Abstract In the past few decades, the development of ultrafast lasers has revolutionized our ability to gain insight into light–matter interactions. The emergence of few-cycle light sources operating from the visible to the mid-infrared spectral range—as well as attosecond extreme ultraviolet and X-ray technologies—provide the possibility to directly observe and control ultrafast electron dynamics in matter on their natural timescale; however, the temporal characterization of few-femtosecond sources in the deep ultraviolet (4–6 eV, 300–200 nm) and the vacuum ultraviolet (VUV; 6–12 eV, 200–100 nm) spectral regions is challenging. Here we fully characterize the temporal shape of microjoule-energy VUV pulses tuned between 160 and 190 nm generated via resonant dispersive wave emission during soliton self-compression in a capillary using frequency-resolved optical gating based on two-photon photoionization in noble gases. The in situ measurements reveal that in most of the cases the pulses are shorter than 3 fs. These findings pave the way toward investigating ultrafast electron dynamics and valence excitation of a large class of atoms and molecules with a time-resolution that has been hitherto inaccessible when using VUV pulses.

We demonstrate compression of few-cycle ultraviolet (UV) resonant dispersive waves (RDWs) generated in a cascaded hollow capillary fiber setup using a Yb laser system. Temporal characterization is performed using both tunneling ionization with a perturbation for the time-domain observation of an electric field (TIPTOE) and self-diffraction frequency-resolved optical gating (SD-FROG), which show good agreement. Through careful dispersion management, we compress the RDW pulse to 6.9 fs at a ∼390-nm central wavelength. This is the first, to our knowledge, measurement of an RDW using the TIPTOE method and demonstrates the viability of this technique to reliably characterize few-cycle UV pulses with μJ pulse energies.

Abstract Attosecond light pulses have revolutionized the study of electron dynamics in materials by enabling the observation of ultrafast processes with unprecedented attosecond temporal resolution. They are primarily generated through the process of high-order harmonic generation (HHG). This paper presents a comprehensive setup for attosecond pulse generation and measurement. Using a 900 nm, 7 mJ, 7 fs femtosecond laser with stabilized carrier-envelope phase (CEP), we employ polarization gating to generate a near single-cycle, linearly polarized pulse that interacts with neon gas to produce a broadband extreme-ultraviolet (XUV) continuum with a cutoff photon energy of ~120 eV. The temporal and spectral characteristics of the generated single attosecond pulses are measured using attosecond streak camera, and the pulse duration is determined to be 59 as through the Frequency-Resolved Optical Gating for Complete Reconstruction of Attosecond Bursts (FROG-CRAB) retrieval algorithm. As part of the Synergetic Extreme Condition User Facility, this setup will facilitate ultrafast research in transient absorption and photoelectron spectroscopy, providing global users with a powerful tool for studying electron dynamics in various materials.

The most widely used frequency-resolved optical gating (FROG) retrieval algorithm solves the trace inversion problem to retrieve the phase distribution of the ultrashort pulse electric field by using preprocessed measured data in each iterative step to improve subsequent guesses. Such algorithms work very well for measurements with high signal-to-noise ratios but can become less reliable in extracting weaker signals buried in noisy data. We introduce the line-search FROG (LSF) algorithm, which enhances noise robustness by treating measurement data passively, using it solely for error evaluation rather than iterative correction. The gradient-free LSF algorithm requires no preprocessing of the measurement data and thus does not make assumptions about the noise in the measured traces. We show that LSF achieves comparable FROG error metrics to a ptychographic retrieval algorithm and COPRA, while producing higher-quality pulse reconstructions with reduced noise contaminations. It is applicable to all FROG geometries, supports blind FROG retrieval, and can reconstruct pulses from incomplete datasets.

