Parameter-free retrieval of subcycle asymmetry of polar molecules by high-order harmonic spectroscopy

Abstract

Retrieving atomic and molecular properties from entangling macroscopic measurements is an ambitious task. In particular, the phase difference between the adjacent attosecond bursts emitted from polar molecules is of interest and has been reconstructed from measured high-order harmonic generation (HHG) when molecules are exposed to an intense laser pulse. However, besides the ratio of intensities between the even and odd harmonic orders which can be measured directly, the suggested construction method requires nonmeasured parameters that can be supplied through theoretical calculations. The complexity or comprehensiveness of this theory clearly affects the accuracy of the retrieval procedure. In this study we propose a robust parameter-free method to retrieve the subtle molecular asymmetry, i.e., both the spectral phase difference and intensity ratio of two adjacent attosecond bursts, from purely experimental measurements of HHG. For this purpose, we first figure out that the phase difference is an intrinsic feature of polar molecules, almost independent of the external laser's parameters. After that, we design a detailed procedure to quantitatively extract these features via the time-frequency profile from the intensity and phase measurements of HHG emitted from partially oriented molecular samples. In this step, we develop the analytical formula to convert the phase difference and intensity ratio of a partially oriented sample to those of a single molecule. The accuracy and robustness of the methods are validated by numerical data.