Investigation of circular Bragg reflection in an azo polymer with photoinduced chirality

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Abstract
There has been recent interest in photoinduced chirality in azo polymers upon irradiation with circularly polarized light. The mechanism of ordering is thought to involve successive twisting of the achiral photoactive azo-containing side chains in response to the handedness of the pump polarization. In this study, a thin film of a liquid-crystalline azo polymer with a room-temperature nematic phase is irradiated with circularly polarized light and a circular Bragg reflection is observed in the absorption spectrum. When probed at normal incidence, the evolution of this selective reflection is monitored with increasing exposure. Then, the probing is carried out for various angles of incidence and the change in peak shape is studied. In the last experiment, the position of the Bragg reflection is monitored as the wavelength of the pump laser is varied. We propose that while the induced chirality is due to the circularly polarized light, the associated pitch may be large; the induced periodicity corresponding to the Bragg reflection may be the result of partial erasure due to a standing wave created by back reflection in thin films.