Temporal Characterization of the Second Harmonic from a Relativistic Plasma Mirror via Spectral Interferometry

Surface second-harmonic generation (SSHG) plays a central role in laser–relativistic plasma interactions. Here we report a temporal characterization of SSHG emitted from a relativistic plasma mirror. We measure the second-harmonic (SH) spectrum and retrieve its spectral phase by spectral interferometry (SI) using a reference SH pulse generated by frequency doubling the fundamental reflected from the same plasma mirror. A birefringent crystal provides a precisely controlled temporal delay, yielding high-contrast spectral fringes suitable for phase reconstruction. The retrieved amplitude and phase agree closely with numerical simulations. These results establish SI-based metrology as a robust tool for characterizing ultrafast frequency-conversion at plasma mirrors and for optimizing relativistic SSHG as a tunable source.