Theoretical studies on the frequency-dependent first hyperpolarizability of a hemicyanine derivative

The frequency-dependent first hyperpolarizabilities β(−2ω; ω, ω) and β(−ω; ω, 0) of the hemicyanine derivative, 4-N,N-dimethylamino-4′-N′-methyl stilbazolium (DAS), have been theoretically investigated by using ab initio and semi-empirical methods at the near-infrared region, respectively. Theoretical results indicate that the first hyperpolarizabilities, dominated by the contribution of the first excited state, show strong frequency dependence on the incident light. With the increase of the incident light frequency in infrared region, the values of β(−2ω; ω, ω) and β(−ω; ω, 0) increase, but β(−2ω; ω, ω) increases more significantly in near-infrared range (λ < 1300 nm) due to the resonant enhancement effect. The influence of the basis sets on the first hyperpolarizabilities was also discussed in time-dependent Hartree-Fock (TDHF) calculation.