Optical Kerr effect , liquid state

A well-known nonlinear process taking place in the liquid state of anisotropic molecules is the optical-field induced birefringence (optical Kerr effect ). This nonlinearity results from the reorientation of the molecules in the electric field of a light beam. In the isotropic phase the optical field perturbs the orientational distribution of the molecules. In the perturbed state more molecules are aligned parallel to the electric field than perpendicularly to it and as a consequence the medium becomes birefringent. On the other hand in liquid crystals the orientational distribution of the molecules is inherently anisotropic. The optical field, just as a d.c. electric or magnetic field, induces a collective rotation of the molecules. This process can be described as a reorientation of the director. 

Fig. 1.2 The experimentally determined spectral density of states of liquid water using the optical Kerr effect -linear response) is shown for comparison [3], Water is one of the most structured liquids in which there is large separation in timescales. The frequency range between 300 and 1,000 cm (1) is related to librational motions (note that the spectrum represented here is artificially truncated at 600 cm due to laser bandwidth hmitations), the peak at 170 cm (2) is hindered translational motion of the heavy O atoms, the 60 cm mode (3) is transverse or shear motion, and below 25 cm (4) corresponds to diffusive relaxation and hydrogen bond breaking [8]. The issue of inhomogeneous to homogeneous interpretations of the dynamic structure of liquid water still holds despite this additional structure. The agreement is quite good with the theoretical calculation of the corresponding spectral density of states for water but the calculations are rather insensitive to basis with respect to this observable. Reprinted with permission from [3]. Copyright 1994, American Chemical Society...

In this book, we collect a series of chapters dedicated to the state-of-art studies of optical spectroscopy in the time domain on complex liquids of different nature. In Chap. 1, a new nonlinear spectroscopic technique, 2D-Raman, is comprehensively reviewed. This is probably tbe most promising experimental tool able to collect truly new information on fast dynamics in liquid matter. Chapter 2 is dedicated to optical Kerr effect techniques and to the investigation of relaxation d5mamics in complex liquids, inclusive of relative slow collective... 

Transient Kerr effect measurements have been used primarily to study large molecules (polymers and biopolymers) or smaller molecules at sufficiently low temperatures as discussed by G. Williams. The limitations are because the relaxation time scale must not be shorter than the time resolutions for available transient field generators and optical detectors. Wntil recently this has set short time limits of 10 ° to 10 seconds at best. These are too large for useful studies of small molecules in liquids at ordinary temperatures but developments in pump and probe laser tectmiques have already made some measurements possible with picosecond resolution (as discussed by Kenney-Wallace In this volume) and it should be possible to do much more with availability of fast solid state switching and optical coupling devices. 

Microcrystals of some diacetylenes, prepared by the reprecipitation method, have been studied as dispersions in liquid media. Interesting behavior has been observed in the solid-state polymerization of diacetylene monomers and with the optical properties of polydiacetylene (PDA) microcrystals. First, the polymerization perfectly proceeded from one end to the other end of the diacetylene microcrystals. Next, the excitonic absorption peak position was found to shift to higher energy side with decreasing size of the PDA microcrystals. The size effect was observed even for crystals as large as 100 nm or more in contrast to conventional quantum effect of inorganic semiconductors where size effect is observed only for microciystals of less than about 10 nm size. In addition, since the microcrystal dispersions in water have low optical loss, the c tical Kerr shutter response of PDA microciystals could be measured, and the non-resonant value was estimated to be on the order of 10 to 10" esu in very low concentrations (ca. 10 M). 

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