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Roaming dynamics in real time Roaming is distinct from conventional reaction channels because of the unusual geometries that chemical systems use to bypass the minimum energy pathway. It is a relatively new phenomenon that is usually determined in experiments through spectroscopic characterization of the roaming products. Using a combination of time-resolved Coulomb explosion imaging and quasiclassical trajectory analysis, Endo et al. report real-time observation of individual fragments of the prototypical reaction of deuterated formaldehyde (D 2 CO) dissociation as they roam on ultrafast time scales. They show that roaming not only occurs several orders of magnitude earlier than previously expected but also that it can terminate in a radical (D + DCO) rather than the well-known molecular (D 2 + CO) product channel. Science , this issue p. 1072

We demonstrate Yb laser (170fs) propagation in large a core hollow-core fiber (HCF) with 97.4% transmission. It serves as a pre-compression stage to achieve 1.3 cycle pulses in a second step with 70% overall efficiency

We developed a high transmission Hollow-core fiber (HCF) with 1mm ID that supports 97% absolute transmission. We were able to achieve about 30 times spectral broadening of 11mJ, 1.5ps pulses from a 275W, 25 kHz InnoSlab multi-pass amplifier with 90% fiber transmission. The spectrum achieved with Kr at 1 bar supports a 50fs TL bandwidth.

A Frequency Resolved Optical Gating instrument accepting s or p polarized input pulses ranging from 205 nm to 2000 nm, durations from 2 cycles to 100 ps, and nano-Joule energies is presented.

To reach high average power operation combined with good conversion efficiency we utilize spectrally selective amplification with multiple nonlinear crystals to yield 12% conversion efficiency with 30W output at 2um wavelength.

To reach high average power operation combined with good conversion efficiency we utilize spectrally selective amplification with multiple nonlinear crystals to yield 12% conversion efficiency with 30W output at 2um wavelength.

Using time-resolved Coulomb explosion imaging (CEI) in an ultraviolet (UV) pump near infrared (NIR) probe experiment, we directly image the different dissociation dynamics in the formaldehyde molecule. Different pathways are distinguished from each other, despite their statistical nature. To extract such dynamics, hidden in a statistical background, calls for an elimination of noise. In our approach, we take advantage of CEI being a quasi-background-free technique., International Conference of Computational Methods in Sciences and Engineering ICCMSE 2020, April 29 – May 3, 2020, Crete, Greece (Virtual), Series: AIP Conference Proceedings

A Frequency Resolved Optical Gating instrument accepting s or p polarized input pulses ranging from 205 nm to 2000 nm, durations from 2 cycles to 100 ps, and nano-Joule energies is presented.

By exploiting stimulated Raman scattering in long nitrogen-filled capillary fibers, we demonstrate continuous tunability of Yb laser systems over the 1.0-1.7 μm range, with conversion efficiency up to 82%, and an up to 10-fold pulse compression.

The results are based on a commercial Yb:KGW regenerative amplifier (Pharos, Light Conversion) delivering 1 mJ, 170 fs pulses at 1 to 6 kHz. This beam is precisely matched to the fundamental mode of a large core HCF ( few-cycle Inc.) of 1 mm inner diameter, by realizing a focal spot diameter of 660 μm at 1/e 2 of intensity (corresponding approximately to a Rayleigh range of 0.3 m).

Nonlinear parametric processes involving ultrashort pulses are typically carried out in time domain, which mathematically corresponds to a convolution of their frequency spectra. In contrast, this spectral convolution changes into a multiplication operation when performing the nonlinear interaction in frequency domain. Here, we extend the scope of frequency-domain nonlinear optics by demonstrating its ability to perform a temporal convolution. Through this approach, nonlinear optical operations that are inaccessible in time domain can be realised: specific optical information can be coherently advanced by picoseconds within a pulse sequence—a newly generated second harmonic pulse carries the amplitude and phase information of two input pulses. This central pulse is isolated when using an input field consisting of two cross-polarized input pulses in combination with type-II second harmonic generation. The effects of nonlinear temporal convolution can be viewed from the aspect of signal processing and pulse shaping, where the nonlinear interaction in the parametric crystal plays the role of a dynamic linear optical filter—in contrast to conventional static filters—with a shaping mask instantaneously adapting to the laser field.

