Highly Efficient, Phase-Matched High-Harmonic Generation by a Self-Guided Laser Beam

We demonstrate a phase-matched high-harmonic generation of 100 fs Ti:sapphire laser pulses by a self-guided beam in 30 Torr of neon gas. Phase matching of high harmonics is experimentally examined by changing the propagation length up to 7 mm. Phase-matched propagation over 7 mm magnifies the conversion efficiency around the 49th harmonic (in the cutoff region) by 40 times. On the other hand, harmonics around the 25th order (in the plateau region) peak at 4 mm, resulting in an enhancement of 4. This scheme enhances the conversion efficiencies in the cutoff region up to 10^-6, corresponding to a few nJ per high-harmonic pulse in the 76 eV region.