Intertube excitonic coupling in nanotube van der Waals heterostructures

Strong intertube excitonic coupling is demonstrated in 1D van der Waals heterostructures by examining the ultrafast response of radial C/BN/MoS$_2$ core/shell/skin nanotubes to femtosecond infrared light pulses. Remarkably, infrared excitation of excitons in the semiconducting CNTs creates a prominent excitonic response in the visible range from the MoS$_2$ skin, even with infrared photons at energies well below the bandgap of MoS$_2$. Via classical analogies and a quantum model of the light-matter interaction we assign these findings to intertube excitonic correlations. Dipole-dipole Coulomb interactions in the coherent regime produce intertube biexcitons, which persist for tens of femtoseconds, while on longer timescales ($>100$\,ps) hole tunneling -- from the CNT core, through the BN tunnel barrier, to the MoS$_2$ skin -- creates intertube excitons. Charge transfer and dipole-dipole interactions thus play prominent roles on different timescales, and establish new possibilities for the multi-functional use of these new nano-scale coaxial cables.