Characterization of photonic colloidal crystals in real and reciprocal space

Research output: ThesisDoctoral Thesis

Abstract

In this thesis, we present experimental work on the characterization of
photonic colloidal crystals in real and reciprocal space. Photonic
crystals are structures in which the refractive index varies
periodically in space on the length scale of the wavelength of light.
Self-assembly of colloidal particles is a promising route towards
three-dimensional (3-D) photonic crystals. However, fabrication of
photonic band-gap materials remains challenging, so calculations that
predict their optical properties are indispensable. Our photonic
band-structure calculations on binary Laves phases have led to a
proposed route towards photonic colloidal crystals with a band gap in
the visible region. Furthermore, contrary to results in literature, we
found that there is no photonic band gap for inverse BCT crystals.
Finally, optical spectra of colloidal crystals were analyzed using
band-structure calculations. Self-assembled photonic crystals are
fabricated in multiple steps. Each of these steps can significantly
affect the 3-D structure of the resulting crystal. X-rays are an
excellent probe of the internal structure of photonic crystals, even if
the refractive-index contrast is large. In Chapter 3, we demonstrate
that an angular resolution of 0.002 mrad is achievable at a
third-generation synchrotron using compound refractive optics. As a
result, the position and the width of Bragg reflections in 2D
diffraction patterns can be resolved, even for lattice spacings larger
than a micrometer (corresponding to approximately 0.1 mrad). X-ray
diffraction patterns and electron-microscopy images are used in Chapter
4 to determine the orientation of hexagonal layers in
convective-assembly colloidal crystals. Quantitative analysis revealed
that, in our samples, the layers were not exactly hexagonal and the
stacking sequence was that of face-centered cubic (FCC) crystals,
though stacking faults may have been present. In Chapter 5, binary
colloidal crystals of organic spheres (polystyrene, PMMA) and/or
inorganic spheres (silica) are introduced as promising templates for
strongly photonic crystals. To prevent melting of the template, we used
atomic layer deposition (ALD) to infiltrate polystyrene and PMMA
templates with alumina, after which chemical vapor deposition (CVD) was
used to further enhance the refractive-index contrast. Binary colloidal
crystals of silica spheres can be infiltrated by CVD directly, but they
often have a layer of colloidal fluid on top. Preliminary etching
experiments demonstrated that it may be possible to etch silica
templates with plasmas or with adhesive tape. As described in Chapter
6, sedimentation of colloidal silica spheres in an external,
high-frequency electric field lead to mm-scale BCT crystals with up to
25 layers. In addition, electric fields were used as an external
control to switch between BCT and close-packed (CP) crystal structures
within seconds. We also developed two procedures to invert BCT crystals
without loss of structure - colloidal particles were immobilized by
diffusion-polymerization or photo-induced polymerization of the
surrounding solvent. Some BCT crystals were even infiltrated with
silicon using CVD. We demonstrate in Chapter 7 that X-ray diffraction
can be used to determine the 3-D structure of such photonic colloidal
crystals at the various stages of their fabrication. Excellent
agreement was found with confocal and electron-microscopy images.
Original languageEnglish
QualificationPh.D.
Awarding Institution
  • Utrecht University, Department of Physics and Astronomy
Supervisors/Advisors
  • Van Blaaderen, Alfons, Supervisor, External person
Award date21 May 2007
Print ISBNs978-90-393-4527-6
Publication statusPublished - 2007

Keywords

  • photonic crystal
  • colloid
  • X-ray diffraction
  • confocal microscopy
  • electric field
  • convective assembly
  • photonic band gap
  • optical spectroscopy
  • chemical vapor deposition
  • electron microscopy

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