Characterization Of Ultrashort Laser Pulses Research Articles

Space–time characterization of ultrashort laser pulses: A perspective

The characterization of ultrashort laser pulses has significantly advanced beyond the standard spatial and temporal diagnostics to now include sophisticated spatio-temporal measurement techniques. In this perspective, we provide an overview of the current state of space–time characterization, discussing the theoretical foundations of ultrashort laser pulses, the various measurement techniques and their design trade-offs, and the challenges and opportunities for future development. We explore the extension of these techniques to different wavelength regimes and delve into the unique challenges posed by the characterization of polarization-structured beams. The potential for data-driven analysis to enhance the information extracted from the measurements is highlighted, along with the need for direct measurement of previously inaccessible field components, such as the longitudinal electric field in tightly focused beams. As these diagnostic tools continue to evolve, we anticipate a future where the intricate space–time structure of light can be analyzed on a routine basis, opening up new frontiers in ultrafast science and technology.

Phase-matching-free ultrashort laser pulse characterization from a transient plasma lens.

A phase-matching-free ultrashort pulse retrieval based on the defocusing of a laser-induced plasma is presented. In this technique, a pump pulse ionizes a rare gas providing a plasma lens whose creation time is ultrafast. A probe pulse propagating through this gas lens experiences a switch of its divergence. The spectrum of the diverging part, isolated by a coronograph, is measured as a function of the pump-probe delay, providing a spectrogram that allows for a comprehensive characterization of the temporal properties of the probe pulse. The method, called PI-FROSt for "plasma-induced frequency-resolved optical switching," is simple, is free of phase-matching constraints, and can operate in both self- and cross-referenced configurations at ultrahigh repetition rate in the whole transparency range of the gas. The assessment of the method demonstrates laser pulse reconstructions of high reliability in both near-infrared (NIR) and ultraviolet (UV) spectral ranges.

Unraveling photodissociation dynamics by sub-femtosecond ultraviolet pulses: insights into fragmental kinetics and carrier-envelope phase characterization

The duration of laser pulses and the carrier-envelope phase (CEP) play a crucial role in shaping kinetic energy release (KER) spectra. In this study, we performed theoretical calculations on pulse duration-dependent KER spectra, ranging from hundreds to sub-femtoseconds, focusing on the MgH+ scenario. Our findings reveal a distinct shift in KER peaks from sub-cycle pulses, deviating from the resonance energy. Utilizing two-level perturbation theory, we identify that this shift is attributable to the energy-dependent transition matrix elements. Moreover, our investigation uncovers a notable CEP effect in KER from sub-cycle pulses, arising from interference between counter-rotating and rotating terms within a single ultraviolet photon transition. To leverage this insight, we propose a novel pump-probe methodology for precise CEP characterization of ultra-short laser pulses. We hope this method would promise advancements in understanding and manipulating ultrafast processes.

Generalizing amplitude swing modulation for versatile ultrashort pulse measurement.

In this work we broaden the amplitude modulation concept applied to the temporal characterization of ultrashort laser pulses with the amplitude swing technique. We theoretically study the effect of diverse types of relative amplitude and phase modulations. This variation of the replicas can be implemented by means of rotating zero-order waveplates to manipulate the delayed pulse replicas produced in a following multi-order waveplate, which can be more practical under certain conditions. We numerically simulate and study different scenarios under different modulations and for different noise levels and pulses. The proposed schemes are validated and compared through the experimental application to compressed and chirped pulses, confirming the applicability of the work. The simplicity, robustness and versatility of this ultrashort pulse measurement benefits the applications of ultrafast optics.

Laser-pulse characterization using strong-field autocorrelation patterns and random-forest-based machine learning

Building on the strategy presented in Opt. Lett. 47, 3992 (2022), we demonstrate an efficient alternative approach for the in situ characterization of ultrashort low-frequency laser pulses. In this context, we employ first-principles quantum-mechanical calculations to model the strong-field ionization of rare-gas atoms and produce autocorrelation patterns for a set of few-femtosecond near-infrared laser pulses. We explore the nonperturbative and nonlinear dependence of the autocorrelation patterns on the pulse characteristics and postulate an analytical function describing these patterns. For every laser pulse considered, we employ the parameters appearing in this analytical function, together with the underlying pulse parameters for supervised machine learning. Specifically, we use the random-forest technique for retrieving key laser pulse parameters from autocorrelation patterns produced via strong-field ionization. The current approach offers advantages for application to experimental data.

