From pore collapse to crystal growth: ultrafast laser-induced stishovite formation in nanoporous silica

Abstract

The crystallization of amorphous solids under ultrafast laser irradiation represents a paradigm of non-equilibrium phase transitions, where the interplay between electromagnetic energy localization and atomic-scale dynamics remains largely uncharted. By using a multiscale framework that couples Finite-Difference Time-Domain simulations of nonlinear light propagation with Molecular Dynamics of the atomic response, we demonstrate that field enhancement at nanopore interfaces confines laser energy and drives a rapid collapse of the surrounding matrix. In the silica structure containing a nanopore of 2 nm radius, corresponding to a porosity of approximately 7%, the enhanced local electromagnetic field led to a final equilibrium temperature 16% higher than for the 1-nm pore (1% porosity), and 20% higher than for the homogeneous medium. Particularly, the heterogeneous energy localization in the 2-nm-pore system created a preferential nucleation site within the dense glass network and led to the ultrafast formation of stishovite, a high-pressure crystalline phase of SiO2, on a sub-nanosecond timescale, significantly faster than in homogeneous silica at the same equilibrium temperature. Such accelerated crystallization enables the phase transition to outpace pressure relaxation, which would otherwise inhibit stishovite formation under identical thermal loading. These predictions align with experimental observations of laser-induced crystallization in confined geometries and establish nanopores as potent catalysts for controlling solid-state transformations via tailored electromagnetic hotspots.

Figures (13)

• Figure 1: Schematic representation of the coupled atomistic-continuum framework used in this work. The upper part depicts the lattice (atomic) subsystem, while the lower part represents the electronic subsystem responsible for laser-induced excitation and free-carrier dynamics.
• Figure 2: Photoionization rate (bottom panel) calculated with Keldysh formalism with dynamic and constant bandgap. The inset in bottom panel shows the smoothened ratio $W_{dyn}/W_{const}$ between ionization rates for dynamic and constant bandgap (8.9 eV), respectively. The top panel shows the free electron density calculated with Keldysh theory for $\lambda=1030$ nm and $\tau=25$ fs for the same intensity range, and this density-corresponding bandgap evolution taken from Ref. tsaturyan2024unraveling to derive the intensity-bandgap relation used in ionization rate calculation in bottom panel. The light gray area indicates the intensity range used in FDTD simulations.
• Figure 3: (a) Amorphous silica nanocuboid obtained after replication and prior to nanopore creation. (b) Central 1-nm-thick slice along the $x$ direction after introducing a nanopore with radius 2 nm. The simulation cell has a volume of $\sim$450 nm$^3$, corresponding to a porosity of about 7%. (c,d) Central slices of the absorbed energy density distribution obtained from FDTD simulations for nanopores with radii of (c) 2 nm and (d) 1 nm, showing strong energy localization near the pore boundary. The laser wavelength, pulse duration, and peak intensity are 1030 nm, 25 fs, and $10^{14}$ W/cm$^2$, respectively. (e) Temporal evolution of the average electronic and atomic temperatures during the first few picoseconds following laser excitation. (f) Snapshots of a central 1-nm-thick slice of the simulation box during the first picosecond, showing the onset of laser-driven nanopore collapse.
• Figure 4: Snapshots of the simulation cell at successive times during crystallization. The first row shows the full simulation box, the second row a central cuboid region of approximately 3 nm per side, and the third row a smaller subsection extracted from this region to highlight the emerging crystalline order. Minor differences between adjacent snapshots may arise from atomic motion across periodic boundaries, which can slightly shift the apparent lattice alignment between frames.
• Figure 5: Temporal evolution of the potential energy per atom and pressure after thermal equilibration. The decrease of both quantities signals the onset of the phase transition. Insets show atomic snapshots illustrating the nucleation and growth of stishovite-like clusters (Si atoms only).