Research of attosecond pulse train generation and phase information reconstruction

Abstract

Attosecond pulses provide higher measurement precision for analyzing ultrafast dynamics in atoms, molecules, and electrons, laying the foundation for studying electronic motion in atomic and molecular systems. The most important method currently is to generate attosecond pulse trains and isolated attosecond pulses through the interaction of femtosecond lasers with gases. The temporal information of attosecond pulses and the dynamic information of electrons can be extracted from spectrograms using attosecond streak camera or the Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBITT) method based on two-photon transition interference. Although phase differences of different high-order harmonics can be directly extracted from the oscillation frequency of sidebands, the iterative algorithm of attosecond streak camera can provide complete phase information of attosecond pulse trains to better support the study of electron dynamics in atoms. <b> Research Purpose:</b> The research presented in this article is dedicated to the investigation of attosecond pulse train (APT) generation, measurement, and characterization, which are essential for probing ultrafast dynamics in atomic, molecular, and electronic systems. The primary focus is on the generation of APTs through interactions between femtosecond lasers and gases, as well as the extraction of temporal and dynamic information from these pulses using advanced spectroscopic techniques such as the RABITT method. <b> Methods:</b> The experimental approach involved the use of a homebuilt femtosecond titanium sapphire regenerative amplifier to produce high-order harmonics, leading to the generation of APTs. The setup included the homebuilt titanium sapphire chirped pulse amplifier and a collinear attosecond pulse generation and measurement beamline, which were used to conduct RABITT experiments. The process entailed the interaction of femtosecond lasers with gas targets to generate high-energy photons in the extreme ultraviolet and soft X-ray spectral ranges. By optimizing the phase-matching conditions within the gas target, strong high-order harmonic signals were observed on an XUV spectrometer. The temporal information of the attosecond pulses was indirectly measured through the photoelectron spectrum produced by the interaction of attosecond pulses with femtosecond lasers. The research also employed the FROG-CRAB algorithm and the ePIE (Extended Phase Retrieval and Iterative Engine) algorithm for the temporal reconstruction of APTs and attempted to use a genetic algorithm to extract phase information. <b> Results:</b> The study yielded three sets of RABITT spectrograms, which were analyzed using the RABITT sideband phase method to directly reconstruct APTs. Fourier transform analysis was applied to extract phase differences between sidebands, offering insights into the phase differences between corresponding high-order harmonics. This method, however, provided an estimation of the phase at the center of each harmonic order, which does not fully represent the actual pulse shape. The FROG-CRAB and ePIE algorithms successfully reconstructed the attosecond pulse trains from the RABITT spectrograms, revealing similar temporal pulse train morphologies. In contrast, the genetic algorithm, despite its potential for high constraint optimization, did not yield satisfactory results, possibly due to the sensitivity of the algorithm to discrepancies between theoretical simulations and experimental data. <b> Conclusion:</b> The research concludes that achieving ideal inversion results for APTs necessitates small time delay steps and a wide scanning range during the experimental data collection process to ensure a rich dataset for inversion. The FROG-CRAB and ePIE algorithms demonstrated effective performance in reconstructing APTs, with ePIE showing higher computational efficiency. The genetic algorithm, while offering a high degree of constraint, faced challenges and requires further refinement. The study underscores the importance of the signal-to-noise ratio in experimental data for the accuracy of inversion results. This work provides significant guidance for future measurements of electron dynamics and the interpretation of their evolution patterns, contributing valuable experimental methodologies and data analysis techniques to the field of attosecond science.