  Atomically thin van der Waals (vdW) films provide a novel material platform
for epitaxial growth of quantum heterostructures. However, unlike the remote
epitaxial growth of three-dimensional bulk crystals, the growth of
two-dimensional (2D) material heterostructures across atomic layers has been
limited due to the weak vdW interaction. Here, we report the double-sided
epitaxy of vdW layered materials through atomic membranes. We grow vdW
topological insulators (TIs) Sb$_2$Te$_3$ and Bi$_2$Se$_3$ by molecular beam
epitaxy on both surfaces of atomically thin graphene or hBN, which serve as
suspended 2D vdW "$\textit{substrate}$" layers. Both homo- and hetero-
double-sided vdW TI tunnel junctions are fabricated, with the atomically thin
hBN acting as a crystal-momentum-conserving tunnelling barrier with abrupt and
epitaxial interface. By performing field-angle dependent magneto-tunnelling
spectroscopy on these devices, we reveal the energy-momentum-spin resonant
tunnelling of massless Dirac electrons between helical Landau levels developed
in the topological surface states at the interface.
