High harmonic generation without tunnel ionization

A high harmonic generation (HHG) scheme, which does not rely on tunnel ionization as the ionization mechanism but rather on single-photon ionization, is theoretically proposed and numerically demonstrated. The scheme uses two driver fields: an extreme-ultraviolet driver which induces the ionization, and a circularly polarized, corotating, two-color infrared driver carried at a fundamental frequency and its second harmonic which induces the recollision. Using classical and time-dependent Schr\"odinger equation simulations of a model argon atom, we show that in this scheme ionization is essentially decoupled from recollision. Releasing the HHG mechanism from being tunneling dependent reduces its degree of nonlinearity, which offers interesting capabilities in attosecond science, such as generation of high harmonics from highly charged ions, or from specific deep core electronic levels. It is shown that the emitted high harmonics involve the absorption of photons of one color of the infrared driver, and the emission of photons of the second color. This calls for future examination of the possible correlations between the emitted high harmonics.