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Spectroscopie pompe-sonde attoseconde

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Spectroscopie pompe-sonde attoseconde

La spectroscopie pompe sonde utilisant des harmoniques d’ordres élevés résolus en polarisation permet une résolution temporelle attoseconde et spatiale de l’angström dans la détection de molécules rotationnellement excitées.

During high harmonic generation from molecules, an electron wavepacket tunnels out from the highest occupied molecular orbital (HOMO) into the continuum, is accelerated by the laser field, is driven back to the vicinity of the parent ion and interferes with the part of the HOMO remained bound. This process is periodic and leads to the emission of attosecond XUV light bursts every half optical cycle.

The spectrum of the emitted light encodes the interference between the returning electron wavepacket and the HOMO. The typical De Broglie wavelength of the electron wavepacket is in the Angström domain. Thus the harmonic signal is a very accurate probe of molecular orbital. This property was recently used to perform the tomographic reconstruction of the HOMO of N2.

We have shown that polarization-resolved pump-probe spectroscopy can be used to increase the contrast in the detection of the harmonic emission from excited molecules.

The experiment was performed on two laser systems : the 1kHz Ti:Sapph. Aurore laser facility from CELIA providing 35 fs, 9 mJ pulses at 800 nm, and the 20Hz Ti:Sapph. Luca laser facility from CEA which delivers 50 fs, 50 mJ pulses. Pulses with a fraction of these energies were focused by a 500 mm lens into a 1kHz pulsed gas jet with a backing pressure of 2 bars. The harmonic spectrum is analyzed by an XUV spectrometer consisting in a grazing incidence grating and a dual MCP in front of a phosphor screen. A silver mirror, placed at 45° between the grating and the MCP acts as a polarizer with an extinction ratio of about 30 deduced from the Malus’ law measured for harmonic H21 generated in argon.

We have applied this method to demonstrate that a signal generated by excited molecules can be extracted from a strong background generated by a carrier gas. For this purpose, we use a 50/50 mixture of Ar and N2 in the gas target. As harmonic generation in Argon is about three times more efficient as in nitrogen, the total yield is strongly dominated by the argon contribution. If the “conventional configuration” is used, the modulation of the harmonic signal induced by the rotational wavepacket is hardly distinguishable as the delay is scanned around the revivals. With the polarization-resolved technique, revivals of molecular alignment can be detected with a contrast of 3.5 .

Pump–probe scans in N2, with a pump intensity of 5×1013Wcm−2
and a probe intensity of 1.2×1014W· cm−2. (a) Evolution of the H21 signal as a function of pump–probe delay in two configurations. Conventional configuration (blue) : the pump and probe pulse polarizations are parallel and the analyzing waveplate is rotated to maximize the reflection on the polarizer (S polarization). Polarization-resolved spectroscopy (red) : the pump-pulse polarization is rotated by 40° with respect to the probe. The analyzing waveplate is rotated so that the probe pulse is P polarized on the polarizer. (b) Evolution of the H19 intensity as a function of pump–probe delay over a full-rotational period of N2, in the polarization resolved spectroscopy configuration.

Polarimetry of high harmonic generation in aligned N2. The colormap
shows the angle between the laser polarization and the harmonic polarization, in degrees. The radial direction is the harmonic order, from 19 to 33. The polar coordinate corresponds to the molecular alignment angle \theta with respect to the
probe laser field ( \theta=0 is along the horizontal direction).

In conclusion, we have shown that polarization-resolved pump-probe spectroscopy can be used to accurately measure the state of polarization of harmonic from rotationally excited molecules.
In pump-probe scans, we have demonstrated an enhancement by a factor 4 of the contrast in the detection of alignment revival in pure nitrogen. We have also shown that polarization-resolved measurements could be used to extract the signal of excited nitrogen molecules from strong background generated by argon atoms.