High-Harmonic Generation in the Water Window from mid-IR Laser Sources

Light sources situated in the water window, which spans the K-edges of carbon and oxygen, can be used to image biological molecules in their natural aqueous environments [1]. These soft X-rays can be produced, e.g., by synchrotron radiation, X-ray free-electron lasers, or high-harmonic generation from mid-IR lasers, with the latter demonstrating the most promise for live-cell imaging with femtosecond time resolution [2]. However, describing harmonic generation in the mid-IR regime is a computationally difficult task, particularly if multi-electron effects are to be included in the calculation. We investigate the harmonic response of neon atoms to mid-IR laser fields (2000−3000 nm) using both a single-active electron (SAE) model [3] and the fully ab initio all-electron R-Matrix with Time-dependence (RMT) method [4]. The laser peak intensity and wavelength are varied to find optimal parameters for high-harmonic imaging in the water window. Comparison of the SAE and RMT results shows excellent agreement between the resulting spectra, and parameters such as the cut-off frequency predicted by the classical three-step model.

[1] R. Neutze et al., Nature 406, 752 (2000).

[2] Y. Fu et al., Commun. Phys. 3, 92 (2020).

[3] T. Pauly et al., Phys. Rev. A 102, 013116 (2020).

[4] A. C. Brown et al., Comp. Phys. Commun. 250, 107062 (2020).