High-order harmonic generation by conduction electrons in carbon nanotube ropes

High-order harmonic generation by metallic carbon nanotubes exposed to an intense ultrashort pulse has been theoretically investigated in the semiclassical approximation. The mechanism of nonlinearity provided by the motion of π electrons has been considered. It has been shown that the high density of conduction electrons in metallic carbon nanotubes and the specific dispersion law inherent in hexagonal crystalline structure result in the efficient generation of high-order harmonics with intensities that fall down smoothly with harmonic number, without cutoff, and with efficiency much larger than in gases with the same pump intensity. We have also studied phase matching for different harmonics in a rope of parallel-aligned carbon nanotubes dependent on the angle of incidence.