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Edificio "E. Fermi"
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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.
|Article: J. Phys. Chem. Lett. (2020).|
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.
|Article: Physical Review X (2020).|
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.
|Article: Journal of the American Chemical Society (2020).|
|Video: Molecular movie of a photoexcited heme.|
An International sChool On Nonlinear vibrational Spectro-microscopy (ICONS) will be held at the Physics Department of "Sapienza" University of Rome, July 30th-August 1st 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.
|More info: ICONS Webpage (2020).|
|Organizers: Giovanni Batignani & Carino Ferrante.|
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.
|Article: J. Phys. Chem. Lett., arXiv (2019).|
|Video(s): Part 1, ISRS in a nutshell; Part 2, Turning ISRS into CISRS.|
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.
|Article: Nature Communications (2019).|
|Video: Coherent anti-Astokes Raman scattering of graphene samples.|
Raman techniques are pivotal in many interdisciplinary aspects of photonics. In this work we use, in turn, fundamentals of non-linear photonics 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.
|Article(s): Annalen Der Physik (2019), Nature Photonics (2015).|
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