalcogenide thin films, by photodarkening with UV light under a standard mask aligner [9]. Photomodification resulted in etch selectivity between irradiated and non-irradiated regions. Unexposed region was chemically etched by mono ethanolamine (MEA) leaving strip channel waveguides of As 2 Se 3. The waveguides were approximately 165 nm thick at their center, with a circle segment cross-section and widths in the range of 800 – 1500 nm. Finally, Polyamide- imide (PAI – n~1.6) polymer [10] was spun cast and cured at 180 °C for one hour. These polymers (BCB and PAI) were selected from several candidate materials due to their compatibility with both silicon and As 2 Se 3. Characterization: Manually cleaved end-facets were generally of high quality, which enabled efficient power coupling from a high numerical aperture (NA) fiber or objective lenses. We did not observe any sign of pealing off or cracking of films, which confirms the compatibility of the selected materials. However, the optical wires had several visual defects along their length, which we attributed to the undercutting during the wet etching. As the absorption length of As 2 Se 3 is in the order of a few tens of nanometers for UV light, most of light is absorbed near the surface and the glass closer to the undercladding is not photomodified to the same extend. The propagation of light was tested with 980 nm, 1480 nm and 1530 nm wavelength light. Even with these defects, the optical wires exhibited strong guiding of TM polarized light. However, the wires did not show strong confinement to TE polarized light. Captured end facet mode images showed the mode field diameter at 980 nm is about 1.1 µm and at 1530 nm is about 6 µm. Due to the dimensions and shape of the wire structure, the modes at longer wavelengths should be nearer to cutoff, resulting in larger mode field diameter. Summary: We have demonstrated a simple, straightforward method of making sub-micron size waveguides in chalcogenide glasses. These glasses have good potential for microphotonic applications due to their flexibility in processing and high index offset. Significant optimization of these optical wires is possible, by tailoring the geometry to increase the light confinement at longer wavelengths, and eliminating processing defects. The loss values are already comparable to those reported for much- studied SOI material system, where losses are mainly attributed to sidewall roughness resulting from the patterning of organic photoresists and from dry etching processes. Fabrication process reported here does not require organic photoresist steps or dry etching, which might be an advantage in reducing the scattering loss. Acknowledgements: The Natural Science and Engineering Research Council of Canada, the Canadian Institute for Photonic Innovations, and TRLabs supported this work. We would like to thank George Braybrook for capturing excellent SEM images. The devices are fabricated at the Nanofab of the University of Alberta. References: [1] Lee, K.K., Lim, D.R., Kimerling, L.C., Shin, J., and Cerrina, F., Opt. Lett., 2001, 26, (23), pp. 1888-1890 [2] Vlasov, Y.A., and McNab, S.J., Opt. Express, 2004, 12, (8), pp. 1622-1631 [3] Nishihara, H., Haruna, M., and Suhara, T., Optical Integrated Circuits (McGraw-Hill, 1989) [4] Viens, J.F., Meneghini, C., Villeneuve, A., Galstian, T.V., Knystautas, E.J., Duguay, M.A., Richardson, K.A., and Cardinal, T., J. Lightwave Technol., 1999, 17, (7), pp. 1184-1191 [5] Lyubin, V., Klebanov, M., Feigel, A., and Sfez, B., Thin Solid Films, (Article in press) [6] Spalter, S., Hwang, H.Y., Zimmermann, J., Lenz, G., Katsufuji, T., Cheong, S.W., and Slusher, R.E., Opt. Lett., 2002, 27, (5), pp. 363-365 [7] van Popta, A.C., DeCorby, R.G., Haugen, C.J., Robinson, T., and McMullin, J.N., Opt. Express, 2002, 10, (15), pp. 639-644 [8] Robinson, T.G., DeCorby, R.G., McMullin, J.N., Haugen, C.J., Kasap, S.O., and Tonchev, D., Opt. Lett., 2003, 28, (6), 459-461 [9] Bryce, R.M., Nguyen, H.T., Nakeeran, P., DeCorby, R.G., Dwivedi, P.K., Haugen, C.J., McMullin, J.N., and Kasap, S.O., JVSTA, 2004, 22, (3), pp. 1044-1047 [10] Bryce, R.M., Nguyen, H.T., Nakeeran, P., Clement, T., Haugen, C.J., Tykwinski, R.R., DeCorby, R.G., and McMullin, J.N., Thin Solid Films, 2004, 458, (1-2), pp. 233-236 SOI has been the main material of interest in most of these works, with a view towards photonic integration with CMOS circuitry. Chalcogenide glasses are promising alternative materials for