Nonlinear optical probe of structures and phase transitions in ultra-thin molecular films

The study of nanometer-thick molecular thin films deposited on a solid surface, due to recent technology applications, has become an important subject. Effective tools for unraveling the intrinsic structure within the molecular films and their growth mechanism, however, are still in the searching. Consequently, little is known about the structure and the most important factors controlling deposition of thin molecular films. This paper summarizes the demonstration showing that the nonlinear optical phenomenon- second harmonic generation- because of its symmetry properties can be used effectively to characterize the structure within the molecular films. Experiments show that disorder-order phase transitions, glass transitions, crystallization kinetics and nucleation processes, and interfacial molecular structure within the thin molecular films can be characterized. The nonlinear optical studies have revealed the mechanisms and established the most important criteria for the deposition and growth of ultrathin molecular films.