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High-order harmonic generation using quantum dots

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2017 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 387127256
 
Final Report Year 2024

Final Report Abstract

We investigated in this joint RFBR-DFG project quasi-phase matching of high-order harmonic generation (HHG) in laser-ablated plasma media. Two different approaches were followed: the illumination of a continuous target by the ablation laser through a slit mask and the use of a target which was structured in such a way that it fulfills the quasi-phase matching conditions. For optimal quasi-phase matching the coherence length in the plasma medium for the generation of a specific harmonic has to be known. We show that for a laser-induced plasma this can conveniently be achieved by a extending a line plasma over the confocal region of the driving laser for HHG. Then a nearly sin2-dependence of the generated harmonic intensity is observed as a function of the length of the plasma. From the distance of two maxima the coherence length within the plasma can be derived. The necessary vacuum spacing between the plasma jets can be calculated from the geometrical phase of the driving beam. The coherence length decreases nearly proportional to 1/q where q is the harmonic order. For the 13th to 25th order and a 800 nm driving beam the length varies from about 0.52 to 0.35 mm, respectively. By choosing the correct thickness of the plasma jets and of their distance the harmonic order which shall be quasi-phase matched can be selected. With four jets we observed an enhancement of the intensity by about a factor of 16 compared to a single jet, as expected. A second set of experiments studied HHG in various sulfide quantum dots as active media in the plasma. Here illumination masks with up to ten narrow slits were used, and thus quasi-phase matching was achieved for harmonic orders as high as 43, corresponding to hν ~ 65 eV. Plasma jets have a sharp density boundary – different from gas jets – and thus can preferably be employed in quasi-phase matching experiments. We also have proven that the obtained HHG intensity for quasi-phase matching is indeed larger than for a standard medium with optimal focusing of the driving beam. We also started to investigate the action of vector and vortex beams in HHG, so far for Ar as active medium. It may also lead to an increase of the HHG yield, and of course to structured light in the XUV. Further thin solid films of quantum dots as harmonic sources are studied. For the low-order 3rd harmonic we achieved quantum control of the yield by the thickness of the film.

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