We discuss the full optical response of twisted bilayer graphene at the
neutrality point close to the magic angle within the continuum model. (i)
First, we define the full optical response consistent with the underlying D3
symmetry, yielding the total, magnetic, and chiral response that transform
according to the irreducible representations A1, A2, and E,
respectively. Then, we numerically calculate the dissipative and reactive
response for twist angles around the magic angle θm and comment on the
possibility of a Condon instability. (ii) Second, we numerically calculate the
full optical response {\it almost at} θm. The total response is
characterized by three universal plateaus which can be obtained from an
analytical calculation. The magnetic and the chiral response, however, is given
by corresponding non-universal plateaus depending on the twist angle θ
via the dimensionless parameter α∼θm−θ. (iii) Following
the discussion on the large magnetic response, we calculate the plasmonic
excitations at the neutrality point inside the optical gap of relaxed twisted
bilayer graphene. We find that acoustic plasmons extend over almost the whole
optical gap and carry the largest oscillator strength. (iv) Finally, we discuss
symmetry relations for the response functions as function of the chemical
potential and highlight the consequences of the approximate particle-hole
symmetry of the continuum model for twisted bilayer graphene. We then discuss a
detailed balance relation where the chiral response at charge neutrality can be
understood in terms of electron (hole) transitions for which the initial
(final) states are energetically closer to charge neutrality than the final
(initial) states.Comment: 17 pages, 7 figure