Large Hyperpolarizabilities of the Second Harmonic Generation Induced by Nonuniform Optical Near Fields

Abstract

We studied the optical selection rules and hyperpolarizabilities of a second harmonic generation (SHG) induced by a nonuniform optical near field (ONF) on the basis of first-principles calculations. The excitations of the symmetric and asymmetric molecules para-dinitrobenzene (pDNB) and para-nitroaniline (pNA), respectively, were investigated according to the time-dependent density functional theory. By calculating the ONF excitation dynamics of symmetric pDNB, we demonstrated that the ONF causes the SHG even in symmetric materials because of nonuniformity of the field in space. To quantitatively evaluate the intensity of the SHG induced by the ONF, we estimated the first hyperpolarizability, β(2ω), of asymmetric pNA for both of the usual far-field light and ONF excitations. Our results showed that β(2ω) for the ONF excitation is 1–2 orders of magnitude larger than that for the far-field excitation when the distance between the source of the ONF and pNA is around 10 Å.