Nanostructured thin plastic foils have been used to enhance the mechanism of laser-driven proton beam acceleration. In particular, the presence of a monolayer of polystyrene nanospheres on the target front side has drastically enhanced the absorption of the incident 100 TW laser beam, leading to a consequent increase in the maximum proton energy and beam charge. The cutoff energy increased by about 60% for the optimal spheres' diameter of 535 nm in comparison to the planar foil. The total number of protons with energies higher than 1 MeV was increased approximately 5 times. To our knowledge this is the first experimental demonstration of such advanced target geometry. Experimental results are interpreted and discussed by means of 2(1/2)-dimensional particle-in-cell simulations.
For the analysis of high-order harmonics generated in a long gas jet by intense chirped femtosecond laser pulses, calculations of the laser propagation and harmonic generation were performed in terms of a nonadiabatic three-dimensional model. The self-guided propagation, observed at the conditions of bright harmonic generation, was confirmed by the model calculations. When using negatively chirped pulses, the calculated distribution of the harmonic field is spatially and spectrally confined, being generated on axis with a narrow spectral profile. The positively chirped pulses generate broad spectral distribution on axis, and narrow off axis, but in the latter case with a large emission angle. The estimation of harmonic beam divergence agreed well with experimentally measured data, showing the lowest divergence at the conditions of the brightest harmonic generation. Spectral, temporal, and spatial modifications of the propagated laser pulse are found to influence decisively the single-atom response and ultimately the harmonic field, providing a coherent picture of harmonic generation.
We present a time-dependent analysis of high-order harmonics generated by a self-guided femtosecond laser pulse propagating through a long gas jet. A three-dimensional model is used to calculate the harmonic fields generated by laser pulses, which only differ by the sign of their initial chirp. The time-frequency distributions of the single-atom dipole and harmonic field reveal the dynamics of harmonic generation in the cutoff. A time-dependent phase-matching calculation was performed, taking into account the self-phase modulation of the laser field. Good phase matching holds for only few optical cycles, being dependent on the electron trajectory. When the cutoff trajectory is phase matched, emitted harmonics are locked in phase and the emission intensity is maximized.
Single-shot nanometer-scale imaging techniques have become important because of their potential application in observing the structural dynamics of nanomaterials. We report here the image reconstruction results obtained using single-shot Fourier transform x-ray holography with an x-ray laser driven by a table top laser system. A minimum resolution of 87 nm was obtained from the reconstructed image. We could also discriminate the aggregates of carbon nanotubes, which shows the feasibility of single-exposure nanoimaging for real specimens using a laser-driven x-ray laser.
We report the manufacturing of an (ultra-)thin foil target made of conjugated polymer, poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT), and the simultaneous observation of laser-accelerated ions and second harmonic radiation, when irradiated with ultrahigh-contrast laser pulse at a maximum intensity of 4 × 1019 W/cm2. Maximum proton energy of 8 MeV is achieved along the target normal direction. Strong second harmonic with over 6% energy ratio compared to fundamental is emitted along the specular direction. Two-dimensional particle-in-cell simulations confirm the simultaneous generation of protons and high-order harmonics, which demonstrates the feasibility of applications requiring particle and radiation sources at once, effectively using the same laser and target.
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