Generation of Bright and Controllable Isolated Attosecond X-Ray Pulses from Synchronized Mid-Infrared and Ultra-short Ultraviolet Laser Fields

Abstract

We investigate, by solving the time-dependent Schrödinger equation in the single-active-electron approximation in helium, a two-color scheme for tabletop high-order harmonic generation (HHG) that combines a mid-infrared (MIR) driving field with an ultrashort ultraviolet (UV) pulse that could be generated via resonant dispersive wave emission in gas-filled hollow-core fibers. This configuration enables the generation of bright, isolated, and tunable attosecond X-ray pulses. In contrast to single-color driving schemes, which suffer from low conversion efficiency, unfavorable wavelength scaling, and limited spectral control, the MIR+UV approach provides a practical and controllable route for advancing tabletop ultrafast spectroscopy and real-time molecular imaging within current experimental capabilities.

Figures (4)

• Figure 1: (a) Schematic of our two-color MIR+UV scheme: the delay between the MIR (red) and UV (blue) pulses is denoted by $\phi_{d}$, and the semiclassical electron trajectories $x(t)$ are shown throughout the MIR pulse. (b,c) UV-intensity-dependent HHG spectra for (b) $\phi_{d} = +1/8$ o.c. and (c) $\phi_{d} = -3/8$ o.c.. The two plateaus, calculated from the semiclassical three-step model corkum1993lewenstein1994 and labeled i and ii, are positioned at $2.22U_{p} + I_{p}$ and $3.17U_{p} + I_{p}$, respectively.
• Figure 2: High-harmonic spectra (left) and Gabor transforms (right) of the dipole acceleration for (a) 3$\mu$m MIR-only, and for MIR+UV at four different values of the phase delay $\phi_{d}$: (b) $+1/8$, (c) $-3/8$, (d) $-1/2$, and (e) $-5/8$, all in optical cycles.
• Figure 3: (a) Peak intensity of the attosecond pulse as a function of the UV intensity. The trendline for intensity below $3.5\times10^{14}$W/cm$^2$ is proportional to $I_{\textsc{uv}}^{\alpha}$, with $\alpha = 2.44$. (b) Normalized isolated attosecond pulses generated via UV-enhanced HHG with increasingly-intense few-cycle UV pulses, synchronized with the MIR such that the short trajectories are enhanced. The colormap indicates the peak intensity of the attosecond pulse, and the arrows the decreasing FWHM.
• Figure 4: Efficiency scaling of the HHG plateau with differing wave lengths measuring from $2.22~U_p$ to $3.17~U_p$ where we compare two different phase delays $\phi_d=-7/16$ and $\phi_d=-3/8$ at two different intensity's $1\times10^{14}$, and $2\times10^{14}$ W/cm$^2$ to that of the MIR only.