Robust Isolated Attosecond Pulse Generation with Self-Compressed Subcycle Drivers from Hollow Capillary Fibers

Abstract

High-order harmonic generation (HHG) arising from the nonperturbative interaction of intense light fields with matter constitutes a well-established tabletop source of coherent extreme-ultraviolet and soft X-ray radiation, which is typically emitted as attosecond pulse trains. However, ultrafast applications increasingly demand isolated attosecond pulses (IAPs), which offer great promise for advancing precision control of electron dynamics. Yet, the direct generation of IAPs typically requires the synthesis of near-single-cycle intense driving fields, which is technologically challenging. In this work, we theoretically demonstrate a novel scheme for the straightforward and compact generation of IAPs from multicycle infrared drivers using hollow capillary fibers (HCFs). Starting from a standard, intense multicycle infrared pulse, a light transient is generated by extreme soliton self-compression in a HCF with decreasing pressure and is subsequently used to drive HHG in a gas target. Owing to the subcycle confinement of the HHG process, high-contrast IAPs are continuously emitted almost independently of the carrier-envelope phase (CEP) of the optimally self-compressed drivers. This results in a CEP-robust scheme which is also stable under macroscopic propagation of the high harmonics in a gas target. Our results open the way to a new generation of integrated all-fiber IAP sources, overcoming the efficiency limitations of usual gating techniques for multicycle drivers.

Figure 1

Schematic of an in-vacuum HHG beamline driven by subcycle self-compressed pulses from a gas-filled HCF with a decreasing pressure gradient. Starting from a multicycle IR pulse (1), a light transient (2) is generated by extreme soliton self-compression in the fiber and subsequently used to produce EUV IAPs (3) in a gas target.

Figure 2

(a) Ratio of output to input peak power of the self-compressed IR pulses as a function of the initial energy U₀ and the equivalent constant pressure p_eq in the negatively pumped 100 μm core-radius, 3 m long HCF filled with Ar. The solid black line represents the contour where L = L_av, which runs along the optimal region for subcycle self-compression. (b, c) Self-compressed IR drivers generated in the HCF for two different sets of (U₀, p_eq, ϕ_CEP): (b) a few-cycle pulse and (c) a subcycle light transient. (d, e) Time-frequency analysis of their corresponding single-atom HHG emissions in hydrogen when their instantaneous peak intensity is set to 1.57 × 10¹⁴ W/cm² (electric field amplitude of 0.067 au). Additional panels (d.1, e.1) show the high-harmonic spectra and (d.2, e.2) their temporal counterpart in the form of an attosecond pulse train or a clean IAP.

Figure 3

(a) Robustness of the IAPs to variations in the CEP of the subcycle drivers which are generated in the HCF for different pairs of pump energy and gas pressure. The highlighted point with a star label refers to the situation shown in Figure 4. (b) Single-atom HHG spectra as a function of the CEP of the IR driving field generated in the HCF for an input pulse energy U₀ = 83.75 μJ and an equivalent Ar pressure p_eq = 179.5 mbar. (c) Temporal profile and (d) fwhm intensity duration and contrast of the corresponding attosecond pulses.

Figure 4

(a) Harmonic spectrum as a function of the divergence angle in the far-field detector obtained by driving the HHG process, in a 1 mm thick low-density hydrogen jet centered at the beam focus, with the subcycle IR pulse obtained from the HCF for U₀ = 58.75 μJ, p_eq = 255.8 mbar, and ϕ_CEP = 5π/8 rad (corresponding to the situation labeled in Figure 3a with a star) and focused and attenuated to an instantaneous peak intensity of 1.82 × 10¹⁴ W/cm² (field amplitude of 0.072 au). (b) Resulting on-axis attosecond pulse and (c) its temporal intensity profile.

Figure 5

(a) Schematic of an all-fiber IAP source, where HHG is directly driven at the HCF end by a self-compressed, high-energy IR subcycle transient. (b) Contrast of the directly emitted IAPs as a function of the CEP of the driving 2.2 fs, 1.3 mJ self-compressed pulse at 1600 nm, generated in the helium-filled HCF. (c) Single-atom 3D-TDSE high-harmonic spectrum in He for the waveform with ϕ_CEP = 3π/8 rad, after transmission through a zirconium filter. (d) Corresponding IAP (fuchsia line) plotted over the main feature of the IR driving field.