Ultrashort high-energy pulses at wavelengths longer than 1 µm are now desirable for a vast variety of applications in ultrafast and strong-field physics. To date, the main answer to the wavelength tunability for energetic, broadband pulses still relies on optical parametric amplification (OPA), which often requires multiple and complex stages, may feature imperfect beam quality, and has limited conversion efficiency into one of the amplified waves. In this work, we present a completely different strategy to realize an energy-efficient and scalable laser frequency shifter. This relies on the continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow-core fibers (HCF). We show a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with a conversion efficiency higher than 70% and high beam quality. The highly asymmetric spectral broadening, arising from the spatiotemporal nonlinear interplay between higher-order modes of the HCF, can be readily employed to generate pulses ( ∼ 20 f s ) significantly shorter than the pump ones ( ∼ 200 f s ) with high beam quality, and the pulse energy can further be scaled up to tens of millijoules. We envision that this technique, coupled with the emerging high-power Yb laser technology, has the potential to answer the increasing demand for energetic multi-TW few-cycle sources tunable in the near-IR.

We demonstrate an efficient approach for enhancing the spectral broadening of long laser pulses and for efficient frequency redshifting by exploiting the intrinsic temporal properties of molecular alignment inside a gas-filled hollow-core fiber (HCF). We find that laser-induced alignment with durations comparable to the characteristic rotational time scale T R o t A l i g n enhances the efficiency of redshifted spectral broadening compared to noble gases. The applicability of this approach to Yb lasers with (few hundred femtoseconds) long pulse duration is illustrated, for which efficient broadening based on conventional Kerr nonlinearity is challenging to achieve. Furthermore, this approach proposes a practical solution for high energy broadband long-wavelength light sources, and it is attractive for many strong field applications.

A conventional hollow core fiber (HCF) scheme is implemented to investigate spectral broadening of Titanium:Sapphire (Ti-Sa) femtosecond laser pulses in saturated hydrocarbon molecules compared to unsaturated ones. While the saturated molecules exhibit a spectral broadening similar to noble gases, for the unsaturated ones with π bonds, broadening towards blue is restrained. Numerical simulations underpin that it is a combination of group velocity dispersion (GVD) and Raman scattering which limits the spectral broadening for the unsaturated molecules. Compression of low energy ∼40fs pulses to ∼8fs using saturated hydrocarbons is demonstrated, suggesting the feasibility of this media for high repetition rate laser pulse compression.

“Roamers” are directly observed in the prototypical roaming reaction in the formaldehyde molecule. Despite their statistical nature, roaming is well discriminated from the radical- and molecular dissociation channels, using Coulomb explosion imaging and theoretical modeling., International Conference on Ultrafast Phenomena 2020, Nov. 16-19, 2020, Washington, D.C.

We present an energy-efficient and scalable technique to realize energetic sub-100-fs pulses, continuously tunable in the 1030÷1730 nm range, by exploiting stimulated Raman scattering in long N2-filled hollow-core fibers.

A conventional hollow-core fiber (HCF) scheme is implemented to investigate the effect of group velocity dispersion (GVD) and Raman scattering in spectral broadening in molecular gases for low energy pulse compression application.

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.

To generalize the applicability of the temporal characterization technique called “tunneling ionization with a perturbation for the time-domain observation of an electric field” (TIPTOE), the technique is examined in the multicycle regime over a broad wavelength range, from the UV to the IR range. The technique is rigorously analyzed first by solving the time-dependent Schrödinger equation. Then, experimental verification is demonstrated over an almost 5-octave wavelength range at 266, 1800, 4000 and 8000 nm by utilizing the same nonlinear medium – air. The experimentally obtained dispersion values of the materials used for the dispersion control show very good agreement with the ones calculated using the material dispersion data and the pulse duration results obtained for 1800 and 4000 nm agree well with the frequency-resolved optical gating measurements. The universality of TIPTOE arises from its phase-matching-free nature and its unprecedented broadband operation range.

Pulse compression based on non-linear propagation in a gas-filled hollow core fiber (HCF) is amongst the common techniques to generate few-cycle laser pulses [1]. Although very efficient, the major disadvantage of this method is the need to use expensive noble gases like krypton or xenon to compress low energy pulses [2–4]. Our latest results confirm that certain molecular gases, hydrofluorocarbons, represent an affordable and efficient alternative to the traditional atomic gases [5,6]. Such gases bear the potential to generalize HCF compression to high repetition rate, low intensity laser systems [7].

Using asymmetric two-color laser fields composed of 1800 and 900nm, we have simultaneously controlled four well identified fragmentation channels in dissociative ionization of the hydrogen molecule, resulting in enhanced electron-localization sensitivities of up to 65%.

