  We show that new low-energy photoluminescence (PL) bands can be created in
semiconducting single-walled carbon nanotubes by intense pulsed excitation. The
new bands are attributed to PL from different nominally dark excitons that are
"brightened" due to defect-induced mixing of states with different parity
and/or spin. Time-resolved PL studies on single nanotubes reveal a significant
reduction of the bright exciton lifetime upon brightening of the dark excitons.
The lowest energy dark state has longer lifetimes and is not in thermal
equilibrium with the bright state.
