The temperature dependence of the surface polarization has been measured for both the planar and homeotropic orientation of a nematic liquid crystal at a solid substrate. A conventional liquid crystal 5CB, pure and doped with an azo-dye, was used in cells with controlled asymmetry for light absorption. The measurements have been made by a pyroelectric technique using short pulses of a YAG laser to create a temperature increment. The latter, in turn, was measured independently by a novel time-resolved "optical thermometer" technique monitoring temperature-dependent birefringence by a He-Ne laser beam. In accordance with the symmetry of the order parameter, the surface polarization has different sign for the two orientations, its magnitude ranges from −4 to +2 pC/m. The same technique has been used for the measurement of the flexoelectric polarization in hybrid cells. The sum of the flexoelectric coefficients is e 1 + e3 = −13 pC/m at 25 • C.

The temperature dependences of the surface polarization have been measured at the interface of a conductive glass with both the homogeneously and homeotropically oriented nematic liquid crystal p-methoxybenzylidene-p(')-butylaniline. The polarization was found in the field-off regime from the pyroelectric response of a cell to a short laser pulse, absorbed in the bulk of the liquid crystal. The temperature increment was calculated from the measurements of the birefringence induced by the same light pulse. It has been shown that the surface polarization at the homeotropic (m(h)) and planar (m(p)) interfaces is directed from an interface into the bulk and from the bulk to an interface, respectively (with a magnitude m(h) approximately -0.3 pC/m and m(p) approximately +0.2 pC/m at 25 degrees C). The experimental data may be explained in terms of the quadrupole model of the order-electric polarization with account of some additional contribution from molecular dipoles. The same technique also allows for the measurements of the z component of the flexoelectric polarization using a pyroelectric response of a hybrid (homeoplanar) aligned nematic cell and proper subtracting of the surface contributions. The flexoelectric polarization has been shown to be opposite to the sum of the surface terms m(h)+m(p) and directed from the planar to homeotropic interface. This means that the sum of the flexoelectric coefficients e=(e(1)+e(3)) is positive (e congruent with 1.7 pC/m at 28 degrees C). The temperature dependence of e has been shown to involve a combination of both the quadrupolar and dipolar contributions.

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The temperature dependencies of surface polarization in the nematic (N) and smectic A (SmA) phases have been measured for both the planar and homeotropic orientations of 4-octyloxy-4′-cyanobiphenyl (8OCB). Polarization was determined in the field- off regime from the pyroelectric response of the cell to a short laser pulse, and the temperature increment was calculated from the in situ measurements of the light induced birefringence. The surface polarization in the N-phase for both orientations is directed from an interface into the bulk at similar magnitudes (about 0.5–0.8 pC/m) but its change at the N–SmA transition was qualitatively different for the two cases. The experimental data are consistent with the quadrupolar model of the order-electric polarization. The field-off measurement technique also allowed for the measurements of the z-component of the flexoelectric polarization of 8OCB in both the N and SmA phases using a hybrid aligned nematic cell. The sum of the flexoelectric coefficients (e 1+e 3) in both phases was shown to be positive (e 1+e 3 ≃7.4 pC/m at 40°C), which is opposite to the case of 4-pentyl-4′-cyanobiphenyl (5CB). On the molecular scale, the difference is accounted for by the different distribution of electric charges in molecular quadrupoles forming the corresponding liquid crystalline phases.