In situ electron microscopy is a powerful tool to investigate materials properties and related processes at atomic resolution. Designing and fabricating ex-novo a new transmission electron microscope (TEM) sample holder for in situ experiments is an exciting challenge to face. By integrating an optical fiber in a sample holder, it is possible to irradiate the specimen with a laser beam inside the TEM and, subsequently, study the effects of this localized laser annealing by electron microscopy. Various issues concerning the motion of the fiber with respect to the sample and the vacuum sealing must be addressed to achieve full operation. In the second part of the talk, recent results from different collaboration will be shown, including laser-induced crystallization process, confined III-V 2D materials and active materials for quantum cascade lasers, highlighting the unique contribution that electron transmission microscopy measurements can provide to each of these topics.