High Harmonic Generation With Focal Cone Geometry

Abstract

High Harmonic Generation (HHG) is a convenient method of producing coherent extreme ultraviolet (XUV) radiation and is typically produced by focusing a laser onto a gas jet. The total intensity of XUV generated in this method is limited by the ionization threshold of the gas and absorption of XUV radiation in the gas. A new geometry of Focal Cone High Harmonic Generation (FCHHG) is examined. This geometry involves focusing the incoming cone of light through a gas sheet rather than placing a gas jet at laser focus. Such a focusing geometry generates a converging cone of XUV radiation producing a high intensity high harmonic hot spot (HHHS) at focus. Using 100 TW to 1 PW laser pulses, high energy, microjoule to millijoule, coherent XUV pulses should be achievable. An experiment to test this concept was carried out at the Centro de L´aseres Pulsados (CLPU) in Salamanca Spain. For this a gas nozzle was designed to produce a rectangular gas sheet capable of providing a uniform density region over which the laser can interact. A number of diagnostics were used to characterize the XUV beam both spectrally and spatially. These included an Xray CCD camera and XUV photodiodes to observe the XUV and measure its intensity, spectral measurements using a transmission grating and knife edge diffraction, and divergence measurements using the knife edge and shadowgraphic imaging of perforated meshes. The conversion efficiency as a function of backing pressure, nozzle distance from laser focus and the laser axis, and intensity is also explored.