Research progress of ultra-short laser pulse characterization

Research progress of ultra-short laser pulse characterization

Artificial intelligence for online characterization of ultrashort X-ray free-electron laser pulses

X-ray free-electron lasers (XFELs) as the world’s brightest light sources provide ultrashort X-ray pulses with a duration typically in the order of femtoseconds. Recently, they have approached and entered the attosecond regime, which holds new promises for single-molecule imaging and studying nonlinear and ultrafast phenomena such as localized electron dynamics. The technological evolution of XFELs toward well-controllable light sources for precise metrology of ultrafast processes has been, however, hampered by the diagnostic capabilities for characterizing X-ray pulses at the attosecond frontier. In this regard, the spectroscopic technique of photoelectron angular streaking has successfully proven how to non-destructively retrieve the exact time–energy structure of XFEL pulses on a single-shot basis. By using artificial intelligence techniques, in particular convolutional neural networks, we here show how this technique can be leveraged from its proof-of-principle stage toward routine diagnostics even at high-repetition-rate XFELs, thus enhancing and refining their scientific accessibility in all related disciplines.

Study of the impact of wavefront aberrations on the characterization of ultrashort laser pulses with GRENOUILLE.

In recent years, GRENOUILLE has emerged as a relatively simple technique to fully characterize the electric field of an ultrashort laser pulse in a single shot. It does so by spatially mapping the delays on the transverse spatial coordinate and by mapping the frequencies on the angular coordinate of the orthogonal direction. Because of this spatial mapping, an aberrated wavefront could distort and affect the measurement of the pulse. It is shown here experimentally how these aberrations can affect the measurement using a deformable mirror to induce various aberrations in the wavefront. This can result in distortions of the spectral or temporal profile of the retrieved pulse, and a decrease of the intensity of the second-harmonic signal generated by the nonlinear crystal. Additionally, the signatures of some of the distortions of the trace resemble those previously identified as being caused by pulse-front tilt or spatial chirp and could be interpreted as such while being in fact caused by aberrations. This can complicate the identification of the real source of the distortions, since a purely spatial effect can cause distortions similar to those created by dispersion-based phenomena or other types of spatiotemporal couplings.

Temporal characterization of broadband femtosecond laser pulses by surface third-harmonic dispersion scan with ptychographic retrieval.

We present a new, to the best of our knowledge, variant of dispersion scan (d-scan) based on surface third-harmonic generation (STHG) and a ptychographic algorithm tailored for full retrieval (amplitude and phase) of broadband laser pulses. We demonstrate the technique by temporally measuring and compressing few-cycle pulses with 7 fs and 2.5 nJ from a Ti:sapphire oscillator, using a sapphire window as the nonlinear medium. The results are in very good agreement with standard second-harmonic d-scan measurements based on a nonlinear crystal. The intrinsically broadband and phase-matching-independent nature of STHG make this technique very suitable for the characterization of ultrashort laser pulses over a broad wavelength range extending into the mid-infrared.

TIGER: TIme‐Gated Electric field Reconstruction

Herein, a novel self-referenced method for the complete temporal characterization (phase and amplitude) of ultrashort optical laser pulses is presented. The technique, called TIme-Gated Electric field Reconstruction (TIGER), measures a second-order nonlinear signal (namely, second-harmonic generation or two-photon absorption) produced by four time-delayed replicas of the input pulse. The delays are spatially encoded in the beam profile using a four-faced pyramid-like optical element. The presented technique enables single shot measurement and does not require any spectral measurements, in contrast with well-known self-referenced characterization methods. Depending on the chosen geometry, the recorded TIGER signal can be either interferometric (i.e., carrier frequency resolved) or intensimetric. This article describes the principle operation of the device together with a detailed theoretical analysis. TIGER measurements of various laser pulse shapes with their reconstructions are then presented, demonstrating the relevance of this original approach.

Simultaneous Characterization of Two Ultrashort Optical Pulses at Different Frequencies Using a WS2 Monolayer.

