Generation of Coherent Soft X Rays at 2.7 nm Using High Harmonics

Ultrafast laser pulses from a Ti:sapphire laser centered at 800 nm, with 26 fs pulse duration, were used to generate coherent soft-x-ray harmonics, at wavelengths down to 2.7 nm (460 eV) in He, and 5.2 nm (239 eV) in Ne. In He, discrete harmonic peaks are observed up to order 221, and unresolved harmonic emission is observed up to order 297. These wavelengths are well within the ``water window'' region of x-ray transmission. Our work represents the shortest wavelength coherent light generated to date. The harmonic cutoff from all the noble gases is consistent with analytic theory.