Decryption of Harmonics: How Spectral Interferometry Reveals the Structure of XUV Pulse Trains

Summary form only given. The sources of isolated ultrashort light pulses are highly requested in modern ultrafast and attosecond science. Even though some experiments with isolated attosecond pulses (APs) have already been demonstrated, such light sources are still quite unique and not easily available while trains of APs are routinely produced in so called high-harmonics generation experiments. Regardless of the generation mechanisms deep understanding and characterisation of these pulse trains is an essential step towards routine generation of isolated attosecond pulses. Working on harmonics generation from relativistic solid density plasma surfaces with a few -cycle laser we have developed a novel spectral-interferometry-like method for analysis of the XUV spectra. Based on the conventional spectral interferometry such a treatment allows deriving a lot of important information about the generation process and partially the temporal pulse train structure purely from the recorded spectra without resorting to any additional diagnostics. Simple Fourier transformation (FT) of the measured spectrum immediately uncovers number of pulses within chosen spectral range in the corresponding XUV burst. It can also reveal the delay between generated pulses, which after some additional analysis allows deriving their intensity ratio. In our experiments it led to identification of a highly-isolated pulse and necessary driving laser field shape. Combining obtained observations with simulations for laser-solids interaction also provided important information about spatial and temporal properties of the APs generation process.