Optical harmonic generation in the vicinity of molecular resonances by mid-IR laser field

We present a novel approach for flexible adjustment of the media nonlinear response for efficient harmonics generation. Molecular resonances located in mid-IR have shown to change refractive index for pump radiation significantly and provide opportunity for effective phase matching. We use mixture containing 48 bar of xenon (Xe) and 1 bar of carbon dioxide (CO₂) to generate eleven harmonics with efficiency of 1% for third harmonic and 0.3% for higher ones (from 5th to 11th) in total. Theoretical calculations predict simultaneous phase matching of large number of harmonics in XUV region paving the way for seeding FELs and pump-probe XUV diagnostic.