H. C. Bandulet

Laser-Induced Electron Tunneling and Diffraction

Molecular structure is usually determined by measuring the diffraction pattern the molecule impresses on x-rays or electrons. We used a laser field to extract electrons from the molecule itself, accelerate them, and in some cases force them to recollide with and diffract from the parent ion, all within a fraction of a laser period. Here, we show that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule. Thus, in one comprehensive technology, the photoelectrons give detailed information about the electronic orbital and the position of the nuclei.

Pulse compression of submillijoule few-optical-cycle infrared laser pulses using chirped mirrors

We report generation of 400 microJ, 13.1 fs, 1425 nm optical parametric amplifier laser pulses. Spectral broadening of a 100 Hz optical parametric amplifier laser source is achieved by self-phase modulation in an argon-filled hollow-core fiber, and dispersion compensation is performed using chirped mirrors. This laser source will be useful for ultrafast time-resolved molecular orbital tomography.

High harmonic cutoff energy scaling and laser intensity measurement with a 1.8 µm laser source

High harmonic generation in gas targets leads to the production of attosecond pulses. The process of high harmonic generation requires that the gas be ionized by an intense femtosecond laser field. The highest photon energy produced is related to the laser intensity times the wavelength squared. This cutoff is reached only if good phase matching is achieved. Using a laser with a wavelength of 1800 nm, we estimate the laser intensity in the gas jet by recording the ion yield, and simultaneously record the high harmonic spectrum. We show that the cutoff energy matches the measured intensity, confirming that good phase matching is achieved to 100 eV. We also use the ion collector to characterize the spatial size of the gas jet and to measure the confocal parameter of the laser beam, parameters that are useful for numerical modelling.

High harmonic generation with a spatially filtered optical parametric amplifier

Numerous applications of high harmonic generation (HHG), such as attosecond pulse synthesis, depend on the ability to increase the electron recollision energy, which is a quadratic function of the driver wavelength. High-energy infrared pulses obtained from an optical parametric amplifier (OPA) are thus attractive for driving the HHG process, thereby offering the opportunity to yield shorter attosecond pulses. However, the increase in driver wavelength is often outweighed by the poor spatial quality of the OPA source. In this paper, we demonstrate that HHG using OPA signal pulses is significantly improved by spatial filtering in a hollow-core fibre prior to focusing in the gas target in comparison with the unfiltered case. Ion yield measurements in combination with beam profile monitoring in the far field enabled control over the interaction volume. For similar interaction volumes, we observe that with less than half the energy per pulse, the HHG yield can increase by one order of magnitude with spatial filtering. The comparison between the harmonic yields in argon and krypton, and their respective dependence on the peak laser intensity, provide experimental evidence that strongly suggests that the enhancement is due to improved phase matching.

NEW ULTRA FAST X-RAY STREAK CAMERA FOR THE ADVANCED LASER LIGHT SOURCE FACILITY

The Advanced Laser Light Source (ALLS) infrastructure is a new state-of-the-art multi-beams femtosecond laser facility currently in operation at INRS near Montreal, Canada. The use of a wide range of energy radiation from hard x-ray up to infrared light on the ultrafast time scale requires the development of ultrafast detector diagnostics tools to study the emission spectrum of these sources. To fulfill these requirements, new streak cameras have been developed for ALLS facility. We present the new FXR streak camera which has been specifically developed for ALLS and which is dedicated to x-ray spectroscopy with sub-picosecond time resolution combined with a very high spatial resolution.

Laser-induced orbital projection and diffraction of O-2 with velocity map imaging

Abstract In a velocity map imaging spectrometer, we measured the electron momentum distributions from the ionization of O molecules with 800 nm wavelength, 40 fs laser pulses at a peak intensity of W cm. The molecules were aligned at 0, 45 and 90 relative to the laser polarization prior to ionization. We show that for all alignments the low momentum region – populated by direct electrons which do not recollide with the parent ion – is consistent with the ionized orbital being filtered and projected onto the continuum electron wave packet. In the high momentum region – populated by rescattered electrons – we observe that the pattern created by diffraction of the recolliding wave packet by the ion core disappears as the alignment gets closer to the laser field axis. We find that a two-slit diffraction model agrees well with the results for molecules aligned at 90, but only partially predicts the decrease in the diffraction signature for smaller alignment angles.

Terahertz nonlinear spectroscopy of free-carriers in direct bandgap semiconductors

Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such interactions is found to be intervalley scattering.

Probing proton dynamics in molecules on an attosecond timescale

A technique for probing ultrafast (attosecond) structural rearrangement in molecules following laser ionization is discussed. The temporal window accessible has recently been extended beyond that previously reported by employing a driving field in the mid-IR.

High-order harmonic generation experiments with IR laser pulses

We report on the first experiments of high-order harmonic generation done with the 100 Hz high-energy optical parametric amplifier (OPA) of the Advanced Laser Light Source. Using krypton and argon as targets, we show that the OPAs signal beam − with a wavelength range from 1200 nm to 1600 nm, 1.3 mJ to 0.8 mJ of pulse energy and 100 fs pulse duration − can generate fully tunable XUV radiation down to a wavelength of 15 nm. We have also started to investigate the use of the OPA pulses for molecular imaging. Inducing molecular alignment with 800 nm, 70 fs pulses, we have measured the high harmonics spectra generated with 1300 nm pulses from nitrogen molecules oriented at various angles with respect to the ionizing field, in order to study for the first time the technique of molecular orbital tomography with a laser wavelength different than 800 nm.

New 100-Hz repetition rate soft x-ray laser plasma source for ultrafast XANES applications

We present here a laser based time resolved ultrafast XANES beam line operating with the 100 Hz ALLS facility laser system (100 mJ and 35 fs at 800 nm wavelength). This system is based on a broadband soft x-ray plasma source, produced with a tantalum solid target, and a grazing flat field incidence grating designed to work in the 1-5 nm range. This femtosecond x-ray absorption spectroscopy experimental set up is used to study ultrafast phase transition in vanadium dioxide (VO2). In this model system we are probing the electronic dynamics occurring during semiconductor to metal phase transition following excitation by a femtosecond laser pulse.