Femtosecond dynamics on the nanoscale of intense laser-induced grating plasma

The complex interaction dynamics of intense femtosecond (fs) pulses and their picosecond (ps)-long leading edge with nanostructured solids occur at both the nanometer and the femtosecond scales, making them extremely difficult to measure directly. Here, we present pump-probe-based measurements that capture the ultrafast evolution of relativistically intense laser-driven grating plasma on fs time and nanometer spatial scales. We measure the transient reflectivity and spectrum of the scattered or diffracted UV-probe pulses from the grating structures with 100s of fs resolution. Our measurements capture the initial onset of the solid-to-plasma transition and the subsequent grating plasma expansion, a few ps before the peak of the intense fs pulse. We measure the instantaneous position of the electron critical surface, its velocity, and its acceleration, which are very crucial for fundamental understanding and applications in ion/electron acceleration and high harmonic generation, while also providing valuable benchmarks for simulations. Particle-in-cell (PIC) simulations corroborate the observations well offering further insight into this process.