Phonic crystals based on chiral liquid crystal

1. Phonic crystals based on chiral liquid crystal Speaker : Huang Chi Chung Teacher :Ja-ho Lin M. Ozaki, Y. Matsuhisa, H. Yoshida, R. Ozaki, and A. Fujii phys. stat. sol. (a) 204, No. 11

2. Outline • Introduction • Twist defect mode in cholesteric liquid crystal • Chiral defect mode induced by partial deformation of helix • Summary

3. Introduction • Liquid crystals (LCs) including chiral molecule have a self-organized helical structure • It can be regard as a one dimensional periodic structure(PC) • In the twist defect mode of CLC • The localization of light caused by the imperfection in the periodic structure • The his applications such as • The low-threshold lasers • The microwaveguides [28.29] [28] T. Hattori, N. Tsurumachi, and H. Nakatsuka, J. Opt. Soc. Am. 14, 348 (1997). [29] R. Ozaki, T. Matsui, M. Ozaki, and K. Yoshino, Jpn. J. Appl. Phys. 41, L1482 (2002).

4. Introduction • The relative studies • Twist Defect in Chiral Photonic Structures[31] • The author have predicted the existence of localized state for single circular polarized light in the twist defect of CLC. • Liquid crystal (LC) • It have a large optical anisotropy • It are sensitive to an external stress (such as electric field). • The 1D PC(LC) does not a complete PBG , • There are plenty of application using extraordinary dispersion of photon • There are localize photonic state in defect layer [31] V. I. Kopp and A. Z. Genack, Phys. Rev. Lett. 89, 033901 (2002).

5. Twist defect mode in cholesteric liquid crystal • In the Fig. (a) , • Laser actions • It was observed at the edge wavelength of the stop band • It was associated with the group velocity anomaly at the photonic band edge. • On the other hand, • Low threshold laser action • It is based on the photon localization at a defect in a periodic structure

6. Figure (c) shows the transmission spectrum of the dye-doped double PCLC composite film It contains a discontinuous defect interface. Figure (d) shows the emission spectrum of the dye-doped double PCLC composite film with the defect interface at pump energy of 200 nJ/pulse. The laser action appears at 611nm It is within the band gap and coincide with the TDM wave.

7. Chiral defect mode induced by partial deformation of helix

8. Introduce chiral defect into the helix structure of CLC • A schematic explanation of the fabrication procedure is shown in Fig. (a). • The right-handed PCLC material doped with 1 wt% of DCM dye was aligned homogeneously in a cell with a gap of 6–7 μm. • In this system, the defect modes can be tuned by modulating the helix pitch in the defect layer upon changing temperature or irradiating with light [51, 52].

9. Berreman’s 4×4 matrix • Figure (b) shows the transmission spectra for right-handed circularly polarized light of the fabricated CLC defect structure. • A single defect mode is observed within the selective reflection band of the CLC. • Figure (c) shows the emission spectrum at high pumping energy of the CLC single-defect structure along with the corresponding transmission spectrum.

10. Summary • They originated from the band edge effect of the one-dimensional photonic band gap and the defect mode within the band gap. • We experimentally demonstrated the twist defect mode (TDM) in the 1D PBG of the CLC film having a twist defect • It was a discontinuity of the director rotation around the helix axis at an interface of two PCLC layers. • The laser action based on the TDM • It was also observed in the dye-doped PCLC composite film with the twist interface. • We also proposed a new type of defect mode • It was based on the chiral defect in which the partial modulation of the helix pitch was induced.