Domains optical effects

When investigating the polar structure by photo-induced light scattering we assume that the largest contribution to the initial optical noise is due to diffraction of the pump beam on optical inhomogeneities located at boundaries of ferroelectric domains [9], Figure 9.12 illustrates this concept schematically. Internal electric fields Ei (random fields) yield local perturbations 5n of the index of refraction via the linear electro-optic effect 5n = - n rssEi. 

Figure 9.12 Seed scattering at refractive index modulations induced by localized internal random fields via the electro-optic effect. The internal fields are also responsible for the formation of a rich ferroelectric domain structure. Here, a periodic sequence of domains with lengths A d is shown. Note, that the grating period of the refractive index modulation As is equal to the lengths of the ferroelectric domains.

Kinoshita S, Kai Y, Ariyoshi T, Shimada Y. Low frequency modes probed by time-domain optical Kerr effect spectroscopy. Int J Mod Phys B 1996 10 1229-1272. 

The inverse proportionality between peak width and mean size stated by the Scherrer equation places practical limits to the range of domain sizes that produce measurable effects in a powder pattern. While the lower bound [a few ( 2)nm, depending on the specific phase] is related to the approximations used, the upper bound depends on the instrumental resolution, i.e. on the width of the instrumental profile. Traditional laboratory powder diffractometers, using standard commercial optics, typically allow the detection of domain sizes up to 200 run. Above this value, domain size effects can hardly be distinguished from the instrumental broadening. This limit, however, can considerably be extended by using suitable high resolution optics, as is the case of many diffractometers in use with synchrotron radiation. In this case the practical limit can reach several micrometres. 

In 1963, Richard Williams observed the formation of very regular patterns or domains in a nematic liquid crystal when the material was subjected to an electric field. This report marked the beginning of a new era in research on the electro-optic properties of liquid crystals, a field which had laid dormant for nearly 30 years. During the remaining years of the 1960 s and the early 70 s, numerous studies of electro-optic effects in liquid crystals were performed, and at the same time, investigations into the synthetic and physical chemistry of these materials were conducted. As a result of these efforts, a whole new display industry evolved. 

Equations (2.27), (2.28), (2.29) and (2.30) show that nonlinear optical effects in organic r-conjugated molecules (i.e. electric field dependent polarizablity) are basically due to electronic rearrangements so it is expected that organic molecules exhibit extremely fast response times leading to the possibility of electro-optic modulation in frequency domains that are prohibited for inorganic crystals presently available in the market. 

If, however, the orientation is originally perpendicular, the first electro-optic effect is the conversion to a planar orientation in the bulk, followed at higher voltage by the domains, whose cylindrical axes may not be correlated over large distances, and clusters result. The diffraction pattern will be a ring if the probe diameter w) is large compared to the cluster size (Fig. 2, lower part). 

In conventional frequency-domain CARS, either o)j or CO2 is scanned so that o)j - (O2 passes through Raman-active resonances. As the difference in frequency between these two beams is tuned to each resonance, a resonance enhancement of the nonlinear optical effect occurs, leading to a peak in the intensity of the output beam. Spectra are produced by plotting the intensity of the output beam as a function of COj - a>2. 

Karapinar R, Neill MO, Hird M (2002) Polymer dispersed ferroelectric liquid crystal films with high electro-optic quality. J Phys D Appl Phys 35(9) 900-903 Kitzerow HS, Molsen H, Heppke G (1992) Linear electro-optic effects in polymer-dispersed ferroelectric liquid crystals. Appl Phys Lett 60 3093 Kossyrev PA, Qi J, Priezjev NV, Pelcovits RA, Crawford GP (2002) Virtual surface, director domain and the Freedericksz transition in polymer-stabilized liquid crystals. Appl Phys Lett 81 2986... 

The development of ordered bands perpendicular to the shear direction in the oriented film of both copolyester compositions is an optical effect observed with liquid-crystalline behaving polymers under mechanical stress conditions and appears to relate to the electro-optical effects observed in nematic liquid crystals. The non-uniform distribution of shear bands in the oriented film of the 20/80 copolyester composition is due to the inhomogeneity of its mesophase which is composed of rich PET and PHBA areas because no shear bands are observed readily in oriented film which is prepared below 310-320 °C, that is when the PHBA domains are not fused. ... 

When used for superresolution, the laser beam is incident on b, which hides the domains in s. During read-out, b is heated and the domains in s are copied to b. The optical system sees only the overlap area between the laser spot and the temperature profile which is lagging behind, so that the effective resolution is increased. Experimentally it is possible to double the linear read-out resolution, so that a four times higher area density of the domains can be achieved when the higher resolution is also exploited across the tracks. At a domain distance of 0.6 pm, corresponding to twice the optical cutoff frequency, a SNR of 42 dB has been reached (82). 

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