Extreme-ultraviolet high-order harmonic pulses in the microjoule range

Abstract

We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation. The generation of the high-order harmonics of intense laser pulses in gases @1# has recently opened new perspectives for probing matter in the extreme-ultraviolet ~XUV! pulses on an unprecedented time scale. The ultrashort harmonic pulse duration is used in pump-probe experiments in atomic @2,3# and molecular @ 4‐6 # spectroscopy, as well as in solidstate physics @7,8#. Combined to the high intrinsic beam coherence, it has allowed ultrafast diagnosis of laser-produced plasmas through XUV interferometry @9,10#. However, the harmonic beam energy is still relatively low and many applications would become possible if the number of generated photons were increased: ultrafast XUV holography, diagnosis of dense bright plasmas, or even study of nonlinear processes in the XUV, limited so far to low harmonic orders @11#. Recently a number of studies have demonstrated high conversion efficiencies, using ultrashort laser pulses focused in hollow core fibers @12‐14# or cells @15,16#. However, these efficiencies were obtained at a very low laser energy ~less than 1 mJ in most cases! that imposed a relatively tight focusing geometry in order to reach saturation intensities of the generating rare gases. This resulted in a low harmonic energy, in the nanojoule range. The fact that much larger energies are now available on ultrashort laser systems raises a number of questions: using higher laser energies and looser focusing, is it possible to achieve similar efficiencies, and thus to generate microjoule harmonic pulses? In particular, how will phase matching be affected by these unusual generating conditions? In this Rapid Communication, we report a thorough study of harmonic generation at a high pumping energy using a long focal length lens. Using a pumping energy of 27 mJ and a f 52 m lens, we study the influence of the beam aperture, the medium length, and the atomic density on the harmonic yield produced in a pulsed gas jet. We identify different saturation regimes, thanks to the excellent quantitative agreement obtained with detailed three-dimensional ~3D! simulations. In particular, we clearly observe the interplay between phase matching, absorption, and defocusing, the main limiting factors of the macroscopic emission. Absolute photon number measurements in the optimal conditions give conversion efficiencies as high as those reported using ultrashort (,20 fs) laser pulses, but now with ten times more energy. Using a f 55 m lens, we show that even higher conversion efficiencies can be obtained, resulting in harmonic energies in excess of 1 mJ. The experiments were performed on the LUCA laser facility with an amplified Ti:sapphire system delivering 60 fs pulses at 800 nm, with an energy of up to 100 mJ at 20 Hz. In our experiment, a pumping energy of 27 mJ was focused with either a f 52 m or 5 m lens in a pulsed gas jet. The nozzle was formed by a slit of dimensions 300 mm3 3m m producing a jet at pressure 10‐100 Torr characterized by Mach-Zehnder interferometry. By rotating the jet relative to the laser axis, we can change the length of the generating medium, while keeping the same peak density. Harmonics produced in the jet are analyzed by an XUV spectrometer without entrance slit @17#, and detected with a calibrated XUV photodiode blinded for diffused IR light with two 100 nm Al filters. The absolute spectrometer response~as well as the filter transmission! was measured using the harmonic radiation as a source further monochromatized with another spectrometer, like a synchrotron beam line. Since the total aperture of the laser beam was 40 mm, the