Fano-Liouville spectral signatures in open quantum systems.

The scattering amplitude from a set of discrete states coupled to a continuum became known as the Fano profile, characteristic for its asymmetric line shape and originally investigated in the context of photoionization. The generality of the model and the proliferation of engineered nanostructures with confined states gives immense success to the Fano line shape, which is invoked whenever an asymmetric line shape is encountered. However, many of these systems do not conform to the initial model worked out by Fano in that (i) they are subject to dissipative processes and (ii) the observables are not entirely analogous to the ones measured in the original photoionization experiments. In this Letter, we work out the full optical response of a Fano model with dissipation. We find that the exact result for the excited population, Raman, Rayleigh, and fluorescence emission is a modified Fano profile where the typical line shape has an additional Lorentzian contribution. Expressions to extract model parameters from a set of relevant observables are given.

Probing Raman enhancement in a dopamine-Ti2O4 hybrid using stretched molecular geometries.

Hybrids consisting of a metal oxide nanoparticle and a molecule show strong enhancement of Raman modes due to an interfacial charge transfer process that induces the formation of midgap states, thereby reducing the effective gap compared to that of the nanoparticle and creating the posibility of an electronic resonance at energies substantially lower than the nanoparticles's band gap. We have developed a simple methodology to mimic the presence of the nanoparticle through a deformation of the bond involved in the chemical binding between the two entities forming the hybrid. The results provide a convincing interpretative frame to the enhancements observed in Raman spectra when all atoms are included. In addition, these enhancements can be correlated to a crossing of excited molecular orbitals that take part in the virtual excitation associated with the Raman process. We illustrate our method for the dopamine-Ti2O4 hybrid using the most acidic molecular O-H bond as the control parameter for the deformation.

Laser-induced nonlinear response in photoassisted resonant electronic transport.

We consider an extension of our previous model for photoassisted electron transport in molecular and atomic junctions to the study of nonlinear processes, induced by a laser field, that are strongly influenced by a resonant level structure associated with the material structure between the two metal electrodes. Using a Floquet transformation to include the radiation field, we calculate the stationary Landauer current as a function of the intensity and frequency of the laser. The emphasis in this work is in the description of the interplay between the optical response of the junction and its transport properties. Our results may be then of importance in understanding the response of molecular junctions to multiphoton excitations, a regime where nonlinearities are dominant.