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Abstract This study examines burst laser-induced pitting (BLIP), an understudied surface modification phenomenon driven by ultrafast laser bursts with sub-picosecond to picosecond inter-pulse delays. Through SEM and AFM analysis, we characterize BLIP as sub-micron pits with polarizationdependent oval shapes, alongside high-fluence melting zones and localized ripple-like structures. Unlike conventional LIPSS, BLIP demonstrates exceptional energy coupling efficiency, evidenced by 10× greater damage areas and a steeper fluence-scaling expansion rate than LIPSS, attributed to transient carrier-mediated processes. Pit density decays exponentially with delay (τ ≈ 6.6-8.9 ps), matching the timescale of self-trapped exciton (STE) relaxation, while spatial statistics reveal a delay-driven transition from field-guided ordering (1-5 ps) to randomized distributions (>10 ps). The resonant-like angular distributions and delay-dependent ellipticity reduction indicate competing mechanisms: optical field enhancement dominates at short delays, while energy dissipation and structure disordering prevail at longer delays. Simulation of nanoplasma excitation reveals near-field optical field enhancements responsible for the ellipticity and ripple-like structures. Beyond their fundamental significance, these BLIP nanostructures offer practical functionalities, including use as anti-reflection coatings and hydrophobic surfaces. These findings establish BLIP as a new paradigm in ultrafast laser-material interactions, where burst parameters selectively activate defect-mediated or field-driven modification pathways in dielectrics. 

Abstract The creation of localized bulk modification using femtosecond pulses inside semiconductors like silicon (Si) is quite challenging, whereas it is not difficult to achieve it for dielectric materials like fused silica (FS). This report addresses the fundamental origin of this issue. By taking a simple numerical approach, it has been found that in FS we can deliver stronger fluence due to self-focusing at higher power levels compared to Si. The origin for the above lies in the spatio-temporal pulse-splitting behavior, which is dominant in the case of FS at the focus, whereas, for Si, it is only effective after focus. We have also considered the influence of plasma and Kerr terms to elucidate the reason behind these nonlinearities. For the FS case, omission of Kerr term dominates, whereas, for Si, the influence of each term does not significantly create self-focusing like FS under a similar focusing condition. This study could provide an important guideline for researchers to understand the complexity of laser-matter interaction in transparent materials specifically being studied by many laser-processing industries. 

The nature of structural changes of nanosecond laser modification inside silicon is investigated. Raman spectroscopy and transmission electron microscopy measurements of cross sections of the modified channels reveal highly localized crystal deformation. Raman spectroscopy measurements prove the existence of amorphous silicon inside nanosecond laser induced modifications, and the percentage of amorphous silicon is calculated based on the Raman spectrum. For the first time, the high-resolution transmission electron microscopy images directly show the appearance of amorphous silicon inside nanosecond laser induced modifications, which corroborates the indirect measurements from Raman spectroscopy. The laser modified channel consists of a small amount of amorphous silicon embedded in a disturbed crystal structure accompanied by strain. This finding may explain the origin of the positive refractive index change associated with the written channels that may serve as optical waveguides. 

The concept for fabrication of waveguides by an in‐volume laser direct writing in single‐crystal silicon is explored using a nanosecond pulse laser. The key innovation of this technology relies on the generation of amorphous silicon, which has a higher refractive index than that of crystalline silicon. Herein, transmission electron microscopy (TEM) together with selected area electron diffraction (SAED) and high‐resolution TEM (HRTEM) characterizations are used to better understand the microstructural evolutions. TEM images reveal the core‐shell structures, while SAED patterns and HRTEM directly observe the presence of amorphous silicon in the core surrounded by a crystalline silicon shell. With a lower laser scanning speed, a higher density of defects yet less amorphous silicon is formed by laser direct writing.