Optical Properties of a Material

In this section, the physical mechanisms and selection rules of IR absorption by bulk material due to vibrations, electronic excitations, and free carriers (electrons and holes) are briefly discussed on the qualitative level from the viewpoint of quantum mechanics. In general, this problem is highly specialized, and for fuller details several standard textbooks [21, 34, 36-50] are reconunended. AU of these mechanisms also apply to ultrathin films, but their appearance, which will be discussed in Chapters 3 and 5-7, is quite specific. Note that although we will discuss absorption by solids, the mechanisms to be considered are also applicable to Uquids. 

If the conduction band is half filled or the valence band overlaps in the energy space with the conduction band, the solids have the maximal conductivity. In such a material under the influence of an electric field, electrons can move to neighboring vacant states causing an electric current to flow. This phenomenon is characteristic for metals. 

In the intermediate case, when the upper band containing electrons has a small concentration of either empty or filled states and the temperature coefficient of the electrical resistance is negative at high temperatures, the substance is classified as a semiconductor. 

ABSORPTION AND REFLECTION OF INFRARED RADIATION BY ULTRATHIN FILMS 

An electron excited into the conduction band and a hole left in the valence band can combine to form an electrically neutral species, called an exci-ton. Energy levels of excitons are located in the forbidden band gap and result in essential changes in the spectrum in the vicinity of the absorption edge. 

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The Electromagnetic Spectrum. The optical properties of a material are related to the electromagnetic spectrum (see Ch. 15, Fig. 15.1). The portions of the spectrum of interest are the following ... 

The non-linear optical properties of a material are evaluated by measuring (using techniques from the field of physics) its molecular hyperpolarizability coefficient (3. 

The visual appearance and optical properties of a material depend on its color and additives, etc., as well as on the nature of its surface. Gloss is a term employed to describe the surface character of a material which is responsible for luster or shine, i.e., surface reflection of a polymeric material. 

Since x(w) is related to the individual atomic or molecular polarizabilities, this simple equation relates a property of light (its speed) to a property of the electron density distribution (the polarizability). Now we can see how the optical properties of a material depend on the electron density distribution, which is dictated by chemical structure. Therefore, as the chemist alters the structure, the optical properties change. 

Another important optical property of a material is the index of refraction. It is defined as the ratio of the speed of light in a vacuum, Uv, to the speed of light in the material Um... 

Fluctuations in the optical properties of a material can induce spatial and temporal inhomogeneities that scatter light. In general, the dielectric tensor is taken as the following function of space and time,... 

Research in electronic materials processing must necessarily revolve around one general question How do structural, electronic, and optical properties of a material or a device depend on the processing and how can they be controlled ... 

Also surface optical properties of a material sometimes need to be changed, for example in making anti-reflection coating for lenses or reflective surfaces for CDs, the magnetic properties may need to be influenced as in the case of giving a ferroelectric surface to a plastic for magnetic recording, and, perhaps most extensively of all, the surface electrical properties need to be controlled in microelectronic devices used in computers and all modern electronic equipment. 

The interaction of light with matter has fascinated people since ancient times. The color of an object is the result of this interaction. In modem terms, this interaction is described as spectroscopy. In this chapter, how the optical properties of a material are the result of its chemical composition and stmcture are examined. Several examples of technologically relevant applications are then presented of the manipulation of the optical properties to achieve a desired performance. 

It is important to have a strong understanding of the effects of processing on the optical properties of a material or structure. In this case, we look to understand how structure affects properties and how processing affects structure one then can decide the best approach for preparing a photonic crystal with a desired performance. 

With the advent of laser technology, and the resulting intense and coherent beams, our ability to study the interaction of crystals with light has been greatly expanded. Just as an electric field can modify the optical properties of a material, so can an intense laser beam. The electric field of the light waves is comparable in magnitude to the electric field that binds electrons in atoms and molecules. 

