Novel control knobs for multidimensional stimulated Raman spectroscopy

Understanding the behavior of complex systems is greatly simplified when the proper energy and time scales over which their evolution occurs are investigated. Consequently, deciphering the dynamics of atoms and molecules requires to access the domain of femtoseconds, and even shorter timescales are involved in the case of electrons. Probing such extreme phenomena is the challenging task at which ultrafast spectroscopy aims. In the last forty years, the development of pulsed laser sources and nonlinear optical techniques has allowed the study of phenomena invisible to electronic devices, through the manipulation of matter macroscopic phases on picosecond and sub-picosecond timescales. This technological leap provided sophisticated and customized ultrashort spectroscopic protocols in a wide energy range, from terahertz to x rays, fully realizing the pioneering view of the ultrafast stroboscope, dreamed by the father of femtochemistry Ahmed Zewail. Indeed, using the proper technique, short flashes of light are currently able to record stop-motion images of a dynamic processes as fast as a chemical reaction. The study of the nonlinear response due to external impulsive optical perturbations has been applied to a wide range of scientific cases, fueling a parallel boost in electronic and vibrational spectroscopies. The frontier in ultrafast sciences is now gradually shifting to tackle the interplay between these two degrees of freedom. Vibronic coupling is considered at the grounds of fascinating processes which connect conceptual topics from the foundation of quantum mechanics, as the breakdown of the Born-Oppenheimer approximation, to technological application, as the coherent energy transfer in biomimic photosynthetic devices or the bewildering effects of strong electron-phonon coupling in novel materials as graphene and third generation semiconductors. Probing electronic and vibrational interactions at the same time is complicated by the time and energy scale separation between the two. Thus, one dimensional spectroscopies are weakened by resolution limits which may partially hamper their use in this direction. Multidimensional techniques can cope this limit spreading the information on separate spectroscopic axes, consequently disentangling the relative resolutions. Couplings between different agents in the microscopic description of the sample dynamics are directly revealed through the presence of cross peaks in the multidimensional maps. In this context, the research presented in this thesis has been devoted to the design, realization and interpretation of novel approaches to multidimensional Impulsive Stimulated Raman Spectroscopy (ISRS). Coherent Raman techniques are indeed able to measure vibrational spectra using visible light, which provides at the same time information about the electronic degrees of freedom when tuned resonant with the absorption edges of the sample. A concerted combination between theory and experiments is the key to successfully probe the quantum properties of the matter on which the vibronic interactions rely. For this reason, the experimental efforts have been flanked by a powerful theoretical toolbox given by the nonlinear response formalism. This framework represents a natural link between theory and experiments and supplies a common language to describe very different techniques, gathering their features to design new experimental protocols. We found that the properties of the probe spectral envelope, the wise tuning of resonant conditions and the choice of the pulses scheme may be used to built multidimensional ISRS maps. The developed schemes have been experimentally tested in three different contexts: the coherent control of ground and excited state vibrations in a liquid solvent, the study of charge photogeneration in a hybrid organic-inorganic perovskite and the vibronic coupling in a prototypical fluorescent protein. The research work presented here is structured in seven chapters and one appendix, which summarize the main theoretical and experimental results achieved during the preparation of this doctoral thesis. The core of the thesis is contained in Chapters 4, 5 and 6, which discuss the application of multidimensional ISRS in different scenarios. Since the investigated scientific problems belong to quite different backgrounds, each of these result chapters is introduced by a brief summary of the relevant field. Specifically: In Chapter 1, we introduce the context in which this thesis is developed. The basics features of ultrafast spectroscopy based on the pump-probe scheme and nonlinear Raman techniques are briefly discussed. We then present the classical mechanism underlying spontaneous and coherent Raman effects, while the detailed, microscopic derivation is postponed to Chapter 2. The remaining part of the chapter is devoted to introduce how multidimensional information can be encoded in the parameters of time and frequency domain stimulated Raman spectroscopies, following the key words in the title of the thesis. As an example, the lineshapes from stimulated Raman spectra measured in hemeproteins are studied as a function of the resonance and the vibrational mode. In Chapter 2, the nonlinear response theory is presented as the unifying framework in which all the different experiments in the thesis are conceived, designed and interpreted. In the first part, the principles of quantum mechanics in the density matrix framework are briefly revised, defining the properties of the Liouville space. Then, the concept of nonlinear polarization is introduced and calculated perturbatively in this space. The light matter interaction is derived from both the classical and quantum treatment of electromagnetism, showing that Feynman diagrams are a convenient way to isolate the relevant terms in the perturbative expansion. Finally, we report the rules to derive expressions for the nonlinear signal in the time and frequency domains directly from the diagrams. In Chapter 3, the experimental setups and the data acquisition are described in detail. We analyze the tools and the physical mechanisms at the base of the generation and handling of the ultrashort pulses used in the experiments and also provide an overview of the data analysis routine applied to the impulsive stimulated Raman measurements presented in the thesis. Chapter 4 is the first of the three chapters presenting the main results of this work. Here, we discuss how the broadband envelope of a supercontinuum probe pulse can be shaped to manipulate vibrational coherences in ISRS. In particular, probe wavelength resolved ISRS maps of a liquid solvent are measured changing the chirp of the probe pulse and interpreted in the light of the diagrammatic framework. As a starting point, the effect of the probe chirp and sample length are investigated to rationalize previously unexplained dependencies of low frequency modes on the dispersed probe wavelength. Then, the probe chirp is demonstrated as a control knob to coherently control ISRS modes and to assign spectral features to specific electronic states. In Chapter 5, broadband ISRS is applied to study electron-phonon coupling in lead halide hybrid perovskites. After briefly revising the field of organic-inorganic perovskite optoelectronics, we present experimental measurements on methylammonium lead bromide thin films, comparing the ISRS response of the system upon excitation above and below the band gap. The results are interpreted in the light of the recently proposed polaronic nature of photocarriers in these materials. In Chapter 6, we present a novel multidimensional ISRS scheme, which combines the capabilities of two dimensional Fourier transform techniques with the structural sensitivity of resonant stimulated Raman. We show how this technique can be used to probe mode couplings between different active sites in molecular compounds and determine the shape of vibrationally structured excited state potential energy surfaces. We apply the diagrammatic approach to design 2D ISRS and assign the origin of the different spectral features in a model system. Then, the proposed scheme is benchmarked by addressing vibronic coupling in Green Fluorescent Protein during the first steps of its photoinduced dynamics. Finally, in Chapter 7, the main results obtained in this work are summarized and analyzed under a common perspective. The appendix reports the calculation of transition integrals for the linearly displaced harmonic model. A list of the publications and contributions to international conferences of the author is included at the end of the thesis.