The precise characterization of ultrashort laser pulses has been of interest to the scientific community for many years. Frequency-resolved optical gating (FROG) has been extensively used to retrieve the temporal and spectral field distributions of ultrashort laser pulses. In this work, we exploit the high, broad-band nonlinear optical response of a WS2 monolayer to simultaneously characterize two ultrashort laser pulses with different frequencies. The relaxed phase-matching conditions in a WS2 monolayer enable the simultaneous acquisition of the spectra resulting from both four-wave mixing (FWM) and sum-frequency generation (SFG) nonlinear processes while varying the time delay between the two ultrashort pulses. Next, we introduce an adjusted double-blind FROG algorithm, based on iterative fast Fourier transforms between two FROG traces, to extract the intensity distribution and phase of two ultrashort pulses from the combination of their FWM and SFG FROG traces. Using this algorithm, we find an agreement between the computed and observed FROG traces for both the FWM and SFG processes. Exploiting the broad-band nonlinear response of a WS2 monolayer, we additionally characterize one of the pulses using a second-harmonic generation (SHG) FROG trace to validate the pulse shapes extracted from the combination of the FWM and SFG FROG traces. The retrieved pulse shape from the SHG FROG agrees well with the pulse shape retrieved from our nondegenerate cross-correlation FROG measurement. In addition to the nonlinear parametric processes, we also observe a nonlinearly generated photoluminescence (PL) signal emitted from the WS2 monolayer. Because of its nonlinear origin, the PL signal can also be used to obtain complementary autocorrelation and cross-correlation traces.

Spectral phase sensitivity of frequency resolved optical switching for broadband IR pulse characterization.

In this work, we demonstrate the sensitivity of the frequency-resolved optical switching (FROSt) technique to detect a small amount of spectral phase shift for the precise characterization of ultrashort laser pulses. We characterized fs pulses centered at 1.75 µm that are spectrally broadened up to 700 nm of bandwidth in a hollow-core fiber and subsequently compressed down to 2.3 optical cycle duration by propagation in the air at atmospheric pressure. By inserting thin fused silica windows of different thicknesses in the beam path, we accurately retrieve group delay dispersion (GDD) variations as small as 10 fs2. Such GDD variations correspond to a change of the pulse duration of only 0.2 fs for a Fourier transform limited 2-cycle pulse at 1.75 µm (i.e., 11.8 fs). The capability to measure such tiny temporal variations thus demonstrates that the FROSt technique has sufficient sensitivity to precisely characterize single-cycle pulses.

Characterization of two-color ultrashort laser pulses using polarization-gating and transient-grating frequency-resolved optical gating

Two-color ultrashort laser pulses have emerging applications in numerous areas of science and technology. In many cases, the slightest change in the combined electric field of a two-color pulse greatly affects its interaction mechanism with the system. Therefore, a precise characterization of the temporal/spectral profile of the combined electric field is of great importance. In this work, we demonstrate that a full characterization is possible using the well-known transient-grating (TG) or polarization-gating (PG) frequency-resolved optical gating (FROG) techniques, and by employing the recently developed Retrieved-Amplitude N-grid Algorithmic (RANA) approach for the retrieval process. We demonstrate the validity of using these techniques and this approach for multi-cycle and few-cycle pulses in the absence and presence of noise.

Broadband mid-infrared phase retrieval for nonlinear microscopy.

Spectral phase characterization of ultrashort laser pulses is essential in nonlinear micro-spectroscopy. Whereas in many applications phases are determined for near-infrared (NIR) pulses, successful mid-infrared (MIR) phase retrieval is rare. The spectral phase of ultra-broadband MIR pulses is determined over more than 1000cm-1 in the presented work. This is accomplished by exploiting the d-scan method in two variants. Both allow for detecting high signals by using the interaction of the MIR and NIR pulses. The two variants differ in imprinting the dispersion. While the dual d-scan imprints phases on both pulses, the Xd-scan method disperses the NIR pulses solely.

Generation and compression of 10-fs deep ultraviolet pulses at high repetition rate using standard optics.

The generation and characterization of ultrashort laser pulses in the deep ultraviolet spectral region is challenging, especially at high pulse repetition rates and low pulse energies. Here, we combine achromatic second harmonic generation and adaptive pulse compression for the efficient generation of sub-10 fs deep ultraviolet laser pulses at a laser repetition rate of 200 kHz. Furthermore, we simplify the pulse compression scheme and reach pulse durations of ≈10 fs without the use of adaptive optics. We demonstrate straight-forward tuning from 250 to 320 nm, broad pulse spectra of up to 63 nm width, excellent stability and a high robustness against misalignment. These features make the approach appealing for numerous spectroscopy and imaging applications.

