  Using terahertz time-domain spectroscopy, the real part of optical
conductivity [$\sigma_{1}(\omega)$] of twisted bilayer graphene was obtained at
different temperatures (10 -- 300 K) in the frequency range 0.3 -- 3 THz. On
top of a Drude-like response, we see a strong peak in $\sigma_{1} (\omega)$ at
$\sim$2.7 THz. We analyze the overall Drude-like response using a
disorder-dependent (unitary scattering) model, then attribute the peak at 2.7
THz to an enhanced density of states at that energy, that is caused by the
presence of a van Hove singularity arising from a commensurate twisting of the
two graphene layers.
