Intrinsic and Extrinsic Defects in Colloidal Photonic Crystal Films

E.W. Vekris, D.D. Perovic, G.A. Ozin, S. Aitchison
Department of Materials Science and Engineering, University of Toronto, Toronto, Canada

Since the proposal of photonic crystal theory in 1987, a large experimental effort has been made to design and fabricate dielectric materials that are structurally periodic on the scale of optical wavelengths. One promising category of photonic crystals are self-assembled colloidal crystal films, which can be grown in a facile and inexpensive manner relative to other forms of photonic crystal. This material, which consists of a close-packed lattice of colloidal spheres, can serve as a template for the infiltration of a material of higher refractive index, such as silicon, for the formation of a material possessing a complete photonic band gap (PBG).
However, the self-assembled nature of colloidal crystal films creates the opportunity for the formation of intrinsic defects. Such defects, disruptions in the periodicity of the colloidal lattice, can be expected to have an influence on the optical properties of this photonic material.
In this work, various intrinsically-formed defects are structurally analyzed by various microscopy techniques. Similarities are drawn between defects in both colloidal and atomic lattices. The optical properties of lattice defects are then individually measured through careful spatially resolved micro-optical spectroscopy. Point, line, and volume defects are found to contribute to incoherent scattering, and the measured optical spectra are shown to be in good agreement with finite-difference time-domain simulations. In all cases, the influence of such defects on the fundamental stop-band is negligible, while influences on the “higher-band” region of the band structure and the PBG are shown to be significant.
Stacking faults, another form of intrinsic defect consisting of variations in the stacking of close packed planes, are shown to occur readily in colloidal crystal films. Through structural and spectroscopic analysis, it is shown that different stacking sequences are associated with a unique optical spectrum. These spectra differ in the higher-band region of the band structure, while leaving the fundamental stopgap unaffected. Additionally, stacking faults are shown to be caused by the incorporation of differently-sized impurity spheres in the lattice, a hard-sphere type mechanism analogous to atomic systems.
Defects can also be deliberately introduced to the colloidal lattice, and through proper design and fabrication can bring desirable photonic functionality to an otherwise bulk, periodic lattice. A fabrication method combining top-down photolithographic patterning with bottom-up self-assembly for the insertion of embedded linear extrinsic defects into the colloidal lattice is presented.
For the first time, a study of individual lattice defects has been successfully carried out in colloidal crystal photonic crystal films. By better understanding of the structure, optical properties, and formation mechanisms of colloidal crystal defects, the quality of self-assembled photonic crystal films can be improved, resulting in more homogeneous optical properties and allowing the full potential of colloidal photonic crystals to be realized.