Phase-matching-free pulse retrieval based on transient absorption in solids

Single-shot complete characterization of synthesized laser pulses and the nonlinear frequency-conversion process

High-energy-synthesized laser pulses through a nonlinear frequency-conversion process with different characteristics, such as polarization, central wavelength, and pulse duration, play important roles in materials science, high-energy physics, and ultrafast optics. In this study, we present an improved transient-grating frequency-resolved optical gating based on a self-referenced and reflective structure, which enables the single-shot complete measurement of complex high-power synthesized laser pulses in the broadband range and analysis of the nonlinear frequency-conversion process of ultrashort pulses. The waveform/spectrum evolution of both the fundamental and second harmonic pulses in a nonlinear frequency-conversion process with different injected energies was studied for the first time using this method. Moreover, the method was numerically and experimentally verified to be able to completely characterize double pulses with spectral and temporal separation, including the relative phase between the two components. This method has considerable potential for studying the complex physical processes of high-power synthesized laser fields.

Single-shot measurement of a laser waveform using plasma fluorescence in ambient air

The temporal characterization of a laser pulse is an important task in studying ultrafast laser-matter interactions. It is ideal to measure the temporal profile of the laser pulse with a single laser shot when the repetition rate is low or its interaction with matter is unstable. Here we report a new approach for the single-shot temporal characterization of a laser pulse, based on the TIPTOE (tunneling ionization with a perturbation of the time-domain observation of an electric field) method. The waveform of the laser pulse is reconstructed from the intensity modulation of plasma fluorescence emission. The result of the single-shot measurement is compared with the time-delay scanning TIPTOE measurements, supporting the validity of the single-shot measurement.

Characterization of ultrashort vector pulses from a single amplitude swing measurement

Ultrashort vector pulses exhibit time- and frequency-dependent polarization, sparking significant interest across various fields. Simple, robust, and versatile characterization techniques are crucial to meet this rising demand. Our study showcases how complete polarization dynamics are encoded within a single amplitude swing trace, demonstrated both theoretically and experimentally. We have developed a reconstruction strategy to effectively extract all this information. The amplitude swing technique's sensitivity to vector pulses offers a robust, compact in-line setup adaptable across diverse pulse bandwidths, durations, and spectral ranges. This self-referenced method offers effective measurement of ultrashort vector pulses, addressing the growing interest in these complex pulses.

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.

Swept coded aperture real-time femtophotography

Single-shot real-time femtophotography is indispensable for imaging ultrafast dynamics during their times of occurrence. Despite their advantages over conventional multi-shot approaches, existing techniques confront restricted imaging speed or degraded data quality by the deployed optoelectronic devices and face challenges in the application scope and acquisition accuracy. They are also hindered by the limitations in the acquirable information imposed by the sensing models. Here, we overcome these challenges by developing swept coded aperture real-time femtophotography (SCARF). This computational imaging modality enables all-optical ultrafast sweeping of a static coded aperture during the recording of an ultrafast event, bringing full-sequence encoding of up to 156.3 THz to every pixel on a CCD camera. We demonstrate SCARF's single-shot ultrafast imaging ability at tunable frame rates and spatial scales in both reflection and transmission modes. Using SCARF, we image ultrafast absorption in a semiconductor and ultrafast demagnetization of a metal alloy.

Determination and correction of spectral phase from principal component analysis of coherent phonons

Measuring the spectral phase of a pulse is key for performing wavelength resolved ultrafast measurements in the few femtosecond regime. However, accurate measurements in real experimental conditions can be challenging. We show that the reflectivity change induced by coherent phonons in a quantum material can be used to infer the spectral phase of an optical probe pulse with few-femtosecond accuracy.

Complete characterization of a Yb-based OPA at a high repetition rate using frequency resolved optical switching

We demonstrate experimentally that frequency resolved optical switching (FROSt) can be used to characterize ultra-broadband pulses at high repetition rates up to 500 kHz. Specifically, we present the complete temporal characterization of an optical parametric amplifier (OPA), from the supercontinuum (SC) to the second stage of amplification. Simultaneous characterization of co-propagating signal and idler pulses enables retrieval of their group delay, as well as their temporal phase and intensity. Our study focuses on an extensive frequency range spanning the infrared region (1.2 to 2.4 µm) and confirms the strength and convenience of FROSt as a single tool for characterizing a wide range of pulses at high repetition rates.

Mid-infrared frequency domain optical parametric amplifier

We report on an optical architecture delivering sub-120 femtosecond laser pulses of 20 µJ tunable from 5.5 µm to 13 µm in the mid-infrared range (mid-IR). The system is based on a dual-band frequency domain optical parametric amplifier (FOPA) optically pumped by a Ti:Sapphire laser and amplifying 2 synchronized femtosecond pulses each with a widely tunable wavelength around 1.6 and 1.9 µm respectively. These amplified pulses are then combined in a GaSe crystal to produce the mid-IR few-cycle pulses by means of difference frequency generation (DFG). The architecture provides a passively stabilized carrier-envelope phase (CEP) whose fluctuations has been characterized to 370 mrad RMS.

Temporal characterization of a two-color laser field using tunneling ionization

The superposition of a fundamental laser pulse and its second harmonic can form an asymmetric laser field that is useful in many applications. The temporal characterization of the two-color laser field becomes necessary. However, the temporal characterization of the two-color laser pulse is a challenging task due to its broad bandwidth and a spectral gap between the two frequency components. Here we demonstrate the temporal characterization of the two-color laser field using multiple ionization yield measurements near the laser focus. This new approach enables the complete temporal characterization of the two-color laser field, including the relative phase between the two frequency components.

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.

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.

