Electro-optical effects

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Blinov LM, Chigrinov VG (1996) Electro-optical effects in liquid crystals. Springer, Berlin Heidelberg New York... 

Figure I. Comparative quantities for selected tensor components of second harmonic generation (left) and the linear electro-optic effect (right) (measured at 1.06 pm wavelength). (The strain free quantity, f, was measured at 0.633 pm wavelength except in the case of GaAs which was measured at - 0.9 pm). (Reproduced with permission from Ref. 8. Copyright 1982, Laser Focus.)...

Figure 9.3 Schematic illustration of second-order nonlinear optical effects, (a) Second-harmonic generation. Two light fields at frequency go are incident on medium with nonvanishing / 2. Nonlinear interaction with medium creates new field at frequency 2 go. (b) Frequency mixing. One light field at frequency GO and one at frequency go2 is incident on nonlinear medium. Nonlinear interaction with medium creates new field at frequency goi + go2. (c) electro-optic effect. Static electric field E (0) applied over nonlinear medium changes phase of an incoming light field.

The proportionality constants a and (> are the linear polarizability and the second-order polarizability (or first hyperpolarizability), and x(1) and x<2) are the first- and second-order susceptibility. The quadratic terms (> and x<2) are related by x(2) = (V/(P) and are responsible for second-order nonlinear optical (NLO) effects such as frequency doubling (or second-harmonic generation), frequency mixing, and the electro-optic effect (or Pockels effect). These effects are schematically illustrated in Figure 9.3. In the remainder of this chapter, we will primarily focus on the process of second-harmonic generation (SHG). 

The electro-optic property of EO polymers comes from the NLO chromophores. When these chromophores are preferentially aligned to break the centrosymmetry of the material, the molecular level microscopic NLO effect of the molecules translates to the macroscopic second-order NLO effect of the polymer material. The poled material exhibits a strong macroscopic electro-optic effect. 

Pockel cells use the linear electro optic effect in crystals to turn the plane of polarization when an external electric field is applied. A detailed description with references is given in... 

Where P is the polarisation and the others the linear (1) and non-linear, second (2) and third order (3) terms. Examples of important second order effects are frequency doubling and linear electro-optic effects (Pockles effect), third order effects are third-harmonic generation, four-wave mixing and the quadratic electro-optic effect (Ken-effect). 

Changes in the refractive index by the electro-optic effect lead to phase encoding of the incident light distribution... 

E. Possible Use in the Activation of Electro-Optic Effects in Liquid... 

Besides the direct electrical induction of electro-optical effects in liquid crystals, their activation by illumination of photoconductors could be of great technical interest. This method might well permit the electro-optical properties of nematic liquid crystals to be used on a larger scale, because photoconductor activation may eventually be applied to light amplification, optical data processing, and projection display systems, or used for recording phase-type holograms. 

Electro-optical effects (Stark effects) of CdS nanocrystallites were investigated 107,108,109... 

As a method to control wavepackets, alternative to the use of ultra-short pulses, I would like to propose use of frequency-modulated light. Since it is very difficult to obtain a well-controlled pulse shape without any chirp, it is even easier to control the frequency by the electro-optic effect and also by appropriate superposition of several continuous-wave tunable laser light beams. 

Hecht, E. Zajac, A. Optics, 2nd ed., Addison-Wesley Reading, MA, 1987. Contains elementary discussion of electro-optical effects and nonlinear optics. 

Historically, the earliest nonlinear optical (NLO) effect discovered was the electro-optic effect. The linear electro-optic (EO) coefficient rij defines the Pockels effect, discovered in 1906, while the quadratic EO coefficient sijki relates to the Kerr effect, discovered even earlier (1875). True, all-optical NLO effects were not discovered until the advent of the laser. 

For both electro-optic and NLO effects we would like the electric field (associated with an external applied voltage for the electro-optic effect and associated with the input light for NLO effects) to have a large effect on properties. 

Beneficial electro-optic effects have also been reported for semiconductor quantum dots doped into nematic liquid crystals. Khoo and Mallouck et al. published one of the earlier reports on suspensions of quantum dots in nematic liquid crystals [331], This work, however, focused on CdSe nanorods and will be discussed in a later section on two-dimensional nanomaterials in liquid crystals. 

---