Subcycle phase matching effects in short attosecond pulse trains

Abstract

Attosecond pulses produced by High-order Harmonic Generation (HHG) in gases driven by intense laser fields have become a cornerstone technique for probing ultrafast electronic motion in matter. These applications require a good knowledge of the temporal and spectral properties of the emitted radiation. In this work, we generate a train of two to three attosecond pulses that we characterize using two-color laser-assisted photoionization. \textcolor{black}{An unexpected spectral behavior, with more pulses at high energies than at low energies, is observed when the carrier-to-envelope phase of the laser field is changed by 90$^\circ$.} HHG simulations indicate that the time-dependent phase matching of the harmonics contributes in a non-trivial way to the structure of the pulse train. Two-color laser-assisted photoionization enables us to unravel the dynamical influence of subcycle phase matching on the spectral properties of the attosecond pulse train, going beyond the predictions of the response of a single atom to a strong laser field.

Figures (11)

• Figure 1: (A) Illustration of the spectral content of attosecond pulses (temporal slits) generated in each half-cycle of a short laser pulse, shown in (B). Each color (purple, blue, and orange) refers to a specific pulse, with a unique cut-off energy $E_c$. The pattern resulting from the interference is shown in gray.
• Figure 2: (A) Schematic of the experimental setup. Laser pulses from a Ti:Sapphire oscillator are amplified using two nonlinear optical parametric amplification (NOPA) stages. The relative CEP of the output pulses is controlled shot-to-shot through the feedback of a stereo Above-Threshold-Ionization (SATI) device to a wedge pair in the oscillator WittmannNatPhys_2009. The CEP-controlled pulses are separated into a probe arm and a variably delayed pump arm by a beam splitter (BS). Short attosecond pulse trains are produced by the IR driving field using HHG and recombined with the probe before being focused into the sensitive region of a 3D electron spectrometer. (B) The controllable relative CEP $\Delta \varphi_{\mathrm{CEP}}$ of the $\leq$ 6 fs laser pulses. (C) Momentum projected along $p_x$ and $p_z$ of the photoemitted electrons. The electrons can be distinguished between those emitted with $p_z>0$ ("Up") and those emitted with $p_z<0$ ("Down"), where $z$ is the axis along the detector and of the polarization of the light. BS: Beam splitter; SATI: Stereo-ATI; AF: Aluminium filter; RM: Recombination mirror; MCP: Micro-channel plate.
• Figure 3: (A and C) Experimental photoelectron spectra as a function of the XUV-IR delay for two different CEP values separated by 90$^\circ$. Only electrons photoemitted in the upper hemisphere around the polarization axis of the light fields are shown. (B and D) Wigner distributions of the attosecond pulses retrieved from the experimental spectrograms through the refined extended ptychographic iterative engine (rePIE) LucchiniAppSci_2018.
• Figure 4: (A and C) Photoionization spectrograms as a function of the kinetic energy of the electrons and of the XUV-IR delay simulated using the 3D model. (B and D) Wigner representation of the simulated attosecond pulse trains. The CEPs are (A and B) $70^\circ$ and (C and D) $160^\circ$.
• Figure 5: (A and B) Total phase mismatch as a function of time in units of the laser period ($T_\mathrm{IR}$) for four harmonic orders $q$ for the CEPs corresponding to Fig. \ref{['fig:fig_3']}B. The horizontal black dashed line corresponds to perfect phase matching. (C and D) Corresponding yield for each harmonic order $q$. The CEPs are 70$^\circ$ (top) and 160$^\circ$ (bottom). Dashed lines indicate the temporal profile of the single-atom response.