A Feynman diagram description of the 2D-Raman-THz response of amorphous ice

Low-frequency anharmonic couplings in crystalline bromoform: Theory

Theoretical calculations of the low-frequency anharmonic couplings of the β-phase of crystalline bromoform are presented based on density functional theory quantum chemistry calculations. The electrical and mechanical anharmonicities between intra- and intermolecular modes are calculated, revealing that the electrical anharmonicity dominates the cross-peak intensities in the 2D Raman-THz response and crystalline, as well as liquid, bromoform. Furthermore, the experimentally observed difference in relative cross-peak intensities between the two intramolecular modes of bromoform and the intermolecular modes can be explained by the C_3v-symmetry of bromoform in combination with orientational averaging. The good agreement with the experimental results provides further evidence for our interpretation that the 2D Raman-THz response of bromoform is, indeed, related to the anharmonic coupling between the intra- and intermolecular modes.

Mapping LiNbO_{3} Phonon-Polariton Nonlinearities with 2D THz-THz-Raman Spectroscopy

Two-dimensional terahertz-terahertz-Raman spectroscopy can provide insight into the anharmonicities of low-energy phonon modes-knowledge of which can help develop strategies for coherent control of material properties. Measurements on LiNbO_{3} reveal THz and Raman nonlinear transitions between the E(TO_{1}) and E(TO_{3}) phonon polaritons. Distinct coherence pathways are observed with different THz polarizations. The observed pathways suggest that the origin of the third-order nonlinear responses is due to mechanical anharmonicities, as opposed to electronic anharmonicities. Further, we confirm that the E(TO_{1}) and E(TO_{3}) phonon polaritons are excited through resonant one-photon THz excitation.

Low-frequency anharmonic couplings in bromoform revealed from 2D Raman-THz spectroscopy: from the liquid to the crystalline phase

Two-dimensional (2D) Raman-THz spectroscopy in the frequency up to 7 THz has been applied to study the crystalline beta-phase of bromoform (CHBr3). As for liquid CHBr3, cross peaks are observed, which however sharpen up in the crystalline sample and split into assignable sub-contributions. In the Raman dimension, the frequency positions of these cross peaks coincide with the intramolecular bending modes of the CHBr3 molecules, and in the THz dimension with the IR active lattice modes of the crystal. This work expands the applicability of this new 2D spectroscopic technique to solid samples at cryogenic temperatures. Furthermore, it provides new experimental evidence that the cross peaks indeed originate from the coupling between intra- and intermolecular vibrational modes.

Equilibrium-nonequilibrium ring-polymer molecular dynamics for nonlinear spectroscopy

Two-dimensional Raman and hybrid terahertz-Raman spectroscopic techniques provide invaluable insight into molecular structures and dynamics of condensed-phase systems. However, corroborating experimental results with theory is difficult due to the high computational cost of incorporating quantum-mechanical effects in the simulations. Here, we present the equilibrium-nonequilibrium ring-polymer molecular dynamics (RPMD), a practical computational method that can account for nuclear quantum effects on the two-time response function of nonlinear optical spectroscopy. Unlike a recently developed approach based on the double Kubo transformed (DKT) correlation function, our method is exact in the classical limit, where it reduces to the established equilibrium-nonequilibrium classical molecular dynamics method. Using benchmark model calculations, we demonstrate the advantages of the equilibrium-nonequilibrium RPMD over classical and DKT-based approaches. Importantly, its derivation, which is based on the nonequilibrium RPMD, obviates the need for identifying an appropriate Kubo transformed correlation function and paves the way for applying real-time path-integral techniques to multidimensional spectroscopy.