Sub-250-mrad, passively carrier-envelope-phase-stable mid-infrared OPCPA source at high repetition rate

Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction

Ubiquitous to most molecular scattering methods is the challenge to retrieve bond distance and angle from the scattering signals since this requires convergence of pattern matching algorithms or fitting methods. This problem is typically exacerbated when imaging larger molecules or for dynamic systems with little a priori knowledge. Here, we employ laser-induced electron diffraction (LIED) which is a powerful means to determine the precise atomic configuration of an isolated gas-phase molecule with picometre spatial and attosecond temporal precision. We introduce a simple molecular retrieval method, which is based only on the identification of critical points in the oscillating molecular interference scattering signal that is extracted directly from the laboratory-frame photoelectron spectrum. The method is compared with a Fourier-based retrieval method, and we show that both methods correctly retrieve the asymmetrically stretched and bent field-dressed configuration of the asymmetric top molecule carbonyl sulfide (OCS), which is confirmed by our quantum-classical calculations.

Laser-induced electron diffraction of the ultrafast umbrella motion in ammonia

Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table-top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH₃) following its strong-field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH₃ ⁺) undergoes an ultrafast geometrical transformation from a pyramidal ( Φ HNH = 107 ° ) to planar ( Φ HNH = 120 ° ) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar ( Φ HNH = 117   ±   5 ° ) field-dressed NH₃ ⁺ molecular structure 7.8 - 9.8 femtoseconds after ionization. Our measured field-dressed NH₃ ⁺ structure is in excellent agreement with our calculated equilibrium field-dressed structure using quantum chemical ab initio calculations.

Temperature-insensitive broadband optical parametric chirped pulse amplification based on a tilted noncollinear QPM design

Ultrafast pulsed laser of high intensity and high repetition rate is the combined requisite for advancing strong-field physics experiments and calls for the development of thermal-stable ultrafast laser systems. Noncollinear phasing matching (PM) is an effective solution of optimizing the properties of optical parametric chirped pulse amplification (OPCPA) to achieve broadband amplification or to be temperature-insensitive. But as a cost, distinct noncollinear geometries have to be respectively satisfied. In this paper, a noncollinear quasi-phase-matching (QPM) scheme of both temperature- and wavelength-insensitive is presented. With the assistance of the design freedom of grating wave vector, the independent noncollinear-angle requirements can be simultaneously realized in a tilted QPM crystal, and the temperature-insensitive broadband amplification is achieved. Full-dimensional spatial-temporal simulations for a typical 1064 nm pumped mid-IR OPCPA at 3.4 µm are presented in detail. Compared with a mono-functional temperature-insensitive or broadband QPM scheme, the presented QPM configuration shows a common characteristic that simultaneously optimizes the thermal stability and the gain spectrum. Broadband parametric amplification of a ∼40 fs (FWHM) pulsed laser is achieved with no signs of gain-narrowing. Both of the beam profiles and the amplified spectra stay constant while the temperature is elevated by ∼100°C. Finally, influence of the QPM grating errors on the gain spectrum is discussed.

Carrier-to-envelope phase-stable, mid-infrared, ultrashort pulses from a hybrid parametric generator: Cr:ZnSe laser amplifier system

Our Cr:ZnSe laser amplifier, seeded by parametric difference mixing, produces 72fs long pulses at the central wavelength of ~2.37µm. The stability of the carrier-to-envelope phase of the amplified seed pulses, attained at the stage of their parametric generation, is preserved through 6 orders of magnitude of laser amplification.

Narrowband, tunable, infrared radiation by parametric amplification of a chirped backward-wave OPO signal

The strict momentum conservation constraints for backward-wave optical parametric oscillators (BWOPOs) gives an inherently narrowband backward-generated wave, even with broadband pumping. Unfortunately, the limited tuning range of this wave restricts potential applications. Here we demonstrate a method to circumvent this restriction and increase the tuning range by more than one order of magnitude. A linearly chirped pump modulation is transferred to the forward-generated BWOPO wave, which is then mixed with an identically chirped pump in a conventional optical parametric amplifier to obtain narrowband (38 GHz), broadly tunable, infrared radiation around 1.86 µm, with an output energy of 19 µJ.

