Development of a refractive plasma lens for focusing attosecond beams

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Refractive lenses, which are widely used for the imaging and focusing of light as well as for microscopy applications, are available in many spectral regions of the electromagnetic spectrum, including the visible, infrared, terahertz, ultraviolet and even X-ray regimes. In the extreme-ultraviolet (XUV) region (covering the wavelength range from 10 to 124 nanometers), which is of high interest for scientific and industrial applications including coherent diffractive imaging of nanoscale structures, attosecond science and nanolithography, refractive lenses have not been available. This can be explained by the large absorption of XUV radiation by solid matter. Recently, we developed a gas-phase lens enabling the focusing of XUV pulses with photon energies in the vicinity of atomic resonances. These gas-phase lenses can be used in the wavelength region between 50 and 124 nanometers, whereas absorption remains a limiting factor for wavelengths below 50 nanometers. The goal of this project is to develop a plasma refractive lens that enables focusing of XUV pulses in the spectral region between 10 and 50 nanometers. The plasma lens exploits the refraction due to free electrons in the plasma, resulting in low chromatic aberration and low dispersion. A plasma refractive lens is therefore ideally suited to focus attosecond pulses, allowing one to maintain an attosecond pulse duration in the focus. Following the development of the XUV plasma lens, we will characterize the XUV focal spot and compare the obtained results with simulations. We will further measure the XUV pulse duration in the focal plane of the plasma lens using the attosecond streaking technique. Compared to mirrors that are typically used to focus XUV beams, important advantages of the XUV plasma lens are the low absorption losses and the possibility to change the focal length in-situ.