Daniil Kartashov

Following a chemical reaction using high-harmonic interferometry

The study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction, electron diffraction or laser-induced recollision—with spectral selection not available for any of these new methods. Here we show that in the case of high-harmonic spectroscopy based on recollision, this apparent limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation. The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique can be used to reconstruct the amplitude and phase of emission from the excited molecules. We then extract structural information from the amplitude, which encodes the internuclear separation, by quantum interference at short times and by scattering of the recollision electron at longer times. The phase records the attosecond dynamics of the electrons, giving access to the evolving ionization potentials and the electronic structure of the transient molecule. In our experiment, we are able to document a temporal shift of the high-harmonic field of less than an attosecond (1 as = 10−18 s) between the stretched and compressed geometry of weakly vibrationally excited Br2 in the electronic ground state. The ability to probe structural and electronic features, combined with high time resolution, make high-harmonic spectroscopy ideally suited to measuring coupled electronic and nuclear dynamics occurring in photochemical reactions and to characterizing the electronic structure of transition states.

Attosecond-recollision-controlled selective fragmentation of polyatomic molecules.

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.

White light generation over three octaves by femtosecond filament at 3.9 µm in argon

We report the first (to our knowledge) experimental results and numerical simulations on mid-IR femtosecond pulse filamentation in argon using 0.1 TW peak-power, 80 fs, 3.9 μm pulses. A broadband supercontinuum spanning the spectral range from 350 nm to 5 μm is generated, whereby about 4% of the mid-IR pulse energy is converted into the 350-1700 nm spectral region. These mid-IR-visible coherent continua offer a new, unique tool for time-resolved spectroscopy based on a mid-IR filamentation laser source.

Mid-infrared laser filamentation in molecular gases

We observed the filamentation of mid-infrared ultrashort laser pulses (3.9 μm, 80 fs) in molecular gases. It efficiently generates a broadband supercontinuum over two octaves in the 2.5-6 μm spectral range, with a red-shift up to 500 nm due to the Raman effect, which dominates over the blue shift induced by self-steepening and the gas ionization. As a result, the conversion efficiency into the Stokes region (4.3-6 μm) 65% is demonstrated.

Ultrafast-laser-induced backward stimulated Raman scattering for tracing atmospheric gases

By combining tunable broadband pulse generation with the technique of nonlinear spectral compression we demonstrate a prototype scheme for highly selective detection of air molecules by backward stimulated Raman scattering. The experimental results allow to extrapolate the laser parameters required for standoff sensing based on the recently demonstrated backward atmospheric lasing.

Two-proton migration in 1,3-butadiene in intense laser fields

Ultrafast proton migration in 1,3-butadiene in an intense laser field (40 fs, 4.5 × 10(14) W cm(-2)) is investigated by using Coulomb explosion coincidence momentum imaging. The spatial distribution maps of a migrating proton reconstructed for the two three-body Coulomb explosion pathways, C(4)H(6)(3+)→ H(+) + CH(3)(+) + C(3)H(2)(+) and C(4)H(6)(3+)→ H(+) + C(2)H(+) + C(2)H(4)(+), reveal that two protons migrate within a 1,3-butadiene molecule, prior to the three body decomposition.

Multi-millijoule few-cycle mid-infrared pulses through nonlinear self-compression in bulk

The physics of strong-field applications requires driver laser pulses that are both energetic and extremely short. Whereas optical amplifiers, laser and parametric, boost the energy, their gain bandwidth restricts the attainable pulse duration, requiring additional nonlinear spectral broadening to enable few or even single cycle compression and a corresponding peak power increase. Here we demonstrate, in the mid-infrared wavelength range that is important for scaling the ponderomotive energy in strong-field interactions, a simple energy-efficient and scalable soliton-like pulse compression in a mm-long yttrium aluminium garnet crystal with no additional dispersion management. Sub-three-cycle pulses with >0.44 TW peak power are compressed and extracted before the onset of modulation instability and multiple filamentation as a result of a favourable interplay between strong anomalous dispersion and optical nonlinearity around the wavelength of 3.9 μm. As a manifestation of the increased peak power, we show the evidence of mid-infrared pulse filamentation in atmospheric air.

Low-energy peak structure in strong-field ionization by midinfrared laser pulses: Two-dimensional focusing by the atomic potential

We analyze the formation of the low-energy structure (LES) in above-threshold ionization spectra first observed by Quan et al. [1] and Blaga et al. [2] using both quasi-classical and quantum approaches. We show this structure to be largely classical in origin resulting from a two-dimensional focusing in the energy-angular momentum plane of the strong-field dynamics in the presence of the atomic potential. The peak at low energy is strongly correlated with high angular momenta of the photoelectron. Quantum simulations confirm this scenario. Resulting parameter dependences agree with experimental findings [1, 2] and, in part, with other simulations [3–5].

Hollow-fiber compression of 6 mJ pulses from a continuous-wave diode-pumped single-stage Yb,Na:CaF 2 chirped pulse amplifier

Here, 200 fs 6 mJ pulses from a cw diode-pumped Yb,Na:CaF(2) amplifier are spectrally broadened in an Ar- or Ne-filled hollow-core fiber and recompressed to 20 fs (Ar) and 35 fs (Ne) using a prism pair. The results of spectral broadening and phase measurement are in excellent agreement with numerical modeling based on the generalized nonlinear Schrödinger equation. The longer laser wavelength of 1030 nm permits favorable energy scaling for the hollow-fiber technique compared to ultrafast amplifiers operating at 800 nm.

Theory of a filament initiated nitrogen laser

We present the theoretical model for a single-pass, discharge-type standoff nitrogen laser initiated by a femtosecond filament in nitrogen gas. The model is based on the numerical solution of the kinetic equation for the electron energy distribution function self-consistently with balance equations for nitrogen species and laser equations. We identify the kinetic mechanisms responsible for a buildup of population inversion in the filament afterglow plasma and determine the dependence of population inversion density and the parameters of nitrogen lasing at a 337 nm wavelength corresponding to the transition between the C3Πu (v = 0) excited and the X1Σg (v = 0) ground electronic states in a nitrogen molecule on the polarization and wavelength of the driver laser pulse used to produce the filament. We show that population inversion is achieved on an ultrafast time scale of ≈10 ps and decays within the time: <100 ps. We derive the low-signal gain 2.2 cm−1 for lasing from a circularly polarized 0.8 μm near-IR filament and 0.16 cm−1 for a linearly polarized 4 μm mid-IR filament. The results of the numerical simulations demonstrate good quantitative agreement with the experimental measurements.