![]() ![]() The best-established technique that exploits these unique symmetry properties is the application of SHG and sum frequency generation (SFG) in studies of surfaces and interfaces, in which only the comparatively few molecules at the interface that exhibit polar order contribute to the coherent detected signals ( 1– 3). However, certain classes of ordered systems allow for coherent addition, rather than cancellation. Essentially, coherent interference between individual SHG-active moieties within the focal volume in an isotropic medium results in cancellation and no net coherent output. In isotropic disordered media, second harmonic generation (SHG), or the frequency doubling of light, is symmetry forbidden. Second-order NLO processes are dictated by unique symmetry conditions, compared with conventional, linear optical effects. Nonlinear optical (NLO) imaging may serve as a powerful complement to the existing suite of measurement tools for fundamental investigations of the initial stages of crystal formation. Tools that can reduce the size scale for crystal characterization can approach such small-scale, short-lived structures, which may allow the description of crystal formation. The greater conformational flexibility and chemical complexity of typical organic molecules, compared with their inorganic counterparts, make these molecules particularly challenging to study and control, yet they arguably represent the most biologically important solid-state materials. Information about the molecular interactions that lead to nucleation is often inferred from observations recorded long after the initial events take place. Fundamental studies of crystallization are often complicated by the so-called needle-in-a-haystack problem: Crystal nucleation is a rare and transient event. Chemical reactivity, stability, and dissolution kinetics are significantly influenced by the nature of the molecular packing and the orientation within the lattice. Crystal formation dictates the physical and chemical properties of solid-state materials. ![]()
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