Topical Review: Optical properties of tilted 2D Dirac systems

Two-dimensional Dirac materials with tilted band structures constitute a representative class of low-symmetry Dirac systems in which reduced rotational symmetry and relativistic-like effects coexist. In this topical review, we survey the theoretical description of optical properties of tilted Dirac systems, and how band structure features manifest in experimentally accessible quantities. We analyze how the degree of tilting, velocity anisotropy, and band gap modifies jointly the Fermi surface, the joint density of states, and optical conductivity, leading to characteristic signatures in optical absorption and dichroism, differential reflectivity, Faraday and Kerr rotations, plasmon propagation, Berry curvature and quantum metric based quantities, valley-dependent effects, and Landau level spectra. The tunability of the optical properties by external fields is also discussed. By relating theoretical predictions to experimental realizations in organic conductors, borophene polymorphs, and related systems, we show how the tilted Dirac model provides a useful effective low-energy framework for describing Dirac physics in complex materials and for guiding future explorations of anisotropic and valley-selective optoelectronic effects.