  We investigate the band structure and the optical absorption spectrum of
twisted bilayer graphenes with changing interlayer bias and Fermi energy
simultaneously. We show that the interlayer bias lifts the degeneracy of the
superlattice Dirac point, while the amount of the Dirac point shift is
significantly suppressed in small rotation angles, and even becomes opposite to
the applied bias. We calculate the optical absorption spectrum in various
asymmetric potentials and Fermi energies, and associate the characteristic
spectral features with the band structure. The spectroscopic features are
highly sensitive to the interlayer bias and the Fermi energy, and widely
tunable by the external field effect.
