Liquid crystal media are characterized by large and tunable dispersive properties and hence allow achievement of large group delays. At the same time, liquid crystals provide large areas and are easily reconfigurable and highly sensitive devices; they are, therefore, well adapted for interferometric applications. Two different ways of achieving slow light in liquid crystals are presented. The first method consists of exploiting photoisomerization-induced transparency in dye-doped chiral liquid crystals, and the second method makes use of two-wave mixing optical resonance in pure nematics. In both mechanisms, two beams are sent to the medium, where they create a grating, either of absorption or of refractive index. Both physical mechanisms are elucidated in the context of slow light, then, as examples of sensing applications, Doppler shift measurements and adaptive holography are presented.
We demonstrate slow light in dye-doped chiral liquid crystals. The medium combines a nematic liquid crystal with methyl-red and chiral agents. When a circularly polarized pump and a pulse beam interact in the cell, the output pulse is delayed thanks to the dispersive properties of the beam coupling occurring in the medium. The origin of the slow-light effect is explained on the basis of photo-isomerization induced transparency of the dye molecules hosted in the chiral liquid crystal structure.
In time resolved photophysics, it is possible to access to transient effects of diffusionnal origin. We present here some classical kinetic models and discuss of their limitations. Particularly, we develop the important case of excimer formation when limited by diffusion.
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