A femtosecond optical switch using molecular two-photon absorption with multi-step charge dissociation

Abstract

Organic semiconductors exhibit strong optical nonlinearity and can be utilized in optical modulation schemes. However, the response speed of such materials is limited by the lifetime of the largely delocalized π-electrons, in particular for application in Kerr type optical switching devices. We report here a thin-film optical switch with a response time shorter than 300 fs in a single-layer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N′-diphenyl)-N,N′-di(p-butylphenyl)-1,4-diamino-benzene)] (PFB), where the unique band structure of PFB that allows strong transient two-photon absorption with high contrast is the main responsible mechanism. The exciton absorption from the bottom of the conduction band to the higher-lying bands is bleached through further excitation by the pump pulses, which induces charge dissociation on higher-lying excited states and largely enhances the contrast of the optical switching signal. The findings here not only supply new insights into the photoelectronics of the conventional polyfluorene semiconductors, but also explore new applications of such organic thin-film materials in nonlinear optics and optical modulation devices.