Exciton lasing in carbon nanotubes below the population-inversion threshold via phonon-mediated stimulated emission

Excitonic optical transitions in carbon nanotubes (CNTs) have been extensively studied for at least two decades. However, exploiting these transitions to produce a lasing effect has proven to be unpractical due to the difficulties of achieving an excitonic population inversion in carbon nanotubes. In this work we show that lasing is theoretically possible at a much lower exciton population threshold by taking advantage of phonon stimulated emission. Specifically, the so-called anti-Stokes transitions, where light is generated by the creation of a phonon and the annihilation of an exciton, can be used to sustain the coherent emission of photons. These transitions have been overlooked for long due to their low intensity as a consequence of the low occupancy of the associated phonon branches. We show that the best conditions to achieve the lasing effect are met in CNTs with (i) excitons of nonzero momentum, (ii) a strong exciton-phonon coupling, and (iii) narrow exciton transitions.