Photoactive monomeric amorphous materials for second-order nonlinear optical applications

摘要

Various push-pull azobenzene molecular derivatives showing glassy properties have been processed as thin monomeric amorphous films using spin-coating and thermal evaporation in vacuum. This latter deposition technique yielded homogenous thin films whose surface RMS roughness has been found less than 0.3 A. Contrarily to spin-coated films requiring external poling for SHG emission, evaporated thin films have proved spontaneously active in second harmonic generation (SHG) when probing at 1.907 μm. Heating above T_g and continuous illumination with an Ar~+ laser in the sample absorption maximum band caused dramatic and irreversible disappearance of the spontaneous SHG signal. Applying an external static electric field by using corona discharge induced SHG reversible modulation whose sign has been tuned by changing the bias from positive to negative. Films obtained by vacuum evaporation under a 4 kV voltage underwent considerable morphology changes characterized by the formation of stable 250 nm high nanoclusters evidenced by AFM imaging. The dramatic decrease in SHG has been attributed to the formed nanopillars whose growth depends apparently on the substrate polarization state and must obey a centrosymmetric organization.