Nontunnelling high-order harmonics from ultra-intense laser-driven tightly bound systems

High-order harmonic emission is investigated by numerical solution of the weakly relativistic, two-dimensional Schrödinger equation for the case of ultra-intense laser-driven tightly bound systems (for example, multiply charged ions such as O⁷⁺ exposed to laser fields of the order of 10¹⁸ W cm^-2 at 248 nm). In contrast to their usual substantial decrease, the low-order harmonics having an energy less than the ionization potential exhibit a high-efficiency (i.e. intense) plateau with a well defined cutoff. The shape of this plateau is found to depend on the shape of the binding potential. A classical `surfing' mechanism for the generation of these harmonics is proposed that does not involve tunnelling and that nevertheless explains the observed cutoff. Thus we call them `nontunnelling harmonics'. The significance of relativistic effects for these harmonics is investigated and found to be small, despite the high laser intensity, because of the absence of tunnelling.