Strong intertube excitonic coupling is demonstrated in 1D van der Waals heterostructures by examining the ultrafast response of radial C/BN/MoS2 core/shell/skin nanotubes to femtosecond infrared light pulses. Remarkably, infrared excitation of excitons in the semiconducting carbon nanotubes (CNTs) creates a prominent excitonic response in the visible range from the MoS2 skin, even with infrared photons at energies well below the bandgap of MoS2. Via classical analogies and a quantum model of the light–matter interaction these findings are assigned 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 MoS2 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 nanoscale coaxial cables.

In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light–matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.