Polarized anisotropic dark-field

Xia and coworkers have demonstrated that the polarization of light plays an important role for nanoparticles with anisotropic shapes, especially with tmncated comers [77]. Optical dark-field mode imaging was employed to identify individual silver nanocubes deposited on silicon substrate as shown in Fig. 15.10a. Raman spectra were collected from the nanocubes, which were oriented in different directions with respect to the laser polarization. Subsequent SEM imaging of the same cubes enabled the authors to directly correlate the orientation of the nanocubes with respect to the light polarization and the SERS enhancement factor. They observed dramatic variation in SERS intensity when the nanocubes were oriented at different angles relative to the polarization of excitation laser as shown in Fig. 15.10b. SERS spectra of 1,4-benzenedithiol adsorbed on Ag nanocubes oriented in different directions showed different intensities with respect to the light polarization direction. The individual nanocubes with sharp comers were the most... 

POM shows that solid films cast from 10 to 12% wtHPC are isotropic (Fig. 15.2a) (dark field under crossed polars). In contrast, solid films cast from a chiral nematic solution of HPC (60% HPC/40% DMAc, w/w) are clearly birefringent (anisotropic) and present a characteristic banded texture perpendicular to the shear direction, observed for thermotropic and lyotropic liquid crystalline phases of HPC after shear. 

In a nematic liquid the director field is not uniform unless an external field of electric, magnetic or mechanical nature is applied. The director may vary in a smooth continuous way around certain points called disclinations, or more abruptly as in domain walls. Characteristic of the nematic structure are the so-called schlieren textures which are readily observed by polarized light microscopy (Fig. 6.23). The dark bands meet at certain points, the disclinations. It should be noted that a dark region indicates that the local molecular director of the optically anisotropic region is parallel to the polarizer or to the analyser, or that the region is optically isotropic. 

Polarizing light microscopy employs crossed polarizers to view the sample. With isotropic specimens, the field of view is dark, while anisotropic, birefringent samples or areas of a sample will appear bright. Polarizing microscopy is employed to view spherulitic structure [66-68] and deformation morphologies (crazes, shear banding) [69] in polymer blends. Samples for... 

PLM is most often used qualitatively as a contrast enhancement tool. For simple observation, the orientations of the polarization states transmitted by the polarizer and the analyzer are set to be perpendicular, and so ideally, if there is no change in polarization due to the specimen, the field is dark. This (0 and 90°) condition is generally called crossed polars (see Figure 1(f)). An anisotropic specimen rotated between crossed polars has four positions of minimum intensity or extinction (condition 2 in Section 2.17.3.1) and four positions of maximum intensity at 45°. In white light, anisotropic stmctures with k>200nm appear brightly colored when viewed in crossed (or in parallel) polars. 

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