High efficiency holographic Bragg grating with optically prolonged memory.

In this paper, we show that photosensitive azo-dye doped Blue-phase liquid crystals (BPLC) formed by natural molecular self-assembly are capable of high diffraction efficiency holographic recording with memory that can be prolonged from few seconds to several minutes by uniform illumination with the reference beam. Operating in the Bragg regime, we have observed 50 times improvement in the grating diffraction efficiency and shorter recording time compared to previous investigations. The enabling mechanism is BPLC's lattice distortion and index modulation caused by the action of light on the azo-dopant; upon photo-excitation, the azo-molecules undergo transformation from the oblong-shaped Trans-state to the bent-shaped Cis-state, imparting disorder and also cause the surrounding BPLC molecules to undergo coupled flow &reorientation leading to lattice distortion and index modulation.

Observation of photorefractive effects in blue-phase liquid crystal containing fullerene-C.

Photorefractive effects manifested in two beam coupling and side diffractions are observed in fullerene-C doped blue-phase liquid crystals (BPLC-C) upon application of a DC bias field. The mechanism at work here is attributed to BPLC lattice distortion by the combined DC (E)+ photorefractive space-charge (E) fields, in addition to the DC + optical field induced effects reported in previous studies of dye-doped system. The first order diffraction efficiency of ∼2×10 and beam coupling gain of over 2% are observed in a 55 μm thick sample with input laser beam power of 5 mW at an applied DC voltage of 160 V.