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Project ID: HPRI-CT-1999-50004
Finanziato nell'ambito di: FP5-HUMAN POTENTIAL
Paese: France

First observation of an ultrafast phase transition

A pioneered application of femtosecond x-ray diffraction has been realized: the characterization of ultrafast melting of semiconductor (InSb) in a time-scale at which such processes occur (100fs). These experiments were done using the ultrafast laser-produced plasma x-ray source. They have shown that an ultrafast atomic transition has been directly observed and characterized using x-rays for the first time. This has been published in the Journal Nature in 2001.

This scientific result shows to the user community that a new range of time scale is available for the study of the dynamics of structures and for the characterisation of a non-thermal processes. These processes are characterised by directed, rather than diffusive, motion of atoms, and are of great current interest in biology, chemistry and physics. However, measurements of these phenomena were only performed at optical wavelengths, probing the state of the valence electrons rather than the positions of the atoms. Thanks to this result, the existence of non-thermal melting of semiconductors was established unambiguously, answering a long-standing question in solid-state physics. This first structural measurement of a non-thermal process is of interest to physicists, chemists and biochemists. Large dissemination has taken place following the publication of the results with significant Press coverage (scientific and non-scientific).

Achievement of this result was seen -at an international level- as the key demonstration experiment that had to be tackled before any further investigations of more complex ultrafast processes. It has played a major role in the design and construction of femtosecond x-ray workstation in laser Facilities by identifying the remaining bottlenecks to be solved. Following the success of this experiment, extended studies have been launched and recently leaded to major publications by laboratories outside of the consortium (Nature & Science 2004) using the same tool.

The results can be described as the following: the sample was excited by a laser delivering pulses of 800nm and 120fs, and the response was probed by x-ray radiation at 7.13 Angtroms generated by focusing a 23mJ, 800nm and 120fs laser pulse from a second arm of the visible laser on a silicon target. The emitted x-rays were collected by a toroïdally bent quartz (100) crystal and focused on the samples

The first order Bragg reflection of InSb in the (111) orientation (72.3° Bragg angle) was detected by a cooled x-ray CCD camera, with the estimated number of diffracted photons reaching about 500 in one laser shot. Both 111, 100 and asymmetrically cut 100InSb crystals were used to change the probe depth l of the x-ray. The optical probe beam is a continuum produced by focusing 20mJ of the infrared laser pulse into a water cell. The reflected p-polarized signal is collected with a cooled 16-byte CCD camera through a spectrometer.

The diffracted x-ray intensity integrated over the whole rocking curve has been recorded as a function of the delay between the 100fs x-ray probe and the 100fs infrared exciting pulse. At 200mJ/cm² for InSb, the x-ray intensity drops by 20 % in 350fs and then remains constant during few picoseconds. We attribute this drop to the melting of 650Angtroms at the surface of the semiconductor. The time-scale of this phase transition is too short to be produced by the usual thermal response of the lattice. Ultrafast melting is believed to arise from a strong modification of the inter-atomic forces due to laser-induced promotion of a large fraction (10%) of the valence electrons to the conduction band. Following excitation of the electrons, the atoms find themselves far from the new equilibrium positions and immediately begin to move, gaining enough kinetic energy to produce very rapid melting.

As the fluence is decreased, we have observed that the duration of the phase transition is getting longer while the thickness of the melted layer is becoming shorter. At 50mJ/cm², it takes almost 1 picosecond to melt less than 100 Angstroms at the surface. This fluence is still larger than the expected threshold of the transition (15mJ/cm²) and the amplitude of the drop is found to be close to the amplitude of the error bars. The use of asymmetrically-cut InSb and thin films of CdTe has allowed us to increase significantly the amplitude of the drop at a fixed exciting fluence. The thickness of the molten layer and the duration of the phase transition have been fully determined as a function of the exciting fluence. At high exciting fluences (200mJ/cm²), we have found that it takes 350fs to melt 700 Angtroms at the surface of the semiconductor.

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