Sébastien Weber

High-order harmonic generation in graphite plasma plumes using ultrashort laser pulses: a systematic analysis of harmonic radiation and plasma conditions

High-order harmonic generation in graphite-ablated plasmas was systematically studied using ultrashort (3.5 and 30 fs) laser pulses. We observed the efficient frequency conversion of 3.5 fs Ti:sapphire laser pulses in the range of 15-26 eV. Stabilization of the harmonic yield at a 1 kHz pulse repetition rate was accomplished using a rotating graphite target. We also show the results of harmonic generation in carbon plasma using 1300 nm, 40 ps pulses, which allowed the extension of the harmonic cutoff while maintaining a comparable conversion efficiency to the case of 780 nm driving radiation. The time-of-flight mass spectrometric analysis of the plasma components and the scanning electron microscopy of plasma debris under optimal conditions for harmonic generation suggest the presence of small carbon clusters (C10-C30 )i n the plasma plume at the moment of femtosecond pulse propagation, which further aggregate on nearby substrates. We present the results of plasma spectroscopy obtained under unoptimized plasma conditions that elucidate the reduction in harmonic signal. We also present calculations of plasma concentration under different excitation conditions of the ablated graphite target. (Some figures may appear in colour only in the online journal)

Comparison of high-order harmonic generation in uracil and thymine ablation plumes

We present studies of high-order harmonic generation (HHG) in laser ablation plumes of the ribonucleic acid nucleobase uracil and its deoxyribonucleic acid counterpart thymine. Harmonics were generated using 780 nm, 30 fs and 1300 nm, 40 fs radiation upon ablation with 1064 nm, 10 ns or 780 nm, 160 ps pulses. Strong HHG signals were observed from uracil plumes with harmonics emitted with photon energies >55 eV. Results obtained in uracil plumes were compared with those from thymine, which did not yield signs of harmonic generation. The ablation plumes of the two compounds were examined by collection of the ablation debris on a silicon substrate placed in close proximity to the target and by time-of-flight mass spectrometry. From this evidence we conclude that the differences in HHG signal are due to the different fragmentation dynamics of the molecules in the plasma plume. These studies constitute the first attempt to analyse differences in structural properties of complex molecules through plasma ablation-induced HHG spectroscopy.

Spatio-temporal characterization of mid-infrared laser pulses with spatially encoded spectral shearing interferometry.

We report on the spatially resolved full amplitude and phase characterization of mid-infrared high intensity laser pulses generated in a three stage OPA. We use a spatially-encoded arrangement (SEA-)SPIDER with spectral filters for ancilla generation for spatially resolved characterization. Using five interchangeable filter sets we are able to characterize pulses from 1 to 2 μm with one single device with minimal adjustments.

Spatio-temporal characterization of intense few-cycle 2 μm pulses

We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 μm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500-650 μJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).

Generation and characterization of few-cycle phase-controlled 1.7 μm pulses

We generate 9 fs 1.7 μm 300 μJ pulses by hollow fiber compression. Complete 3D simulations show good agreement across a range of fiber pressures and elucidate the role of shock formation and Kerr effect saturation.

Dynamical coupling of molecular rotation and Coulomb explosion

Summary form only given. The first observation of 1/8<;sup>th<;/sup> rotational revival in impulsively aligned CO<;sub>2<;/sub> molecules probed by Coulomb explosion is reported. Such a dynamic gives insight in the coupling arising between rotation wavepacket and strong field dynamic. Field free molecular rotational alignment is nowadays a routine technique to achieve structural and dynamical measurements in the molecular frame [1]. It is a unique tool to study electron recollision in strong field physics as in High Harmonic Generation (HHG) spectroscopy and recollision-induced double ionisation and fragmentation. A rotational wavepacket is launch by an ultrashort pulse leading to periodic alignment of the molecules, so called “revivals”. Various experimental techniques are able to give insight in the rotational dynamic of the wavepacket as for example the direct visualization of the molecular bond orientation using velocity map imaging [2]. While the temporal evolution of such alignment is well represented by the expectation value <;cos<;sup>2<;/sup> θ> (0 is the angle between molecular bond and electric field polarisation), the experimental measurement of rotational periodic alignment can lead to significantly distorted revivals, especially with the presence of higher order terms as <;cos<;sup>n<;/sup> 0>with n=4,6. This has been shown especially with HHG, where the coupling of rotational wavepacket and angular yield of HHG is modulated differently, depending the molecule structure [3,4].

