High Harmonic Generation from a Microplasma Source

Abstract

The nonlinear process of High Harmonic Generation (HHG) has enabled table-top ultrafast sources of coherent radiation, mainly in the extreme ultraviolet (XUV) spectral region, with extensive use in a large variety of domains, such as industry [1], biology [2], and photoemission science [3]. In particular, the latter would greatly benefit from tunable high repetition rate XUV sources, to map the electronic band structure of materials over a large range of the Brillouin Zone with fast data acquisition. Generally, in HHG experiments, the required peak intensity is reached with the use of femtosecond lasers with a pulse energy in the range of 10 µJ – 1 mJ, typically focused down to a spot diameter of 10 µm – 100 µm, respectively. Focusing the beam tighter than a few microns, thus generating a microplasma, would enable the use of sub-µJ sources. However, a microplasma HHG source is expected to lead to a very poor XUV photon flux. This assumption originates from the phase matching condition in an ultra-tight focusing geometry, where the significant contribution of Gouy phase factor cannot be balanced out. Prior HHG work based on micron-scale spot diameter [4] has shown an XUV photon flux multiple orders of magnitude lower than the requirement for photoemission experiments [3]. Here, we demonstrate experimentally the high efficiency of a microplasma XUV source [5], with a photon flux that exceeds 1011 photons/s/harmonic, driven by a pulse energy lower than 1 µJ. As a driving source, we use a noncolinear optical parametric amplifier system operated at 4 MHz repetition rate, from which we tune the driving wavelength between 700 nm - 900 nm. In Fig. 1, we show the measured XUV spectra, with two regimes of tunability: a) a discrete harmonic tunability ranging from 20 to 50 eV and b) a continuous tunability of about 9 eV. The generation of efficient XUV radiation with sub-µJ class sources could open a new era for spectroscopy and material science.