XFROG phase measurement of threshold harmonics in a Keldysh-scaled system

Attosecond pulses are created (by Fourier synthesis) from a comb of odd-order high harmonics resulting from the non-perturbative interaction of intense near-visible laser light with an atomic gas(1-3). When produced by a mid-infrared laser, harmonics can have simultaneously high order, visible wavelength, and photon energy below the ionization threshold, I(p), of the generating atom. Methods requiring photon energies greater than I(p) have been developed(4) that measure the spectral amplitude and phase necessary for temporal reconstruction of the harmonic radiation. Here we report the temporal characterization of below-threshold harmonics using sum frequency generation cross-correlation frequency resolved optical gating(5) (SFG XFROG), a technique sensitive to the relative delay between orders(6), coupled with a novel approach that makes use of the Keldysh scaling(7) in strong-field physics. The results surprisingly suggest non-perturbative generation of below threshold harmonics, providing a potential alternative to existing vacuum-ultraviolet frequency comb generation methods(8,9).