Single cycle midIR pulse: Spatial, temporal and absolute phase characterisation

Summary form only given. 1.7 cycles midIR pulse has been measured via a tunable SEA-F-SPIDER arrangement providing both temporal and spatial characterisation. The intrinsic CEP stability of the pulse is monitored and stabilised from a simple interferometric measurement.A significant part of future development in attosecond science [1] and especially molecular high harmonic generation spectroscopy [2] is likely to require the use of intense and ultrashort infrared laser pulses in the mid-infrared (mid-IR) spectral range. Secondary sources based on Ti:Sapphire laser pumped Optical Parametric Amplifier (OPA) in nonlinear crystals are readily available as commercial systems and their extension to few-cycles mid-IR sources have been demonstrated [3,4]. MidIR pulses at 1.7um are generated by self phase modulation (SPM) of the idler pulse of a HE-TOPAS (Light Conversion) in an Argon filled hollow core fibre and subsequent compression in Fused Silica. The carrier envelope phase (CEP) of such short pulses is of prime interest for strong field physics experiment. Its pulse to pulse passive stability is here assured by the difference frequency generation mechanism in the last OPA stage [4]. Providing a stable pump source (~1% pulse to pulse energy fluctuation) the main source of CEP fluctuation is due to the temporal interferometric jitter between pump and seed in the DFG process.Our temporal characterisation relies on SEA-SPIDER [6] with direct spectral filtering for ancilla preparation (SEA-F-SPIDER [7,8]). A spatio-temporal reconstruction of our single cycle pulse is shown in fig.1 (inset) from which a lineout has been extracted (red curve) and compared to the Fourier Limit (in black). Apulse duration of 9.9fs has been obtained representing 1.7 cycles at 1.7um. Interference fringes at ~700nm are produced from SHG of the seed for one part and SPM of the pump in the last OPA crystal for the other part. The interference of these two almost balanced paths (about 3 meters each) creates fringes that are stabilised and controlled by a piezo actuator added on the pump arm. The evolution of the interferogram as a function of time with a slow loop stabilisation turned on is visible on figure 1 b) for different relative set phases. The stability in each interval is excellent and on the order of 100 mrad standard deviation. Since the stability of this interferometer is directly coupled to the CEP stability of the generated pulse, this measurement is a monitor of an important source of the CEP fluctuations.

Development of High Power Infrared Optical Parametric Amplification Laser System Seeded by Self-difference Frequency Generation Pulses

We report on our recent development of a femtosecond infrared laser system based on self-phase-stabilized seed by difference-frequency generation of supercontinuum obtained with so-called hollow fiber compression technique and two-stage optical parametric amplification. After the final amplifier, we obtained pulses with duration of 500 μJ and as a first demonstration of this new laser system, we generated high-harmonics up to 47th order in a tube target filled with Xe gas.

High-Order Harmonic Generation in Stabilized Plasma Plumes Using the 800 and 1300 nm Femtosecond Pulses

We show the advantages of using the rotating targets for plasma harmonic generation, which allowed the dramatic improvements of harmonic stability in the case of resonance enhancement and application of 1300 nm radiation.

Controlling ionisation and fragmentation processes in CO2 via laser driven inelastic electron recollisions

For the first time, the angular dependence of nonsequential double ionisation and dissociation induced by laser driven inelastic electron rescattering was investigated experimentally in aligned CO2. A strong dependence on the recollision angle was found.