Temporal Characterization of Below-Threshold Harmonics in a Scaled Keldysh System

We report an experimental and theoretical study of below-threshold high harmonics created by the scaled Keldysh system interaction of a $3.6\mu\textrm{m}$ laser and a cesium target with Keldysh parameter $\gamma\sim 1$; in the Keldysh picture this interaction is the scaled equivalent of an 800nm laser illuminating an argon target. Harmonic orders $5-13$ were temporally characterized using cross-correlation frequency-resolved optical gating (XFROG); this scheme is sensitive to the relative delay between harmonic orders, $\textrm{d}\phi/\textrm{d}q$, and also provides a complete reconstruction of the individual harmonic orders. Through the use of unconventional dispersion management we completely account for the dispersion in our heat pipe exit window, allowing access to $\textrm{d}\phi/\textrm{d}q$ and the spectral phase $\phi(\omega-\omega_q)$ inside the heat pipe. We show that below-threshold harmonics exhibit negative dispersion. Comparison with a time-frequency analysis of harmonic emission times performed on 1-D TDSE simulation results will be presented. The observed $\textrm{d}\phi/\textrm{d}q$, pulse durations for individual harmonic orders, and intensity scaling strongly suggest non-perturbative below-threshold harmonic generation.