Imaging the Renner-Teller effect using laser-induced electron diffraction

Structural information on electronically excited neutral molecules can be indirectly retrieved, largely through pump-probe and rotational spectroscopy measurements with the aid of calculations. Here, we demonstrate the direct structural retrieval of neutral carbonyl disulfide (CS₂) in the [Formula: see text] excited electronic state using laser-induced electron diffraction (LIED). We unambiguously identify the ultrafast symmetric stretching and bending of the field-dressed neutral CS₂ molecule with combined picometer and attosecond resolution using intrapulse pump-probe excitation and measurement. We invoke the Renner-Teller effect to populate the [Formula: see text] excited state in neutral CS₂, leading to bending and stretching of the molecule. Our results demonstrate the sensitivity of LIED in retrieving the geometric structure of CS₂, which is known to appear as a two-center scatterer.

Generation of sub-two-cycle CEP-stable optical pulses at 3.5 μm by multiple-plate pulse compression for high-harmonic generation in crystals

Multiple-plate pulse compression of femtosecond mid-infrared pulses is demonstrated using YAG and Si windows. With this robust compression scheme, we produce sub-two-cycle, CEP-stable optical pulses and observe CEP-dependent high harmonic generation in crystals.

Highly stable, 15 W, few-cycle, 65 mrad CEP-noise mid-IR OPCPA for statistical physics

We demonstrate a 100 kHz optical parametric chirped-pulse amplifier delivering under 4-cycle (38 fs) pulses at ~3.2 µm with an average power of 15.2 W with a pulse-to-pulse energy stability <0.7% rms and a single-shot CEP noise of 65 mrad RMS over 8h. This source is continuously monitored, by using a fast 100 kHz data acquisition device, and presents an extreme stability, in the short and long terms.

High-power OPCPA generating 1.7 cycle pulses at 2.5 µm

We present a high-power mid-infrared (mid-IR) optical parametric chirped-pulse amplifier (OPCPA) generating 14.4 fs pulses centered at 2.5 µm with an average power of 12.6 W and a repetition rate of 100 kHz. The short pulses are obtained without nonlinear pulse compression. This is in contrast to most few-cycle systems operating in the mid-IR. In our case, the ultrashort pulse duration is enabled by a careful design of the gain profile of each amplification stage as well as a precise control of the signal dispersion throughout the system. A pulse shaper is used in the seed beam to adjust the spectral phase at the output of the OPCPA system. This approach allows for a clean temporal profile leading to a high peak power of 6.3 GW.

10 W CEP-stable few-cycle source at 2 µm with 100 kHz repetition rate

We developed a high repetition rate optical parametric chirped-pulse amplification (OPCPA) laser system based on fiber-laser-seeded Innoslab to generate few-cycle pulses around 2 µm with passively stable carrier-envelope phase (CEP) by difference frequency generation (DFG). Incorporating a piezo mirror before the DFG stage permits rapid CEP control. The OPCPA system is seeded by a stable supercontinuum generated in bulk material with the picosecond Innoslab pulses. Few-cycle pulses with durations of 17 fs and energies of over 100 μJ were produced in a single OPCPA stage. Three different nonlinear crystals: BBO, BiBO, and LNB were tested in the final parametric amplifier, and their average power related limitations are addressed.