multi-function monolithic integrated optics circuits [3]. These glasses, Have low characteristic phonon energies  high transparency from the visible to mid-infrared region [4]. Have large photosensitivity  direct patterning of optical devices by light exposure. Exhibit selective etching after photomodification [5]  fabrication of optical waveguide structures without using organic photoresists or dry etching. Have high third order nonlinearities  promising for all-optical switching[6] Can be deposited at room temperature  might enable photonic integration with CMOS circuitry. Have high refractive indices (in the range of 2 to 4)  high confinement of light leading to single mode propagation in nanometer scale ‘optical wires’. In this work we have fabricated photonic wire waveguides using As 2 Se 3 chalcogenide glass and compatible polymers. As 2 Se 3 is a commercially available glass, exhibits large photo-induced index changes [7-9], and has the highest reported (amongst the chalcogenide glasses) third order nonlinearity at communication wavelength [6]. Losses were estimated by various methods in waveguide samples of 2.4 mm long, and can be summarized as:  Scattered light measurement at 980 nm  ~11 dB/cm  Scattered light measurement at 1480 nm  ~15 dB/cm  Fabry-Perot loss measurement at 1530 nm  ~15 dB/cm  Transmission loss at 1530 nm calculated from the following data  ~10 dB/cm  Total insertion loss with high NA fiber at both input and output side  ~7.4 dB  Total insertion loss with high NA fiber input and objective lens output  ~5 dB This is comparable to the loss values reported for similar waveguides in the SOI material system [2]. Material Waveguide size (height x width), nm Wavelength, nm Loss, dB/cm Ref. SOI220 x 46515003.5 All these values are taken from [2] SOI220 x 44515003.6 SOI270 x 47015505 SOI300 x 30015506 SOI 220 x 500 1550 2.4 220 x 4507.4 220 x 40033.8 SOI320 x 400155025 SOI200 x 500154032 As 2 Se 3 165 x 900153010This work PAI film Silicon BCB As 2 Se 3 thin film Mask UV light Silicon BCB As 2 Se 3 thin film (on BCB undercladding) was exposed to UV light for about 500 s, under a standard mask aligner. Exposed glass is photomodified, which increases the refractive index and the etch resistance to MEA. Due to high absorption at UV wavelength, the film nearer to the surface is photomodified to a greater extent than the region closer to BCB layer. Unexposed glass was etched by mono ethanol amine (MEA) Etching at about 40 °C for about 60 min resulted in circular segment cross section waveguides. PAI was spun cast as uppercladding and cured at 180 °C for one hour. This polymer layer improved the end facet quality upon dicing and protects the waveguides from damage due to handling and exposure to moisture. Wafers were manually cleaved into chips containing waveguides of different lengths. End facets are generally of high quality without the need for polishing. These chips were used in light propagation experiments. Scanning electron microscope (SEM) pictures ((c) back scattered image) of waveguides showing the end facet quality and the shape of the realized optical wire waveguides. Scale bar length is given in larger font for clarity. (a) Microscopic image (100X) and (b) SEM image (back scattered) of defects in the waveguides. On average there was about one defect per 400 µm waveguide length. We attribute these defects to the undercutting of the chalcogenide glass during etching. 10 µm (a) 100 nm (b) 1 µm (c) (a) (b) Coupling station used for the characterization of the optical wire waveguides. The station has 5 degrees of freedom on either side to precisely align the input and output coupling of light. Polarization controller was used to control the polarization of the input light. (a) (c) (b)(d) End facet mode images were captured using an objective lens and CCD camera (placed behind the objective) at (a) 980 nm and (c) 1530 nm. The modes were near circular in shape. From the intensity plots ((b) & (d))mode field diameter was estimated to be 1.1 µm for 980 nm and about 6 µm for 1530 nm. Fiber at the input Test chip Objective lens at the output Polarization controller CCD Camera (a) Scattered light image at 980 nm captured by the CCD camera placed above the waveguide and (b) the intensity variation with length plot. From the plot, the loss was estimated as about 11 dB/cm. Transmission loss at 1480 nm was estimated to be 15 dB/cm from scattered light. (a) (b)


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