Z-scan measurements of the light-induced thermal contribution to the optical nonlinearity of dye-doped liquid crystal

Abstract

The large optical nonlinearity of nematic liquid crystals makes them promising materials for various applications such as dynamic holography, image processing and adaptive optics. The optical nonlinearity in nematic liquid crystals is larger by as much as six orders of magnitude than conventional nonlinear materials (like CS₂) and is appropriately referred to as a giant optical nonlinearity (GON). When the liquid crystal is made sensitive to visible light by adding small amounts of dye (typically less than 1% by weight), the nonlinearity is further enhanced by as much as two orders of magnitude. Sensitivity to light intensity as low as 40 μW/cm² has been observed in methyl-red doped liquid crystals. Different contributing mechanisms have been proposed to explain the nonlinearity in dyed-nematics such as: i) conformational changes; ii) intermolecular torque; iii) space-charge fields; and iv) thermal effects. Dye-aided enhancement of the optical nonlinearity is not limited to doped liquid crystals but has been also observed in other molecular guest-host systems such as dyes in liquid solutions and polymer matrices, side chain polymers and Langmuir-Blodgett films.
In this research, the thermal contribution to the enhancement of the optical nonlinearity is studied via the z-scan technique.