Genesis of Nanogratings in Silica Bulk via Multipulse Interplay of Ultrafast Photo‐Excitation and Hydrodynamics

Abstract

Structuring below diffraction limit is key to developing new laser processing technologies as well as to understanding light-induced processes on mesoscopic scales, notably self-organization. Here, an advanced numerical perspective on the generation of embedded self-arranged sub-wavelength periodic patterns is developed, describing multipulse ultrafast laser interaction with bulk silica glass. Combining light and material dynamics, the approach couples self-consistently nonlinear propagation, electronic excitation, and fluid dynamics resulting in irreversible phase transitions and localized damage. With increasing the number of applied pulses, the modification changes from localized nanovoids and elongated random nanopatterns towards regular void nanogratings dominantly covering the spot of the focused laser beam. Driven by local and collective scattering events, the order imposed by electric field patterns is then amplified and stabilized by the material response. The model predicts the gradual evolution of the optical properties considering the complex interplay between material arrangement and the electromagnetic field distribution. It allows thus to define light transport optical functions optimizing losses and anisotropic effects.

Ultrafast bandgap narrowing and cohesion loss of photoexcited fused silica

“…12,13 A recent trend in ultrashort laser processing relies on the use of few-cycle pulses, predicted to achieve ultimate precision, thus preparing a new generation of micro-processing tools. 14,15 They enable an extended processing control by tuning the interplay between multiphoton excitation and strong field ionization, 16 where the cycling of the laser pulse becomes critical.…”

Ultrafast laser volume nanostructuring; a limitless perspective

“…This case, that equally combines light and matter reactions, has an underlying physics which is different from the direct structuring approaches mentioned above, based on single spot irradiation events or small number of accumulated pulses. Starting from a collection of stochastically-arranged scattering centers that diffuse light, corrugated matter determines first distributed light patterns through a coherent superposition of near-field and far-field scattered components (Rudenko et al, 2021). The patterns push the excited matter to rearrange.…”

100‐Layer Error‐Free 5D Optical Data Storage by Ultrafast Laser Nanostructuring in Glass

“…This could indicate that moderate free electron concentration (N e ) of 10 18 -10 19 cm −3 [31,32] is responsible for elongated nanopore formation where the pulse energy is just above the threshold of avalanche ionization. The higher free carrier density of about 10 20 cm −3 [33,34] with larger pulse energy is required for the formation of nanogratings (type 2). Similar pulse energy dependence of birefringent modifications was observed for 500 fs pulses (Figure 2b) and different pulse numbers (Figure S3, Supporting Information).…”