Brewster polarizers

Figure 3.2-6 Principle of the Brewster polarizer. A beam reflected at the Brewster angle

Brewster polarizers were widely used in infrared spectroscopy in two varieties. The first of them directly utilizes the reflected beams that contain the pure perpendicular component while the second type functions in transmission. Frequently used materials for Brewster polarizes are selenium (in old designs), silver bromide, KRS-5, germanium and silicon. The general requirement for the material used in these types of polarizers is to have a large refractive index. The higher the latter is the better parallel and peipendicular components of radiation separate. 

The reflection polarizers have the disadvantage of declination of the beam direction from the optical axis of the instrument, and therefore, additional reflection elements must be introduced in order to restore the initial beam direction. Fig. 3.2-7a, b shows two possible designs of reflection Brewster polarizers. Their obvious advantage is the higher polarization purity achieved by one or two active reflection elements. 

Brewster polarizers functioning in transmission consist of several plates or foils of high refractive index ordered in a holder at the Brewster angle. The plates (foils) consequently eliminate the perpendicular component of radiation thus filtering the parallel component. Old constructions with fragile selenium foils are known. Other constructions use silver bromide or KRS-5. The side displacement of the beam by the plates is compensated for, making use of the symmetric constructions displayed in Fig. 3.2-7c. 

Brewster polarizers are almost not used anymore in IR spectroscopy since their relatively big sizes represent a definite disadvantage. Nowadays they are practically replaced by the convenient grid polarizers. 

Figure 3.2-7 Brewster polarizers with one reflection (a), with two reflections (b) and in transmission (c) M - plane mirrors, BP - Brewster plates,

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

By placing these windows in a plane set at the Brewster angle with respect to the Ught beam, maximum transmission is assured (minimum reflection). The above considerations regarding polarization of refracted and reflected beams must be modified for nontransparent media such as metals, for which there is little polarization of a reflected beam. 

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). 

This concept has been known for over a century. Expressed as Brewster s Constant law, it states that the index of refraction in a strained material becomes directional, and the change of the index is proportional to the magnitude of the stress (or strain) present. Therefore, a polarized beam in the clear plastic splits into two wave fronts in the X and Y directions that contain vibrations oriented along the directions of principal stresses. An analyzing filter passes only vibrations parallel to its own transmitting plane (Chapter 4, TRANSPARENT AND OPTICAL PRODUCT, Polarized Lighting). 

Lipp, M.M., Lee, K.Y.C., Waring, A., and Zasadzinski, J.A. Fluorescence, polarized fluorescence, and Brewster angle microscopy of palmitic acid and lung surfactant protein B monolayers. Biophys. J. 1997, 72, 2783-2804. 

Brewster-angle microscopy dispensed with the need for a probe molecule [41,42], Brewster-angle microscopy is based solely on the reflectivity properties of p-polarized light. The reflectance of p-polarized light at the water-air interface vanishes at 53.1° (using refractive indices at 20°C) if the interface is perfectly sharp in reality there is a deep minimum near zero. The presence of a monolayer gives an intervening layer of different re-... 

The method is based on the fact that plane-polarized light does not reflect from the interface when it is incident at the Brewster angle, determined by the equation... 

Sir David Brewster 1781-1868, Scottish physicist famous for his researches on tire absorption, reflection, refraction, and polarization of light, and on doubly refracting crystals. One of the founders of the British Association for die Advancement of Science. He invented the kaleidoscope and improved the stereoscope. His optical researches led to great improvement in the construction of lighthouses. 

Bodor and Brewster (1983) first used the term CDS, in describing the use of dihydropyridine ester- (or amide)-linked prodrugs such as 27 (X-OH is the parent) which can partition readily into the CNS, there to be oxidized to pyridinium salts (28), which are effectively trapped in the biophase because of their extreme polarity, and which then undergo enzymic or chemical hydrolysis of the now very labile ester link to release active drug. 

Brewster window A flat optical window tilted at an angle such that light whose electric vector is polarized parallel to the plane of the window is 100% transmitted. Light polarized perpendicular to the window is partially reflected. It is used on the ends of a laser to produce light whose electric field oscillates perpendicularly to the long axis of the laser. 

BREWSTER ANGLE. The Brewster angle, or polarizing angle, of a dielectric is that angle of incidence for which a wave polarized parallel to the plane of incidence is wholly transmitted (no reflection). An unpolarized wave incident at this angle is therefore resolved into a transmitted partly-polarized component and a reflected perpendicularly-polarized component. See Fig. I. 

Fig. 1. Brewster angle. A, incident plane (plane of polarization or plane of magnetic vector, after reflection) B, plane of vibration (plane of electric vector, after refleclion) C, reflecting surface (dielectric)...

A typical set-up for a FTIR ATR experiment is described in Figure 1. A Brewster angle rotating Ge polarizer is place in the IR beam, in front of the C face of an ATR crystal, which can be made of Si, Ge, or ZnSe. A separate background spectra are recorded for the s- and p-polarization, and the crystal is used as a substrate either for LB or SA monolayer samples. 

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