Enhancement of high-order harmonic generation at tuned wavelengths through adaptive control.
An adaptive learning loop enhances the efficiency and tuning of high-order harmonic generation. In comparison with simple chirp tuning, we observe a broader tuning range and a twofold to threefold enhancement in integrated photon flux in the cutoff region. The driving pulse temporal phase varies significantly for different tunings and is more complicated than a simple chirp. We compare our experimental results with a one-dimensional, time-dependent model that incorporates the intrinsic atomic response, the experimental pulse temporal phase, ionization effects, and transverse coherence of the spatial mode of the laser. The model agrees with our experimental results and indicates that a specific quantum path coupled with ionization effects determines the optimized harmonic spectrum.