Design and characterization of extreme-ultraviolet broadband mirrors for attosecond science

International audienceA novel multilayer mirror was designed and fabricated based on a recently developed three-material technology aimed both at reaching reflectivities of about 20% and at controlling dispersion over a bandwidth covering photon energies between 35 and 50 eV. The spectral phase upon reflection was retrieved by measuring interferences in a two-color ionization process using high-order harmonics produced from a titanium: sapphire laser. We demonstrate the feasibility of designing and characterizing phase-controlled broadband optics in the extreme-ultraviolet domain, which should facilitate the manipulation of attosecond pulses for applications

Attosecond Electron Wave Packet Interferences

Attosecond pulses offer a new route to produce temporally localized electron wave packets (EWPs) that can easily be tailored by altering the properties of the attosecond pulses. In this thesis we will present different experiments, supported by theory, where attosecond EWPs are created in presence of a strong infrared (IR) field. By tuning the central frequency of the attosecond pulses and/or changing the target gas, the initial energy of the wave packets is set to be either above, or below the ionization potential. In a first set of experiments, trains of free attosecond EWPs separated by half a laser period are created by single photon ionization. Depending on the timing of ionization compared to the external IR field, a shear and/or a phase difference between the consecutive EWPs is induced. Because the EWPs are created coherently, interferences depending on their phase difference will occur. The analysis of the interferograms enables to retrieve information about the phase of the EWPs. In a second set of experiments, bound electron wave packets are created below the ionization potential of a target gas. In the case of a train of bound EWPs, we find that the ionization is greatly enhanced by the presence of the infrared laser field and that this enhancement strongly depends on the timing between the attosecond pulses and the laser field. We show that this effect can be attributed to interference between consecutive wave packets. In the case of a single bound EWP, we are able to probe its time evolution with a short IR pulse

Momentum shearing interferometry of electron waves

We analyse recent experiments on momentum shearing interferometry of electron wave packets1,2 by using an optical analogy with shearing interferometry for optical waves. This analogy offers a convenient point of view to discuss the capabilities and difficulties of this technique used to access the phase of electron wave packets

Effects of Interval Aerobic Training Program with Recovery bouts on cardiorespiratory and endurance fitness in seniors

International audienceInterval aerobic training programs (IATP) improve cardiorespiratory and endurance parameters. They are, however, unsuitable to seniors as frequently associated with occurrence of exhaustion and muscle pain. The purpose of this study was to measure the benefits of an IATP designed with recovery bouts (IATP-R) in terms of cardiorespiratory and endurance parameters and its acceptability among seniors (≥70 years). Sedentary healthy volunteers were randomly assigned either to IATP-R or sedentary lifestyle. All participants performed an incremental cycle exercise and 6-minute walk test (6-MWT) at baseline and 9.5 weeks later. The first ventilatory threshold (VT1 ); maximal tolerated power (MTP); peak of oxygen uptake (VO2peak ); maximal heart rate (HRmax ); and distance walked at 6-MWT were thus measured. IATP-R consisted of 19 sessions of 30-minute (6 × 4-min at VT1 + 1-minute at 40% of VT1 ) cycling exercise over 9.5 weeks. With an adherence rate of 94.7% without any significant adverse events, 9.5 weeks of IATP-R, compared to controls, enhanced endurance (VT1 : +18.3 vs -4.6%; HR at baseline VT1 : -5.9 vs +0.2%) and cardiorespiratory parameters (VO2peak : +14.1 vs -2.7%; HRmax : +1.6 vs -1.7%; MTP: +19.2 vs -2.3%). The walk distance at the 6-MWT was also significantly lengthened (+11.6 vs. -3.1%). While these findings resulted from an interim analysis planned when 30 volunteers were enrolled in both groups, IATP-R appeared as effective, safe, and applicable among sedentary healthy seniors. These characteristics are decisive for exercise training prescription and adherence

Angularly resolved electron wave packet interferences

We study experimentally the ionization of argon atoms by a train of attosecond pulses in the presence of a strong infrared laser field, using a velocity map imaging technique. The recorded momentum distribution strongly depends on the delay between the attosecond pulses and the laser field. We interpret the interference patterns observed for different delays using numerical and analytical calculations within the strong field approximation

Short XUV pulses to characterize field-free molecular alignment

We present experiments on field-free molecular alignment of N-2 and CO2 probed with short XUV pulses that are obtained via high-harmonic generation. The XUV pulses induce a dissociative ionization or a Coulomb explosion of the molecule, where the fragment ion recoil (measured using the velocity map imaging technique) provides information on the alignment of the parent molecule at the time of ionization. We discuss how photoelectron detection may in future allow the determination of molecular-frame photoelectron angular distributions and molecular structure

Attosecond Electron Wave Packet Dynamics in Strong Laser Fields

International audienceWe use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (∼20  eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes

Experimental studies of attosecond pulse trains

We present experimental measurements of attosecond pulse trains. The characterization of the pulses uses a spectral interferometry technique that is implemented with a Mach-Zehnder interferometer. This allows us to manipulate independently the pump and probe pulses for a wide range of applications. By letting the attosecond pulses pass through metallic films, we can in particular compensate for the intrinsic chirp of the attosecond pulses corresponding to the plateau spectral region, thus getting pulses as short as 170 attoseconds- only 1.2 optical cycles at the central frequency. The measurement technique is also applicable for determination of the group delay of thin XUV-transparent films and relative delay in the photoionization process. Our experimental method is applied to attosecond pulse trains created by 35- and 9-fs laser pulses, and the shortest train observed consists of three or four pulses