| Abstract |
Photonic crystals are optical materials with periodically distributed refractive indices. Their photonic bandgap effect on the flow of photons makes them appear colored like the wings of a Morpho butterfly. They have potential for use in nano lasers, photovoltaics, nonlinear optics, and other applications. Most of artificial photonic crystals are made by photolithography. For three-dimensional photonic crystals having their photonic bandgaps in the visible spectrum, the required precision and accuracy of fabrication are extremely high. Blue phases (BP) are a class of particular liquid crystal (LC) phases located between the isotropic and cholesteric phase. Such materials in their blue phases (BPI and BPII) organize spontaneously into three-dimensional cubic nanostructures. By controlling the chirality of the material or applying external fields, the color (bandgap) of a BPLC can be modulated. The conventional way of growing BP crystals is to cool the sample slowly and homogeneously from the isotropic state to the blue phase, resulting in polycrystalline texture. The diameter of each single crystal can range from 3 to 200 μm depending on the cooling rate. BP polycrystals often lack uniform lattice directions, sufficient number of periods, and grain boundaries cause optical scattering, therefore hindering their applications.
In this study, we created spatial temperature gradient by using two temperature-controlled stages. One stage was set at a temperature above the clearing point (the ISO stage) and the other at a temperature in the BP range (the BP stage). The BP crystal growth was conducted by moving the sample from the ISO stage to the BP stage. The single BP crystals thus grew along the moving direction. The resulting single crystals are considerably larger than those grown by the conventional approach. In this thesis, we discussed the parameters that affect the grain size, such as material, temperatures of stages, moving rate, and the addition of external fields. In the experiments, we examined the grain size and other properties of the BP crystals by means of microscopic images, Kössel diagrams, spectra and voltage-transmittance diagrams.
By adopting the proposed strategy for crystal growth, we have achieved millimeter-sized BP single crystals, enabling a wider range of applications and more in-depth fundamental research. |
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