Nanospheres which all equal in size will be self-organized from a suspension into orderly, hexagonally packed crystalline lattices in two or three dimensions, called colloidal crystals (see Figure 1). These lattices can show unusual optical insulating behavior against light wave in certain range of wavelengths (Photonic Band Gaps, PBG) like semiconductors in electronic microcircuit, which are called 'photonic bandgap materials.' Such behavior of photonic bandgap materials arises from cooperation of periodic scatterers in every lattice point (multiple light scattering).
According to theories, optical properties of such photonic crystals could be significantly influenced by characteristics of each scatterer including size, shape and refractive index distribution. Due to practical limitation thus far, monodisperse nanospheres of isotropic scattering points have been used for demonstration of colloidal opal structures or their inverse structures as three dimensional photonic crystals (see Figure 2). However, it is still challenging to create practically useful structures for photonic crystals application in spite of a few successful experimental fabrication of 3D photonic crystals of spherical building blocks. This is because such dielectric crystals of spherical lattices do not exhibit a complete but just pseudo PBG behavior and their inverse structure have also shown narrow photonic band gaps, which may be practically meaningless for highly integrated photonic chips. Therefore, it is necessary to produce more complex building blocks and novel fabrication route for their assembly, which is promising for photonic crystals of practical significance.
During last a few years, we have conducted research on the fabrication of such novel building blocks and their assemblies and developed a number of clever approaches including soft-microfluidics, electrospraying or aerosol generation, micropipette injection, etc. (Figure 3,4)