  We present a theoretical study of the effects of heterostrain and lattice
relaxation on the optical conductivity of twisted bilayer graphene near the
magic angle, based on the band structures obtained from a continuum model. We
find that heterostrain, lattice relaxation and their combination give rise to
very distinctive spectroscopic features in the optical conductivity, which can
be used to probe and distinguish these effects. From the spectrum at various
Fermi energies, important features in the strain- and relaxation-modified band
structure such as the bandgap, bandwidth and van Hove singularities can be
directly measured. The peak associated with the transition between the flat
bands in the optical conductivity are highly sensitive to the direction of the
strain, which can provide direct information on the strain-modified flat bands.
