Bulk Optical Properties

The history of optical science closely parallels the history of the development of optical glasses. Development of early telescopes and microscopes immediately forced a search for new optical glasses with appropriate refractive index and optical dispersion characteristics. It can be argued that the development of modem astronomy, biology, and medical science were controlled by the ability of glass makers to develop glasses with the appropriate optical properties. 

In this category are methods which can be applied to transparent solids, such as refractive index measurement, refraction anisotropy, and rotary dispersion. Details regarding the use of the polarizing microscope may be found in many excellent textbooks, such as those by Hartshorne and Stuart S and Wahlstrom.  

The optical character (uniaxial or biaxial) and sign (positive or negative) can be ascertained by conoscopic examination between crossed Nicols. The angle between optical axes (2V) and dispersion of optical axes are also readily determined under these conditions. 

Precise measurement of the refractive index, or, more frequently, of the three principal indices of the indicatrix, is frequently used to characterize transparent materials. High precision is possible using monochromatic light with temperature-controlled immersion methods, giving the index to + 0.0005. The method is tedious and not completely unequivocal as an identification tool. However, routine immersion techniques are capable of determining the refractive indices in the higher-symmetry systems to +0.002 in a matter of minutes and are an invaluable simple characterization tool. The determinative tables collected by Winchell and Berman are helpful in identifying unknown phases. 

Detailed evaluation of the optical indicatrix can give useful information on the crystal symmetry. Cubic crystals and amorphous solids are optically isotropic trigonal, tetragonal, and hexagonal crystals are uniaxial orthorhombic, monoclinic, and triclinic crystals are biaxial. Extinction directions vary with wavelength in monoclinic and triclinic crystals. 

These data together with the refractive indices provide one of the most effective structural characterizations of a solid. Optical methods are particularly sensitive in detecting pseudosymmetric forms of a higher symmetry phase and, therefore, for detecting phase changes with temperature in ferroelectric, antiferroelectric, and transparent ferrimagnetic crystals. 

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Equation (16.1) is an approximation it assumes that the measured spectral contribution from Pa is independent of the overall composition of the sample. In reality, the bulk absorption and scattering properties of different samples will cause variations in how much Raman signal is collected. In the case of urine and blood serum specimens, the variation is often negligible. For whole blood and turbid tissue specimens, the influence of bulk optical properties is more important. Methods of correcting for their effects and returning to the simple linearity assumptions of (16.1) are mentioned briefly in the next section. 

Chapter 1 of the present volume provides the basic concepts related to the properties and characterization of the centres known as shallow dopants, the paradigm of the H-like centres. This is followed by a short history of semiconductors, which is intimately connected with these centres, and by a section outlining their electrical and spectroscopic activities. Because of the diversity in the notations, I have included in this chapter a short section on the different notations used to denote the centres and their optical transitions. An overview of the origin of the presence of H-related centres in crystals and guidelines on their structural properties is given in Chap. 2. To define the conditions under which the spectroscopic properties of impurities can be studied, Chap. 3 presents a summary of the bulk optical properties of semiconductors crystals. Chapter 4 describes the spectroscopic techniques and methods used to study the optical absorption of impurity and defect centres and the methods used to produce controlled perturbations of this absorption, which provide information on the structure of the impurity centres, and eventually on some properties of the host crystal. Chapter 5 is a presentation of the effective-mass theory of impurity centres, which is the basis for a quantitative interpretation... 

The objections to the Hamaker theory were overcome by Lifshitz and his coworkers [Lifshitz 1956, Dzyaloshinskii 1961] using the bulk optical properties of the interacting bodies. The approach employed by Lifshitz uses the so-called Lifshitz-van der Waals constant h that depends only on the materials involved provided the separation distance is relatively small. Under some conditions the constant h can be related to the Hamaker constant by... 

Table 13.5 Bulk optical properties of amorphous Si02 and crystalline Cap2 at the 193-nm wavelength.

L. Spinelli, A. Torricelli, A. Pifferi, P. Taroni, R. Cubeddu, Bulk optical properties and tissue componenty in the female breast from multiwavelength time-resolved optical mammography, J. Biomed. Opt. 9, 1137-1142 (2004)... 

Ghoroghchian PP, FraQ PR, Li G, Zupancich JA, Bates FS, Hammer DA, Therien AMJ (2007) Controlling bulk optical properties of emissive polymersomes through intramembra-nous polymer-fluorophore interactions. Chem Mater 19 1309-1318... 

The Surface Plasmon Resonance technique has proved to be very versatile, finding a number of applications where small changes in the properties of a material characterise a physical change in the vicinity of the measurement [2]. As a technique for investigating the linear and non-linear bulk optical properties of novel materials in thin film form, the SPR method is both accurate and sensitive [3]. For non-linear organic materials, particularly in LB film form, information on film thickness, refractive index and electro-optic coefficient may be straightforwardly obtained by computer analysis, even for a molecular monolayer with thiclmess of the order of only 3 nm or less. 

The preconditions for the use of polymer liquid crystals in display applications are that they exhibit bulk optical properties dependent on the molecular orientation in the mesophase and that this orientation may be altered on application of an external field. In this chapter we shall be concerned with electric or optical fields only. The particular optical property, i.e. (a) the birefringence, (b) the dichroism or (c) the scattering power, defines the display construction in terms of the use of polarized (a and b) or non-polarized (b and c) light, whereas the ability to switch from one orientation to another depends on the anisotropic electric permittivity and the orientational elastic constants. The dynamics of the induced orientation will depend, additionally, on the viscosity constants of the material. 

L.E. Brus, On the development of bulk optical-properties in small semiconductor crystallites. J. Lumin. 31, 381-384 (1984)... 

The optical properties of metal monolayers differ markedly from those of the respective bulk material, as we have just seen in the previous section. Hence, it will be interesting to study the gradual transition from monolayer to bulk properties and to determine the minimum film thickness which is necessary for the deposit to acquire bulk optical properties. Besides this, reflectance measurements may be employed to determine the structure and the quality of thin metal overlayers, e.g., whether a thin film is continuously deposited on a substrate or whether it consists of clusters. 

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