Characterisation of Photonic Colloidal Crystals in Real and Reciprocal Space: A Tale of Two Cities

Although not a literary masterpiece like Charles Dickens's 1859 historical novel, my doctoral thesis could be described as a 'Tale of Two Cities'. It deals with the fabrication and characterisation of photonic crystals. The latter are structures in which the refractive index varies periodically in space. They can be considered as the optical analogues of electronic semiconductors. Possible applications include, but are not limited to, infrared telecommunications and optical integrated circuits. A promising route towards the fabrication of photonic band-gap materials is by self-assembly of colloidal particles. We used two self-assembly techniques to grow crystals from dispersions of silica spheres: close-packed crystals were grown by vertical controlled drying and body-centred tetragonal crystals were fabricated by sedimentation in an external electric field. Nearly all of these crystals were grown in Utrecht (the Netherlands). There, they were also characterised in real space using optical, confocal and electron microscopy. Most of the reciprocal-space characterisation was performed using microradian X-ray diffraction at the ESRF in Grenoble (France). Combining data from real and reciprocal space, we have been able to probe the 3D structure of close-packed and non-close-packed crystals, even after infiltration with silicon. Furthermore, we showed that controlled-drying crystals are not perfect FCC crystals.