Photonic Crystals from Emulsion Templates**

The emergence of the field of photonic crystals has led to a surge of research on the fabrication of macroporous materials. Calculations show that a complete optical photonic bandgap may be observed if such materials have a sufficiently high index of refraction and contain ordered pores with periodicity on the order of the wavelength of visible light. With a complete gap there is a range of frequencies for which electromagnetic waves cannot propagate through the material. This property leads to a plethora of possible applications in optical devices. The challenge of producing a three-dimensionally ordered material with lattice constants on the order of micrometers or below has led several groups to monodisperse colloids, which are easily produced at the requisite length scales and selfassemble into the face-centered-cubic (fcc) crystal structure under certain conditions. Such aartificial opalso, as they are known, can be infiltrated with a second material and transformed to an ordered macroporous material, or air-sphere crystal, after the removal of the opal template. The resulting ainverse opalo structure has certain optical advantages over the original template. Commonly used opal templates include colloidal crystals of silica spheres, which can be removed by dissolution in hydrofluoric acid, or polystyrene spheres, which can be removed by calcination. In photonic crystals designed for the visible region of the spectrum the infiltrated phase is typically titanium dioxide (titania), introduced as either a liquid sol or a particulate suspension. For an fcc lattice of pores the index of refraction of the solid phase must be approximately 2.9 for the material to have a complete bandgap. Titania is one of the few non-absorbing dielectrics suitable for visible wavelengths; however, only one crystalline phaseÐthe rutile phaseÐhas such a high index. Unfortunately there are processing difficulties with retaining the pore structure of the material while converting the matrix phase to the equilibrium rutile phase from the kinetically favored (but lower index) anatase phase. Here we discuss a fabrication scheme, emulsion templating, that employs monodisperse liquid droplets instead of solid particles as the colloidal template and that can be used to produce porous rutile titanium dioxide. We discuss the advantages and disadvantages of the approach, the properties of the materials we have produced, and the prospects for building photonic bandgap materials by emulsion templating. Although the method may be used to produce a variety of inorganic and organic porous materials, we discuss only the preparation of porous titania.