An Omnidirectional Reflector and Microcavity Resonatorvia the Sol-Gel Method
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ABSTRACT Thin films of sol-gel Si0 2 and TiO 2 were used to fabricate two types of onedimensional photonic crystals: an omnidirectional reflector and microcavity resonator. The reflector consisted of six Si0 2/TiO 2 bilayers, designed with a stopband in the near infrared. Reflectance measurements over an incident angle range of 0-80' showed an omnidirectional band of 70 nm, which agrees with theoretical predictions for this materials system. The microcavity resonator consisted of a TiO 2 Fabry-Perot cavity sandwiched between two SiO2/TiO 2 mirrors of three bilayers each. We have fabricated a microcavity with resonance at Xcavity = 1500nm and achieved a quality factor of Q=35. We measured a modulation in the cavity resonance frequency with a change of defect layer thickness and incident angle of light. This work demonstrates the feasibility of fabricating photonic crystals via the sol-gel method. INTRODUCTION The fabrication techniques typically used for solid state microphotonic components are well-suited for deposition onto wafer-sized surface areas (-cm 2). Techniques such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and sputtering, because of their vacuum requirements, exhibit a parabolic fabrication cost function that scales with deposition surface area.
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When deposition onto large areas (-cm to mi ) is required, other
techniques that are better suited for macrophotonic applications may provide practical advantages. An example of such a technique is the well-known sol-gel technique'. The solgel technique is an inexpensive, simple wet-chemistry process that has been widely used in both micro- and macrophotonic realms to deposit a variety of oxide films onto various lb substrates. Examples of sol-gel applications include antireflection (AR) coatings , selective 2 3 4 filters , Er-doped optical waveguides , and Bragg grating waveguides . Sol-gel films are typically deposited by dip- or spin-coating, so that fabrication cost functions scale linearly with deposition surface area. Moreover, the sol-gel technique offers compositional flexibility over a range of oxides, piezoelectrics, and ferroelectrics. This variety of materials makes it possible to explore the use of composite sol-gel structures as photonic bandgap devices. As optical analogs to semiconductors, photonic crystals possess a periodic refractive index that can give rise to bands of forbidden photon propagation states in the material 5 . With appropriate design, the presence of a defect in the periodic structure can introduce localized photonic states within the photonic bandgap. Examples of one-dimensional photonic crystals are an omnidirectional reflector 6 and a waveguide microcavity 7. Interference filters and microcavities fabricated from solid-state materials have been well-studied and characterized. In this work, we wish to demonstrate the feasibility of using the sol-gel technique to fabricate a one-dimensional photonic crystal both with and without a defect-a microcavity 75 Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Ma
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