High-Order Harmonic Generation in the Highly Relativistic Regime

The mechanisms of high-order harmonic generation (HOHG) by the reflection of an intense laser pulse from an overdense plasma surface can provide information about the complex laser-plasma coupling. Under p-polarized interactions, “vacuum heating” has been demonstrated to drive the coherent wake emission (CWE) mechanism responsible for radiation extending to the plasma frequency. However, in s-polarized interactions, robust harmonic generation has not been observed, as relativistic intensities are required for the JxB force to be efficient. In our experimental campaign at the Advanced Laser for Extreme Photonics (ALEPH) facility, high brightness radiation at integer multiples of the fundamental frequency were observed in the specular reflection of highly intense, ultra-high contrast, and s-polarized laser pulses. For copper foils irradiated at ∽10²⁰ W/cm², the radiation spectra displayed a cutoff at the plasma frequency, resembling the CWE mechanism. Spectra obtained from particle-in-cell simulations show how the JxB force can generate radiation at and beyond the plasma frequency.

This work was supported by the NSF under awards PHY-2308982, PHY-1753165, and DMR-1548924; and by the DOE under award DE-SC0023504.