We have successfully produced an ultrathin freely suspended GO film, which is a biomimetic structure inspired by the transparent dragonfly wing structure. Based on a colloidal self-assembly process over a large area, solvent evaporation was applied within a limited opening geometry. The free-standing GO film shows a significant enhancement of the nonlinear optical absorption, where saturable absorption and photoinduced absorption were observed at dramatically decreased excitation fluence compared with other work on GO films dispersed on substrates. Surprisingly, we also found that free-standing GO films are beneficial for compressing femtosecond pulses around 800 nm. Using a frequency-resolved optical gating as well as an open aperture Z-scan method, the origin was found to be associated with two effects. While the pulse shortening results from saturable absorption, the chirp effect is also suppressed due to the presence of an inflection point around 800 nm in the refractive index spectrum of free-standing GO film.

Abstract The creation of structured electronic wave packets (EWPs) energetically close to Fano resonances has been achieved with ultrafast extreme ultraviolet coherent light sources. However, direct real-time observations of EWP evolution and full reconstructions of the quantum properties of EWPs, including both amplitude and phase, are lacking. Here we introduce and demonstrate a comprehensive approach for the direct measurement and complete characterization of structured EWPs created within a prototypical Fano resonance. Because of its analogy with frequency-resolved optical gating (FROG), we named the method photoelectron FROG. The correlated EWP is initiated by a carefully engineered extreme UV pump pulse. A weak near-infrared laser field, serving as a probe pulse, samples the evolution of the EWPs in the time domain, as well as in the frequency domain. The amplitude and phase of the EWPs are obtained via a time-dependent reconstruction algorithm based on a short-time Fourier transformation. Given the excellent agreement between our experimental results and time-dependent reconstructions, we expect this method to be broadly applicable to the study of ultrafast processes, especially electronic ones, in complex systems, as well as the coherent control of such systems on their fundamental timescales.

Accurate retrieval of ultrashort laser pulses using frequency-resolved optical gating (FROG) is often hindered by local convergence in existing algorithms. We introduce the Sigma Check, a novel supervised evaluation step, to our knowledge, designed to detect and mitigate erroneous convergence to local minima. This method evaluates the difference between measured and retrieved spectrograms, applying targeted perturbation when significant discrepancies are detected. Numerical simulations and experimental measurements demonstrate that the Sigma Check enhances retrieval accuracy across multiple algorithms, including line-search FROG, extended ptychographic iterative engine, and principal components generalized projections algorithm, providing a systematic approach to improving FROG pulse reconstruction.

The precise spatiotemporal characterization of broadband ultrafast laser beams is essential for accurate laser control and holds significant potential in photochemistry and high-intensity laser physics. Existing methods for spatiotemporal characterization, such as frequency-resolved optical gating (FROG) and compressed ultrafast photography (CUP), are often spatially averaged or suffer from limited spatial resolution. Recent advances in imaging techniques based on multiplexed ptychography have enabled high-spatial-resolution diagnostics of ultrafast laser beams. However, the discrete spectral assumption inherent in multiplexed ptychographic algorithms does not align with the continuous spectral structure of ultrafast laser pulses, leading to significant crosstalk between different wavelength channels (WCs). This paper presents a method to reduce the bandwidth of each wavelength channel through spectral modulation, followed by the discretization of the continuous spectrum using interference techniques, which significantly improves the convergence and accuracy of the reconstruction. Using this method, the experiment accurately measured chromatic dispersion, spatial chirp, and other spatiotemporal coupling effects in femtosecond laser beams, achieving a spatial resolution of 9.4 μm, close to the pixel size resolution limit of the angular spectrum method.

High-repetition-rate targets present an opportunity for developing diagnostic tools for on-demand calibration at high-power laser facilities for consistent performance and reproducibility during experimental campaigns. The non-linear change in transmission associated with a laser-driven plasma mirror, based on high-repetition rate targets, has been used in a Frequency Resolved Optical Gating (FROG) configuration to analyze the spectral phase for near-infrared pulses far from the Fourier limit. Three types of targets were compared for characterizing pulses in the 1–8 ps range: a glass slide, a polymer tape, and a thin liquid sheet created by two impinging micrometer-scale jets. The thin liquid film had the best mechanical stability and introduced the least spectral distortion, allowing the most robust reconstruction of the temporal intensity profile. The spectral phase was reconstructed using a non-iterative algorithm, which reproduced the second-order phase distortions induced with an acousto-optic programmable dispersive filter with an RMS error of 6.2%, leading to measured pulse durations with an RMS deviation ranging from 1% for pulses of 6.8–7.8 ps up to 7.5% for pulses around 1 ps.

