Unveiling The Microstructural Mechanism of Tuning Optical Properties of Densified Silica Glass Via High Pressure And Ultrafast Laser Excitation

01 Introduction

Silica glass exhibits various structural configurations accompanied by volume densification under different temperature and pressure conditions, significantly affecting its macroscopic optical properties, such as refractive index. However, the fundamental microstructural evolution mechanisms behind densification induced by high-pressure high-temperature (HPHT) physical compression and femtosecond laser direct writing (FLDW) photoelectric excitation have not been fully resolved. A deep understanding of the non-equilibrium glass network reorganization process under these extreme conditions is crucial for the flexible design and development of new multi-core fibers, quantum integrated chips, and advanced optoelectronic fusion devices.

02 Overview

This study systematically compared the structural phase transitions and optical responses induced in silica glass by HPHT treatment, FLDW, and a combination of both processes. The study found that while both methods achieve a linear increase in refractive index and macroscopic densification, there are essential differences in the micro-reorganization paths. The core difference is that the glass structure in the laser-irradiated area evolves to a characteristic high fictive temperature (1600–2000 K) state under extreme photothermal effects, accompanied by the generation of non-bridging oxygen (NBO) defects closely related to edge-sharing SiO4​ tetrahedra, thereby inducing photoluminescence behavior distinctly different from simple high-pressure treatment.

03 Figure Analysis

Figure 1 shows the photoluminescence (PL) spectral characteristics of silica glass under different treatment processes. Research shows that after femtosecond laser irradiation, the sample shows significant enhancement in the red light (650 nm) and green light (530 nm) bands, which is closely related to the generation of isolated NBO and self-trapped exciton defects. In contrast, HPHT treatment only enhanced green light emission. This indicates that laser direct writing breaks chemical bonds and promotes the formation of isolated NBO through nonlinear ionization, while high-pressure compression treatment tends to promote the structural relaxation of NBO or enhance its interaction with the surrounding network structure, leading to a significant divergence in optical response.

Figure 2 deeply analyzes the evolution of Si−O−SiSi−O−Si bond angles and network topology in silica glass based on confocal Raman spectroscopy. The study found that HPHT treatment shifts the Raman main frequency band to higher wavenumbers and narrows it, marking physical densification with a narrowed distribution. However, surprisingly, regardless of whether prior high-pressure densification treatment was performed, once femtosecond laser irradiation is applied, the Raman characteristics of all samples eventually cross the original density limit and asymptotically converge to the same specific position. Conversion between Raman parameters and fictive temperature reveals that the local ultrafast heating-quenching effect of the laser always drives the glass structure toward a high fictive temperature non-equilibrium state of 1600–2000 K.

Figure 3 reveals the deep reorganization mechanism at the atomic scale using machine learning molecular dynamics (MLMD) simulations trained on first principles. Local heating (simulating femtosecond laser excitation) not only broadens the distribution of ring structure sizes in the glass network but, more importantly, induces the generation of characteristic edge-sharing SiO4 tetrahedral configurations (i.e., binary ring structures) in both the original and high-density glass models under local heating. This extreme topological deformation directly corresponds to the large increase in non-bridging oxygen observed in experiments, perfectly explaining the unique mechanism of ultrafast laser modification distinct from traditional high-pressure physical reorganization.

04 Conclusion

This paper systematically reveals the similarities and differences between high-pressure physical compression and femtosecond laser direct writing in densified silica glass. Although both can achieve permanent densification and refractive index enhancement, on a microscopic level, electromagnetic-excited ultrafast lasers uniquely drive the glass network evolution toward a high fictive temperature state and locally induce unique micro-crystallographic defects such as bond breaking, NBO, and edge-sharing tetrahedra, which are absent in high-pressure treatment relying on rigid unit mode (RUM) deformation. This discovery not only clarifies the boundaries of non-crystalline network topological deformation under different energy injection modes but also provides a solid theoretical basis for designing new photonic devices through local structural customization in the future.

Original link: https://doi.org/10.1038/s41427-026-00649-4