Alloharmonics in Burst Intensification by Singularity Emitting Radiation

Abstract: Burst Intensification by Singularity Emitting Radiation (BISER) in underdense relativistic laser plasma is a bright source of coherent extreme ultraviolet (XUV) and x-ray radiation. In contrast to all harmonic generation mechanisms, high-resolution experimental BISER spectra in the XUV region contain spectral fringes with separation much finer (down to 0.12 eV) than the initial driving laser frequency (~1.5 eV). We show that these fringe separations result from two main factors: laser frequency downshift (redshift) due to the quasi-adiabatic energy loss to the plasma waves, and spectral interference of different harmonic orders from different emission moments, i.e. alloharmonics [Pirozhkova et al., arXiv:2306.01018]