  Excitons play major roles in optical processes in modern semiconductors, such
as single-wall carbon nanotubes (SWCNTs), transition metal dichalcogenides, and
2D perovskite quantum wells. They possess extremely large binding energies
(>100~meV), dominating absorption and emission spectra even at high
temperatures. The large binding energies imply that they are stable, that is,
hard to ionize, rendering them seemingly unsuited for optoelectronic devices
that require mobile charge carriers, especially terahertz emitters and solar
cells. Here, we have conducted terahertz emission and photocurrent studies on
films of aligned single-chirality semiconducting SWCNTs and find that excitons
autoionize, i.e., spontaneously dissociate into electrons and holes. This
process naturally occurs ultrafast (<1~ps) while conserving energy and
momentum. The created carriers can then be accelerated to emit a burst of
terahertz radiation when a dc bias is applied, with promising efficiency in
comparison to standard GaAs-based emitters. Furthermore, at high bias, the
accelerated carriers acquire high enough kinetic energy to create secondary
excitons through impact exciton generation, again in a fully energy and
momentum conserving fashion. This exciton multiplication process leads to a
nonlinear photocurrent increase as a function of bias. Our theoretical
simulations based on nonequilibrium Boltzmann transport equations, taking into
account all possible scattering pathways and a realistic band structure,
reproduce all our experimental data semi-quantitatively. These results not only
elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in
SWCNTs but also suggest promising routes toward terahertz excitonics despite
the orders-of-magnitude mismatch between the exciton binding energies and the
terahertz photon energies.
