Dichroic absorbing polarizer

Institute of Technology, 1949 [28]. (b) Schematic diagram of sheet polarizer. Reproduced with permission from the Optical Society of America. 

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B) Data Analysis. Irradiation was performed with a high pressure mercury lamp (250W, Ushio Electrics Co.). The two dichroic absorbance components of HNMA and Cyclophane at 393.5tun and 402nm were monitored at room temperature after irradiating the sample over different time intervals with linearly polarized light at 365nm. The reorientation dynamics of these molecular tracers were obtained from the analysis of the time-dependent induction efficiency qft) which is defined as" r (t) = 100. ODx(t) - ODii (t) /ODo (1)... 

PI-1 and PI-2 films exhibit dichroic absorbance after polarized irradiation. Figure 4.15 shows polar plots of the absorbance of linearly polarized probe light (at 488 nm) as a function of the angle, between the polarization of the probe and irradiation light (532 nm 30 mW/cm ). Nonpolar orientation is clearly shown for both PI-1 (left) and PI-2 (right). The highest absorption is observed when the probe and irradiation beams have perpendicular polarizations,... 

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anthraquinone dyes have been found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are appHed. Anthraquinone compounds have also been found to be usehil dichroic dyes for guest-host Hquid crystal displays when the substituents are properly selected to have high order parameters. These dichroic dyes can be used for polarizer films of LCD systems as well. Anthraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be appHcable in semiconductor laser recording. 

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

A medium is said to be circularly dichroic—it absorbs differently according to the state of circular polarization of the light—if kL — kR 0 it is circularly birefringent, which is manifested by optical rotation, if nL — nR = 0. Optical rotation and circular dichroism are not independent phenomena, but are connected by Kramers-Kronig relations ... 

Assuming that the transition dipole does not absorb light polarized perpendicular to its principal axis ( a2 = 0 ), the dichroic ratio takes on the simple form,... 

The first manifestation of VCD in the optical train of a spectrometer is the modulation of the intensity of the infrared beam in synchronization with the modulation of the polarization as the beam passes through the circular dichroic sample. The phase of the synchronization is opposite for negative and positive VCD bands. More specifically, in reference to the definition of VCD in Eq. (1), there will be a synchronization between larger transmission (smaller absorbance) and right CP radiation for positive VCD bands, and between larger transmission and left CP radiation for negative VCD bands. 

Next, we consider the absorbance due to a dichroic adlayer adsorbed onto the waveguide surface with the optical constants as indicated in Fig. 4. The optical properties of the dichroic layer are described by the different extinction coefficients k, ky, and k in each Cartesian direction. The reflectance of the waveguide-adlayer-cover system follows the analysis found in Macleod [9] with the anisotropic coefficients taken from Horowitz and Mendes [10]. By assuming a thin and weakly absorbing adlayer, the following expressions are obtained for the absorbance as measured through a guided mode at each polarization [8] ... 

A system of such particles is considered in which the particle axes are distributed in the electric field according to the Boltzmann distribution, and absorbance of the polarized llj t is calculated, neglecting both polarization of the polypeptide molecule by such a low external field (0-300 V/cm) and molecular fields for such a long particle 23). The dichroic ratio of the band in questicm is given in the form ... 

Figure 3.6-10 Schematic diagram of a femtosecond time-resolved CARS apparatus. YAG, cw mode-locked Nd YAG laser ML, mode locker PL, polarizer A s, apertures LP, laser pot DM, dichroic mirror DLl, femtosecond dye laser SA, saturable absorber CLFB, cavity-length feedback system DL2, picosecond dye laser W, tuning wedge E, etalon FD, fixed delay VD, variable delay BS, beam splitter P s, half-wave plates (when necessary) F s, filters S, sample MC, monochromator PMT, cooled photomultiplier. (Okamoto and Yoshihara, 1990).

The dichroic ratio R defined as R = A/// Aj, is commonly used to characterize the degree of optical anisotropy A// and Aj being the measured absorbances of the investigated band for an incident radiation polarized respectively along and perpendicular to the stretching direction. 

The infrared spectra were recorded on a Nicolet 7199 or a Nicolet 205 Fourier transform infrared spectrometer at a resolution of 2 cm l with a total number of scans of up to 128 for low-orientation samples. The infrared beam was polarized using a SPECAC gold wire-grid polarizer. Samples, rather than polarizer, were rotated 90° in order to obtain the two polarization measurements. The dichroic ratio was calculated from the measured absorbance at the maximum of the infrared band. 

Figure 3.14 shows the dichroic spectra observed in films of DE/PMMA and SP/PMMA. The insets in Figure 3.15A are expanded views of both the UV and the visible absorptions of the DE chromophore. These spectra were obtained 30 s after polarized UV irradiation (irradiation dose 78 mj/cm ). It is clear that Abs// and Abs are different in other words, the irradiated samples show anisotropic absorbance upon polarized UV irradiation. Identical spectra were recorded for Abs// and Absj before UV irradiation, demonstrating that the samples were in-plane isotropic at that time. 

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

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