In this talk, I will start from introducing a general picture of light-matter interactions in nanoscale materials and the first-principles many-body perturbation approach for calculating excited states, such as quasiparticles and excitons. Then I will focus on many-electron interactions in two-dimensional (2D) moiré crystals. We propose a way to enhance the crucial moiré potential by manipulating the coupling between quasiparticles and carrier plasmons. Our calculation shows that this mechanism can effectively enhance the quasiparticle moiré potential by a few times of magnitude, providing an excellent opportunity to control and observe correlated Hubbard physics in 2D moiré crystals. In the second part of the talk, I will switch to the second-order light-matter interactions in PT-symmetric materials. Using the quantum perturbation theory, we predict that light with different polarizations can selectively drive sizable charge current or pure spin current in emerging antiferromagnetic 2D structures or topological materials.