Quadratic electrooptical measurements

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.

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. ... 

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... 

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) ... 

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