Berlin 2005 – wissenschaftliches Programm
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CPP: Chemische Physik und Polymerphysik
CPP 8: SYMPOSIUM: Polymer networks and beyond: From molecular structure to materials and biological functions III
CPP 8.3: Vortrag
Samstag, 5. März 2005, 15:15–15:30, TU C243
Cholesteric Liquid Crystal Elastomers as Mechanically Tunable Photonic Band Gap Materials — •Jürgen Schmidtke1, Werner Stille1, Simon Kniesel2, and Heino Finkelmann2 — 1Physikalisches Institut, Albert-Ludwigs-Univ., Freiburg — 2Institut für Makromolekulare Chemie, Albert-Ludwigs-Univ., Freiburg
We have studied the effects of uniaxial and biaxial strain on the photonic band structure of an elastomer with cholesteric liquid-crystalline order. Cholesteric liquid crystals (CLCs) act as polarization-sensitive one dimensional photonic crystals. Due to the elastic coupling of the liquid-crystalline phase structure and the polymer network, a CLC elastomer allows for a mechanical tuning of the optical properties.
On application of biaxial strain, the photonic stop band is shifted to shorter wavelengths. The number of director turns along the film normal is found to be a conserved quantity. Using a sample doped with a laser dye, photonic band edge lasing can be mechanically tuned over a wavelength range of about 100 nm.
Application of uniaxial strain perpendicular to the helical axis results in a modification of the photonic band structure, due to a deformation of the cholesteric helix. However, contrary to theoretical predictions, no polarization-independent photonic stop band emerges. Again, the shift of the stop band is affine to the compression along the helical axis. The anisotropic molecular order of the elastomer leads to an anisotropic elastic modulus: Perpendicular to the applied stress, contraction is not isotropic, but to some degree hindered along the direction of the cholesteric helix.