Strong-field coherent control of isolated attosecond pulse generation

Attosecond pulse generation, primarily through high-order harmonic generation (HHG) in gaseous media, has revolutionized our understanding of light-matter interactions. This technique allows the exploration of charge dynamics on sub-femtosecond timescales, revealing phenomena like valence electron motion in atoms and electron charge migration in molecules. While HHG-based sources are widely accessible, limitations exist: low photon flux hinders the study of low cross-section processes, and limited spectral tunability restricts the ability to selectively address specific electronic transitions. This necessitates the development of attosecond sources with both higher photon flux and improved spectral tunability. The inherent relationship between the driving laser field's shape and the characteristics of the generated high harmonics suggests a path toward enhanced control over the generated attosecond pulses. Previous research demonstrated that tailored optical waveforms can increase HHG conversion efficiency. However, experimental exploration of waveform shaping for controlling attosecond pulse central energy, spectral shape/bandwidth, and duration remained largely unexplored. This research investigates the generation of tunable isolated attosecond pulses (IAPs) with precisely controlled spectral-temporal characteristics, achieved through parametric waveform synthesis. The goal is to demonstrate the coherent control of free-electron trajectories within the HHG process by shaping the driving optical waveform, thereby improving the performance of attosecond light sources.

The generation of attosecond pulses using HHG has been extensively studied, with methods focusing on pulse trains and isolated pulses. Gating techniques such as double optical gating and amplitude gating have been employed to isolate single attosecond pulses. However, the low photon flux and limited spectral tunability of conventional HHG sources pose significant experimental challenges. The electric field's influence on ionization, electron excursion, and recombination suggests the potential for controlling HHG characteristics through waveform shaping. Previous studies indicated that specific waveforms could enhance HHG conversion efficiency. This work addresses the gap by experimentally exploring waveform control over attosecond pulse parameters, contrasting with previous studies that focused on macroscopic parameter adjustments (gas pressure, gas cell position, driving pulse energy).

The study employed a parametric waveform synthesizer (PWS) to generate tailored, octave-spanning waveforms. The PWS combines near-infrared (NIR) and infrared (IR) pulses, allowing control over the relative phase (RP) between the two spectral regions and the overall carrier-envelope phase (CEP). A cryogenic Ti:Sapphire laser pumped optical parametric amplifiers (OPAs) to produce the individual NIR and IR channels. These channels were then coherently combined and compressed. The RP and CEP were precisely controlled and stabilized, enabling reproducible waveform synthesis. The synthesized waveforms, focused into a gas cell, generated high-harmonic radiation. The HHG spectra were recorded for a large range of RP and CEP values by systematically scanning both parameters. The resulting four-dimensional dataset (photon energy, spectral intensity, RP, CEP) was analyzed to reveal the relationship between waveform and HHG output. Attosecond streaking measurements were conducted for selected waveforms to determine the temporal structure of the IAPs. The Volkov Transform Generalized Projection Algorithm (VTGPA) was utilized for pulse retrieval, handling the broadband nature of the generated continua and complex streaking waveforms. Classical trajectory analysis and quantum single-atom response simulations (using HHGMax) were performed to understand the impact of waveform shaping on the HHG process and to complement experimental results. Argon and neon were used as target gases to explore the tunability across a broader spectral range.

The study demonstrated comprehensive control over the spectral and temporal characteristics of isolated attosecond pulses. By varying the RP and CEP of the driving waveform, the researchers produced attosecond continua spanning 30-110 eV in argon and extending to 200 eV in neon. The central photon energy, bandwidth, and duration of the IAPs were all substantially tuned without additional gating or spectral filtering techniques. The HHG spectra exhibited a rich dependence on both RP and CEP. For instance, a wide range of RP values generated IAPs free of spectral fringes. The highest IAP energy reached ~500 pJ for broadband IAPs and ~100 pJ for narrowband continua. Attosecond streaking measurements confirmed that the generated continua corresponded to IAPs, with pulse durations varying from 80 as to 240 as depending on the waveform parameters. The consistency between measured and reconstructed photoelectron spectrograms validated the accuracy of the pulse retrieval method. Simulations qualitatively reproduced the main experimental observations, revealing the connection between waveform shape and the characteristics of the generated IAPs. For example, simulations showed how different waveform shapes could lead to either narrowband or broadband emission, depending on the intensity and timing of the optical half-cycles that drive electron trajectories. This highlights that the control over the IAP characteristics was directly linked to the precise shape of the driving waveform, enabling selection of waveforms that favor either broad- or narrow-band emission.

The results significantly advance attosecond source technology by demonstrating unprecedented control over IAP generation. The ability to precisely tune the spectral and temporal properties of IAPs solely by adjusting the driving waveform's shape opens new avenues in attosecond science experiments. This surpasses the limitations of existing gating techniques, which often trade off bandwidth for pulse duration. The demonstrated control is highly versatile, extending the range of possible experiments significantly. The use of a PWS allows for a high degree of control over the waveform, resulting in precise tuning of the attosecond pulse parameters. The good agreement between experimental results and numerical simulations provides strong support for the interpretation of the experimental findings. The ability to access a wide range of attosecond pulse durations and bandwidths is particularly noteworthy, enabling a broad range of experimental applications.

This research demonstrated strong-field coherent control over isolated attosecond pulse generation, achieving significant advancements in spectral tunability and photon flux. Precise control over pulse duration and bandwidth, achieved solely through waveform shaping, expands the capabilities of attosecond science experiments. Future research might explore further refinement of waveform synthesis techniques to optimize IAP characteristics and investigate the application of this approach to different target materials and experimental setups.

While the study demonstrated remarkable control over IAP parameters, limitations exist. The simulations, although providing qualitative agreement with experimental findings, are simplifications of a complex process. The computational cost might limit the exploration of a broader parameter space. The experimental characterization through attosecond streaking measurements was resource intensive, restricting the number of waveforms characterized temporally. Further studies should aim for quantitative agreement between simulations and experiments.