Generation of sub-two-cycle CEP-stable optical pulses at 3.5 µm from a KTA-based optical parametric amplifier with multiple-plate compression

We demonstrate the generation of 21 fs (1.8 optical cycle), 45-µJ, carrier-envelope phase (CEP)-stable optical pulses with an octave-spanning spectrum from 2.2 to 4.9 µm. Multi-cycle output pulses (120 fs, 149 µJ, 3.5 µm, and 300 Hz) from a KTA-based optical parametric amplifier are compressed down to the sub-two-cycle regime using YAG and Si plates. The single-shot CEP stability of the compressed pulses is measured to be 283 mrad for 500 s. This robust and compact scheme realizes a field strength of 282 MV/cm with f/11 focusing, opening a new regime of attosecond strong-field physics in solids.

Highly stable, 54mJ Yb-InnoSlab laser platform at 0.5kW average power

We report on the leap of Yb-InnoSlab laser technology towards high pulse energies of 54mJ combined with high average power exceeding half a kW. The system features pulse durations of 1.5 picoseconds (ps) at 10kHz repetition rate with excellent beam properties (M² of 1.1) combined with superb power and pointing stability in the sub-% range. It provides different output ports to facilitate optical synchronization for pumping parametric amplifiers. Tunable, femtosecond seed pulses are derived directly from the ps Yb pump pulses. We investigate the long term stability of this ps driven white light continuum and demonstrate 100-fold pulse compression down to 10fs duration. Ultra-broadband IR spectra centred at 2µm wavelength are subsequently generated via difference frequency generation of selected white light components.

4-W, 100-kHz, few-cycle mid-infrared source with sub-100-mrad carrier-envelope phase noise

We demonstrate an optical parametric chirped-pulse amplifier delivering 4-cycles (38-fs) pulses centered around 3.1 µm at 100-kHz repetition rate with an average power of 4 W and an undersampled single-shot carrier-envelope phase noise of 81 mrad recorded over 25 min. The amplifier is pumped by a ~1.1 ps, Yb-YAG, thin-disk regenerative amplifier and seeded with a supercontinuum generated in bulk YAG from the same pump pulses. Carrier-envelope phase stability is passively achieved through difference-frequency generation between pump and seed pulses. An additional active stabilization at 10 kHz combining 2f-to-f interferometry and a LiNbO₃ acousto-optic programmable dispersive filter achieves a record low phase noise.

Influence of orbital symmetry on diffraction imaging with rescattering electron wave packets

The ability to directly follow and time-resolve the rearrangement of the nuclei within molecules is a frontier of science that requires atomic spatial and few-femtosecond temporal resolutions. While laser-induced electron diffraction can meet these requirements, it was recently concluded that molecules with particular orbital symmetries (such as πg) cannot be imaged using purely backscattering electron wave packets without molecular alignment. Here, we demonstrate, in direct contradiction to these findings, that the orientation and shape of molecular orbitals presents no impediment for retrieving molecular structure with adequate sampling of the momentum transfer space. We overcome previous issues by showcasing retrieval of the structure of randomly oriented O2 and C2H2 molecules, with πg and πu symmetries, respectively, and where their ionization probabilities do not maximize along their molecular axes. While this removes a serious bottleneck for laser-induced diffraction imaging, we find unexpectedly strong backscattering contributions from low-Z atoms.

2D IR spectroscopy at 100 kHz utilizing a Mid-IR OPCPA laser source

We present a 100 kHz 2D IR spectrometer. The system utilizes a ytterbium all normal dispersion fiber oscillator as a common source for the pump and seed beams of a MgO:PPLN OPCPA. The 1030 nm OPCPA pump is generated by amplification of the oscillator in cryocooled Yb:YAG amplifiers, while the 1.68 μm seed is generated in a OPO pumped by the oscillator. The OPCPA outputs are used in a ZGP DFG stage to generate 4.65 μm pulses. A mid-IR pulse shaper delivers pulse pairs to a 2D IR spectrometer allowing for data collection at 100 kHz.

