Ionization-assisted Deposition of Polyurea Thin Films for NLO Applications

Abstract

Polymeric material needs a poling process to align the dipole moment for activating optical nonlinearity. The major problem is the relaxation of dipole orientation, which is to be solved by designing a molecular structure of higher rigidity. However, it leads to a contradiction that the thermally stable polymer, like the linear main-chain NLO polymer, is difficult to align the dipole orientation by the poling procedure. This paper will propose a new concept in film formation of NLO polymer, integrating the dipole orientation process with the polymerization and film deposition. The authors have investigated the film formation of organic materials by using the ionization-assisted deposition method. It is a unique method that involves the ionization of evaporated material in the course of vacuum deposition, and grows the films with an assistance of the ionized particles that are accelerated toward the substrate. It is reported that polymeric thin films can be synthesized on the substrate surface by co-deposition of bifunctional monomers. If the polymerization proceeds under an electric field, the dipole orientation could take place much effectively, because the electric field influences the highly mobile monomers. Our objective is to control the dipole orientation in the film formation process by utilizing the electric field that is generated by the substrate bias voltage and by the electric charge carried by the deposition ions. Polyurea (PU) was chosen for the film material because of its thermal stability and optical transparency. Moreover, its dipole moment is built in the backbone, making this material attractive as a stable optical nonlinear material.