  The surface of a three-dimensional topological insulator (TI) hosts
two-dimensional massless Dirac fermions (DFs), the gapless and spin-helical
nature of which yields many exotic phenomena, such as the immunity of
topological surface states (TSS) to back-scattering. This leads to their high
transmission through surface defects or potential barriers. Quantum corrals,
previously elaborated on metal surfaces, can act as nanometer-sized electronic
resonators to trap Schr\"odinger electrons by quantum confinement. It is thus
intriguing, concerning their peculiar nature, to put the Dirac electrons of TSS
to the test in similar circumstances. Here, we report the behaviors of TSS in a
triangular quantum corral (TQC) fabricated by epitaxially growing Bi bilayer
nanostructures on the surfaces of Bi2Te3 films. Unlike a circular corral, the
TQC is supposed to be totally transparent for DFs. By mapping the electronic
structure of TSS inside TQCs through a low-temperature scanning tunneling
microscope in the real space, both the trapping and de-trapping behaviors of
the TSS electrons are observed. The selection rules are found to be governed by
the geometry and spin texture of the constant energy contour of TSS upon the
strong hexagonal warping in Bi2Te3. Careful analysis of the quantum
interference patterns of quasi-bound states yields the corresponding wave
vectors of trapped TSS, through which two trapping mechanisms favoring momenta
in different directions are uncovered. Our work indicates the extended nature
of TSS and elucidates the selection rules of the trapping of TSS in the
presence of a complicated surface state structure, giving insights into the
effective engineering of DFs in TIs.
