Optically induced alignment

Optical properties of organic compounds are determined by the polarization of electrons along the induced electric field. On the other hand, photochemical reactions change the chemical structures and electronic states of organic materials. Many researchers are interested in photoisomerization of azo dyes because it changes orientations of dye molecules and can be controlled with light irradiation. This effect is utilized for optically-induced alignment of liquid crystals(i, 2) and... 

The Optical-Induced Freedericksz Transition of Nematic Liquid Crystal (5CB) doped with l%(w/w) of 5, 10, 15, 20-tetraphenylporphyrinatozinc( II) (ZnTPP) were studied. Excited by Ti Sappire laser with the 82MHz repetition rate and lOOfs pulse duration, the optical Freedericksz threshold of a 23.6pm-thickness planar alignment sample occurred at an intensity level of0.35mW/mm in contrast to the normally observed 83mW/mm value for pure 5CB. The coordination-bonding interaction between 5CB and ZnTPP were discussed by UV-vis and fluorescence spectra. We attribute the reduction of the optical Freedericksz threshold to the coordination-bonding interaction. 

Kim, D. Y., Kumar, J., and Tripathy, S. K. Liquid crystal alignment using optically induced surface relief gratings on azo polymer films. Polymer Preprints 1998, 319, pp. 1107-1108. 

If such a retarder is placed with its optic axis aligned exactly with the x or y directions, however, no change in the polarisation is induced, i.e. linearly polarised light stays linearly polarised. A combination of linear polariser and quarter wave plate can therefore be used to create any state of polarisation by proper orientation of their relative azimuths. 

Let us mention a few more capabilities created in this growing field. Laser-induced reorientation of the optical axis is possible in PLC combs optically induced trans—cis isomerization occurs. Erasable holograms can be created in PLC materials—as discussed by Eich and Wendorff [100] and pursued since by many. The mbbing of a polymer leads to an anisotropic surface morphology, since the LC molecules become aligned [101]. Scanning force microscopy can be used to create in a controlled way areas with a similar anisotropy and with a desired refractive index patterns. 

FIGURE 28 (a) Random molecular orientation in a liquid with anisotropic molecules produces an isotropic refractive index, (b) Partial alignment with a laser beam produces a nonlinear index and optically induced birefringence. 

E. Santamato, G. Abbate et ai, Optically Induced Twist Freedericksz transitions in Planar-Aligned Nematic Liquid Crystal, Phys. Rev. A (1987) 174... 

Optically induced twist Freedericksz transitions in planar-aligned nematic liquid crystals... 

M. B. Feller, W. Chen, and Y. R. Shen, Investigation of surface-induced alignment of liquid crystal molecules by optical second-harmonic generation, Phys. Rev. A, 43, 6778 [1991). 

A sensitive approach for the analysis of molecular structures is the measurement of rotation time constants and optical anisotropy coefficients by relaxation electrooptical methods.The experimental procedure used for electrooptical investigations is relatively simple samples are subjected to electric field pulses, and the response due to field-induced alignment or field-induced reactions is recorded by spectrophotometric techniques. In the dichroism experiments, the absorbance of polarized light is measured under electric field pulses. The measured quantity, linear dichroism (LD), means that anisotropic absorption of plane or linearly polarized light has taken place. 

Laiho, A. and Ikkala, O. (2007) A rheo-optical apparatus for real time kinetic studies on shear-induced alignment of self-assembled soft matter with small sample volumes. Review of Scientific Instruments, 78,015109-015106. 

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