Spatio-spectral characterization of ultrashort laser pulses with a birefringent delay line

We utilize a birefringent delay line to perform spatially-resolved Fourier transform spectroscopy at multiple planes around the focus of an ultrashort laser pulse. Combining these measurements at multiple planes with phase-retrieval, i.e. the INSIGHT technique, allows for the determination of the three-dimensional laser field, in both amplitude and phase, lacking only the knowledge of a spatially-uniform spectral phase. We use the device to simultaneously measure two low-order spatio-temporal couplings resulting from a misaligned chromatic lens doublet. Besides being a new implementation of spatio-spectral characterization of ultrashort pulses, this technique allows for more accuracy and stability in spatio-spectral characterization and a push to shorter wavelengths, and may ultimately enable sparse measurements.

Few-cycle optical pulse characterization under phase-mismatching.

The phase-matching bandwidth of nonlinear crystal is of great significance in ultrashort laser pulse characterization. In order to satisfy the phase-matching bandwidth, ultra-thin nonlinear crystals are generally required. However, the significantly reduced conversion efficiency, as well as the machining difficulties, limits its applications. Here, we show that sufficient spectrum bandwidth response can be achieved for a thick crystal when the phase-matching wavelength is tuned outside of the spectral window of the measured pulse. By applying this phenomenon to a single-shot second-harmonic generation frequency resolved optical gating (SHG-FROG) device, we successfully characterized a few-cycle pulse using a 150µmβ-barium borate (BBO) crystal. The accuracy of the method was verified by comparing the conventional pulse retrieving approach with a 5µm BBO crystal, which has a sufficient phase-matching bandwidth.

Robustness and capabilities of ultrashort laser pulses characterization with amplitude swing

In this work we firstly study the influence of different parameters in the temporal characterization of ultrashort laser pulses with the recently developed amplitude swing technique. In this technique, the relative amplitude of two delayed replicas is varied while measuring their second-harmonic spectra. Here we study the retrieval of noisy traces and the implications of having different delays or phase retardations (relative phases) between the two replicas. Then, we study the capability of the technique to characterize the pulses when the second-harmonic signal is spectrally uncalibrated or incomplete, presenting the analytical calculation of the marginal, which is used to calibrate the traces and to perform the pulse retrievals. We experimentally show the retrieval of different pulses using diverse delays and phase retardations to perform the amplitude swing trace and demonstrate that, from an uncalibrated trace, both the pulse information and the response of the nonlinear process can be simultaneously retrieved. In sum, the amplitude swing technique is shown to be very robust against experimental constraints and limitations, showing a high degree of soundness.

In situ temporal measurement of ultrashort laser pulses at full power during high-intensity laser–matter interactions

In laser–matter interaction experiments, it is of paramount importance to characterize the laser pulse on target (in situ) and at full power. This allows pulse optimization and meaningful comparison with theory, and it can shed fundamental new light on pulse distortions occurring in or on the target. Here we introduce and demonstrate a new technique based on dispersion-scan using the concurrent third harmonic emission from the target that permits the full (amplitude and phase), in situ, in-parallel characterization of ultrashort laser pulses in a gas or solid target over a very wide intensity range encompassing the 10 13 – 10 15 W c m − 2 regime of high harmonic generation and other important strong-field phenomena, with possible extension to relativistic intensities presently inaccessible to other diagnostics.

Experimental focusing in the characterization of ultrashort chirped pulses in a simplified device

Grating eliminated no-nonsense observation of ultrafast incident laser light e-fields is an experimental technique for the full characterization of ultrashort laser pulses. It requires the nonlinear conversion of broadband ultrashort optical pulses into their second harmonic generation (SHG) that is relatively tightly focused on a thick SHG crystal. In this paper, the laser focusing problem is experimentally presented and demonstrated in the measurements of 780 nm chirped pulses emitted from a Ti:sapphire oscillator. In this experimental study, it has been shown how the focusing effect can cause distortions in the trace of the chirped pulses. It is demonstrated that the measurement of broadband pulses requires that the beam have a wide angular divergence. Therefore, a tight focusing is essential. A long focal length leads to a smaller range of angles incident on the crystal causing an insufficient divergence angle. Finally, the ability of this device in the characterization of double and complicated chirped pulses is described. The broadening of complex chirped pulses after passing through a thick BK7 glass is measured and the group velocity dispersion of the BK7 glass is calculated as about 47.3 fs2 mm−1. Also the train of double chirped pulses created using a Michelson interferometer is measured and the spectral fringes marked by phase jumps are observed. The experimental results of the retrieved phases and intensities are in good agreement with independently measured data.