A. Zair

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.

Polarization gated high order harmonic generation approaching the single attosecond pulse emission

The confinement of high-harmonic-generation pulse train by modulation of the fundamental beam polarization is studied temporally and spectrally. We observe a clear effect, being the maximal confinement compatible with l-or-2 subfemtosecond pulse emission.

Confinement of attosecond train pulses by using a modulated polarization IR pulse

We study the temporal and spectral behaviour of high order harmonics generated by pulses with temporally modulated polarization. We observe a harmonic temporal confinement and a harmonic spectral broadening, compatible with 1-or-2 attosecond pulse emission.

Controlling the duration of XUV high order harmonic pulses

We modulate temporally the polarization of a - 30 fs, SO0 nm IR pulse and use it to generate high order harmonics. The harmonic emission can clearly be confined and the XUV pulse duration can be continuously tuned from - 5 - 7 fs to 50 fs. There are curently two ways to generate a single sub-femtosecond pulse via high order harmonic emission (HHG). The first one is to use ultrashort linearly polarized pulses and to confine the harmonic emission to that of the cutoff harmonics. The second one, valid for plateau harmonics also, is to use a relatively long pulse (-15 fs) and to temporally modulate its polarization. By performing two feasibility experiments, we show that by modulating the polarization of a (T = 30 - 35 fs) long pulse, one can continuously control the harmonic pulse duration. The technique used for temporally modulating the ellipticity of a short pulse is simple and robust. By using two quartz plates, we can transform a pulse (of duration T) into a flat top pulse (duration 2 T) which is linearly polarized at the center (t=O) ofthe pulse and elliptically polarized at the begining and at the end of the pulse. The HHG is extremely sensitive to the ellipticity of the fundamental pulse and typically a 10% ellipticity reduces the efficiency by a factor 2. By temporally modulating the ellipticity of the fundamental pu1se;we therefore create a gate where the ellipticity is smaller than 10% inside which the harmonic emission is confined. In our experimental conditions, the minimum gate width is T / 6 (narrow gate). It can also be increased up to -2s (large gate) by rotating one of the plates. The high order harmonic pulse duration was estimated in two ways. In a first experiment (performed at the CELIA laboratory), the harmonic spectra were recorded and showed a clear dependance on the gate width. For the cutoff harmonics, the spectra broadened as the gate width was decreased as expected when a temporal confinment occurs. In contrast, for the plateau harmonics, the spectral width decreased with the gate width. Also counterintuitive, this is also consistent with a confinement because of the importance of the intensity dependent atomic,dipole phase. In both cases, the spectra were consistent with a confinement of the harmonic emission down to 5-7 fs. We could also observe that a temporal confinment does not always reduce the HHG efficiency and allows to optimize the phase matching. In a second experiment (performed at the Lund Laser Center), we measured directly the duration of harmonics created in Argon by doing a cross correlation of the harmonic pulse with an ultrashort (7-10 fs) 800 nin pulse. The cross correlation signal was the photoelectron peak corresponding to absorption of one harmonic photon plus absorption (or emission) of one photon of the ultrashort probe pulse. Because of a non-collinear geometry, the resolution of this measurement is larger than 10 fs but was sufficient to clearly observe an evolution of the harmonic pulse profile. For instance for the sideband 18 shown on the figure, the duration was 57 fs in the large gate configuration, 43 fs without any gate and 26 fs in the narrow gate situation. The confinment of HHG in the narrow gate configuration is therefore clearly visible, even for the plateau harmonics considered bere.