  When light is incident on a medium with spatially disordered index of
refraction, interference effects lead to near-perfect reflection when the
number of dielectric interfaces is large, so that the medium becomes a
"transparent mirror." We investigate the analog of this effect for electrons in
twisted bilayer graphene (TBG), for which local fluctuations of the twist angle
give rise to a spatially random Fermi velocity. In a description that includes
only spatial variation of Fermi velocity, we derive the
incident-angle-dependent localization length for the case of
quasi-one-dimensional disorder by mapping this problem onto one dimensional
Anderson localization. The localization length diverges at normal incidence as
a consequence of Klein tunneling, leading to a power-law decay of the
transmission when averaged over incidence angle. In a minimal model of TBG, the
modulation of twist angle also shifts the location of the Dirac cones in
momentum space in a way that can be described by a random gauge field, and thus
Klein tunneling is inexact. However, when the Dirac electron's incident
momentum is large compared to these shifts, the primary effect of twist
disorder is only to shift the incident angle associated with perfect
transmission away from zero. These results suggest a mechanism for
disorder-induced collimation, valley filtration, and energy filtration of Dirac
electron beams, so that TBG offers a promising new platform for Dirac fermion
optics.
