  Excitons dominate the optics of atomically-thin transition metal
dichalcogenides and 2D van der Waals heterostructures. Interlayer 2D excitons,
with an electron and a hole residing in different layers, form rapidly in
heterostructures either via direct charge transfer or via Coulomb interactions
that exchange energy between layers. Here, we report prominent intertube
excitonic effects in quasi-1D van der Waals heterostructures consisting of
C/BN/MoS$_2$ core/shell/skin nanotubes. Remarkably, under pulsed infrared
excitation of excitons in the semiconducting CNTs we observed a rapid
(sub-picosecond) excitonic response in the visible range from the MoS$_2$ skin,
which we attribute to intertube biexcitons mediated by dipole-dipole Coulomb
interactions in the coherent regime. On longer ($>100$ps) timescales hole
transfer from the CNT core to the MoS$_2$ skin further modified the MoS$_2$'s
absorption. Our direct demonstration of intertube excitonic interactions and
charge transfer in 1D van der Waals heterostructures suggests future
applications in infrared and visible optoelectronics using these radial
heterojunctions.
