Frequency-domain nonlinear Raman spectroscopy

We have developed a Femtosecond Stimulated Raman Scattering (FSRS) setup for mapping ultrafast photo-physical events. The FSRS experimental scheme requires three pulses: a femtosecond actinic pump (AP) that triggers the dynamics of interest, a Raman pulse (RP), and a broadband probe pulse whose joint action coherently stimulates and records Raman oscillations, providing the chance to follow photoreactions with uncompromised temporal precision (down to 50 fs) and spectral resolution (a few wavenumbers). By taking advantage of widely tunable AP (266 nm, 400 nm, 500-750 nm) and RP (350-800 nm) pulses, resonant Raman responses can be explored across the entire visible spectrum. The main research lines investigated by FSRS are both studies of ultrafast chemical phenomena, as for instance the case of photolyzed ligand dynamics in heme proteins, or the sub-picosecond manipulation of solid-state systems, as for the case of femtomagnetism.

Video: Molecular movie of a photoexcited heme

Time-domain impulsive Raman spectroscopy

We developed an Impulsive Stimulated Raman Scattering (ISRS) setup to measure molecular vibrational responses directly in the time-domain. Within the ISRS experimental scheme, two temporally separated laser fields, conventionally referred to as Raman pulse (RP) and probe pulse (PP), are exploited to stimulate and read out the vibrational signatures of the system under investigation. When the RP is shorter than the period of a normal mode, it can generate a localized wave packet that coherently oscillates and evolves as a function of time. The photoexcited wave packet modulates the transmissivity of the sample at the frequencies of the stimulated Raman modes, which can hence be detected by monitoring the PP transmission as a function of both temporal delay T between the pulses and the probe wavelength. Upon Fourier transformation over T, ISRS yields the Raman spectrum of the system of interest. This approach can be exploited to investigate molecular as well as solid state samples.

Videos:

• Part 1, ISRS in a nutshell

• Part 2, Turning ISRS into CISRS (see also the related paper)

Multi-dimensional Raman spectroscopy

We recently introduced 2-dimensional Impulsive Stimulated Raman Scattering (2D-ISRS) scheme to selectively probe vibrational mode couplings between different active sites in molecular compounds. Three temporally delayed pulses generate nuclear wave-packets whose evolution reports on the underlying potential energy surface, which can be deciphered using a diagrammatic approach assigning the measured spectroscopic signatures to the corresponding excited-state molecular properties (such as molecular displacements along normal coordinates or Duschinsky rotation).

Raman spectro-microscopy of 2D-materials

We recently addressed the out of equilibrium dynamics of 2D materials from the phonon perspective. Specifically, we focus on the electron-phonon coupling and energy vs charge transfer processes in graphene and transition metal dichalcogenides, studied using picosecond laser excitation.

Video: Coherent anti-Astokes Raman scattering of graphene samples

Funded by the Graphene Flagship European initiative, we developed a laboratory for multimodal non-linear imaging embedding the label free capabilities of the Raman spectroscopy. The project aims at developing a turn-key platform introducing a new method to simplify the generation of the necessary synchronized laser pulses thanks to a graphene based saturable absorber.

Non-linear Vibrational Imaging in biosystems

Taking advantage of a Coherent Anti-Stokes Raman Scattering (CARS) setup we perform label free live imaging in biosystems. Examples include lipid accumulation and metabolism in Hepatocytes, HDAC inhibitors and their role for tumor development. Neurodegenerative diseases, localization of Amyloid-beta Plaques in Alzheimer's Disease Brain. Nanoparticle uptake in Arabidopsis plants for drug delivery and toxicity evaluation. Impact of short-chain fatty acid (2-HIBA) on obesity in C-elegans model.

The investigation of extremely short-lived molecular species often implies understanding complex signals and is ultimately hampered by unwanted nonlinear effects once the femtosecond time resolution limit is approached. Deciphering coherent Raman lineshapes requires a careful modelling for retrieving the structural information encoded in transient measurements. Our research activities include the development and applications of theoretical tools to numerically calculate nonlinear Raman response occurring under these extreme conditions.

We investigate relaxation processes of density fluctuations in liquids and amorphous materials, and their evolution across the glass formation. In particular, the connection between equilibrium properties of the liquid side (such as viscosity) and vibrational properties of the corresponding glass (velocity and attenuation of sound waves, vibrational density of states, non-ergodicity factor). Experimental methods are based on photon-in photon-out methods in the broadest sense, ranging from Inelastic X-ray Scattering to Brillouin Light Scattering and interferometric photo-acoustics.

Relaxations in simple liquids

Collective dynamics of simple liquids (liquid metals, Lennard Jones systems and related binary mixtures) have been studied characterizing the nature of their relaxation processes. Beside the diffusive motion, vibrational motion around “instantaneous” equilibrium position can be identified in a liquid. This latter survives the structural arrest occurring at the glass transition and it is ultimately ruled by the structural disorder.

Glass transition and aging

We focus on the relation between the fragility of liquids and microscopic aspects of the glassy dynamics. Inspired by popular urban legends concerning the apparent flow of stained glass windows in medieval cathedrals, the connection between equilibrium viscous flow and non-equilibrium vibrational properties has been recently extended (experimentally) below and (numerically) above the glass transition temperature to verify the finite temperature divergence of the relaxation time predicted by several glass transition theories.

Thermodynamics of supercritical fluids

According to textbook definitions, there exists no physical observable able to distinguish a liquid from a gas beyond the critical point. By looking at sound waves propagation, we demonstrated how the extension of the coexistence line identifies two regions reminiscent of gas-like and liquid-like behavior.