Optical cooling of single-walled carbon nanotubes as revealed by their anti-Stokes Raman spectra

Abstract

Semiconducting single-walled carbon nanotubes resonantly excited by the interband E(22)(S) electronic transitions (at 1064 nm) display for the two components of the radial Raman band-one associated with the isolated tubes and the other associated with the bundled tubes-an anti-Stokes/Stokes Raman intensity ratio (I(aS)/I(S)) which deviates oppositely from the predictions of the Maxwell-Boltzmann formula. A cooling and heating vibration process, evidenced by an enhancement and diminishment of (I(aS)/I(S)), appears in the isolated and bundled nanotubes, respectively. Here we confirm a cooling process, observed only for semiconducting nanotubes, which emerges from the relaxation of the E(22)(S) excited state by the electronic relaxation from E(22)(S) to E(11)(S) that precedes the spontaneous luminescence emission at E(11)(S). Metallic nanotubes do not exhibit luminescence and no cooling effect is observed. Both semiconducting and metallic nanotubes show for the bundled component of the radial Raman band an enhancement of (I(aS)/I(S)) such as is frequently observed in a coherent anti-Stokes Raman scattering process.