We report a strong dependence of the soft-x-ray spectra generated by high harmonic emission on the chirp of the excitation pulse, when an ultrashort laser drives the process. For identical pulse durations, distinct harmonic peaks can be observed for positively chirped excitation pulses, while for negatively chirped pulses, the harmonic peaks become irregular. This behavior is explained by simulations that combine the chirp of the laser with the intrinsic phase shift of the harmonics. This work resolves an outstanding discrepancy between theory and experiment by demonstrating that high-order harmonic generation driven by short-duration pulses can result in distinct harmonic peaks. This work conclusively demonstrates the role of the intrinsic phase in determining harmonic emission spectra, and control this phase during the emission process.

In this paper, we report the development of rare-earth high-entropy alloys (RE-HEA) with multiple principle elements randomly distributed on a single hexagonal close-packed (HCP) lattice. Our work demonstrated that it is the entropy, rather than other atomic factors such as enthalpy, atomic size and electronegativity, that dictates phase formation in the current rare-earth alloy system. The high configuration entropy stabilized the crystalline structure from phase transformation during cooling, whereas a second-order magnetic phase transition occurred at its Neel temperature. The quinary RE-HEA exhibited a small magnetic hysteresis and the largest refrigerant capacity (about 627 J kg-1 at the 5T magnetic field) reported to date, along with respectable mechanical properties. Our analysis indicates that the strong chemical disorder resulted from the high configuration entropy makes magnetic ordering in the HEA difficult, thus giving rise to a sluggish magnetic phase transition and enhanced magnetocaloric effect. Our findings evidenced that RE-HEAs have great potential to be used as magnetic refrigerants and the alloy-design concept of HEAs can be employed to develop novel high-performance magnetocaloric materials.

By using adaptive optics to correct the wave-front distortion of a 21-fs, 0.7-mJ, 1-kHz laser, we are able to focus the pulses to a 1-mum spot with an f/1 off-axis parabolic mirror. The peak intensity at the focal position is 1.5x10(18) W/cm(2) , which is to the authors' knowledge the first demonstration of generating relativistic intensity at a kilohertz repetition rate.