Nonlinear photoluminescence from van Hove singularities in multiwalled carbon nanotubes.

Van Hove singularities (VHS) in the density of states of multiwalled carbon nanotubes (MWNTs) have been observed by optical spectroscopy for the first time to the authors' knowledge. Additionally, unexpected visible luminescence has been observed on photoexcitation at 1064 nm. This luminescence is believed to arise from radiative transitions between energy states in the VHS. A mechanism describing the underlying processes, which involve multiphoton absorption followed by upconverted luminescence from VHS energy states, is presented. Our results show that the presence of VHS readily facilitates efficient optical transitions in