Comment on "Large optical nonlinearity of semiconducting single-walled carbon nanotubes under resonant excitations"

In a recent Letter Maeda et al. studied the optical nonlinearities of single-walled carbon nanotubes (SWNT) films using both ??Z-scan?? and ??pump and probe?? (PP) techniques with ~100 fs time resolution [1]. They found that SWNTs have a large resonant third-order nonlinear optical susceptibility (X(3)), which was interpreted in terms of ??coherent process rather than by saturation of absorption,?? possibly due to ??optical Stark effect and stimulated emission.?? This interpretation was based on degenerate PP measurements, in which an ultrafast response was found for t < tp (pulse duration), superimposed on a slower response with ~1 ps time constant. Importantly, the ultrafast response was not found in nondegenerate PP measurements [1,2].