Hybrid Femtosecond Laser and Ion-Implantation Processing for Controlled, Deep, High-Efficiency Ablation in Fused Silica

Femtosecond laser modification of fused silica enables precise surface tailoring for the fabrication of micro-optical components such as microlenses and diffractive elements. However, the process is governed by laser-matter interactions where the local fluence determines the processing depth, often limiting control over feature geometry and efficiency. Here, we present a hybrid approach combining localized Au implantation (1.8 MeV Au2+ ions) into SiO2 samples with femtosecond laser irradiation (250 fs), effectively tuning the laser-matter interaction and resulting morphology. At both 515 nm and 1030 nm irradiation wavelengths, single-shot femtosecond pulses produce cylindrical craters with sharp edges and flat-bottom profiles. Independently of the fluence, these craters exhibit a constant depth of 550 nm, corresponding to the region of maximum Au concentration. The effect manifests already at moderate fluence (app. 4 J/sq.cm) and yields high ablation efficiency, up to 15 cubic micrometers per microjoule. The hybrid method also works effectively at lower implantation doses that preserve the excellent transmission of fused silica, offering a promising pathway for the high-quality fabrication of flat optical components such as binary phase masks, phase lenses, or fused-silica micromolds.

💡 Research Summary

In this work the authors introduce a hybrid processing technique that combines localized MeV‑scale Au ion implantation with single‑pulse femtosecond laser irradiation to achieve depth‑controlled, high‑efficiency ablation in bulk fused silica. Fused‑silica wafers were implanted with 1.8 MeV Au²⁺ ions at fluences of 1 × 10¹⁵ ions cm⁻² and 1 × 10¹⁶ ions cm⁻². After annealing at 800 °C for five hours, the Au ions formed a Gaussian‑shaped concentration profile peaking at ~595 nm below the surface with a full‑width at half‑maximum of ~260 nm. Rutherford back‑scattering confirmed the depth distribution, while optical spectroscopy showed a pronounced plasmonic absorption peak near 520 nm for the high‑dose sample, whereas the low‑dose sample remained essentially transparent (<2 % absorption).

The laser system delivered 250 fs pulses at either 515 nm (second harmonic) or 1030 nm (fundamental) with spot radii of ~13 µm and ~16 µm, respectively. Single‑shot experiments spanned peak fluences from ~1 J cm⁻² up to 22 J cm⁻². In pristine silica, the resulting craters displayed the familiar quasi‑Gaussian profile whose depth increased with fluence, reaching only 300–380 nm even at the highest fluence. By contrast, both Au‑implanted samples produced cylindrical craters with sharply defined edges and a flat bottom whose depth was essentially constant at ~550 nm, independent of the incident fluence or wavelength. This depth coincides with the depth of maximum Au concentration, indicating that the implanted layer acts as an internal “absorption sheet.”

Quantitative analysis using interferometric microscopy and atomic‑force microscopy revealed crater volumes of 190–290 µm³ for the various conditions, corresponding to an ablation efficiency of up to 15 µm³ µJ⁻¹. Notably, the low‑dose (10¹⁵ ions cm⁻²) sample retained the excellent transmission of bulk silica while still exhibiting the flat‑bottomed crater morphology, making it suitable for optical component fabrication.

The authors propose a physical picture in which Au implantation creates a region of enhanced linear absorption (via plasmonic resonance at 515 nm, color‑center formation, and local densification) and a high density of seed electrons. During the femtosecond pulse, free‑carrier absorption (inverse Bremsstrahlung) and impact ionization are strongly amplified within this region, generating a steep electron‑density gradient that concentrates the deposited energy beneath the surface. Subsequent rapid heating and pressure buildup lead to spallation of the thin surface layer, producing the observed flat‑bottomed crater. This mechanism mirrors the “thin‑film” ablation behavior previously reported for dielectric films on reflective substrates, but here it is realized inside a bulk transparent material.

The hybrid approach offers several advantages: (1) depth control that is decoupled from the incident fluence, (2) markedly higher material removal efficiency compared with conventional femtosecond ablation of silica, (3) preservation of optical quality at low implantation doses, and (4) applicability to both resonant (515 nm) and off‑resonant (1030 nm) wavelengths. Consequently, the technique is well suited for the fabrication of high‑precision flat optical elements such as binary phase masks, phase lenses, and micromolds directly in fused silica, without the need for additional coating or sacrificial layers.

In summary, by engineering an internal Au‑rich absorption layer, the authors transform the interaction of ultrashort laser pulses with fused silica from a fluence‑limited, gradually deepening process into a deterministic, shallow‑to‑moderate depth ablation with cylindrical geometry and high efficiency. This work opens a new pathway for scalable, high‑resolution laser micromachining of transparent dielectrics.