6 - Molecular crystals and thin films for photonics

Abstract

Organic materials with second-order nonlinear optical (NLO) activity have interesting potential for many applications in optics and optoelectronics, such as light-frequency conversion, terahertz-wave generation, electric-field detection, and electro-optic (EO) modulation. These materials are based on NLO molecules (chromophores) with a high molecular nonlinearity, which are most often dipolar. In order to achieve a macroscopic second-order NLO response, such molecules need to be arranged in a noncentrosymmetric way in a material. This can be done by incorporating the chromophores in a polymer matrix and subsequent electric-field poling, molecular self-assembly into amorphous acentric structures, or self-assembly into single-crystalline acentric structures. In a single-crystalline form, these materials feature a high thermal and photochemical stability, which is important for the majority of device implementations using organic materials. This chapter discusses state-of-the-art, second-order NLO, single-crystalline organic materials, including molecular and crystal engineering approaches, as well as processing in bulk and thin-film single-crystalline forms. We also present the most promising photonic applications of single-crystalline organic NLO materials, including integrated EO devices, terahertz-wave generation, and terahertz-wave detection.