Spectral interferometric transient complex refraction spectroscopy with extreme ultraviolet double attosecond pulses

Understanding optical properties in the extreme ultraviolet (XUV) region requires access to the dynamical electronic wave functions across a broad energy range. However, time-resolved spectroscopic tools to probe the real and imaginary parts of electric susceptibility in a wide portion of the XUV region remain limited. Here, we demonstrate transient complex refraction spectroscopy using double isolated attosecond pulses (DAPs) to investigate dynamical autoionization in argon. By combining DAPs-based spectral interferometry with complex Fourier analysis, we characterized the relative amplitude and phase of electronic dipoles excited from inner subshells to autoionizing states, which are modified by a few-cycle near-infrared femtosecond pulse. By precisely tuning the temporal gate width between DAPs, we could unveil the temporal behavior of the complex refractive index. Our spectroscopic scheme with DAPs will thus help to reveal the phase-resolved ultrafast electron dynamics and paves the way for attosecond coherent control in a broad XUV spectral range. Understanding the transient changes in complex refractive index due to XUV excitation requires obtaining both intensity and phase responses. In this study, the authors measure the transient relative changes in complex refractive index during the autoionization of argon by using a spectral interferometry with double attosecond pulses