Second electrooptic measurements

This high glass transition temperature polymer exhibits large electrooptic coefficients, -14-35 pm fV. The second harmonic measurements indicate long-term stability of the dipole orientation (> 800h at 100 °C). 

As shown in previous sections of this chapter, when an external perturbation is applied to the polymer film (such as irradiation), the ATR guided modes shift their angular positions and the reflectivity is modulated (Fig. 31b). These angular shifts are very small in the case of electrooptic experiments they correspond to refractive index variations of the order of 10 . One has then to modulate the measuring electric field at a low frequency Q( = cos fit) and to detect the modulated signal with lock-in amplifiers. The lock-in signals detected at the modulation frequency and its second harmonic give, respectively, the linear (or Pockels) and the quadratic (or Kerr) electrooptic effects. The amplitude of the modulation of the thickness and the refractive indices is evaluated by a computer fit, and allows the determination of Pockels (r) and Kerr (s) coefficients (Eqs. 28) ... 

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). 

Phases having biaxial symmetry (tilted smectic phases) exhibit dielectric biaxiality in particular. At frequencies of 1 MHz and below, the biaxiality becomes important and critically influences the electrooptic switching behavior of the SmC phase. It is therefore important to be able to measure the biaxiality at these frequencies. Being a symmetrical second rank tensor, the dielectric permittivity can always be diagonalized in a proper frame and described by three components along the principal directions. The three principal values can then be expressed by a single subscript and can be determined by three independent measurements performed at three different orientations of the director relative to the measuring electric field. In practice, this may not be that... 

Here ai is the frequency of the first strongly absorbing electronic transition in the molecule, and la and at are the fundamental wavelengths in second harmonic generation and for electrooptic coefficient measurements, respectively. The electrooptic effect (Pockels effect) is related to the corresponding second-order NLO susceptibility and by knowing the SHG coefficients, one can also estimate the electrooptic coefficients. 

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