Imaging an aligned polyatomic molecule with laser-induced electron diffraction

Laser-induced electron diffraction is an evolving tabletop method that aims to image ultrafast structural changes in gas-phase polyatomic molecules with sub-Ångström spatial and femtosecond temporal resolutions. Here we demonstrate the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C–C and C–H bond lengths in aligned acetylene. Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based on the combination of a 160 kHz mid-infrared few-cycle laser source with full three-dimensional electron–ion coincidence detection. Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas-phase molecules with atto- to femto-second temporal resolution.

Laser-induced electron diffraction can provide structural information on gas-phase molecules with high spatial and temporal resolution. Going beyond previous diatomic cases, Pullen et al. apply this approach to acetylene and show that it can be used to measure bond lengths for polyatomic molecules.

Relative carrier-envelope phase stabilization of hybridly synchronized ultrafast Yb and Er fiber-laser systems with the feed-forward scheme

Stabilization of the relative carrier-envelope (CE) phase for hybridly synchronized two-color fs Yb and Er fiber-laser systems is demonstrated for the first time by utilizing the feed-forward scheme based on an acousto-optic frequency shifter. The slow drift issues arising from the feed-forward scheme are solved by adding the in-loop relative CE frequency coarse stabilization via modulating the laser pump current. Sub-fs timing locking between the two-color pulses is still maintained due to the fast response and large locking range of hybrid synchronization. The approach provides an alternative way to obtain phase-stable synchronized two-color pulses with higher pulse energies.

Spectral and spatial full-bandwidth correlation analysis of bulk-generated supercontinuum in the mid-infrared

We report the measurement of spectral and spatial correlations in supercontinua generated by focusing microjoule pulses from a femtosecond ytterbium-doped fiber amplifier laser in bulk YAG. The measurement is full-bandwidth at a repetition rate of 1 MHz owing to the use of time-stretch dispersive Fourier transform technique. In contrast with fiber-based supercontinuum generation, our results show an excellent stability of the spectral and spatial properties of the output supercontinuum, with an essentially correlated behavior in the 1.4-1.7 μm wavelength range. These results provide strong ground for the development of supercontinuum-seeded ultrafast optical parametric amplifier systems in the mid-infrared using ytterbium lasers as pump sources.

1.9 octave supercontinuum generation in a As₂S₃ step-index fiber driven by mid-IR OPCPA

Using a 3.1-μm optical parametric chirped-pulse amplifier (OPCPA), we generate a supercontinuum in a step-index chalcogenide fiber that spans from 1.6 to 5.9 μm at the -20  dB points. The rugged step-index geometry allows for long-term operation, while the spectral bandwidth is limited by the transmission of the As2S3 fiber.

Generation of few-cycle infrared pulses from a degenerate dual-pump OPCPA

A degenerate dual-pump optical parametric chirped-pulse amplifier (OPCPA) for generation of few-cycle intense pulses centered at 1.6 μm is theoretically investigated. By adding the optimized linear chirp to the two pump pulses from Ti:sapphire source and carefully adjusting the delays between the two pumps and seed, the long- and short-wavelength components of the seed pulse are efficiently amplified during the parametric process. Our simulations show that a broadband spectrum spanning from 1.3 μm to 2.1 μm is attained with a conversion efficiency of 22.6%. Signal pulse with a near transform-limited (TL) duration of 10.1 fs can be achieved by simply removing the linear chirp from the output signal. Besides, the compressed signal beam manifests good quality both spectrally and temporally, which allows tightly focusing the signal beam for further use.

Temporal and spatial effects inside a compact and CEP stabilized, few-cycle OPCPA system at high repetition rates

We present a compact and ultra-stable few-cycle OPCPA system. In two non-collinear parametric amplification stages pulse energies up to 17 µJ at 200 kHz repetition rate are obtained. Recompression of the broadband pulses down to 6.3 fs is performed with chirped mirrors leading to peak powers above 800 MW. The parametric amplification processes were studied in detail employing (2 + 1) dimensional numerical simulations and compared to experimental observations in terms of spectral shapes, pulse energy, spatial effects as well as delay dependent nonlinear mixing products. This gives new insights into the parametric process and design guidelines for high repetition rate OPCPA systems.

