Journal of Nonlinear Optical Physics & Materials Vol. 11, No. 3 (2002) 261–274 c  World Scientific Publishing Company ALTERNATIVE EXPERIMENTAL DETERMINATION OF WEAK LOCALIZATION OF LIGHT IN NANOSTRUCTURED MATERIALS KOEN CLAYS, KURT WOSTYN, YUXIA ZHAO and ANDR ´ E PERSOONS Centre for Research on Molecular Electronics and Photonics, Department of Chemistry, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Received 11 July 2002 An alternative experimental technique for the determination of weak localization of light in partially ordered nanostructured materials is proposed. The technique is based on the criterion for weak localization of light that the transport mean free path length of multiply scattered photons is reduced down to shorter than the wavelength of the light. This mean free path is calculated from the experimental dwell time of the photons in the scattering structure and by applying the photon random walk model using the diffusion approximation. The dwell time is experimentally determined by multifrequency phase- fluorimetry. This technique is capable of providing corroborative intensity demodulation data that can be linked to the wavelength dependent transmission (optical bandgap) of colloidal crystals. Keywords : Localization; order; nanostructure; scattering; photonic crystal (PC), photonic bandgap (PBG). 1. Introduction Nanostructuring of materials is seen as an important technique to impart new functionality to existing materials, or to extend existing capabilities to smaller structures. Examples of new functionality include quantum size effects in colloids, where new optical and electronic properties are induced by the quantum con- finement in small structures. 1 Examples of miniaturizing existing techniques are micromachining 2 and micro(opto)electromechanical systems. 3 Two fundamentally different approaches towards nanostructuring are now being used. The “top-down” approach starts from nonstructured bulk material. Structure can then be imparted by milling or drilling, basically well-known macro techniques but extended with micron and sub-micron tools; by chemical vapor deposition; or by lithographic or etching techniques. The advantage of all these techniques is that they are mature and readily available. The disadvantage is often the cost and time effort and the increased difficulty with decreasing the size, apart from the intrinsic difficulty to produce three-dimensional structure. 4 The “bottom-up” approach starts from 261