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Planar high index-contrast photonic crystals for telecom applications

: März, R.; Burger, S.; Golka, S.; Forchel, A.; Hermann, C.; Jamois, C.; Michaelis, D.; Wandel, K.


Busch, K.:
Photonic crystals. Advances in design, fabrication, and characterization
Weinheim: Wiley-VCH, 2004
ISBN: 3-527-40432-5
Book Article
Fraunhofer IOF ()
high contrast photonic crystal; technology; modelling; superprism; dispersion compensation

The pioneering work on photonic crystals (PhCs) dates back more than 20 years when R. Zengerle [1], S. John [2] and E. Yablonovitch [3] reported theoretical results an dispersion, light localization and modification of spontaneous emission in media with periodic refractive index patterns in two and three dimensions. A long period of great progress in modeling, patterning, etching, and characterization was required to demonstrate photonic crystals of sufficient quality, showing fundamental gaps in the visible and near infrared region. Parallel to the fundamental research, a spectrum of proposed applications offering new and attractive features of photonic crystals, has been and is still enlarging. It includes microwave antennas, photonic crystal fibers, cosmetic and food colors, car lacquers, various types of sensors, high-brightness LEDs, as well as active and passive components for telecommunications. The assessment of their feasibility is currently the subject of numerous research projects throughout the world. For telecommunications, photonic crystals other the following attractive features which mainly utilize two medium-term perspectives, compactness and photonic mode engineering: ·strong interaction between light and matter allowing for on-chip resonators of high finesse, and optical components/circuits being orders of magnitude smaller than currently fabricated devices. ·reduction of the group velocity by orders of magnitude, permitting the optimization of the chromatic dispersion and its slope. ·enhancement of tunability by concentrating the optical field in regions infiltrated with the tuning agents (especially for high index-contrast photonic crystals). ·reduced number of fabrication steps (e.g., mask layers and epitaxial steps) easing the integration of active and passive photonic functions (e.g., layers, waveguides, filters, switches ) an a single chip on the basis of simplified processing. This chapter describes ongoing work within the "HIPhoCs" consortium which tackles telecom-oriented components based an high-contrast Ph Cs with refractive index steps An, > 2 between air holes and dielectric material. Photonic crystals of this type exhibit very large photonic bandgaps for both polarizations. The material systems under investigation include the two group III -V compounds InGaAsP/InP and InGaAs/GaAs, allowing for both active and passive components as well as the silicon-on-insulator (SOI) material system which is well suited for the realization of tunable passive components. Photonic crystals with an average refractive index n, > 2 operating at 1.55 µm require elementary cells with minimum feature sizes in the sub-100 nm range. The in-plane diameters of the eigenmodes of defect waveguides with one missing row (W1) are also several hundred nanometers. The vertical index contrasts Onv between the refractive index step core and cladding layers, differ from one high-contrast material system to another. The PhC-components based an the III-V compounds exhibit a moderate vertical index contrast (Onv < 0.5) in order to be compatible with both optical and optoelectronic components, whereas the SOI material system provides high refractive index contrast (Onv 2) between silica and silicon. Consequently, photonic crystals based on SOI discussed in the following can be (and are always) operated below the light-line, whereas the III-V PhCs have to be operated above, where the propagation losses are much more sensitive to the shape and depth of the photonic crystal structures. The challenging specifications for aspect ratio and surface quality require the development of novel etching technologies and equipment for all material systems. We selected a set of active and passive components presented in the following such that the feasibility of the key features from the telecommunication point of view compactness, integrability, dispersion management, and tunability can be assessed. Special focus is also given to fiber-to-chip coupling which is of particular importance for applications in telecommunications. The organization of this chapter reflects the above discussion. We start with an investigation of theoretical aspects including the light-cone problem and from that derive theoretical losses for both types of material systems. In addition, new modeling tools allowing for tolerancing of PhC-components are presented. Great emphasis will be given to crucial patterning steps such as lithography and etching along with the required equipment. The second part deals with the concepts of some irnportant components as well as component-oriented characterization techniques. The chapter ends with a discussion of fiber-chip coupling of photonic crystal components and chips, and the basic components required for it.