Highly uniform silicon nanopatterning with deep-ultraviolet femtosecond pulses

The prospect of employing nanophotonic methods for controlling photon-electron interactions has ignited substantial interest within the particle accelerator community. Silicon-based integrated dielectric laser acceleration (DLA) has emerged as a viable option by leveraging localized photonic effects to emit, accelerate, and measure electron bunches using exclusively light. Here, using highly regular nanopatterning over large areas while preserving the crystalline structure of silicon is imperative to enhance the efficiency and yield of photon-electron effects. While several established fabrication techniques may be used to produce the required silicon nanostructures, alternative techniques are beneficial to enhance scalability, simplicity and cost-efficiency. In this study, we demonstrate the nano-synthesis of silicon structures over arbitrarily large areas utilizing exclusively deep ultraviolet (DUV) ultrafast laser excitation. This approach delivers highly concentrated electromagnetic energy to the material, thus producing nanostructures with features well beyond the diffraction limit. At the core of our demonstration is the production of silicon laser-induced surface structures with an exceptionally high aspect-ratio -reaching a height of more than 100 nm- for a nanostructure periodicity of 250 nm. This result is attained by exploiting a positive feedback effect on the locally enhanced laser electric field as the surface morphology dynamically emerges, in combination with the material properties at DUV wavelengths. We also observe strong nanopattern hybridization yielding intricate 2D structural features as the onset of amorphization takes place at high laser pulse fluence. This technique offers a simple, yet efficient and attractive approach to produce highly uniform and high aspect ratio silicon nanostructures in the 200-300 nm range.