Generation of high-energy high-order harmonics by use of a long interaction medium

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Abstract
We investigate the energy scaling of high-order harmonics in Ar, using a loosely focused beam in a phase-matched condition. Adjusting the argon gas density and the pump laser focusing condition produces a total output harmonic energy as high as 0.7 μJ in the spectral region 34.8–25.8 nm (the corresponding orders of the 23rd–31st harmonics); 27th-order harmonic (29.6-nm) energy attained is as high as 0.3 μJ with an almost perfect spatial profile. High efficiency and good spatial quality are achieved simultaneously.