Low Refractive Index Contrast Research Articles

Blue phases of cholesteric liquid crystals as thermotropic photonic crystals

The study of dye-doped low pitch cholesteric liquid crystals in their blue phases as an example of tunable "weak" photonic crystals is proposed and demonstrated. The presence of the blue phases in cholesterics can be tuned with temperature, and this allows for an easy in situ comparison of the emission and/or absorption of the dyes with or without an enwrapping lattice of disclination lines. The fluorescence emission of the dyes is shown to be affected by the presence of the blue phases. Although unlikely to be suitable for real applications (due to the natural low refractive index contrast), these systems may represent unique examples of tunable photonic crystals. It is proposed that single crystals of dye-doped blue phases should provide a very interesting testing ground for the study of optical emission anisotropies which can, on the other hand, be controlled by an external parameter.

Photonic band-structure effects in the visible and near ultraviolet observed in solid-state dielectric arrays

Two-dimensional photonic band-structure effects have been observed in the visible and near-uv regions, between 350 and 800 nm. These effects appear in all-solid-state arrays of glass cylinders in a glass matrix. The arrays have center-to-center nearest-neighbor separations between 0.188 and 0.278 μm, perfect long-range order, and low refractive index contrast. For light propagating perpendicular to the cylinder axes, very narrow attenuations appear in the transmission spectra as a result of the photonic band structure. The positions of these attenuations are determined by the periodicity, composition, and symmetry of the arrays.

Device equivalence in integrated optics

The concept of device equivalence is introduced. In equivalent devices, the light propagation can be described by identically evolving modal expansions, resulting in identical power transfer ratios. By first applying this concept to a z-invariant structure with a low refractive-index contrast it is shown how a normalized coordinate space can be defined in which equivalent structures have exactly the same geometry. Subsequently it is shown how this normalized coordinate space can be defined for z-variant integrated optical devices, again provided that the lateral refractive-index contrast is small. This normalization makes it possible to perform numerical device simulations in normalized coordinate space, the results being applicable to a large set of equivalent devices. Furthermore, starting from a known design, it simplifies redesigning that device for use at another wavelength or using other materials significantly, the resulting device being equivalent to the original one. >