Few-Cycle and Sub-Cycle Metrology for the Characterization of High Harmonics.

Abstract

The rapid advances in the generation of ultra-short optical pulses in recent decades have often outstripped the ability of metrologists to accurately measure the pulses' temporal profiles. With each reduction in pulse duration, existing measurement techniques must be re-evaluated and often times partially or completely replaced with newer schemes providing the required temporal sensitivity. Frequency or time-domain metrology performed after a short pulse interaction with a physical system can provide volumes of information about the governing physics of the system. Two new techniques for the temporal characterization of few-cycle and sub-cycle radiation are presented, along with experimental results and analysis. A dispersion-free autocorrelator designed to characterize attosecond pulses generated through relativistic laser-plasma interactions is demonstrated. As opposed to other dispersion-free autocorrelation designs, this device is capable of measuring a linear autocorrelation as well as a nonlinear autocorrelation, and hence is suitable for complete characterization of ultrafast pulses in-situ. Experimental results demonstrate that this autocorrelator produces pulse reconstructions that are in good agreement with measurements performed using an alternative time-resolved technique.

In the strong field regime, a cross-correlation frequency-resolved optical gating scheme is presented. The XFROG is designed for characterizing harmonics generated by a scaled system: a 3.6micon laser driving a cesium source. Unlike more widely-used time-domain measurements, this scheme is sensitive to the relative arrival time between harmonic orders. A novel technique employing the XFROG itself to completely characterize the unknown dispersive properties of the cesium heat pipe output window is demonstrated, allowing the removal of the window dispersion from the data and the reconstruction of the harmonics inside the heat pipe. Error analysis demonstrates that the XFROG is sensitive to the relative delay between harmonic orders to within 180as. The observed negative dispersion on the harmonics' spectral phase and the observed harmonic yield versus frequency are shown to be qualitatively consistent with TDSE calculations.

Additional measurements are presented demonstrating self-compressed, spectrally broadened pulses emerging from filamentary propagation at both 800nm and 2micron with high energy transmission. The 2micron self-compressed pulses are shown to maintain carrier-envelope phase stability through the filamentary propagation process with pulse durations < 3 optical cycles.