Broadband extreme ultraviolet dispersionmeasurements using a high-harmonic source

We demonstrate direct dispersion measurements of various thin films at extreme ultraviolet (EUV) wavelengths, using a table-top laser-driven high-harmonic generation (HHG) source. In this method, spatially separated identical EUV pulses are generated through HHG with a pair of phase-locked infrared pulses. The EUV pulses are re-imaged to a sample plane using a single toroidal mirror, such that one pulse illuminates the target thin film, while the other pulse passes through a reference aperture. By comparing the EUV interference with and without a sample, we are able to extract the dispersion properties of the sample, integrated over the full film thickness. We have measured thin films of titanium, nickel, copper, and silicon nitride, demonstrating that this technique can be applied to a wide range of materials, only requiring a film thin, enough for sufficient EUV transmission.