Laser actions in nano-helical-structured liquid crystals

Optically pumped laser action has been observed at the edge wavelength of the stop band in a dye-doped flexible freestanding film of photopolymerized cholesteric liquid crystal (PCLC), which originates from band edge effect of the one-dimensional photonic band gap. On the other hand, defect mode laser action has also been experimentally demonstrated at the defect mode wavelength within the band gap. This laser action is based on the photon localization at the twist defect of the composite film consisting of two PCLC layers. Twist defect mode (TDM) is induced by the introduction of twist defect which is a discontinuity of the director rotation around the helix axis at an interface of two PCLC layers. We also propose a new type of tunable defect mode based on the chiral defect in which the partial modulation of the helix pitch is introduced.