  We theoretically study the optical absorption property of twisted bilayer
graphenes with various stacking geometries, and demonstrate that the
spectroscopic characteristics serve as a fingerprint to identify the rotation
angle between two layers. We find that the absorption spectrum almost
continuously evolves in changing the rotation angle, regardless of the lattice
commensurability. The spectrum is characterized by series of peaks associated
with the van Hove singularity, and the peak energies systematically shift with
the rotation angle. We calculate the optical absorption in two different
frameworks; the tight-binding model and the effective continuum model based on
the Dirac equation. For small rotation angles less than $10^\circ$, the
effective model well reproduces the low-energy band structure and the optical
conductivity of the tight-binding model, and also explains the optical
selection rule analytically in terms of the symmetry of the effective
Hamiltonian.