The optimization of photoinjector brightness is crucial for achieving the highest performance at X-ray free-electron lasers. To this end, photocathode laser pulse shaping has been identified as a key technology for enhancing photon flux and lasing efficiency at short wavelengths. Supported by beam dynamics simulations, we identify transversely and longitudinally truncated Gaussian electron bunches as a beneficial bunch shape in terms of the projected emittance and 5D brightness. The realization of such pulses from chirped Gaussian pulses is studied for 514 nm and 257 nm wavelengths by inserting an amplitude mask in the symmetry plane of the pulse stretcher to achieve longitudinal shaping and an aperture for transverse beam shaping. Using this scheme, transversely and longitudinally truncated Gaussian pulses can be generated and later used for the production of up to 3 nC electron bunches in the photoinjector. The 3D pulse shape at a wavelength of 514 nm is characterized via imaging spectroscopy, and second-harmonic generation frequency-resolved optical gating (SHG FROG) measurements are also performed to analyze the shaping scheme’s efficacy. Furthermore, this pulse-shaping scheme is transferred to a UV stretcher, allowing for direct application of the shaped pulses to cesium telluride photocathodes.

ABSTRACTThis study systematically investigates the characterization and analysis of ultrashort laser pulses by simulating the virtual frequency‐resolved optical gating (FROG) setup. Using LabII tools within the LabVIEW environment, virtual experiments were performed to generate ideal ultrashort pulses, introduce controlled phase distortions using Taylor's equation, and evaluate their temporal and spectral properties. The simulation framework serves as an efficient, interactive, and accurate platform for analyzing the effects of dispersion, including group delay dispersion (GDD), third‐order dispersion (TOD), and fourth‐order dispersion (FOD), on pulse dynamics. The FROG simulations illustrate how these dispersion parameters alter pulse symmetry, broaden durations, and induce chirp. Various FROG configurations, such as SHG‐FROG, SD‐FROG, PG‐FROG, and THG‐FROG, explored chirp evolution and its relationship with tilt, skewness, and fragmentation in FROG traces. Chirp evolution was calculated, and pulse duration results from the virtual FROG setup were validated by comparison with commercial FROG pulse‐retrieval software. The findings confirm that higher‐order dispersion introduces increasingly complex distortions, affecting the spatiotemporal coherence of the pulses. The study underscores the robustness of virtual FROG diagnostics, presenting them as a reliable alternative to physical experimentation for understanding and optimizing laser pulse dynamics. Our simulation results emphasize the importance of virtual diagnostics in accurately quantifying dispersion‐induced distortions, enhancing the design and performance of laser systems.

The ability to control the amplitude and phase of extreme ultraviolet (XUV) and X-ray free-electron laser (FEL) pulses can allow for the extension of optical techniques, such as multidimensional spectroscopy or coherent control, to higher photon energies, for probing and controlling core electronic transitions. However, this requires the ability to make single-shot, and complete, electric field measurements of potentially complex FEL pulses, in order to develop, and verify, pulse shaping strategies. Here, we present direct, single-shot measurements of XUV pulses generated under special operating configurations for producing specific pulse shapes from a laser-seeded XUV FEL. To do this, we built upon our past work using transient grating (TG) cross correlation frequency-resolved optical gating (FROG), where an optical reference pulse is diffracted from an XUV TG produced by a pair of interfering FEL pulses. The resulting nonlinear signal versus frequency and delay, i.e., the FROG trace, contains the electric field of the FEL pulse. The FEL pulse electric field is reconstructed from the FROG trace using a phase retrieval algorithm. Here we confirmed three different pulse shaping strategies for generating chirped, double and multiple FEL pulses by tuning the seed laser and FEL parameters and measuring the resulting shaped FEL pulses with TG XFROG. This work paves the way for generating on-demand pulse shapes with a seeded FEL by improving the characterization of FEL pulses, for transform-limited to more complex shapes.

