One of the most interesting research fields in laser–matter interaction studies is the investigation of effects and mechanisms produced by nano- or micro-structured targets, mainly devoted to the enhancing of laser–target or laser–plasma coupling. In intense and ultra-intense laser interaction regimes, the observed enhancement of x-ray plasma emission and/or hot electron conversion efficiency is explained by a variety of mechanisms depending on the dimensions and shape of the structures irradiated. In the present work, the attention is mainly focused on the lowering of the plasma formation threshold which is induced by the larger absorptivity.

Flat and nanostructured targets were here irradiated with an ultrashort laser pulse, in the range 1 × 10^17–2 × 10^18 W µm^2 cm^−2. The effects of structures on laser–plasma coupling were investigated at different laser pulse polarizations, by utilizing x-ray yield and 3/2ω harmonics emission. While the measured enhancement of x-ray emission is negligible at intensities larger than 10^18 W µm^2 cm^−2, due to the destruction of the structures by the amplified spontaneous emission (ASE) pre-pulse, a dramatic enhancement, strongly dependent on pulse polarization, was observed at intensities lower than ~3.5 × 10^17 W µm^2 cm^−2. Relying on the three-halves harmonic emission and on the non-isotropic character of the x-ray yield, induced by the two-plasmon decay instability, the results are explained by the significant lowering of the plasma threshold produced by the nanostructures. In this view, the strong x-ray enhancement obtained by s-polarized pulses is produced by the interaction of the laser pulse with the preplasma, resulting from the interaction of the ASE pedestal with the nanostructures