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We investigate the effect of dynamic electron correlation on high-harmonicgeneration in helium atoms using intense visible light (\lambda=390nm). Twocomplementary approaches are used which account for correlation in anapproximate manner: time-dependent density-functional theory and asingle-active-electron model. For intensities I~10^{14} W/cm^2, the theoriesare in remarkably good agreement for the dynamic polarization and harmonicspectrum. This is attributed to a low-frequency collective mode together with ahigh-frequency single-electron response due to the nuclear singularity, both ofwhich dominate electron correlation effects. A time-frequency analysis is usedto study the timing and emission spectrum of attosecond bursts of light. Forshort pulses, we find a secondary maximum below the classical cut-off. Theimprint of the carrier-envelope-phase, for the time-integrated spectral densityappears at frequencies above the high-frequency drop-off, consistent withprevious studies in the infrared \lambda~800nm.

Dynamic correlation in high-harmonic generation for helium atoms in intense visible lasers