Lez #43+44 FSRS I Principles and applications (Heme proteins)

 We introduced the Femtosecond Stimulated Raman Scattering (FSRS) experimental scheme, where SRS is exploited to track the evolution of a system upon the photoexcitation of an Actinic pump pulse, preceding the Raman and Stokes pair. The experimental layout has been introduced and the temporal/spectral resolutions of FSRS have been compared with the time-resolved spontaneous Raman counterpart. Then we started to discuss the paradigmatic case of Myoglobin, where FSRS has been exploited to: i) interpret transient absorption experiments to discriminate between two possible contrasting scenarios related to the existence of transient electronic states vs vibrational cooling. ii) determine how the absorbed photon energy, initially stored in delocalized low frequency heme modes, is very efficiently funneled into specific high frequency modes prior to slow dissipation through the protein 

Lez #41+42 IRS signal, resonance effect in the blue-side SRS response

We investigated the IRS1 response under various resonance conditions, demonstrating that the resulting lineshapes - positive, negative, or dispersive- depend on the Raman pump wavelength relative to the sample absorption and the probed vibrational mode.

An expression for the IRS Raman gain was first derived for arbitrary spectral profiles of the Raman pump and probe pulses. The response was then analytically evaluated in the case of a monochromatic pump and an impulsive probe.

Lez #39+40 SRS signals

  Still on chi3 processes. We evaluated the SRS (RRS1 diagram) response in resonant and non resonant case, for monochromatic Raman pump and probe beams.

Lez #37+38 Double sided Feynman diagrams: linear absorption and pump-probe (3rd order)

  We introduced the double sided Feynman diagrams to describe the perturbative expansion of the polarization, end explicitly evaluated the linear absorption of a two level system.

We started to extend the same approach to third order processes. Among them, pump-probe experiments with the first 2 simultaneous interactions with the pump, and the third interaction with a probe. This originates stimulated emission (SE), bleaching and excited state absorption (ESA). We commented on the differences between bleaching and SE. 

Lez #35+36 Radiation - matter, rabi oscillations, perturbative expansion

   Today we recalled the basic concepts of quantum mechanics, introducing the density matrix. We stressed the importance  of density matrix to describe statistical mixtures, and how this is not possible through an eigenstates superposition (for instance a two level system).

We discussed how to solve Bloch equation and Rabi oscillation, introducing the rotating wave approximation to neglect some terms in the integration.

We then introduced Shroedinger, Heisenberg and Interaction schemes (in this latter both eigenfunctions and operators evolve, and the temporal evolution related to the unperturbed Hamiltonian is removed from the wavefunction). Then it makes sense a perturbative expansion of the eigenfunctions and of the density matrix in the Interactions scheme. Finally, we went back to the Shroedinger scheme.

Please refer to the first part of the notes added to classroom.

Lez #33+34 Introduction to spontaneous vs stimulated Raman processes, classical approach

  We introduced the field-induced polarization, in both the spontaneous and stimulated cases. We discussed the origin of spontaneous Raman, and why it is incoherent, linear and homodine. As for the selection rules we recalled some basic notions from struttura della materia class. Then we moved to the stimulated Raman, a third order process for which we evaluated different contributions to the polarization. See the material on classroom.

Lez #31+32 Laser Principles II

 We first finalized the laser lecture introducing the solid state laser principle. Then mode locking for pulsed laser generation, active and passive methods, gain and losses compensation.