O. Tcherbakoff

Extracavity compression technique for high-energy femtosecond pulses

We introduce a new extracavity pulse-compression technique suitable for generating high-energy femtosecond pulses. This technique is based on spectral broadening by self-phase modulation in bulk media followed by far-field spatial filtering, which provides a uniform spectral broadening over the spatial profile combined with a transmission of 50%. In principle, this technique allows compression of pulses with energy up to ∼100 mJ by a factor of 3–5. In a proof-of-principle experiment, we compressed a 42-fs, 480-μJ pulse to a 14-fs, 220-μJ pulse.

Single attosecond pulse production with an ellipticity-modulated driving IR pulse

We theoretically study attosecond pulse production via high-harmonic generation using a driving laser pulse with a time-dependent ellipticity. The theoretical approach produces results that agree with our experimental data obtained using 35 fs driving laser pulses and is further used to study the generation of single attosecond pulses with shorter laser pulses. We find an equation for the duration of the temporal window created by the time-varying driving laser polarization in which high-harmonic emission can occur. We formulate the necessary requirements concerning the driving laser field in order to confine the high-harmonic emission in the form of a single attosecond pulse. Indeed, we show that using incident 12 fs laser pulses single attosecond pulses can be produced for certain carrier-envelope phase (CEP) values of the driving pulse. For 6 fs incident laser pulses, single attosecond pulses are produced for all values of the CEP (the intensity of the attosecond pulse still depends on the actual value of the CEP). If implemented with state-of-the-art 5 fs laser pulses, this technique can even lead to the production of sub-100 as pulses.

Evolution of angular distributions in two-colour, few-photon ionization of helium

Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

Carrier-envelope phase control by linear electro-optic effect in LiNbO 3

In a dispersive medium, phase and group velocity are different, inducing a slippage of the carrier frequency wave inside the envelope. This slippage is usually of no concern for long pulses but for ultra-short pulses, which contain few optical cycles, physical phenomena can strongly depend on the electric field and not only on its envelope [1]. It is, in this case, of prime importance to control the Carrier-Envelope Phase (CEP). Due to environmental effects, which are linked to variations of parameters of the system induced mainly by perturbations (vibrations, thermal drift…), laser systems emitting ultra-short pulses do not generate a train of pulses with the same value of the CEP. Different techniques already exist to stabilize the CEP of the amplified pulses of a CPA laser system seeded by a CEP stabilized mode locked oscillator. They are mainly based on a slow feedback loop containing a f−2f interferometer [2]. These techniques can be split into two categories. In the first one, the correction is made by modifying parameters inside the cavity of the mode-locked oscillator [3]. In the second one, the corrections are made outside the mode-locked oscillator [4–7].

Carrier-Envelope Phase stabilization of a grating based chirped pulse amplified laser at 17 mJ, 1 kHz using regenerative and cryo-cooled amplifiers

Over the last several years, a lot of work has been completed to develop Carrier Envelope Phase (CEP) stabilization of chirped pulse amplification (CPA) laser systems. There is now a large variety of systems. Those using prisms or transmission gratings based stretchers-compressors are limited to pulse energy lower than 5 mJ [1,2]. Those using reflection grating with multi-stages amplifier [3–7] (pulse energy higher than 10 mJ) are scalable to higher energies but more difficult to stabilize.

XUV induced non-linear transitions

We observe two photon transitions induced by high order harmonics of a Titanium Sapphire laser with harmonic orders larger than 11 (i. e. h < 73 nm). We generate high order harmonics by focussing an 800 nm, 50 fs pulse into a I cm gas cell (filled with Xe, Kr or Ar) and use this harmonics to observe two XUV photon processes. After spectral filtering of the harmonic beam (by transmission through a thin Aluminium filter that only transmits harmonics with orders higher than 1 I), the XUV beam is focussed onto a second rare gas jet with a 5 cm focal length, platinium coated mirror. This gaz jet is located inside the sensitive zone of a time of flight mass spectrometer that allows us to detect singly and doubly charged ions. When the XUV beam is focussed into the sensitive zone of the spectrometer, one could observe doubly charged ions (we could observe A?, K? and Xe” by using Ar, Kr or Xe). This signal disapears when the XUV beam crosses the sensitive zone without being focussed which shows that the observed processes are intensity dependent. Among the several pathes that could lead to observation of such doubly charged ions, only two photons transitions involving harmonic order larger than 1 I are consistent with our observations. These transitions can be sequential or direct but both can help measuring the XUV pulse duration via autocorrelation like techniques. 0-7803-7734-6/03/

Time-gated high-order harmonic generation

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