Unique optical changes in cholesteric liquid crystals using guest mediated single laser beam excitation

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Abstract
The dynamics of three distinctively different optical changes of three different cholesteric liquid crystal mixtures of different pitch doped with a guest molecule are seen for the first time utilizing single laser beam excitation. The guest molecule acted as a light absorber and subsequent fast heat radiator. Two of the three cholesteric mixtures had opposite directions of pitch change from the third with increasing temperature. In the first optical process (also the fastest), just after laser excitation, ≤200 ns, the reflectance band always moved to longer wavelength irrespective of the magnitude of the original pitch or the directional sensitivity of the pitch change to increasing temperature. This nanosecond time domain phenomenon is assigned as originating from formation of a transient grating caused by modulation of the index of refraction associated with heat instantaneously (≤10 ps) released by the guest molecules to their immediate neighborhood. The second process, in the time domain of 30–600 μs, is mainly the result of the characteristic pitch change of the helix of the cholesteric mixtures with corresponding reflection band shift to longer or shorter wavelengths according to the type of mixture. The final process (>1000 μs) is the consequence of the pitch of the cholesteric returning to the original value after the heat is dissipated.