Abstract This article presents a detailed simulation study of a multi-pass amplifier in the chirped pulse amplification (CPA) system using Lab2 femtosecond virtual tools in LabVIEW to model and optimize the pulse amplification process. The virtual experimental setup includes Gaussian pulse generation, pulse stretching using an Offner-triplet stretcher, multi-pass amplification, and final pulse compression using a double-grating compressor. Various multi-pass amplifier configurations (4-pass, 8-pass, and 12-pass) are simulated, and their effects on pulse amplification, phase stability, and energy gain are analyzed. The study demonstrates that the 8-pass configuration achieves the most optimized performance in terms of energy gain, spectral and temporal stability, and pulse duration, providing a more balanced pulse compression. Diagnostics via frequency-resolved optical gating confirm these results, showing that the 8-pass Amplifier delivers shorter pulse durations (∼36 fs) and better phase control compared to the 4-pass and 12-pass amplifiers, which experience higher nonlinearity and less stability. The article underscores the critical role of compressor grating separation in minimizing pulse duration and optimizing laser performance. Using the Lab2 virtual tools in LabVIEW, this simulation provides valuable insights into the dynamic optimization of multi-pass amplifiers in CPA systems.

Devices that measure the presence of instability in the pulse shapes in trains of ultrashort laser pulses do not exist, so this task necessarily falls to pulse-measurement devices, like Frequency-Resolved Optical Gating (FROG) and its variations, which have proven to be a highly reliable class of techniques for measuring stable trains of ultrashort laser pulses. Fortunately, multi-shot versions of FROG have also been shown to sensitively distinguish trains of stable from those of unstable pulse shapes by displaying readily visible systematic discrepancies between the measured and retrieved traces in the presence of unstable pulse trains. However, the effects of pulse-shape instability and algorithm stagnation can be indistinguishable, so a never-stagnating algorithm—even when instability is present—is required and is generally important. In previous work, we demonstrated that our recently introduced Retrieved-Amplitude N-grid Algorithmic (RANA) approach produces highly reliable (100%) pulse-retrieval in the second-harmonic-generation (SHG) version of FROG for thousands of sample trains of pulses with stable pulse shapes. Further, it does so even for trains of unstable pulse shapes and thus both reliably distinguishes between the two cases and provides a rough measure of the degree of instability as well as a reasonable estimate of most typical pulse parameters. Here, we perform the analogous study for the polarization-gating (PG) and transient-grating (TG) versions of FROG, which are often used for higher-energy pulse trains. We conclude that PG and TG FROG, coupled with the RANA approach, also provide reliable indicators of pulse-shape instability. In addition, for PG and TG FROG, the RANA approach provides an even better estimate of a typical pulse in an unstable pulse train than SHG FROG does, even in cases of significant pulse-shape instability.

The ptychographic reconstruction algorithm is a commonly utilized method for pulse phase retrieval due to its super-resolution and robustness, allowing it to retrieve pulses from incomplete traces. However, the algorithm's performance can be hindered by occasional convergence stagnation caused by local minima in the gradient descent strategy. To address this issue, we propose a pulse reconstruction approach for frequency-resolved optical gating (FROG), which employs a multi-grid flexible sampling and parallel extended ptychographic iterative engine (ePIE), ultimately converging to the global FROG trace. The approach can effectively escape from the local minima and demonstrates extremely stable convergence without any prior information. We demonstrate, numerically and experimentally, that this approach can converge well to the correct pulse, especially for complex pulse reconstruction, even in cases of high noise and highly incomplete traces.

