Nonresonant frequency dispersion of the electronic second hyperpolarizability of alltrans polysilane chains: An ab initio TDHF oligomeric approach

Abstract

The frequency-dependent electronic second hyperpolarizability of increasingly large polysilane chains is computed for the most common nonlinear optical (NLO) processes at the time-dependent Hartree–Fock level with the 6-31G atomic basis set. Due to σ-conjugation, the longitudinal component (γLe) turns out to be dominant. Its nonresonant dispersion relations are described by the coefficients of the power expansion formula, γLe(−ωσ; ω1, ω2, ω3)=γLe(0; 0, 0, 0)[1+AωL2+BωL4+CωL6+···], where ωL2=ωσ2+ω12+ω22+ω32 and γLe(0; 0, 0, 0) is the static limit value. In the infinite chain length limit, the CHF/6-31G static longitudinal electronic second hyperpolarizability per Si2H4 unit cell is estimated to attain 463±10×103 a.u. whereas the A coefficient reaches 27.8±0.9 a.u. The accuracy that could be reached from using this power expansion expression for estimating the second hyperpolarizability for other optical frequencies is discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 751–761, 1998