Quantum states and optical responses of low-dimensional electron–hole systems

Quantum states and their optical responses of low-dimensional electron–hole systems in photoexcited semiconductors and/or metals are reviewed from a theoretical viewpoint, stressing the electron–hole Coulomb interaction, the excitonic effects, the Fermi-surface effects and the dimensionality. Recent progress of theoretical studies is stressed and important problems to be solved are introduced. We cover not only single-exciton problems but also few-exciton and many-exciton problems, including electron–hole plasma situations. Dimensionality of the Wannier exciton is clarified in terms of its linear and nonlinear responses. We also discuss a biexciton system, exciton bosonization technique, high-density degenerate electron–hole systems, gas–liquid phase separation in an excited state and the Fermi-edge singularity due to a Mahan exciton in a low-dimensional metal.