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We report here on results of experimental-theoretical investigation of high-order harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference bulk CdSe thin film. We observe a strong decrease in the efficiency, up to complete suppression of HHG with energies of quanta above the bandgap for the smallest dots, whereas the intensity of below bandgap harmonics remains weakly affected by the dot size. In addition, it is observed that the ratio between suppression of above gap harmonics versus below gap harmonics increases with driving wavelength. We suggest that the reduction in the dot size below the classical electron oscillatory radius and the corresponding off the dots wall scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency of the emission of highest-order harmonics. Our results demonstrate a new regime of intense laser-nanoscale solid interaction, intermediate between the bulk and single molecule response., Comment: 32 pages, 19 Figures

The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties have limited the availability of nonlinear XUV and x-ray spectroscopies to free electron lasers (FEL). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source for the first time by observing SHG at the Ti M2,3-edge with a high harmonic seeded soft x-ray laser (HHG-SXRL) [1,2]. Further, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations are used to confirm the surface specificity and resonant enhancement of the SHG signal. The realization of XUV-SHG on a table-top source with femtosecond temporal resolution opens up tremendous opportunities for the study of element-specific dynamics in multi-component systems where surface, interfacial, and bulk-phase asymmetries play a driving role in smaller-scale labs as opposed to FELs., Comment: 16 pages, 3 main text figures, 1 supplementary table, 6 supplementary figures

From fusion dynamics in stars, to terrestrial lightning events, to new prospects of energy production or novel light sources, hot dense plasmas are of importance for an array of physical phenomena. Due to a plethora of correlations in highly excited matter, direct probing of isolated dynamics remains challenging. Here, the 32.8-nm emission of a high-harmonic seeded laser-plasma amplifier (LPA), using eight-fold ionized Krypton as gain medium, is ptychographically imaged in longitudinal direction in the extreme ultraviolet (XUV). In excellent agreement with ab initio spatio-temporal Maxwell-Bloch simulations, an overionization of krypton due to nonlinear laser-plasma interactions is observed. This constitutes the first experimental observation of the laser ion abundance reshaping a laser plasma amplifier. The findings have direct implications for upscaling plasma-based XUV and X-ray sources and allow modeling light-plasma interactions in extreme conditions, similar to those of the early times of the universe, with direct experimental verification.

We report on experimental results in a new regime of a relativistic light-matter interaction employing mid-infrared (3.9-micrometer wavelength) high-intensity femtosecond laser pulses. In the laser generated plasma, the electrons reach relativistic energies already at rather low intensities due to the fortunate lambda^2-scaling of the kinetic energy with the laser wavelength. The lower intensity suppresses optical field ionization and creation of the pre-plasma at the rising edge of the laser pulse efficiently, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. In our experiments about 80% of the incident laser energy has been absorbed resulting in a long living, keV-temperature, high-charge state plasma with a density of more than three orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate.

We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. The measured electron energy distribution was in a good agreement with results of Particle-in-Cell (PIC) simulations. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electrons production in the case of the high laser contrast. By application of Ti-foils covered with nm-thin Fe-layers we demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature EOS, wide-range description of all transport and optical properties, ionization, electron and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy firstly deposited into the nm-thin skin-layer is then transported into the target depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons., Comment: 15 pages, 10 figures

We present a table-top coherent diffraction imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth and high degree of spatial coherence allow for ultra-high sub-wavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 13.6 nm, very close to the actual Abbe-limit of 12.4 nm, which is the highest resolution achieved from any table-top XUV or X-ray microscope. In addition, 20.5 nm resolution was achieved with only 3 sec of integration time bringing live diffraction imaging and 3D tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nm resolutions are in reach and will find applications in many areas of science and technology., Comment: 5 pages, 5 figures

We present few-femtosecond shadowgraphic snapshots taken during the non-linear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma wave period is induced by a lengthening of the first plasma period. Three dimensional particle in cell simulations support our observations., Comment: 5 pages, 4 figures