Relativistic Dirac and Weyl fermions were extensively studied in quantum field theory. Recently they emerged in the nonrelativistic condensed-matter setting as gapless quasiparticle states in some types of crystals. Notable examples of 2D systems include graphene and surface states in topological insulators such as Bi2Se3. Their 3D reincarnation is Dirac and Weyl semimetals that were recently discovered experimentally. Most of the research has been focused on their topological properties and electron transport. However, their optical and plasmonic properties are no less exciting. Optical phenomena can provide valuable insight into the fascinating physics of these materials. Moreover, their unique optical properties can be utilized in future optoelectronic devices. I will discuss several examples illustrating these points. They include plasmons and polaritons in Weyl semimetals, nonlinear optical response of graphene and topological insulators in the infrared and THz range, nonlinear generation of THz plasmons, and optical properties of chiral Dirac/Weyl fermions in a quantizing magnetic field.
High-quality thin films of highly aligned semiconducting single-wall carbon nanotubes have been recently demonstrated. They have excellent absorption and photoluminescence properties; however, fast nonradiative recombination of carriers prevents their use as a gain medium in lasers. Here we predict that such films can operate as efficient sources of ultrashort radiation pulses under the conditions of superfluorescence, i.e. cooperative interband recombination of injected electrons and holes. Superfluorescence develops much faster than nonradiative recombination and leads to high-intensity, coherent pulses of near/mid-infrared radiation.