The optical properties of ceramics are useful in the ultraviolet, visible, and infrared ranges of the electromagnetic spectrum, and one key quantity used to describe the optical property of a material is the refractive index, which is a function of the frequency of the electromagnetic radiation. Other quantities used to characterize optical performance are absorption, transmission, and reflection these three properties sum to unity and are also frequency dependent. The last three properties govern many aspects of how light interacts with materials in windows, lenses, mirrors, and filters. In many consumer, decorative, and ornamental applications, the esthetic qualities of the ceramic, such as color, surface texture, gloss, opacity, and translucency, depend critically on how light interacts with the material. 

As everyone knows, the optical properties of a material are expressed in two optical constants, the refractive index n and the absorption coefficient x. It is the purpose of spectroscopy to determine experimentally one or both of these optical constants as a function of frequency. This can be done by measuring reflection or transmission. If we were able to measure amplitudes or electrical fields (magnitude and phase) in an optical investigation, it would generally be possible to deduce both optical constants from one measurement of either reflection or transmission. However, we are only able to measure intensities where the magnitude of the field is determined and the phase information is lost. Thus, in general, from one item of information only one optical constant is obtained, and two measurements are necessary to determine both. There are a few exceptions to this rule, e.g. the... 

The phenomenon of electrochromism can be defined as the change of the optical properties of a material due to the action of an electric field. The field reversal allows the return to the original state. In practice, when the material is polarized in an electrochemical cell, the change of colour is conelated to the insertion/extrac-tion of small ions H" ", Li" ". This insertion/extraction is monitored by the passage from cathodic to anodic polarization which allows to go from bleached (or coloured) state to coloured (or bleached) one. This property belongs to all (or almost all) transition metal oxides it corresponds to the change of valency of the cation Ni "> Ni " O . .. accompanied... 

Electrochromism is the reversible change in optical properties of a material caused by redox reactions. The redox reactions can be initiated when the material is placed on the surface of an electrode. When the electro-chromic material is capable of showing several colors, it is addressed as polyelectrochromic. 

The magnetic, electric, and optical properties of a material are all related mathematically through the Maxwell equations. 

As mentioned above, photorefractivity is a multifunctional property which is produced by the combined action of photoconductivity and nonlinear optical properties of a material (24). It is a result of the functions of photogeneration, transport, charge trapping, and the electro-optic effect. 

Chromatic changes caused by electrochemical processes were originally described in the literature in 1876 for the product of the anodic deposition of aniline [271]. However, the electrochromism was defined as an electrochemically induced phenomenon in 1969, when Deb observed its occurrence in films of some transition metal oxides [272]. Electrochromism in polypyrrole was first reported by Diaz et al. in 1981 [273]. Electrochromism is defined as the persistent change of optical properties of a material induced by reversible redox processes. Electronic conducting polymers have been known and studied as electrochromic materials since the initial systematic studies of their electronic properties. 

Optical properties The most important optical properties of a material are its transparency and refractive index. Transparency is the physical property allowing the transmission of light through a material. It is important for many practical applications of polymer nanocomposites. The refractive index is the ratio of speed of the light in vacuum to the speed of light in the medium. It is the most important property of optical systems that use refraction, and its can be measured by a refractometer. 

Photochromism is the reversible change of optical properties of a material caused by its exposure to electromagnetic radiation, typically UV light. During the change, not only... 

The optical properties of a material are defined in terms of refractive index, clarity or transparency, haze and gloss. The refractive index of PP is 1.49. The remaining catalyst residue in the resin may affect the opacity of the PP resin and produce yellowness. Different catalyst systems may have different effects on the transparency and yellowness of the resin. Hence, the optical properties of equivalent grades of PP may be different. 

Optical characterization techniques are usually non-destructive, fast, and of simple implementation, most requiring very little sample preparation. These techniques explore the change on intensity, energy, phase, direction, or polarization of the light wave after interaction with the object being studied. Many of them can be performed at room temperature and atmosphere, dispensing the use of complex vacuum chambers. That, allied to the fact that the optical properties of a material... 

The optical properties of a material are characterized by the real and imaginary parts of the index of refraction N+iK) as a fimction of wavelength. These optical parameters are obtained from the square root of the dielectric function, which may also be complex. 

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