Ultrafast laser-based micromanufacturing is essential for fabricating high-precision quantum devices such as quantum gates, photonic circuits, and waveguides. Besides the area of micromachining, recent applications of laser beams also include quantum imaging and sensing. The laser beams have been characterized for their wavelength and pulse durations. The techniques include homemade setups of intensity autocorrelation, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric-field reconstruction (SPIDER) besides the commercial spectrographs. Beams obtained from non-collinear optical parametric amplifier (NOPA) capable of delivering tunable ultrafast pulses are also described. The dark core beams through Hermite Gaussian (HG) modes and thermal effects are also generated. All these beams are capable of generating polarization-entangled photons by the nonlinear-optical phenomenon of spontaneous parametric down-conversion (SPDC). After describing the construction of the entangled photon source (EPS), an interferometric setup for imaging based on extracting the phase and intensity of an object is discussed. The application areas of the EPS for quantum imaging, multi-photon applications and adaptive imaging are also highlighted. We propose that quantum imaging can overcome the existing challenges of real-time feedback during micromanufacturing and help prevent the wastage of resources.

This paper presents RecNet, an innovative convolutional neural network designed for reconstructing cecond harmonic generation frequency-resolved optical gating (SHG-FROG) traces. Unlike conventional approaches, RecNet incorporates noiseless sample constraints through a domain knowledge embedded loss function, enhancing the network's robustness to noise and interpretability. The encoder-decoder architecture is intentionally selected to match the dimensions of the trace diagram with intermediate representations, facilitating the effective application of these constraints. Comparative studies show that RecNet surpasses classical algorithms like PCGPA and network models that do not incorporate domain knowledge constraints in reconstruction accuracy and convergence ratio. Experimental results further confirm the superiority of RecNet.

Abstract Time-domain characterization of ultrashort pulses is essential for studying interactions between light and matter. Here, we propose and demonstrate an all-optical pulse sampling technique based on reflected four-wave mixing with perturbation on a solid surface. In this method, a weak perturbation pulse perturbs the four-wave mixing signal generated by a strong fundamental pulse. The modulation signal of the four-wave mixing, which is detected in the reflection geometry to ensure a perfect phase-matching condition, directly reflects the temporal profile of the perturbation pulse. We successfully characterized multi-cycle and few-cycle pulses using this method. The reliability of our approach was verified by comparing it to the widely employed frequency-resolved optical gating method. This technique provides a simple and robust method for characterizing ultrashort laser pulses.

Intense twin beams generated by high-gain parametric downconversion exhibit simultaneous correlations in time and frequency. To characterize the simultaneous correlations, it is important to determine the temporal correlation profile and the corresponding phase. Although sum frequency generation (SFG) is often used to characterize the temporal correlation of the twin beams, the SFG spectra reconstruct the pump intensity spectrum shape used to generate the twin beams, so useful information for phase reconstruction cannot be obtained from the SFG spectra. We propose and demonstrate frequency-resolved optical gating measurement for ultranarrow temporal correlation of twin beams (referred to as quantum FROG measurement). Quantum FROG measurement for twin beam correlations is realized using a cross-correlator based on four-wave mixing (FWM). We show theoretically that although the quantum FROG trace exhibits a different functional form obtained by the conventional FROG trace for ultrashort pulses, it is sensitive to phase dispersion. In experiments, we observed a shift of the FWM frequency, i.e., frequency chirping, within a correlation time of hundreds of femtoseconds, and the sign of the chirp was also identified.

Abstract Ultrafast pulse characterisation is crucial for studying processes that occur at femtosecond timescales and below. Because of this, various methods have been developed to recover a pulse’s electric field profile at these durations, with the frequency-resolved optical gating (FROG) technique being the most common. However, this approach is computationally expensive and suffers from limitations in terms of robustness and reliability. In this regard, recent publications have demonstrated that applying machine learning towards ultrafast pulse recovery can alleviate these issues, providing more accurate retrievals. Inspired by these works, we propose an encoder–decoder scheme for a FROG system which exploits dual harmonic generation in low-index thin films. Specifically, we demonstrate enhanced reliability and accuracy of ultrafast pulse recovery when compared to machine learning approaches using second or third harmonic signals independently. As the amount of information used to train each neural network is kept constant, this study demonstrates and benchmarks the technological advantages of contextual information analysis involving multiple nonlinear processes.