Ionization with low-frequency fields in the tunneling regime

Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (γ = 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95 eV. Such level of high-precision measurements at only 1 meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.

Compact dual-crystal optical parametric amplification for broadband IR pulse generation using a collinear geometry

A novel compact dual-crystal optical parametric amplification (DOPA) scheme, collinearly pumped by a Ti:sapphire laser (0.8 μm), is theoretically investigated for efficiently generating broadband IR pulses at non-degenerate wavelengths (1.2 μm~1.4 μm and 1.8 μm~2.1 μm). By inserting a pair of barium fluoride (BaF(2)) wedges between two thin β-barium borate (BBO) crystals, the group velocity mismatch (GVM) between the three interacting pulses can be compensated simultaneously. In this case, the obtained signal spectrum centered at 1.3 μm is nearly 20% broader and the conversion efficiency is increased, but also the pulse contrast and beam quality are improved due to the better temporal overlap. Furthermore, sub-two-cycle idler pulses with carrier-envelope phase (CEP) fluctuation of sub-100-mrad root mean square (RMS) can be generated. Because a tunable few-cycle IR pulse with millijoule energy is attainable in this scheme, it will contribute to ultrafast community and be particularly useful as a driving or controlling field for the generation of ultrafast coherent x-ray supercontinuum.

Anomalous isotopic effect on electron-directed reactivity by a 3-μm midinfrared pulse

We have theoretically studied the effect of nuclear mass on electron localization in dissociating H₂⁺ and its isotopes subjected to a few-cycle 3-μm pulse. Our results reveal an anomalous isotopic effect in which the degree of electron-directed reactivity can be even higher for heavier isotopes in the intense midinfrared field. We show, for the first time, the pronounced electron localization can be established through the interferences among the multi-photon coupling channels. Due to the relative enhancement of higher-order coupling channels with growing mass, the interference maxima at different kinetic energy of the spectra gradually become in phase, ultimately resulting in the larger dissociation asymmetries of heavier isotopes.

Carrier-envelope phase stable sub-two-cycle pulses tunable around 1.8 µm at 100 kHz

We present a simple and efficient concept for the generation of ultrashort infrared pulses with passively stabilized carrier-envelope phase at 100 kHz repetition rate. The central wavelength is tunable between 1.6 and 2.0 µm with pulse durations between 8.2 and 12.8 fs, corresponding to a sub-two-cycle duration over the whole tuning range. Pulse energies of up to 145 nJ are achieved. As a first application we measure the high nonlinearity of multiphoton photoemission from a nanoscale metal tip.

Multi-octave supercontinuum generation from mid-infrared filamentation in a bulk crystal

In supercontinuum generation, various propagation effects combine to produce a dramatic spectral broadening of intense ultrashort optical pulses. With a host of applications, supercontinuum sources are often required to possess a range of properties such as spectral coverage from the ultraviolet across the visible and into the infrared, shot-to-shot repeatability, high spectral energy density and an absence of complicated pulse splitting. Here we present an all-in-one solution, the first supercontinuum in a bulk homogeneous material extending from 450 nm into the mid-infrared. The spectrum spans 3.3 octaves and carries high spectral energy density (2 pJ nm⁻¹–10 nJ nm⁻¹), and the generation process has high shot-to-shot reproducibility and preserves the carrier-to-envelope phase. Our method, based on filamentation of femtosecond mid-infrared pulses in the anomalous dispersion regime, allows for compact new supercontinuum sources.

Broadband coherent light sources are crucial for numerous applications, such as imaging and spectroscopy. Using filamentation of mid-infrared laser pulses in bulk crystals, Silva et al. generate supercontinuum spectra over three octaves, from 4.5 μm to 450 nm, with carrier-envelope phase stability.