A. Amani Eilanlou

Direct observation of an attosecond electron wave packet in a nitrogen molecule.

Attosecond electron wave packet in a molecule is captured by the pump-probe method with a-few-pulse attosecond pulse train. Capturing electron motion in a molecule is the basis of understanding or steering chemical reactions. Nonlinear Fourier transform spectroscopy using an attosecond-pump/attosecond-probe technique is used to observe an attosecond electron wave packet in a nitrogen molecule in real time. The 500-as electronic motion between two bound electronic states in a nitrogen molecule is captured by measuring the fragment ions with the same kinetic energy generated in sequential two-photon dissociative ionization processes. The temporal evolution of electronic coherence originating from various electronic states is visualized via the fragment ions appearing after irradiation of the probe pulse. This observation of an attosecond molecular electron wave packet is a critical step in understanding coupled nuclear and electron motion in polyatomic and biological molecules to explore attochemistry.

Resolving vibrational wave-packet dynamics of D2+ using multicolor probe pulses

We demonstrate the generation and real-time observation of the vibrational wave packet of D(2)(+) by using a sub-10-fs extreme UV high-harmonic pump pulse and a three-color probe laser pulse whose wavelength ranges from near-IR to vacuum UV. This multicolor pump-probe scheme can provide us with a powerful experimental tool for investigating a variety of wave packets evolving with a time scale of ~20 fs.

Sub-10-fs control of dissociation pathways in the hydrogen molecular ion with a few-pulse attosecond pulse train

The control of the electronic states of a hydrogen molecular ion by photoexcitation is considerably difficult because it requires multiple sub-10 fs light pulses in the extreme ultraviolet (XUV) wavelength region with a sufficiently high intensity. Here, we demonstrate the control of the dissociation pathway originating from the 2pσu electronic state against that originating from the 2pπu electronic state in a hydrogen molecular ion by using a pair of attosecond pulse trains in the XUV wavelength region with a train-envelope duration of ∼4 fs. The switching time from the peak to the valley in the oscillation caused by the vibrational wavepacket motion in the 1sσg ground electronic state is only 8 fs. This result can be classified as the fastest control, to the best of our knowledge, of a molecular reaction in the simplest molecule on the basis of the XUV-pump and XUV-probe scheme.

Femtosecond laser pulses in a Kerr lens mode-locked thin-disk ring oscillator with an intra-cavity peak power beyond 100 MW

We report 860 W average power, ∼440 fs laser pulse generation inside a 15.2 MHz thin-disk ring oscillator under development for intra-cavity high-order harmonic generation (HHG). This has been achieved by placing a Kerr lens mode-locking setup in a low-pressure chamber. The intra-cavity pulse energy is ∼57 µJ with an estimated peak power of 130 MW, which is the highest, to the best of our knowledge, in ring type oscillators. By focusing these laser pulses, we can expect a peak intensity high enough for demonstration of intra-cavity HHG at the multi MHz repetition rate to facilitate experiments in photoelectron spectroscopy.

Settling time of a vibrational wavepacket in ionization

The vibrational wavepacket of a diatomic molecular ion at the time of ionization is usually considered to be generated on the basis of the Franck–Condon principle. According to this principle, the amplitude of each vibrational wavefunction in the wavepacket is given by the overlap integral between each vibrational wavefunction and the ground vibrational wavefunction in the neutral molecule, and hence, the amplitude should be a real number, or equivalently, a complex number the phase of which is equal to zero. Here we report the observation of a non-trivial phase modulation of the amplitudes of vibrational wavefunctions in a wavepacket generated in the ground electronic state of a molecular ion at the time of ionization. The phase modulation results in a group delay of the specific vibrational states of order 1 fs, which can be regarded as the settling time required to compose the initial vibrational wavepacket.

Frequency modulation of high-order harmonic fields with synthesis of two-color laser fields

We report periodical frequency modulation of high-order harmonic fields observed by changing the delay between the driving two-color laser fields consisting of the fundamental and its second harmonic (SH) field. The amplitude of modulation has been up to ∼0.4 eV, which is larger than the bandwidth of the fundamental field. Experimental results show that the intensity and chirp of the fundamental field can control this phenomenon. Numerical analysis by solving the time-dependent Schrödinger equation approves of these results and shows that anharmonic frequency components of the SH field have a crucial role in this phenomenon.

Frequency-resolved optical gating technique for retrieving the amplitude of a vibrational wavepacket.

We propose a novel method to determine the complex amplitude of each eigenfunction composing a vibrational wavepacket of / molecular ions evolving with a ~10 fs time scale. We find that the two-dimensional spectrogram of the kinetic energy release (KER) of H+/D+ fragments plotted against the time delay of the probe pulse is equivalent to the spectrogram used in the frequency-resolved optical gating (FROG) technique to retrieve the complex amplitude of an ultrashort optical pulse. By adapting the FROG algorithm to the delay-KER spectrogram of the vibrational wavepacket, we have successfully reconstructed the complex amplitude. The deterioration in retrieval accuracy caused by the bandpass filter required to process actual experimental data is also discussed.

Attosecond Frequency Resolved Momentum Imaging of Two-Photon Dissociative Ionization Dynamics of Nitrogen Molecule

Two-photon dissociative ionization processes of nitrogen molecule are investigated with attosecond nonlinear Fourier transformation spectroscopy. Nonlinear absorption spectrum of nitrogen molecule are extracted by measuring interferometric autocorrelation of a-few-pulse attosecond pulse train using non-sequential two-photon processes of nitrogen molecule. The frequency resolved momentum images (FRMI) are reconstructed from the delay dependent momentum images of fragment ion \( {\text{N}}^{ + } \) and the resultant FRMIs show the attosecond nonlinear response of nitrogen molecule.

Material survey for a novel beam splitter separating high-order harmonics from high-average-power fundamental pulses

We have investigated novel beam splitter (BS) materials to relax thermal issues under irradiation of a high-average-power visible and near infrared (VIS–NIR) laser beam. The material should efficiently reflect a high-order harmonic beam in the extreme ultraviolet (XUV) spectral region and transmit a VIS–NIR fundamental laser beam at a Brewster incidence for the latter beam. We have investigated optical and thermo-mechanical properties of crystalline silicon carbide (SiC), diamond, and rutile since they exhibit high transparency and high index of refraction for a VIS–NIR light. We also address thermal issues of conventional opaque BS materials by measuring their thermal distortion. We have shown that crystalline SiC is the most promising candidate among the other materials because of a high reflectivity for XUV light, as well as an ignorable thermal distortion. The surface flatness of a crystalline SiC plate must be improved to realize an applicable BS in the XUV spectral region.

Kerr lens mode-locked Yb:Lu2O3 bulk ceramic oscillator pumped by a multimode laser diode

We report a Kerr lens mode-locked Yb:Lu₂O₃ ceramic oscillator with a pulse energy of 23.5 nJ, which is the highest value in bulk Yb:Lu₂O₃ ceramic oscillators, to the best of our knowledge.