Verification of generalized Kramers-Kronig relations and sum rules on experimental data of third harmonic generation susceptibility on polymers

Abstract

We present the first analysis of harmonic generation data where the full potential of the generalized nonlinear Kramers-Kronig (K-K) relations and sum rules is exploited. We consider two published sets of wide spectral range experimental data of third harmonic generation susceptibility on different polymers, the polysilane (frequency range: $0.4-2.4$ $eV$) and the polythiophene (frequency range: $0.5-2.0$ $eV$). We show that, independent truncated dispersion relations connect the real and imaginary part of the moments of the third harmonic generation susceptibility $ω^{2α}χ^{(3)}(3ω; ω, ω,ω)$, with $α$ ranging from 0 to 3, in agreement with the theory, while there is no convergence if we choose $α=4$. We repeat the same analysis for $ω^{2α}[χ^{(3)}(3ω; ω, ω,ω)]^{2}$ and show that a larger number of independent K-K relations connect the real and the imaginary part of the function under examination. We also compute the sum rules for the suitable moments of the real and imaginary parts, and observe that only considering higher powers of the susceptibility the correct vanishing sum rules are more precisely obeyed. Sum rules providing explicit information about structural properties of the material seem to require wider spectral range. These constraints are expected to hold for any material and provide fundamental tests of self-consistency that any experimental or model generated data have to obey.