Visualization of Polarization in Organic Thin Films by Optical Second Harmonic Generation and Maxwell-displacement Current

Probing and modeling of carrier motions and polarization in thin films is a fundamental research subject in science and electronics. According to the Maxwell electromagnetic field theory, carriers are a source of electric field. Therefore, by probing dielectric polarization caused by the electric field arising from moving carriers and dipoles, we can find a way to visualize the carrier motions and polarizations in organic thin films. The techniques used here are an electrical Maxwell-displacement current (MDC) measurement and a novel optical method based on optical second harmonic generation (SHG) measurement. The MDC measurement probes changes of induced charge on electrodes, while the SHG and electric field-induced optical second harmonic generation (EFISHG) probes nonlinear polarization induced in organic thin layers due to coupling of electron clouds of molecules and electro-magnetic waves of incident laser beam. In this presentation, the concept for probing electron and hole transport in solids by using the EFISHG is discussed in comparison with the conventional time of flight (TOF) measurement. We then visualize carrier transit in organic field effect transistors. We also show that visualization using EFISHG microscopic image is a novel way for characterizing the anisotropic carrier transport in organic thin films and to visualize tribo-electrification. We further discuss the concept for the observation of polarization formed in monolayers by means of the MDC and optical SHG measurements, and discuss domain shapes and textures in monolayer films in comparison with the analytical solutions of shape equations of monolayers.