Objective Physical fields in femtosecond laser-induced water breakdown by Al/SiO₂ core/shell nanostructure were calculated. By the interaction of a femtosecond laser pulse and nanostructures, the near-field of nanostructures along the laser polarization direction was enhanced, which leads to the breakdown of water in the neighborhood. The physical model for the femtosecond laser-induced breakdown includes the electromagnetic field model, two-temperature model, plasma model, and heat transfer model. Calculation of these four physical fields was realized in this paper. Size corrections on the optical properties of the aluminum nanoparticle by modifying the critical point model were considered. This provided more accurate results of dielectric function for aluminum nanoparticles under femtosecond laser irradiation.

Methods This study employed the radio-frequency module, electromagnetic waves, and frequency domain interface of COMSOL to model electromagnetic wave propagation in different media and structures. A two-temperature model for the evolution of the lattice temperature of nanoparticles and the finite heat diffusion at the aluminum-silica-water interface during a femtosecond laser pulse irradiation was solved. It was coupled to the electromagnetic model through the resistive loss during the laser-pulse interaction with nanostructures. The plasma rate equations from the Keldysh theory for multiphoton ionization, the tunneling effect, avalanche ionization, diffusion, and recombination losses were also solved and used to calculate the dynamics of the free-electron plasma density around nanoparticles. The plasma dynamics model was coupled with the electromagnetic model through the electric field value and the change in the dielectric function of water due to the free-electron plasma formation. During the nanoparticle laser-pulse interaction, free-electron plasma generation occurs outside the nanoparticle, whereas a nanoparticle with silica nanoshell, free-electron plasma is generated in silica and water. The morphology of the monomer, dimer, and trimer of nanoparticles with silica shell was investigated. To account for the separation by coupling medium molecules or surfactant on the surface of the particles, assemblies of particles were spaced several nanometers (d_g) apart, keeping a strong plasmonic coupling effect. The structure of aluminum nanoparticles is shown in Fig. 1, with d representing the diameter of a nanoparticle, d_s representing the thickness of the silica shell, and d_g representing the distance between adjacent nanoparticles.

Results and Discussions Near-field enhancement of nanoparticle monomer, dimer and trimer, femtosecond laser breakdown threshold, the evolution of the lattice temperature, and water plasma temperature were considered. The relative electric near-field enhancement, |E|/E₀, for different morphologies of Al/SiO₂core/shell nanoparticles in water (Fig. 4) shows the maximum of the relative electric field enhancement for the monomer was 2.23 times. This was located on both sides of the monomer, along the polarization direction (Z-axis) of the incident laser electric field, as shown in Fig. 4 (a1). That of dimer and trimer was 4.23 times and 4.38 times, respectively, as shown in Figs. 4 (b1) and (c1). The maximum electric field lies between nanoparticles and is along the polarization direction of the incident laser electric field, as shown in Figs. 4 (b2) and (c2). The maximum electric field of dimer and trimer was doubled compared with the monomer. This indicated that the near-field enhancement of the polymerized Al/SiO₂ core/shell nanoparticles was stronger. The extinction cross-sections of monomer, dimer, and trimer at the incident wavelength of 200--600 nm are shown in Fig. 6. The resonance peak was 16735 nm ² at 230 nm for trimer, 4433 nm ² at 200 nm for dimer, and 2734 nm ² at 230 nm for monomer. In this example, the extinction cross-sections of the trimer, dimer, and monomer at incident wavelength λ=580 nm were 4115, 209, and 74 nm ², respectively. These were in the non-resonant state. The evolution of plasma electron density for different morphologies of Al/SiO₂ core/shell nanoparticles is shown in Fig. 7. The laser fluence required by plasma electrons to reach the breakdown threshold decreased gradually for monomer, dimer to trimer, due to improvement in the near-field enhancement of nanoparticles. The femtosecond laser breakdown threshold of the pure water was 1428 mJ/cm ², as with the parameters (wavelength and pulse width) in this paper. This indicated that the Al/SiO₂ core/shell nanostructure reduces the required laser intensity. The lattice temperature of different morphologies of Al/SiO₂ core/shell nanoparticles at corresponding laser breakdown fluence of t = 1200 fs is shown in Fig. 8. Similarly, the evolution of lattice temperature of different morphology of Al/SiO₂ core/shell nanoparticles was shown in Fig. 9.

Conclusions The calculation results showed that Al/SiO₂core/shell nanostructure significantly reduces the water breakdown threshold of the femtosecond laser. Also, the near-field enhancement of polymeric nanoparticles was stronger than that of the monomer. Furthermore, the lattice temperature of monomer was higher than the melting point, while that of dimer and trimer was lower. In sum, polymeric Al/SiO₂ core/shell nanostructures are endowed with great potential for applications in fields such as femtosecond laser-induced cavitation in water and cell transfection, among others.