In this Letter, we investigate the energy-scaling rules of hollow-core fiber (HCF)-based nonlinear pulse propagation and compression merged with high-energy Yb-laser technology, in a regime where the effects such as plasma disturbance, optical damages, and setup size become important limiting parameters. As a demonstration, 70 mJ 230 fs pulses from a high-energy Yb laser amplifier were compressed down to 40 mJ 25 fs by using a 2.8-m-long stretched HCF with a core diameter of 1 mm, resulting in a record peak power of 1.3 TW. This work presents a critical advance of a high-energy pulse (hundreds of mJ level) nonlinear interactions platform based on high energy sub-ps Yb technology with considerable applications, including driving intense THz, X-ray pulses, Wakefield acceleration, parametric wave mixing and ultraviolet generation, and tunable long-wavelength generation via enhanced Raman scattering.

We introduce hydrofluorocarbon molecules as an alternative medium to noble gases with low ionization potential like krypton or xenon to compress ultrashort pulses of relatively low energy in a conventional hollow core fiber with subsequent dispersion compensation. Spectral broadening of pulses from two different laser systems exceeded those achieved with argon and krypton. Initially 40 fs, 800 nm, 120 μJ pulses were compressed to few optical cycles duration. With the same approach a compression factor of more than 10 was demonstrated for an ytterbium-based laser (1030 nm, 170 fs, 200 μJ) leading to 15.6 fs.

High harmonics from laser-ablated plumes are mostly generated from ionic species. We demonstrate that with ultrashort infrared (∼1.82 μm) driving lasers, high harmonics from laser-ablated manganese are predominantly generated from neutral atoms, a transition metal atom with an ionization potential of 7.4 eV. Our results open the possibility to advance laser-ablation technique to study the dynamics of neutral atoms of low ionization potential. Moreover, as manganese contains giant autoionizing resonance, intense and broadband high harmonics have been demonstrated from this resonance at energies from 49 to 53 eV. This opens the possibility to generate intense attosecond pulses directly from the giant resonances, as well as to study these resonances using high-harmonic spectroscopy.

We study high-order harmonic generation from laser ablated Indium plumes below the ionization potential at various driving laser wavelengths. Apart from intense harmonic from the autoionizing state, we also observe enhanced harmonic generation below the ionization potential.

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.

We introduce hydrofluorocarbon molecules as a valuable alternative to conventional noble gases to compress 100 pJ level ultrashort pulses. We demonstrate efficient compression down to 15.6 fs for 170 fs, 1030 nm, 200 gJ initial pulses.

Structural isomers, molecules having the same chemical formula but with atoms bonded in different order, are hard to identify using conventional spectroscopy and mass spectrometry. They exhibit virtually indistinguishable mass spectra when ionized by electrons. Laser mass spectrometry based on photoionization of the isomers has emerged as a promising alternative but requires shaped ultrafast laser pulses. Here we use transform limited femtosecond pulses to distinguish the isomers using two methods. First, we probe doubly charged parent ions with circularly polarized light. We show that the yield of doubly charged ortho-xylene decreases while para-xylene increases over a range of laser intensities when the laser polarization is changed from linear to circular. Second, we probe high harmonic generation from randomly oriented isomer molecules subjected to an intense laser field. We show that the yield of high-order harmonics varies with the positioning of the methyl group in xylene isomers (ortho-, para- A nd meta-) and is due to differences in the strength of tunnel ionization and the overlap between the angular peaks of ionization and photo-recombination.

Owing to the increasing significance of high harmonic generation (HHG) as a tabletop coherent x-ray source and the coming of age of intense infrared (IR) lasers, the development of high brightness soft x-ray beamlines is gaining a lot of attention. We discuss the self-guided propagation of high energy IR pulses around 1.8 mu m centre wavelength being loosely focused into a long, high-pressure gas cell. A bright x-ray beam with photon energies extending up to the oxygen K-edge at 543 eV is achieved with a flux of 2.9 x 10(3) photons/shot/1% bandwidth around the carbon K-edge (280 eV). We provide experimental and numerical evidence of an ionization steady state condition in the generation medium causing self-channelling and intensity clamping of the driving field. While the later limits the HHG cut-off energy for a given driving field wavelength, self-channelling increases the HHG flux through a longer, phase-matched, interaction length and provides a well-collimated HHG beam covering more than three octaves from

