Nonlinear bleaching in the selective reflection of nonabsorbing chiral-nematic liquid-crystal thin films

High-intensity, circularly polarized light beams tuned to selective-reflection conditions in chiral-nematic liquid-crystal layers made possible the first observation of a light-induced drop in the chiral reflection coefficient of liquid-crystal layer. The dependence of the effect on intensity (and its corresponding absence at low intensities) permits one to connect it mechanistically with the chiral nematic's helix pitch dilation up to spiral untwisting. Up to now, this effect has been observed only in static and low-frequency electric and magnetic fields. Under the current, specifically chosen experimental irradiation conditions, nonlinear bleaching of the medium's reflectivity could be observed over time intervals that allow a distinction in the driving mechanism of nonlinear bleaching between optical-field-induced and thermal processes.