In 2023, Pierre Agostini, Ference Krausz and Anne L'Huillier had been awarded the Nobel Prize in Physics for their contribution in experimental methods of generating attosecond pulses of light for the study of electron dynamics in matter. Based on their pioneering work of high harmonics generation (HHG), generation and characterization of attosecond pulse trains (APTs) and isolated attosecond pulses (IAPs), a whole new physics research field named attosecond science was opened up. With the rapid development of attosecond science in the past two decades, extremely short IAPs have been generated and applied in photon spectroscopy and attosecond transient absorption spectroscopy (ATAS), providing researchers more powerful tool to study the ultrafast electron dynamics in atoms, molecules and condensed matter than ever with attosecond temporal resolution. These ultrafast processes include the photoionization time delay in atoms, ionization difference of polar and non-polar molecules, electrons migration in multi-atomic molecules, measurement of Auger decay process, inner-shell transition and probing of multielectron dynamics.
Thanks to the progress of the ultrafast laser techniques as pumping lasers, multiple methods for gating, and fine spectral chirp for compensation in the past two decades, the spectrum of the IAP has expanded from tens of electron volts to hundreds of electron volts and its pulse duration record is getting compressed. Although many research groups have succeeded in achievement of broadband spectrum and appropriate dispersion compensation, generating sub-100 as (
For the accurate measurement of such short attosecond pulses, the attosecond streaking camera scheme is adopted from the femtosecond pulses measurement in 2002. Based on the cross-correlation scheme, the IAP photoionized electrons are modulated in the presence of the delay controllable near-infrared (NIR) light field. And both the spectral phase and intensity distribution of IAP and NIR are encoded in the detected frequency and delay time two-dimensional measurement, denoted as spectrogram, which permits full reconstruction of the IAP and NIR.
Based on the attosecond streaking camera, many techniques have been proposed to retrieve the spectral phase and then reconstruct the temporal electric field of IAP and NIR. Developed by Mairesse et al., the frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) is commonly used for attosecond pulse characterization. But it uses high intensity streaking fields, resulting in the above-threshold ionized electrons that could overlap with streaked electrons. Much worse is the central momentum approximation (CMA) used to apply the iterative algorithms in femtosecond laser measurement, which limits the IAP bandwidth to few electron volts. For circumventing the CMA, Chini et al. proposed the phase retrieval by omega oscillation filtering (PROOF) for broader bandwidth and shorter IAP. PROOF applies weak field approximation (WFA) to modulate the photoelectrons and therefore focuses on the oscillation component of the dressing laser frequency, while WFA limits the streaking and retrieval application and its genetic algorithm has the problems of huge time cost and fatal shortcomings of multiple solutions in the iterative process. The quick version of PROOF (qPROOF) proposes a new error function to improve the retrieval accuracy and can be solved by the steepest descent method, improving the speed 5000 times faster than genetic algorithm. Moreover, qPROOF algorithm is numerically tested and proved to be robust against the pulse duration and intensity of streaking NIR, time-of-flight (TOF) electron detection noise, pump-probe delay jitter and large scanning step.
Multiple methods also have been proposed to avoid the CMA, WFA and slowly varying envelope approximation. The Volkov transform generalized projections algorithm (VTGPA) based on the Volkov states is developed to bypass the commonly used Fourier transform, making this method more applicable for complex IAP electric field waveform. Also, many groups have come up with novel approaches such as phase retrieval of broadband pulse (PROBP) and PROBP-autocorrelation (PROBP-AC), as well as ptychographic algorithm for attosecond reconstruction, and even the neural network and machine learning techniques are adopted to inject new solutions for attosecond measurement.
Since the advent of IAP generation, extensive efforts have been devoted to IAP experimental generation, measurement and characterization algorithm research mainly based on attosecond streaking camera scheme, paving the way for further attosecond application, such as ATAS and attosecond photoelectron spectroscopy.
With the development and application of high-repetition, high pulse energy mid-infrared laser, the attosecond streaking camera faces theoretical flaws as its energy resolution and photoionization cross-section of the gas medium decrease with the increase of photon energy. Also streaking camera based characterization algorithm should be verified and developed under these novel experimental conditions. Both theoretically and experimentally, there is urgent need for a new approach to accurately characterize the spectral and temporal properties of IAP with the latest driving laser and measurement techniques. And the single shot measurement and characterization of IAP is also of vital importance in high-energy laser drive facility with relatively lower repetition rate.