The powerful technique of optical parametric amplification (OPA) experienced a huge advance with the invention of optical parametric chirped pulse amplification (OPCPA) and later noncollinear OPA. In this paper, we describe a radically different approach of performing parametric interaction in the frequency domain instead of the time domain. The frequency domain is reached via optical Fourier transformation, which provides a separation ansatz. It allows breaking down a big task into smaller ones which ultimately enables simultaneous up-scaling of peak power and bandwidth. The first proof-of-concept experiment yielding 1.4 mJ, two cycle pulses at 1.8-μm wavelength is complemented by a high gain (>2000) setup at 800-nm wavelength with 2.5 kHz repetition rate. A very low amount of parasitic superfluorescence without degradation of the picosecond pulse contrast within the pump pulse duration has been observed upon high gain conditions.

Broadband optical parametric amplification in the IR region has reached a new milestone through the use of a non-collinear Frequency domain Optical Parametric Amplification system. We report a laser source delivering 11.6 fs pulses with 30 mJ of energy at a central wavelength of 1.8 μm at 10 Hz repetition rate corresponding to a peak power of 2.5 TW. The peak power scaling is accompanied by a pulse shortening of about 20% upon amplification due to the spectral reshaping with higher gain in the spectral wings. This source paves the way for high flux soft X-ray pulses and IR-driven laser wakefield acceleration.

We measure the ellipticity dependence of high-harmonic generation (HHG) from unaligned gas-phase ensembles of the five-membered ring molecules 2,3-dihydrofuran (${\mathrm{C}}_{4}{\mathrm{H}}_{6}\mathrm{O}$), furan (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O}$), and thiophene (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S}$). As is normally the case, the HHG emission is suppressed for increased ellipticity since the recollision wave packet leading to HHG is steered away from the parent ion for large ellipticity. However, through comparison with computations of the first step in HHG, namely, strong-field ionization (SFI), we show that the observed differences in the ellipticity dependence for the three molecular species reflect the lateral momentum distributions of the continuum electron responsible for HHG, which in turn provides information about the particular orientation between the molecular axis and the laser field that maximizes the SFI probability. Strikingly, and contrary to the conventional wisdom in the strong-field community, we find that for furan and thiophene the most probable orientation for SFI occurs when the electric field of the laser is aligned near a nodal plane of the corresponding ionizing orbital. 2,3-dihydrofuran, on the other hand, follows the standard expectation that the most probable orientation for SFI occurs when the electric field is aligned away from any nodal plane.

Using a non-collinear FOPA, a source delivering 1.8 μm, 30 mJ, 13 fs laser pulses is demonstrated. This is the first step towards 100 mJ for ∼10 TW. This laser opens the way for high brightness soft X-ray attosecond pulses.

In linear optics, light fields do not mutually interact in a medium. However, they do mix when their field strength becomes comparable to electron binding energies in the so-called nonlinear optical regime. Such high fields are typically achieved with ultra-short laser pulses containing very broad frequency spectra where their amplitudes and phases are mutually coupled in a convolution process. Here, we describe a regime of nonlinear interactions without mixing of different frequencies. We demonstrate both in theory and experiment how frequency domain nonlinear optics overcomes the shortcomings arising from the convolution in conventional time domain interactions. We generate light fields with previously inaccessible properties by avoiding these uncontrolled couplings. Consequently, arbitrary phase functions are transferred linearly to other frequencies while preserving the general shape of the input spectrum. As a powerful application, we introduce deep UV phase control at 207 nm by using a conventional NIR pulse shaper.

FNO enables deep UV pulse shaping through direct phase transfer of conventionally shaped NIR pulses at 830nm to their 4th harmonic. We demonstrate first time pulse characterization with a transient grating FROG at 207nm.

We demonstrate controlled amplitude and phase transfer of an ultrashort laser pulse through second-harmonic generation in the frequency domain. Sequential conversion allowed pulse shaping down to the deep UV at 207 nm.

Frequency-domain Nonlinear Optics (FNO) merges the simplicity of linear optics with the power of nonlinear optics. Employing FNO, we demonstrate pulse shaping in the deep UV and amplification of two-cycle infrared pulses to 2.5 TW., Nonlinear Optics, 17-21 July 2017, Waikoloa, Hawaii

A non-collinear Frequency domain Optical Parametric Amplifier is used to produce a 1.8 μm, 30 mJ, 13 fs laser source, leading to 2.5 TW peak power. This laser opens the way for high brightness soft X-ray attosecond pulses., CLEO: QELS_Fundamental Science, May 14-19, 2017, San Jose, California, USA

We present a system for extremely broadband terahertz (THz) generation based on an Ytterbium (Yb) amplified laser emitting 170-fs-long pulses centered at 1030 nm. The pulses are first spectrally broadened in an Ar-filled hollow-core capillary fiber (HCF) and then recompressed down to ∼18 fs with a chirped-mirror pair. Extreme broadband THz pulses of bandwidths up to 60 THz and peak electric field as high as 55 kV/cm are obtained via two-color plasma generation. The combination of high-power Yb laser systems with gas-filled HCF opens the path towards the realization of the next generation high-repetition-rate, extremely broadband, and intense-field THz time-domain spectroscopy systems.

