Polarizers electrooptic measurements

Today the electrooptical properties of liquid crystals form well-developed branches both in the physics and technology of liquid crystals. In addition, electrooptical measurements are the basis of a number of precise methods for determining the physical parameters of a material, such as its elastic and viscosity coefficients, optical anisotropy, spontaneous polarization, flexoelectric coefficients, anchoring energies at interfaces, etc. 

Figure 69. (a) The experimental setup for electrooptic measurements. The sample is placed between crossed polarizers, (b) The RC-cir-cuit used in the switching experiments. The current response from the sample is measured across the resistor R (from Hermann [136]). 

Summarizing the information from the optical, electrooptical, and dielectric measurements we can draw several structures which are consistent with the crystallographic data but differ in the tilt and polarization distribution (Fig. 9). There is no direct method which would enable one to determine unambiguously which structure is correct, and we are left with some freedom for speculation. It seems... 

The measurement of optical activity in Raman spectra is most often done by measuring the circular intensity difference (CID) and the circular intensity sum (CIS). Using this technique the linearly polarized la.ser beam used for excitation is modulated with a KD P electrooptical modulator resulting in an excitating beam which changes its state of polarization from left to right circularly polarized at a frequency of about 1/3 Hz. The Raman intensities are then collected separately and the dimensionless Raman CID calculated using ... 

Assuming a factor of 2-2.5 for the resonance-enhancement factor in the electrooptic Pockels measurements (this range for the enhancement factor has been reported by many authors who measured the dispersion of the electrooptic Pockels coefficient in dc field poled PMMA-DRl) [88,89], and assuming similar NLO molecular densities for both the DRl-polyimide and PMMA-DRl polymers, one can see that the polar order achieved by PEP cycle in the DRl-polyimide high polymer is comparable to the one obtained in a flexible main chain DRl-PMMA copolymer. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

Optical Kerr Effect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an intensity-dependent refractive index change. The sample is placed between crossed polarizers and a weak, typically tunable, continuous wave (cw) probe laser (usually at a different wavelength and polarized at 45° to the pump pulse) overlaps the pumped region. The increased transmission of the probe beam when the pump pulse arrives is proportional to (Xeff), a combination of elements of the tensor. Many... 

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. 

The apparatus described in [18] allows not only the observation of the sample with a polarizing microscope but also includes the application of optical spectroscopy electrooptic and electrical measurements. 

Experiments in which the parameters in the equation of motion are measured may be carried out with an electrooptic experimental setup, usually based on a polarizing microscope. In the setup, the sample is placed between crossed polarizers and an electric field (t) is applied across the cell. The optical and electric responses of the sample to the field are then measured. 

Approximately 20 different smectic phases have been identified up to now [3]. Eight among them consist of so-called tilted phases, Le.. the long axes of the moleciiles are tilted with respect to the layer normal (, Sb Sr. So. Sh, Si, Sk. and Sm). If these latter mesophases consist of chiral molecules, they in principle match the requiremeni for intrinsic ferroelectric polarization. In six of these tilled chiral phases (denoted herein by an asterisk), spontaneous polarization has been measured (Sr. Sr. , So St. Sy ). For technological application in electrooptical devices, the chiral smectic C phase Sc is prominent due to its lowest ordering a hence highest fluidity, making reorientation processes caused by electric fields very find. 

Optical Kerr Eftect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an... 

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