We describe the necessary steps towards the realization of ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals. These include the development of hollow air-core – dense-silica-shell core-shell monodisperse and spherical nanoparticles; introducing of a pattern of hydrophilic regions in a hydrophobic surrounding; and the combination of these two approaches by selective oxygen plasma etching of hollow core-shell nanospheres. The pattern imprinted by the difference in only surface property remains invisible in normal conditions of static environmental humidity. The hydrophilic regions in the patterns are reversible and immediately unveiled by dynamic humid flow. The specific properties of a human breath in terms of relative humidity and vapor flow are ideal for optimal revealing in terms of the spectral shift of the photonic bandgap of the colloidal crystal. The revealing of the pattern is determined by the surface tension of the vapor, while the color of the imprinted pattern is independently determined by its refractive index.
Colloidal photonic crystals are photonic crystals made by bottom-up physical chemistry strategies from monodisperse colloidal particles. The self-assembly process is automatically leading to inherently three-dimensional structures with their optical properties determined by the periodicity, induced by this ordering process, in the dielectric properties of the colloidal material. The best-known optical effect is the photonic band gap, the range of energies, or wavelengths, that is forbidden for photons to exist in the structure. This photonic band gap is similar to the electronic band gap of electronic semiconductor crystals. We have previously shown how with the proper photonic band gap engineering, we can insert allowed pass band defect modes and use the suppressing band gap in combination with the transmitting pass band to induce spectral narrowing of emission. We show now how with a high-quality narrow pass band in a broad stop band, it is possible to achieve photonic crystal lasing in self-assembled colloidal photonic crystals with a planar defect. In addition, with proper surface treatment in combination with patterning, we prepare for addressable integrated photonics. Finally, by incorporating a water in- and outlet, we can create optomicrofluidic structures on a photonic crystal allowing the optical probing of microreactors or micro-stopped-flow in the lab-on-an-optical-chip.
We demonstrate a facile method for fabrication of colloidal crystals containing a planar defect by using
PS@SiO2 core-shell spheres as building blocks. A monolayer of solid spheres was embedded in
core-shell colloidal crystals serving as the defect layer, which formed by means of self-assembly at the
air/water interface. Compared with previous methods, this fabrication method results in pronounced
passbands in the band gaps of the colloidal photonic crystal. The FWHM of the obtained passband is only
~16nm, which is narrower than the previously reported results. The influence of the defect layer
thickness on the optical properties of these sandwiched structures was also investigated. No high-cost
processes or specific equipment is needed in our approach. Inverse opals with planar defects can be
obtained via calcination of the PS cores, without the need of infiltration. The experimental results are in
good agreement with simulations performed using the FDTD method.