  Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the
density of states that can be tuned by changing the twisting angle $\theta$. A
$\theta$-defined tBLG has been produced and characterized with optical
reflectivity and resonance Raman scattering. The $\theta$-engineered optical
response is shown to be consistent with persistent saddle-point excitons.
Separate resonances with Stokes and anti-Stokes Raman scattering components can
be achieved due to the sharpness of the two-dimensional saddle-point excitons,
similar to what has been previously observed for one-dimensional carbon
nanotubes. The excitation power dependence for the Stokes and anti-Stokes
emissions indicate that the two processes are correlated and that they share
the same phonon.
