Optical absorption coefficient, phase

Ferroelectric materials are capable of being polarized in the presence of an electric field. They may exhibit considerable anomalies in one or more of their physical properties, including piezoelectric and pyroelectric coefficients, dielectric constant, and optoelectronic constant. In the latter case, the transmission of light through the material is affected by the electric field, which produces changes in refractive index and optical absorption coefficient. Varying the applied field changes the phase modulation. 

CZTS is composed of abundant and non-toxic elements and has a 1.45-1.51 eV band gap with a high optical absorption coefficient (> 104 cm ), which makes it suitable for solar cell absorber layers. CZTS also shows promising thermoelectric properties, with ZT values of up to 0.36 at 700 K. Control of the materials composition has been shown to be fundamental for optimization of its functional properties. Solution processed CZTS absorber layers have provided photovoltaic efficiencies much higher than those obtained by vacuum-deposition techniques. This may be attributed to the better control of the composition and crystal-phase homogeneity by solution processing. Therefore, solution based routes for the preparation of solar absorber materials and solar cells are moving more and more into focus of scientific and industrial research. 

Various polymorphs have been reported for SnS with band gap widths in the range 1.0-1.5 eV, depending on the preparation method. The a-SnS (herzenbergite) is the most frequently occurring phase and is a p-type semiconductor with a direct optical transition at 1.3 eV and a high absorption coefficient (> 10" cm ). The orthorhombic S-SnS phase possesses a direct gap between 1.05 and 1.09 eV. 

Reaction chambers fitting the Harrick Praying Mantis mirror optics are available commercially, and sketches or images are presented in the product description (Harrick, 2006), in the work of Weckhuysen and coworkers (Weckhuysen and Schoonheydt, 1999 Weckhuysen et al., 2000 Weckhuysen, 2002 Weckhuysen, 2003 Weckhuysen, 2004) and in a handbook article by Sojka et al. (2008). A low-pressure and a high-pressure version, suitable at pressures up to 202-303 kPa or 3.4 MPa (500 psi), are available they are characterized by a dome with either three flat, circular windows or a dome with a single quartz half-sphere shaped quartz block with a small (also half-sphere shaped) volume above the catalyst. Evacuation to pressures less than 1.33 x 10-6 hPa and a maximum temperature of 873 K (under vacuum) are specified. A low-temperature version is specified for 123-873 K and up to 202-303 kPa. In the low-pressure versions, there are several centimeters of beam path through the gas phase, so that gas phase contributions are more likely to be observed than in experiments with cells holding the sample directly at the window (this depends on the gas phase concentrations and molar absorption coefficients). 

The temperature dependences of optical properties of organic conductors beyond the phase-transition region have not been investigated sufficiently so far. The quantitative temperature studies of the e-mv coupling are very difficult and possible only for some selected low-dimensional salts. It was shown [94,95] that an analysis of T dependence of the IR spectra of the salts composed of isolated dimers (TCNQ)2- makes it possible to pinpoint the main mechanisms responsible for thermal evolution of the IR spectra and changes in the absorption coefficients. Among other things it was... 

Several authors have addressed the determination of the optical properties of aqueous titanium dioxide suspensions in the context of photoreactor modeling (Brandi et al., 1999 Cabrera et al., 1996 Cured et al., 2002 Salaices et al., 2001, 2002 Satuf et al., 2005 Yokota et al., 1999). Among the determined properties are extinction, scattering, and absorption coefficients, as well as the asymmetry parameter of the scattering phase function. In general the procedures involve fitting of a radiative transfer model to the experimental results for reflectance and transmittance of radiation. 

The discrete ordinates method in a S4-approximation is used to solve the radiation transport equation. Since the intensity of radiation depends on absorption, emission and scattering characteristics of the medium passed through, a detailed representation of the radiative properties of a gas mixture would be very complex and currently beyond the scope of a 3D-code for the simulation of industrial combustion systems. Thus, contributing to the numerical efficiency, some simplifications are introduced, even at the loss of some accuracy. The absorption coefficient of the gas phase is assumed to have a constant value of 0.2/m. The wall emissivity was set to 0.65 for the ceramic walls and to a value of 0.15 for the glass pane inserted in one side wall for optical access. 

Tlie usual experimental techniques developed to study the optical Kerr effect in materials have already been described in a preceding chapter of this book. We only mention here the methods which have especially been used for nanocomposite materials as colloidal solutions or thin films Degenerate four-wave mixing (DFWM) and optical phase conjugation, which provide the modulus of x only and may be completed by Interferometry techniques to get its phase as well, optical limiting, optical Kerr shutter, and z-scan, which is probably the most common technique used in recent years due to its ability to provide simultaneously the nonlinear refraction and absorption coefficients of the same sample point [118],... 

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... 

---