Ultrafast light excitation is expected to trigger new states in solids, which are not accessible by varying the pressure, temperature, or doping in standard thermodynamic equilibrium conditions. Ultrafast terahertz (THz) spectroscopy has been blooming for the last 20 years. Developments of new intense THz sources provide the tools to trig not only the linear response but also the nonlinear response of matter offering new perspectives in exploring extreme matter behavior at the picoseconds timescale. In solid state physics, different degrees of freedom have been addressed dynamically by driving electrons, phonons or spins well out-of-equilibrium with ultrashort THz pulses. The EPHONO project is focus on fundamental aspects of the light induced ultrafast lattice dynamics in condensed matter. It aims at unraveling the complexity of coherent phonon-phonon interactions with high THz field at the sub-picoseconds timescales. In the EPHONO project, I will apply the nonlinear phononics framework in order to explore the optical phonons ultrafast dynamics into the nonlinear regime. As optical phonons are mediating matter properties (the “soft mode” in phase transition material, the ferroelectric mode in multiferroics material to name a few), it is crucial to develop experimental and theoretical methods to study, explain and predict the excitation pathways in this extreme nonlinear regime. To achieve this, I propose to tackle this problem in two connected ways by experimental and theoretical investigations. The first one will be dedicated to the development of state-of-art nonlinear ultrafast THz spectroscopy and the second one to ab-initio calculations within the Density Functional Theory. Within this context, the EPHONO project will be at the core of developments both experimentally (2D THz spectroscopy) and theoretically (ab-initio calculations). The main idea of the experimental approach is based on the fact that “regular” ultrafast nonlinear spectroscopy in the THz range is not enough to fully understand the ultrafast lattice dynamics: single-pulse excitation does not fully unravel the complexity of phonon-phonon interaction. That is why, extending the experimental technique to multidimensional ultrafast THz spectroscopy associated to a THz frequency tunability, will allow distinguishing the dominant optical phonon mode excitation pathway. More precisely, this technique is based on using a multiple pump pulses sequence, which allows decoupling the excitation frequency to the detected frequency by looking at their correlation in the frequency domain. This powerful method can help to unravel the different coupling pathways leading to the excitation of one specific optical phonon mode. It can be implemented in two ways: either by measuring the optical properties change with a delayed optical probe (2D THz-Raman) or by measuring the total transmitted THz electric field by an electro-optic sampling technique (2D THz Spectroscopy). The experimental aspects of the project EPHONO will be supported by “in-house” theoretical approach with ab-initio calculations (Density Functional Theory) within the framework of the local density approximation with Spin-orbit coupling included. The purpose of these calculations is to understand the obtained experimental results in the case of a Bi2Te3 nanofilm excited with a strong THz pulse. The aim is to be able to have a quantitative comparison between experimental and high end first principles calculations. The strategy will be focused on continuing investigating topological insulator like nanofilm of Bi2Te3. This type of material possesses strong anharmonicities and, thus, makes them ideal for studying nonlinear phononics. In a second step of this project, other materials will be studied such as multiferroic and ferroelectric materials in which another degree of complexity exists. Applying the multidimensional spectroscopy will provide new insights in this multi-orders material.