Optical Materials High-Frequency Properties

Dielectrics and Electrooptics I 4.2 Optical Materials High-Frequency Properties 825... 

Optical Materials High-Frequency Properties 4.4.2.1 Crystal Optics General... 

Crystals with one of the ten polar point-group symmetries (Ci, C2, Cs, C2V, C4, C4V, C3, C3v, C(, Cgv) are called polar crystals. They display spontaneous polarization and form a family of ferroelectric materials. The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property - the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties [16, 388],... 

From these results, As the particular tensor component of DAD molecular crystal is expected to be very high, more than 2-methyl-4-nitroaniline(MNA)(20), it would become one of the most suitable materials of highly efficient optical device for frequency doubler by using phase matching with optical wave guide. Electro-optical properties as well may be interesting. 

The dielectric tensor describes the linear response of a material to an electric field. In many experiments, and particularly in optical rheometry, anisotropy in is the object of measurement. This anisotropy is manifested as birefringence and dichroism, two quantities that will be discussed in detail in Chapter 2. The nonlinear terms are responsible for such effects as second harmonic generation, electro-optic activity, and frequency tripling. These phenomena occur when certain criteria are met in the material properties, and at high values of field strength. 

Figure 3.58 shows a curve of the variation of the dielectric constant (relative permittivity) with firequency for a hypothetical solid dielectric having aU four mechanisms of polarization. Note that except at high frequencies the electronic mechanism makes a relatively low contribution to permittivity. However, in the optical range of firequencies, only this mechanism and the ionic mechanism operate they therefore strongly influence the optical properties of materials. 

In our context, it is of course not the properties of vacuum but the permittivity of biological materials that are of interest. It is not always clear whether s is the relative permittivity 8r (dimensionless, often called the dielectric constant) or, as in this book, the complete expression Sj. bq. For vacuum, Sj = 1. There are a lot of subscripts in use for different cases, bq does not mean permittivity at zero frequency, but the permittivity of vacuum. 8s means static permittivity. This is not the same as 8dc because static is electrostatic with no current flow, whereas DC means current flow but at zero frequency. 8m is the permittivity at very high frequencies. And indeed here it may be high frequencies up in the optical range. The relationship between the optical refractive index n and the permittivity is roughly n = /b. ... 

The optical properties of liquid crystals determine their response to high frequency electromagnetic radiation, and encompass the properties of reflection, refraction, optical absorption, optical activity, nonlinear response (harmonic generation), optical waveguiding, and light scattering [1], Most applications of thermotropic liquid crystals rely on their optical properties and how they respond to changes of the electric field, temperature or pressure. The optical properties can be described in terms of refractive indices, and anisotropic materials have up to three independent principal refractive indices defined by a refractive index ellipsoid. 

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