Phase-matched emission of few high-harmonic orders from a helium gas cell

We report on femtosecond-laser-based generation of a few phase-matched high-order harmonics in a helium semi-infinite gas cell. The harmonic beam consists of effectively four to six intense orders with wavelengths around 9.5 nm. By varying the effective interaction length, we observe the effects of coherence length related to plasma formation and the Gouy phase shift on the output. The atomic scattering factors for photoabsorption, the effective propagation lengths (leff,He ≈1 mm, leff,Ar ≈7.2 mm), and the energy conversion efficiencies (εHe ≈ 10-7, εAr ≈5× 10-7) are deduced for harmonic generation in helium (77th to 95th orders) and also argon (21st to 27th orders) for comparison.