Bottom-Up Fabrication of Photonic Defect Structures in Cholesteric Liquid Crystals Based on Laser-Assisted Modification of the Helix

摘要

Cholesteric liquid crystals (ChLCs) with chiral constituents have their so-called director, defining the averaged molecular orientation, assume a helical periodic structure along a particular helical axis, with a periodicity called the pitch. They thus form a twisted anisotropic medium with their dielectric tensor varying periodically along the helical axis, exhibiting a forbidden bandgap for photons only with the same circular polarization as the material itself. Because of this property, known as selective reflection, ChLCs are regarded as pseudo-photonic bandgap (PBG) materials, from which promising photonic devices can be realized. Recent interest has focused on the investigation and utilization of the photonic properties of such flexible, self-organizing PBG materials. One of the extensively studied topics is the gain enhancement observed at the band-edge of the selective reflection band, where the group velocity approaches zero. Tunable lasers utilizing the thermosensitivity of the ChLC pitch have been realized in low-molecular-weight ChLCs, while flexible free-standing dye lasers were proposed using polymeric ChLC (PChLC) materials. Another topic of particular interest is the investigation and utilization of photonic properties exhibited when a structural defect is introduced in the perfect lattice. It is known that photons localize at the structural defects introduced in PBG materials, leading to the realization of many applications, such as narrow-bandpass filters, low-threshold lasers, low-loss waveguides, and optical add and drop filters. This is also the case in ChLCs, and numerous theoretical and experimental studies have been made. A particularly interesting defect characteristic of ChLCs is the twist defect mode, which is a discontinuous phase shift introduced in the director of the ChLC. Experimental demonstration of a defect mode transmission peak and low-threshold lasing was reported by stacking two PChLC films with a certain twist angle. The technique to stack PChLCs has lead to the realization of other defect structures: for example, by introducing a nematic liquid crystal layer between two PChLC films, the optical diode effect or low-threshold lasing have been reported. The stacking technique, however, is a top-down method which limits the size of the structure to be fabricated, and only structures with defect layers of a few to several micrometers have been reported in combined PChLC structures. If the helical structure of the ChLCs could be manipulated at a smaller scale, sophisticated structures with controlled optical characteristics would be realized.