Origin of the photoinduced optical second harmonic generation arising in N-phenyl microcrystalline films

Photoinduced second-order nonlinear optical effects, particularly optical second harmonic generation (SHG) of N-phenyls with different numbers of aromatic rings deposited on glass substrates were studied. As a fundamental beam, a 5-ps pulsed Nd:YAG laser was used. Quantum chemical time-dependent density functional theory (TDDFT) simulations of the nonlinear optical properties were performed. The first-order hyperpolarizabilities of isolated molecules were calculated, under the influence of a polarized pumping beam, to evaluate the role played by the nanointerfaces separating the microcrystallites and the amorphous environment. Consideration was performed within a framework of steady-state Langevin order parameters for amorphous-like films. A strong dependence of the photoinduced SHG versus the number of aromatic rings determining the degree of film crystallinity was shown. A comparison of experimental data and theoretically evaluated results shows that for the photoinduced first-order nonlinear optical effect the dominant contribution is an amorphous-like structural component, unlike the transport properties, where the crucial role is played by the nanointerface region. This may reflect a specific feature of the multiphoton processes in such types of nanointerfaces because of nanoconfined effects.