Organic Nonlinear Materials for Frequency Conversion: Property Trade-off and Recent Applications

Abstract

Frequency doubling of short wavelength diode lasers has significant technological applications in imaging and optical recording. One of the major hurdles towards achieving efficient doubling remains to develop highly nonlinear materials with appropriate linear optical properties such as birefringence and transparency. One approach is to engineer materials by doping or grafting nonlinear organic molecules into transparent polymeric matrixes. Second-order nonlinearity is obtained by the acentric alignment of these molecules along their dipole moments under an external electric field. With this approach, several molecular properties are important, including large molecular dipole moment and hyperpolarizability inner product (μβμ) as well as very low absorption at the first and second harmonic wavelengths of diode lasers (near 800 and 400 nm). An established guideline in identifying organic molecules with large hyperpolarizabilities is by the presence of low-lying strong charge-transfer (CT) electronic transitions. However, such an approach is in conflict with the requirement of transparency.