Field Effect Polarization Modulators

The application of an external field onto many materials will induce optical anisotropy. If the applied field oscillates, a time-dependent modulation of the polarization of the light transmitted by the device will result. Modulators of this sort include photoelastic modulators (PEM) [30,31], Faraday cells [32], Kerr cells [32], and Pockel cells. 

Typically quite large retardations can be achieved (S n) and at frequen- 

The Faraday effect refers to the induction of circular birefringence following the 

The coefficient K is a measure of the Faraday effect and is proportional to the applied 

The Kerr effect is the result of applying an electric field to produce birefringence. This phenomena is commonly observed for both colloidal and polymeric liquids and is used in the characterization of the structure of these materials. Alternatively, by using an AC electric field, a modulation of the polarization of light can be affected. Such devices have rarely been used as modulators but do have the potential of allowing higher frequencies than the more common photoelastic devices. 

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Based on the results obtained in the investigation of the effects of modulation of the electron density by the nuclear vibrations, a lability principle in chemical kinetics and catalysis (electrocatalysis) has been formulated in Ref. 26. This principle is formulated as follows the greater the lability of the electron, transferable atoms or atomic groups with respect to the action of external fields, local vibrations, or fluctuations of the medium polarization, the higher, as a rule, is the transition probability, all other conditions being unchanged. Note that the concept lability is more general than... 

A thorough discussion Is given of the field modulation technique, a new stationary relaxation method based on electric field perturbation of Ionic equilibria. Concomitantly the theory of electric field effect In Ionic systems Is reviewed especially stressing their Importance for conductance phenomena In low polar solutions. 

The measurement of the polarization properties of light can be automated and improved by introducing a modulation of the polarization. Here a regular, time-dependent variation is introduced onto the optical properties of certain devices within either (or both) the PSG or PSA sections of the instrument. The modulation can be one of two types rotation of an optical element with fixed optical properties, or the modulation of the optical properties (retardation, for example) of an element with a fixed orientation. These are referred to as rotary modulators or field effect modulators, respectively. The latter name reflects the use of external fields (stress, electric or magnetic) to impart the modulation in these devices. In any case, a periodic oscillation is introduced into the signals that are measured that can effectively isolate specific optical properties in the sample. 

Consider a plane-parallel condenser of capacitance C whose plates are a p-type semiconductor (e.g., a CP) and a metal, and polarize the latter negatively. Excess positive charges (i.e., holes) appear at the surface of the semiconductor, and since its conductivity is low, they are in fact distributed over a certain thickness within the material. These excess holes, or at least part of them, should take part in the conduction. Applying a voltage to an external electrode not in contact with the semiconductor modulates its conductivity. This is the principle of the field effect, and clearly this control of the current through a gate electrode opens the possibility of transistor action without requiring the existence of p-n junctions. 

Figure 2.7 Artistic view of electron-phonon scattering. Lattice motions involving the displacement of polar modes can scatter the electron inelastically. The polar fluctuations create dipolar fields that can modulate the electron distribution. The electron responds to these stochastic fluctuations in local fields with a change in its energy and effective momentum transfer to the lattice. This process is depicted by comparing (a) and (b) to visualise the motion of the lattice atoms, leading to a change in direction or momentum of the electron from its initial path shown in (a).

Secondary effects can introduce small errors in this method of measuring twist. Dust and surface scatches can, of course, seriously affect local alignment. More subtle are perturbations apparently produced by variations in the flow pattern (hydrodynamic domains) of neutral molecules. In the cells used here hydrodynamic instabilities become quite pronounced at field frequencies around 5 Hz at 17v and resemble the patterns described by Sussman.9 They were not visible at 0.3 Hz and 17v but Fig. 11 shows evidence of their presence. In this case, the 0 orientation of the cell was fixed and the cell was translated relative to the beam while applying 17v at 0.3 Hz. A slow variation in polarity modulation amplitude is expected because of the effects discussed in connection with Fig. 3. Superimposed are local oscillations of modulation amplitude possibly attributable to hydrodynamic domains. It may be surprising that neutral molecule flow would not exhibit a more pronounced effect however, the flow adjacent to the cell walls must be parallel to the walls and exert a minor orienting force on the thin layer of molecules with significant homogeneous component at 17v. 

An interdigital transducer deposited on the surface of a piezoelectric substrate (i.e., hthium niobate) launches a surface acoustic wave (Fig. 3.13). The interdigital transducer has a combhke structure that establishes an alternating electric field whose polarity is spatially alternating. This alternation produces an electrically induced strain in the medium via the piezoelectric effect that excites an acoustic wave. The acoustic wave modulates the index of refraction of the medium that diffracts the optical beam. Note that this diffraction effect is not only due to the modification of the index of refraction, but also to the physical deformation of the medium produced by the acoustic wave. 

In a motor-driven self-nuUing ellipsometer, the dc feedback currents are used to drive servomotors instead of being passed through the Faraday cell. As an alternative to the polarizer modulation described above, crystals with electro-optic effect (e.g., Pockels effect) can be used to modulate the relative phase retardation by an electric ac field. Independent modulations of polarizer... 

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