Compression of low energy ultrashort laser pulses using a hollow core fiber (HCF) filled with 1-1 difluoroethane (C2H4F2), also known as R152a, is demonstrated. A 45 fs, 16 µJ Ti:Sapphire laser pulse is spectrally broadened in an HCF filled with R152a and subsequently compressed to ∼9 fs with ∼6 µJ pulse energy, which corresponds to a fivefold compression. This affordable and easily accessible gas is promising to compress new generation high average power lasers in conjunction with a flexible HCF setup. Additionally, the influence of the group velocity to the spectral broadening dispersion and its pressure dependence was numerically simulated. At pressures higher than 2.5 bar, the spectral broadening is affected by an interplay between two effects: while it is promoted by a denser gas medium leading to higher nonlinearity, it is hampered at the same time by the lower intensity resulting from the pulse stretching, which causes an overall limitation at higher pressures.

The time-dependent field of an electromagnetic pulse can be measured if there is a fast enough gate. For terahertz radiation, femtosecond photoinjection of free carriers into a semiconductor in the presence of the terahertz radiation can serve as the gate1. For visible or infrared radiation, attosecond photoionization of a gas target in the presence of the optical field is a direct analogue2,3,4,5,6,7,8. Here, we show that nonlinear optical mixing9,10,11,12,13 in a medium in which attosecond pulses are being generated can also be used to measure the time-dependent field of an optical pulse. The gate is the phase accumulated by the recollision electron during the subcycle time interval between ionization and recombination. We show that the instantaneous field of an unknown pulse is imprinted onto the deflection of the attosecond extreme ultraviolet pulse using an all-optical set-up with a bandwidth up to 1 PHz. A new laser-field measurement technique is demonstrated that exploits nonlinear optical mixing in a gas in which attosecond pulses are being generated. The instantaneous field of an unknown pulse is imprinted onto the deflection of an attosecond pulse using an all-optical set-up with a bandwidth of up to 1 PHz.

We report mid-infrared (MIR) nonlinear absorption in As

The influence of cyclic electron delocalization associated with aromaticity on the high-order-harmonic generation (HHG) process is investigated in organic molecules. We show that the aromatic molecules benzene $({\mathrm{C}}_{6}{\mathrm{H}}_{6})$ and furan $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O})$ produce high-order harmonics more efficiently than nonaromatic systems having the same ring structure. We also demonstrate that the relative strength of plateau harmonics is sensitive to the aromaticity in five-membered-ring molecules using furan, pyrrole $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{NH})$, and thiophene $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S})$. Numerical time-dependent Schr\"odinger equation simulations of total orientation-averaged strong-field ionization yields show that the HHG from aromatic molecules comes predominantly from the two highest $\ensuremath{\pi}$ molecular orbitals, which contribute to the aromatic character of the systems.

In high-order harmonic generation, resonant harmonics (RH) are sources of intense, coherent extreme-ultraviolet radiation. However, intensity enhancement of RH only occurs for a single harmonic order, making it challenging to generate short attosecond pulses. Moreover, the mechanism involved behind such RH was circumstantial, because of the lack of direct experimental proofs. Here, we demonstrate the exact quantum paths that electron follows for RH generation using tin, showing that it involves not only the autoionizing state, but also a harmonic generation from dressed-AIS that appears as two coherent satellite harmonics at frequencies ±2Ω from the RH (Ω represents laser frequency). Our observations of harmonic emission from dressed states open the possibilities of generating intense and broadband attosecond pulses, thus contributing to future applications in attosecond science, as well as the perspective of studying the femtosecond and attosecond dynamics of autoionizing states., High-energy photons in XUV range and attosecond pulses are generated from infrared laser pulses through high harmonic generation in gases and solids. Here, the authors demonstrate the microscopic origin of resonant harmonic generation involving the autoionizing states of Sn in plasma plumes.