Enhanced high-order-harmonic generation in a carbon ablation plume

We report on the observation of enhanced high harmonics from a carbon plasma using sub-5-fs laser pulses. The efficiency of harmonic generation in the range of 14-25 eV was up to five times higher in the case of a plasma medium (graphite ablation) compared with gas (argon) under similar experimental conditions. The harmonic enhancement can be attributed to the presence of carbon nanoparticles in the ablation plumes.