The attribute dielectric of a medium indicates that electric fields can penetrate such a medium. (The Greek word dia stands for through, and dia-electric was contracted to become dielectric.) Such field penetration (at least for DC fields) is possible only for electrically insulating materials, because in electrical conductors some of the electrical charges can freely move until the generated space charges completely screen the externally applied electric field. Therefore, only quite weak electric fields can occur within metals, for example, and then always drive an electric current.
Dielectrics can in principle be solid, liquid or gaseous substances (only not plasmas), but solid dielectrics are most common in technical applications.
Polarization by Electric Fields
In simple physical models, dielectric materials (= dielectrics) also contain electrical charges, which however are bound to their atoms. They can thus, in reaction to an applied electric field, only produce some local polarization, quantified as a density of electric dipoles (called the polarization P, not to be confused with the polarization of light). A characteristic property of a dielectric medium is its electrical susceptibility χ, which is directly related to its relative permittivity (= dielectric constant): χ = εr − 1.
In linear optics, the induced polarization is given by P = ε0 χ E, where the susceptibility is taken for the optical frequency (not for low frequencies as in electronics), and is in the most general case (for anisotropic materials) described as a tensor. It can generally be complex, with the imaginary part indicating absorption losses, but these are often quite weak. In nonlinear optics, one also considers nonlinear contributions to the polarization with terms which contain products (or squares) of electric field strength components.
In some application areas, dielectrics are explicitly meant to be substances with high electric susceptibility, as are useful e.g. for fabricating electric capacitors. However, materials with low susceptibility are preferred in other cases, e.g. for minimizing stray capacitances.
The Band Gap
In solid state physics, one considers all possible electronic states of electrons, having different energies, and finds that there can be band gaps, i.e., regions of energy for which no electronic states exist. It turns out that dielectric materials exhibit such a band gap of significant width (large compared with the thermal energy at room temperature) between their valence band and their conduction band. As a consequence, the valence band is essentially completely filled, while the conduction band is essentially not populated at all – unless there are certain external influences such as incident ultraviolet light or an applied extremely strong electric field, causing an electrical breakdown.
Band structures are more easily calculated for materials with periodic microscopic structures, but also exist for amorphous materials like glasses.
The described kind of band structure is required for an electrical insulator, and is also essential for the optical properties of dielectrics, in particular for their transparency for near-infrared and visible light. Only with a sufficiently high photon energy, carriers from the valence band can be excited into the conduction band by absorption of light (unless at very high optical intensities where multiphoton absorption processes are possible). That level of photon energy is usually reached only in the ultraviolet region, or at the short-wavelength edge of the visible region (blue light).
Absorption of light with rather long wavelengths, e.g. in the mid-infrared, is it still possible – not based on the mentioned excitation of electrical carriers, but on the excitation of phonons, i.e., quantized lattice vibrations. Therefore, one has some transparency region between the ultraviolet absorption based on electrons and the infrared absorption based on phonons. Frequently, the propagation losses due to absorption and scattering of light are very low, which is important e.g. for optical fibers and for dielectric supermirrors. However, the UV and infrared absorption influence the refractive index in the transparency region, including its frequency dependence (→ chromatic dispersion).
Dielectric Materials for Optics
A wide range of dielectric materials is used in optics, usually exploiting their optical transparency in the relevant wavelength regions:
- There are optical crystals, often single crystals, i.e., with a very wide-range microscopic order. Classical examples are quartz, calcite, diamond and sapphire. Some of them (e.g. with a cubic lattice structure) are optically isotropic, while others exhibit pronounced anisotropic properties and birefringence, i.e., a polarization-dependent refractive index. Polycrystalline materials (e.g. ceramics) are less often used in optics, since they usually exhibit substantial scattering of light.
- Optical glasses, having an amorphous microscopic structure and therefore usually optically isotropic properties, have been used from the very beginning of technical optics. Depending on their dispersive properties, they are divided into crown glasses and flint glasses. Many of them can be made with good optical quality and without requiring extraordinarily difficult fabrication techniques. However, one also uses ultra-pure chemically processed glasses like fused silica, e.g. for use in fiber optics.
- There are well transparent polymer materials, which can be used in plastic optics.
Some dielectrics are used as bulk materials (e.g. for lenses and prisms), while others are used as dielectric coatings, usually having a quite small thickness. There are also cases where one starts with a dielectric bulk material and locally modifies that to obtain a waveguide, or structures it to obtain a diffraction grating. Further, some photonic metamaterials are made from dielectrics; here, nanostructures are used to substantially modify the optical properties.
Many different optical properties can be relevant for the use of dielectric materials, for example:
- the degree of transparency, limited by residual absorption and scattering of light
- the refractive index at relevant wavelengths
- the chromatic dispersion through the wavelength-dependence of refractive index
- possibly anisotropic behavior and birefringence
- nonlinear optical properties, for example the nonlinear index of refraction
- thermo-optic properties, e.g. the temperature dependence of refractive index
- thermo-mechanical properties, e.g. piezo-optic coefficients
In addition, there is a range of possibly relevant non-optical properties:
- the chemical durability and resistance
- the density
- the mechanical hardness and rupture strength
- the ease of grinding and polishing
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