Quadratic electrooptics

An attempt is made to fit quadratic electrooptic (QEO) results to a two-level model for the microscopic third order susceptibility, y. 

DIRK AND KUZYK Quadratic Electrooptic Effect in Small Molecules... 

We report the largest known useful microscopic NLO susceptibilities for the linear and quadratic electrooptic effects. The QEO susceptibility of 9 might be large enough to explore simple primitive QEO modulation experiments, though perhaps is not nearly large enough to be of commercial importance. 

X(3) ( — (1) 0), 0,0) Quadratic electrooptic effect Variable-phase retardation, liquid-crystal displays... 

In electric-field-induced birefringence (or electrical birefringence, quadratic electrooptic effect, Kerr effect), an isotropic transparent substance becomes birefringent when placed in an electric field . The sample assumes the characteristics of a uniaxial crystal, the optical axis of which is parallel to the direction of the applied field. When the sample is illuminated normal to , the resulting two indices, and wj (which can be thought of as He and ), are associated with the parallel and perpendicular orientations, respectively. 

Explain why BaTiOs is a linear electrooptic material below E but a quadratic electrooptic material above E. 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2 > = > - - third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect co = co + 0), or the quadratic electrooptic (Kerr) effect ( > = > - - 0 -I- 0). These electrooptic effects are discussed extensively in the article Electrooptical Applications (qv). EFISH is... 

For every nonlinear optical effect, one would expect that there is a measurement technique to characterize it. Not all of NLO effects, however, are subject to measurements that are convenient or informative for comparative purposes or device applications. This section highlights a few of the more common test methods for NLO organic molecules and polymers, and provides references for more detailed explanations of these techniques. Significant omissions here are techniques based on the linear and quadratic electrooptic effect, which are discussed in the article Electrooptical Applications. 

Optical Kerr Effect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an intensity-dependent refractive index change. The sample is placed between crossed polarizers and a weak, typically tunable, continuous wave (cw) probe laser (usually at a different wavelength and polarized at 45° to the pump pulse) overlaps the pumped region. The increased transmission of the probe beam when the pump pulse arrives is proportional to (Xeff), a combination of elements of the tensor. Many... 

Third-order nonlinear optical properties (namely, quadratic electrooptic coefficient) of this polymeric system at different dye concentrations have been measured using a modified Michelson interferometer at 633nm. For this measurement, the polymer film was spin-coated from the filtered solution on an indium-tin-oxide (FTO) coated glass plate where the ITO layer was used as an electrode. A gold layer of 500A thickness was thermally evaporated onto the polymer film to form the second electrode. The experimental set-up of this modified Michelson interferometer is schematically shown in (Figure 7). 

Figure 7. Experimental set-up for the measurement of quadratic electrooptic coefficient.

The first observation of natural optical anisotropy was made in 1669 by Bartolinius in calcite crystals, in which light travels at different velocities depending on the direction of propagation relative to the crystal structure. The electrooptic effect, electric-field-induced anisotropy, was first observed in glass in 1875 by J. Kerr. Kerr found a nonlinear dependence of refractive index on applied electric field. The term Kerr effect is used to describe the quadratic electrooptic effect observed in isotropic materials. The linear electrooptic effect was first observed in quartz crystals in 1883 by W. Rontgen and A. Kundt. Pockels broadened the analysis of this relationship in quartz and other crystals, which led to the term Pockels effect to describe linear behavior. In the 1960s several developments... 

Optical shutters utilize the properties of quadratic electrooptic materials as shown in Fig. 3.53. Switching from zero field to the field required for half-wave retardation results in a light shutter. If the electrooptic material is placed between crossed polarizers, polarized... 

Broussoux and Mkheron [17] reported electrooptic and elastooptic effects in stretched PVDF. The linear EO coefficients in PVDF were measured at room temperature and constant stress at the He-Ne laser wavelength. The refractive indices n, 1.444. A = 1.436, and n, 1.42S and electrooptk coefficients rn = 0.10 pmA, 0.21 pm/V were OKasured. The secondary electrooptic effect estimated for the longitudinal configuration was ZS% of the measured free electrooptic coefficient. The quadratic electrooptic coefficients at constant strain and stress were gu 0.02 m /C and ga s 0.01 m /C. ... 

NLO organic molecules and polymers, and provides references for more detailed explanations of these techniques. Significant omissions here are techniques based on the linear and quadratic electrooptic effect, which are discussed in the article Electrooptical Applications. 

Optical Kerr Eftect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an... 

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