ABSTRACT
Although “hyper-Rayleigh” scattering does not refer to any specific higher-order scattering, it is usually reserved for the scattering at the second-harmonic frequency (Clays and Persoons 1991). Since there is no static field involved, hyper-Rayleigh scattering is a second-order nonlinear optical effect, governed by the second-order nonlinear susceptibility. As for any even-order phenomenon, there is the noncentrosymmetric requirement. This means that no second-harmonic generation can come from a centrosymmetric medium. Only noncentrosymmetric media (dipolar or
octupolar) will give a second-order response. We should keep in mind that this rule is applied on the microscopic, molecular scale as well as on the macroscopic, bulk scale. At the molecular scale this problem can be solved by using electron donor and acceptor substituted conjugated D-p-A dipolar or octupolar molecules (CCl4 for example; Terhune et al. 1965). However, when all such molecules are also randomly oriented, no second-order signal will occur from a bulk isotropic medium. A polar order should be induced on the macroscopic scale. This can be achieved by, for example, electric-field poling, noncentrosymmetric crystal growth, the X-or Z-type Langmuir-Blodgett deposition technique, or the more stable Y-type deposition but then in combination with alternate deposition (Prasad and Williams 1991), again to overcome the centrosymmetry arrangement (see Chapter 3).
