Two open post-doc positions!

The "Femtoscopy" group @ Sapienza (Rome, Italy) has two open post-doc positions in the following areas of research:

• Ultrafast Raman spectroscopy

• Coherent Raman Imaging

Further details can be found in the attached files.

Candidates with hands on background in linear and non-linear optics are encouraged to apply.

Previous experiences with ultrafast lasers and/or coherent and/or time-resolved Raman spectroscopy and/or transient absorption are highly appreciated.

Both the appointments are initially for 12 Months with possible extension upon mutual consent.

Physical and chemical reactions driven by light absorption are determined by multidimensional excited-state potential energy surfaces (PESs). Nature has shaped excited-state PESs ad-hoc displaced with respect to the ground-state along specific nuclear reaction coordinates to drive the system photochemistry, inducing specific structural rearrangements which define the biological function by positive vs negative bond length modifications and torsional re-orientations, up to formation or rupture of chemical bonds. Such displacements are encoded in the Franck-Condon overlap integrals, which in turn determine the resonant Raman response. Conventional spectroscopic approaches only probe their square moduli, and hence cannot access the sign of ES displacements.

By introducing an experimental scheme, based on broadband time-domain impulsive Raman, we shown how to determine the most elusive aspect of the excited-state molecular displacements, namely its sign relative to the ground-state. The key to achieve this task is in the signal linear dependence on the Frank-Condon overlaps, brought about by non-degenerate resonant probe and off-resonant pump pulses, which ultimately enables time-domain sensitivity to the phase of the stimulated vibrational coherences .

Intense light-matter interactions and unique structural and electrical properties make van der Waals heterostructures composed by graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunneling transistors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics, and quantum light emitting devices (QLEDs). An efficient energy harvesting of photoexcited hot carriers is critical for the performances of such devices. In this respect, the way initially photogenerated excitons in the TMD are converted into an electric current in Gr is a highly controversial issue. Exploiting a picosecond pump pulse, resonant with the WS2 monolayer absorption, we injected photo-carriers in the TMD and then monitored the graphene response with a temporally delayed probe pulse. By tracking the picosecond dynamics of the G and 2D Raman bands, we determined the electronic temperature profile of Gr in response to TMD photoexcitation, unveiling that the fundamental mechanism boosting the carrier injection in Gr from the TMD monolayer is an energy transfer occurring on the picosecond scale. This indicates the existence of an additional conversion mechanism bridging such ultrafast energy transfer with the much slower charge transport involved in optoelectronics applications.

Spontaneous Raman spectra depend on the displacement along the normal coordinates between ground and excited potential energy surfaces, and hence the relative intensities of the measured Raman bands encode information on the PESs relative displacement. Critically, in order to extract such molecular information, several spectra have to be recorded scanning the Raman pump wavelength across the absorption profile, with the detection of the experimental signals that is typically hampered by the overwhelming fluorescent background. Most importantly, spontaneous Raman spectroscopy cannot be applied to monitor ultrafast chemical reactions on electronically excited states. Recenlty we have shown how to circumvent these limitations introducing an approach based on time-domain impulsive Raman scattering: a femtosecond pulse impulsively launches nuclear wave packet motions in the system under investigation and then their couplings with an arbitrary excited state potential is measured by a resonant Raman process enabled by a delayed femtosecond probe pulse. A perturbative treatment of the scattering process, validated by time-dependent density functional theory calculations, reveals that the signal is generated by the interference between multiple quantum pathways resonant with the excited state manifold. The relative phase of such components is experimentally tuned by varying the probe chirp and we demonstrate how to decode the nuclear displacements along the different normal modes from the experimentally detected impulsive Raman excitation profiles, thus revealing the multidimensional potential energy surfaces.

Assigning the measured vibrations to the pertaining ground or excited electronic states represents a demanding task for interpreting vibrational spectra recorded by time-resolved spectroscopic experiments. Stimulated Raman scattering can coherently stimulate and probe molecular vibrations with optical pulses, but it is generally restricted for the study of ground state properties. Two-pulse Femtosecond Stimulated Resonant Raman Scattering (FSRRS) can be exploited for mapping excited state molecular properties: tuning the optical pulses to be in resonance with an electronic transition enables promoting the system to a targeted electronic state, creating both excited state populations as well as excited state vibrational coherences. Combining an experimental setup, which takes advantage of the the relative time delay between Raman and probe pulses for controlling the excited state contributions, with a diagrammatic formalism able to dissect the concurring pathways that generate the Raman signal, ground and excited state properties of the system under investigation can be simultaneously reconstructed.

Light-induced processes in molecules rely on the efficient and directed conversion of photon energy into electronic and atomic motions. This conversion is controlled by the underlying multidimensional energy surfaces, which describe how the potential energy of the system changes with modifications in the vibrational and electronic configurations. Mapping the potentianl energy surfaces over multiple vibrational dimensions discloses the ultrafast evolution of the system but is typically hampered by the need of spectroscopic probes detecting different energy scales with high temporal and frequency resolution. Two-Dimensional (2D) coherent Raman spectroscopy, by exploiting three temporally delayed femtosecond pulses to coherently generate and track excited-state vibrational wavepackets, is able to probe vibrational correlations pertaining to a targeted electronically excited state. The evolution of the wavepackets is determined by the shapes of the vibrationally structured potential energy surfaces of the system. Couplings and correlations appear as cross peaks in 2D Raman maps, whose origin can be assigned by using a diagrammatic perturbative expansion in terms of the field-matter interactions, mapping the multidimensional energy surfaces.