Optical Brewster angle

It was estabhshed ia 1945 that monolayers of saturated fatty acids have quite compHcated phase diagrams (13). However, the observation of the different phases has become possible only much more recendy owiag to improvements ia experimental optical techniques such as duorescence, polarized duorescence, and Brewster angle microscopies, and x-ray methods usiag synchrotron radiation, etc. Thus, it has become well accepted that Hpid monolayer stmctures are not merely soHd, Hquid expanded, Hquid condensed, etc, but that a faidy large number of phases and mesophases exist, as a variety of phase transitions between them (14,15). 

Brewster angle microscopy (BAM) As imaging ellipsometry. Restricted to interfaces exhibiting a Brewster angle of incidence. Resolution is that of optical microscopy. 

Using optieal microscopie teehniques (fluorescence microscopy, Brewster angle microseopy and microscopic ellipsometry) the formation, size and shape of domains in the LE-LC coexistence region of phospholipid monolayers have been studied extensively. Furthermore, structures within condensed domains and phases have been visualized, the contrast resulting from the optical anisotropy caused by long-range tilt orientational order. 

In the present study, the water surface is being surveyed by two optical methods, i.e., Brewster Angle Microscopy [BAM] and Infrared Reflection-Absorption Spectroscopy [IRRAS], A schematic sketch of the BAM... 

If the incidence angle of / -polarised radiation is equal to the Brewster angle aB, the reflectivity from the pure (i.e., slick-free) water surface is close to zero. As a consequence, the water surface appears to be dark. In the presence of a film-forming substance, however, the slick patches represent a different optical medium that gives rise to a measurable reflectivity, which in turn makes the slick domains visible by their lighter appearance. This effect can be recorded by a Charge-Coupled-Device [CCD]-camera. 

Consider an optically homogeneous thin film. optically homogeneous, we mean one that is thin enough so that no scattering can occur. If abeam of photons is incident to the surface at a given angle (but less than that where all of the beam is transmitted- the so-called Brewster angle), part of the beam will be reflected and part will be absmbed. Ihis is shown as follows ... 

There are numerous polarizing optical components available. Their operation is based on birefringence, polarization at mirrors at the Brewster angle, optical dichroism, etc. Precision achieved with sheet polarizers is inadequate, thus devices like Glan-Thompson, Glan-Foucault or Rochon prisms have to be used. 

Brewster angle spectroscopy (BAS), developed originally for the contactless characterization of electronic defects in semiconductors [131], was extended, shortly after the introduchon of the method, to high-sensitivity surface optical characterization [132]. Recently, the method was used in situ to follow surface changes in real time upon conditioning of semiconductors [133,134]. The latter technique is described here. 

BAS measures the optical response of a sample using % and Rp( >B). The optical response is obtained as the wavelength (or frequency)-dependent complex dielectric function e = e-i+ (where i and 2 are the real and imaginary parts of the dielectric function, respectively), and analytical expressions have been derived to determine e(co) from rp and Rp(%). For a sample with purely dielectric properties, for example, = 1, the Brewster angle is given by... 

In this case, the optics of the interface is slightly complicated in view of the birefringence of the surface induced pre-nematic phase. The interface is now consisting of (i)the isotropic bulk liquid crystal, (ii) the surface induced, weakly optically uniaxial layer with a thickness of the nematic correlation length and with the optical axis perpendicular to the surface, and, (iii) an isotropic substrate (such as glass). If the optical anisotropy and the thickness of the anisotropic layer are small, the eUipticity coefficient at the Brewster angle> PB, can be calculated in the Drude approximation [1]... 

Brewster angle ellipsometry (BAE) and surface optical second harmonic generation (SHG, see Chap. 5) were used to study the growth of 8CB films, evaporated in air onto glass (BK7) substrates, covered with a 15 nm thick film of poly(vinyl cinnamate) (PVCN) [48]. As the thickness of 8CB on PVCN layers was far below the optical wavelengths, the Drude formula for the ellipsometric coefficient at the Brewster angle, ps, (4.3), was used. The ellipticity coefficient of the 8CB adsorbate was calculated as... 

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