Temporal characterization of two-octave infrared pulses by frequency resolved optical switching

We present the temporal characterization of infrared pulses with spectra extending from 0.55 to 2.5 μm by using the frequency resolved optical switching (FROSt) technique. The pulses are obtained by broadening femtosecond pulses at 1.75 μm central wavelength in a two-stage hollow core fiber setup. This work demonstrates the capability of the FROSt technique to temporally characterize pulses with ultra-broadband spectra. Being free of phase-matching constraints, it enables the characterization of pulses with very low energy at the limit of the detection threshold and with arbitrary long pulse duration. This strength of the FROSt technique is illustrated by the characterization of supercontinua pulses whose spectra span over two octaves and with only 150 nJ energy that is spread temporally over almost 40 ps. The FROSt capabilities provide a versatile tool for the characterization of sub-cycle pulses and to study nonlinear processes such as supercontinuum generation.

Polarization-independent pulse retrieval based on frequency resolved optical switching

We demonstrate experimentally that the frequency resolved optical switching (FROSt) method is independent of the polarization direction of the pulse to be characterized. In this perspective, it is employed to characterize two or three co-propagating pulses linearly polarized in orthogonal directions, enabling to retrieve simultaneously their temporal intensity and phase profiles together with their group delay. This technique is also applied to track a simple nonlinear process involving different polarization states: type-I second harmonic generation (SHG). We are able to characterize the depleted fundamental pulse along with the generated second-harmonic pulse, thus demonstrating that the FROSt technique is a practical and powerful tool to observe nonlinear processes both in the temporal and spectral domains even if it involves different polarization states.

Measuring an ultrashort, ultraviolet pulse in a slowly responding, absorbing medium

Frequency-resolved optical gating (FROG) is a common technique for measuring ultrashort laser pulses using an instantaneous, nonlinear-optical interaction as a fast time-gate to measure the pulse intensity and phase. But at high frequencies, materials are often absorbing and it is not always possible to find a medium with a fast nonlinear-optical response. Here we show that an ultrashort, ultraviolet (UV) pulse can be measured in a strongly absorbing medium, using the absorption as the nonlinear-optical time-gate. To do this, we build on our recent implementation of FROG, known as induced-grating cross-correlation FROG (IG XFROG), where an unknown, higher-frequency pulse creates a transient grating that is probed with a lower-frequency, more easily detectable reference pulse. We demonstrate this with an 800 nm reference pulse to characterize 400 nm or 267 nm pulses using ZnS as the nonlinear-optical medium, which is absorptive at and below 400 nm. By scanning the delay between the two UV pulses which create the transient grating, we show that the phase-sensitive instantaneous four-wave-mixing contribution to the nonlinear signal field can be detected and separated from the slower, incoherent part of the response. Measuring a spectrally-resolved cross-correlation in this way and then applying a simple model for the response of the medium, we show that a modified generalized projections (GP) phase-retrieval algorithm can be used to extract the pulse amplitude and phase. We test this approach by measuring chirped UV pulses centered at 400 nm and 267 nm. Since interband absorption (or even photoionization) is not strongly wavelength-dependent, we expect IG XFROG to be applicable deeper into the UV.

Measuring the frequency chirp of extreme-ultraviolet free-electron laser pulses by transient absorption spectroscopy

High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of an ionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties.

Free-electron laser pulses generated from self-amplification of spontaneous emission scheme vary from one another in their characteristics. Here the authors demonstrate a transient absorption spectroscopy method to characterize the frequency chirp of the FEL pulses.

Single-shot phase-matching free ultrashort pulse characterization based on transient absorption in solids

The frequency-resolved optical switching (FROSt) method developed for ultrashort pulse characterization is implemented for single-shot measurements. In this basic demonstration, the delay axis of the spectrogram is spatially encoded by the pump beam having a small incident angle with the photoexcited material. We present the calibration procedure for spectrograms acquired in single-shot and the temporal characterization of 44 fs pulses with central wavelength at 800 nm both in scanning and single-shot FROSt configurations. The retrieved pulses are compared by means of the root-mean-square field error. Finally, the pulses are propagated through a known dispersive material to measure the added group-delay dispersion.

Discrete dispersion scan setup for measuring few-cycle laser pulses in the mid-infrared

In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

Characterizing the carrier-envelope phase stability of mid-infrared laser pulses by high harmonic generation in solids

We present a novel approach for measuring the carrier-envelope phase (CEP) stability of a laser source by employing the process of high harmonic generation (HHG) in solids. HHG in solids driven by few-cycle pulses is very sensitive to the waveform of the driving pulse, therefore enabling to track the shot-to-shot CEP fluctuations of a laser source. This strategy is particularly practical for pulses at long central wavelength up to the mid-infrared spectral range where usual techniques used in the visible or near-infrared regions are challenging to transpose. We experimentally demonstrate this novel tool by measuring the CEP fluctuations of a mid-infrared laser source centered at 9.5~μm.

High-field mid-infrared pulses derived from frequency domain optical parametric amplification

We present a novel, to the best of our knowledge, approach for scaling the peak power of mid-infrared laser pulses with few-cycle duration and carrier-to-envelope phase stabilization. Using frequency domain optical parametric amplification (FOPA), selective amplification is performed on two spectral slices of broadband pulses centered at 1.8 µm wavelength. In addition to amplification, the Fourier plane is used for specific pulse shaping to control both the relative polarization and the phase/delay between the two spectral slices of the input pulses. At the output of the FOPA, intrapulse difference frequency generation provides carrier-envelope phase stabilized two-cycle pulses centered at 9.5 µm wavelength with 25.5 µJ pulse energy. The control of the carrier-envelope phase is demonstrated through the dependence of high-harmonic generation in solids. This architecture is perfectly adapted to be scaled in the future to high average and high peak powers using picosecond ytterbium laser technologies.