Intensities of two-photon absorptions to low-lying even-parity states in linear-chain conjugated polymers

Rashba splitting in organic-inorganic lead-halide perovskites revealed through two-photon absorption spectroscopy

The Rashba splitting in hybrid organic-inorganic lead-halide perovskites (HOIP) is particularly promising and yet controversial, due to questions surrounding the presence or absence of inversion symmetry. Here we utilize two-photon absorption spectroscopy to study inversion symmetry breaking in different phases of these materials. This is an all-optical technique to observe and quantify the Rashba effect as it probes the bulk of the materials. In particular, we measure two-photon excitation spectra of the photoluminescence in 2D, 3D, and anionic mixed HOIP crystals, and show that an additional band above, but close to the optical gap is the signature of new two-photon transition channels that originate from the Rashba splitting. The inversion symmetry breaking is believed to arise from ionic impurities that induce local electric fields. The observation of the Rashba splitting in the bulk of HOIP has significant implications for the understanding of their spintronic and optoelectronic device properties.

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wave functions of the optical singlets, the lowest triplet exciton, and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against earlier and very recent experimental transient absorptions in the near- and mid-infrared, we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 2¹A_g^- in linear polyenes.

Hyperpolarizabilities for the one-dimensional infinite single-electron periodic systems. I. Analytical solutions under dipole-dipole correlations

The analytical solutions for the general-four-wave-mixing hyperpolarizabilities chi(3)[-(omega1 + omega2 + omega3);omega1,omega2,omega3] on infinite chains under both Su-Shrieffer-Heeger and Takayama-Lin-Liu-Maki models of trans-polyacetylene are obtained through the scheme of dipole-dipole correlation. Analytical expressions of dc Kerr effect chi(3)(-omega;0,0,omega), dc-induced second-harmonic generation chi(3)(-2omega;0,omega,omega), optical Kerr effect chi(3)(-omega;omega,-omega,omega), and dc electric-field-induced optical rectification chi(3)(0;omega,-omega,0) are derived. By including or excluding nabla(k) terms in the calculations, comparisons show that the intraband contributions dominate the hyperpolarizabilities if they are included. nabla(k) term or intraband transition leads to the break of the overall permutation symmetry in chi(3) even for the low-frequency and nonresonant regions. Hence it breaks the Kleinman [Phys. Rev. 126, 1977 (1962)] symmetry that is directly based on the overall permutation symmetry. Our calculations provide a clear understanding of the Kleinman symmetry breaks that are widely observed in many experiments. We also suggest a feasible experiment on chi(3) to test the validity of overall permutation symmetry and our theoretical prediction. Finally, our calculations show the following trends for the various third-order nonlinear optical processes in the low-frequency and nonresonant region: chi(nonres)(3)(-3omega;omega,omega,omega)>chi(nonres)(3)(-2omega;0,omega,omega)>chi(nonres)(3)(-omega;omega,-omega,omega)>chi(nonres)(3)(-omega;0,0,omega) > or = chi(nonres)(3)(0;omega,-omega,0), and in the resonant region: chi(res)(3)(-omega;0,0,omega)>chi(res)(3)(-omega;omega,-omega,omega)>chi(res)(3)(-2omega;0,omega,omega)>chi(res)(3)(0;omega,-omega,0)>chi(res)(3)(-3omega;omega,omega,omega).