  A review work is done for electronic and optical properties of graphene
nanoribbons in magnetic, electric, composite, and modulated fields. Effects due
to the lateral confinement, curvature, stacking, non-uniform subsystems and
hybrid structures are taken into account. The special electronic properties,
induced by complex competitions between external fields and geometric
structures, include many one-dimensional parabolic subbands, standing waves,
peculiar edge-localized states, width- and field-dependent energy gaps,
magnetic-quantized quasi-Landau levels, curvature-induced oscillating Landau
subbands, crossings and anti-crossings of quasi-Landau levels, coexistence and
combination of energy spectra in layered structures, and various peak
structures in the density of states. There exist diverse absorption spectra and
different selection rules, covering edge-dependent selection rules,
magneto-optical selection rule, splitting of the Landau absorption peaks,
intragroup and intergroup Landau transitions, as well as coexistence of
monolayer-like and bilayer-like Landau absorption spectra. Detailed comparisons
are made between the theoretical calculations and experimental measurements.
The predicted results, the parabolic subbands, edge-localized states, gap
opening and modulation, and spatial distribution of Landau subbands, have been
verified by various experimental measurements.
