Physical and chemical reactions driven by light absorption are determined by multidimensional excited-state potential energy surfaces (PESs). Nature has shaped excited-state PESs 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, we have 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 (PESs), 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.

Identifying the structural rearrangement and the active sites in photo-induced reactions is a fundamental challenge to understand from a microscopic perspective the ultrafast dynamics of heme proteins. Among the different types of hemes, those able to form the iron atom sixth bond with an internal residue, namely six-coordinate hemes, represent one of the most elusive systems for ultrafast spectroscopies. As dissociation of the internal residue is followed by a rapid rebinding embedded with several picosecond and femtosecond concurring processes, such as structural reconfiguration, energy redistribution and relaxation on intermediate excited states, the natural hindrance for their study revolves around the simultaneous need of two key ingredients: high temporal and spectral resolutions, which are mutually compromised due to the Heisenberg principle. By exploiting femtosecond stimulated Raman spectroscopy (FSRS) to follow the ultrafast evolution of different six-coordinate hemes with combined high spectral resolution and femtosecond time precision, we have determined the whole spectrum of vibrational amplitudes with extremely high temporal resolution, allowing us to animate the molecular normal modes and to visualize how the different structural and thermal relaxation processes syncretize on the coherent picture of the reaction pathway.

An International sChool On Nonlinear vibrational Spectro-microscopy (ICONS) will be held at the Physics Department of "Sapienza" University of Rome, July 30^th-August 1^st 2020. ICONS will be structured around comprehensive review talks from major world leading experts in complementary areas of nonlinear Raman spectroscopy both on theory, experiments and applications. The aim of the school is to provide high level-expertise for young researchers, graduate students interested in nonlinear Raman spectroscopy, applied to access dynamical and microscopic properties at the molecular and condensed matter levels. Specifically, the school will be three days of classes, with an introduction on the peculiarities of nonlinear Raman approaches with respect to linear Raman spectroscopy, discussing the technical requirements and the main tools that are conventionally exploited in modern experimental laboratories to manipulate and control optical pulses. The following two days will be focused on time-resolved and microscopic techniques, with principles and examples. The program, supported by world leading experts in the field of Raman nonlinear photonics, aims to have a balance between theoretical and experimental advances.

Impulsive stimulated Raman scattering (ISRS) is a powerful technique able to real time monitor vibrational oscillations in photo-excited systems: by combining a pump and a probe pulse for stimulating and then probing Raman coherences, ISRS can directly access atomic motions and molecular properties. Critically, at odd with frequency-domain Raman approaches, ISRS typically requires long acquisition times since the system response has to be measured scanning a sequence of time delays between pump and probe pulses. Introducing a chirp in the probe pulse, we have introduced a novel experimental scheme for the realization of Chirped-based ISRS (CISRS), for recording the time-domain Raman information without scanning the pump and probe delay: since different probe wavelengths interact with sample at different time delays, the evolution of the stimulated vibrational coherences is encoded in the probe spectrum.

Spontaneous Raman spectroscopy is a powerful characterization tool for graphene research. Its extension to the coherent regime, despite the large nonlinear third-order susceptibility of graphene, has so far proven challenging. Due to its gapless nature, several interfering electronic and phononic transitions concur to generate its optical response, preventing to retrieve spectral profiles analogous to those of spontaneous Raman. We have recently reported stimulated Raman spectroscopy of the G-phonon in single and multi-layer graphene, through coherent anti-Stokes Raman Scattering (CARS). The nonlinear signal is dominated by a vibrationally non-resonant background, obscuring the Raman lineshape. We demonstrate that the vibrationally resonant coherent anti-Stokes Raman Scattering peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the vibrationally non-resonant background. Modelling the spectra, for taking into account the electronically resonant nature of both, we have shown how CARS can be used for graphene imaging with vibrational sensitivity.

Raman techniques are pivotal in many interdisciplinary aspects of photonics. In turn, fundamentals of non-linear photonics can also be used as a key enabling tools to disclose sub-100 fs magnetic excitations dynamics exploiting coherent Raman spectroscopy. The relevance of probing the ultrafast dynamics of the exchange interaction strikes a chord of interest from both the fundamental and applied science standpoint. On the one hand one wonders on how fast angular momentum can be exchanged, and if it may become possible to reach the timescale of the spin-orbit interaction. On the other hand, the study of the fundamental and practical limits of the speed of manipulation of the magnetic ordering is obviously of great importance for magnetic recording and information processing technologies.