Optical probes of strongly driven quasiparticles in solid state platforms

High-order harmonic generation (HHG) and high-order sideband generation (HSG) are non-linear optical phenomena which rely on the non-equilibrium and non-perturbative dynamics of quasiparticles. First demonstrated in atomic gasses, HHG has expanded to solid-state platforms and developed into a probe for the physics of quasiparticles driven by large laser fields. Microscopic theories have laid the framework with conceptual descriptions and quantitative predictions for the phenomenon in a variety of condensed matter systems. Around the same time, the related process of HSG was observed in semiconducting systems. Likewise, semiclassical and fully quantum microscopic theories have been developed to explain experimental results. For both, increased understanding of the quasiparticle dynamics have enhanced experimental understanding, connecting the observed spectra to the quantum behavior of the quasiparticles responsible for the generation of high order photons. This review presents common theoretical descriptions and highlights experimental triumphs of the two phenomena, and outlines future research questions which can be addressed by these complementary all-optical methods.