Spectral detuning of relativistic surface harmonics

Relativistic surface harmonics driven by high-intensity lasers are considered a promising future light source, as their bandwidth and brightness are scalable with the driving laser power. Typically, the emission frequencies of these sources are limited to integer multiples of the laser fundamental frequency. In this Letter, we describe how the generation dynamics of these harmonics may enable spectral detuning of their frequencies. A dent in the plasma surface driven by the radiation pressure of the laser field grows during the interaction and varies the harmonic beam divergence. When a temporal chirp is added to the driving laser pulse, different instantaneous fundamental frequencies drive the harmonic beam to different cone angles, resulting in an overall spectral detuning of the high harmonics beam. We present experimental measurements of the dependence of the spectral detuning on the temporal chirp of the driving pulse. We show how these results are reproduced by our model, and conclude with its predictions for higher intensity laser systems.