Refractive indices intensity dependence

However, the linear response of a dielectric to an applied field is an approximation the actual response is non-linear and is of the form indicated in Fig. 8.6. The electro-optic effect has its origins in this non-linearity, and the very large electric fields associated with high-intensity laser light lead to the non-linear optics technology discussed briefly in Section 8.1.4. Clearly the permittivity measured for small increments in field depends on the biasing field E0, from which it follows that the refractive index also depends on E0. The dependence can be expressed by the following polynomial ... 

The photorefractive effect is a physical mechanism where the change in the intensity-dependent refractive index is dependent on the spatial variations of intensity. It is a non-local process, because unlike most processes, the change in the refractive index is not dependent on the magnitude of the intensity that produces such change. 

A material is said to possess third-order optical nonlinearity if its refractive index (n) depends, reversibly, on the intensity of incident light (/), that is,... 

Attenuated total reflection, on which atr—ftir is based, occurs when the rarer medium is absorbing and is characterized by a complex refractive index (40). The absorbing characteristics of this medium allow coupling to the evanescent field such that this field is attenuated to an extent dependent on k. The critical angle in the case of attenuated total reflection loses its meaning, but internal reflection still occurs. Thus, if the internally reflected beam is monitored, its intensity will reflect the loss associated with the internal reflection process at the interface with an absorbing medium. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

Quantification at surfaces is more difficult, because the Raman intensities depend not only on the surface concentration but also on the orientation of the Raman scat-terers and the, usually unknown, refractive index of the surface layer. If noticeable changes of orientation and refractive index can be excluded, the Raman intensities are roughly proportional to the surface concentration, and intensity ratios with a reference substance at the surface give quite accurate concentration data. 

The refractive index of the sample can be written as a complex number 2 = n2 — ik2. At wavelengths where the sample is not absorbing, 2, the absorption constant, equals zero. However, kj is non-zero at wavelengths where the sample is absorbing. In transmission spectroscopy, the intensity of an absorption band depends almost entirely on k2 while in ATR the intensity of the same band is a complex function of 2 and 2- Nevertheless, the statement made previously still holds. There will be absorption bands in ATR at wavelengths where 2 0. Thus, bands are expected at the same wavelengths in transmission and in ATR but their intensities may be dissimilar. 

The resolution of the instrument in the vertical direction depends upon the wavelength of visible light (450 to 850 nm), the oil refractive index, and the difference between the maximum and the minimum interference intensity as follows [5,18] ... 

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

---