MANUFACTURING with short pulse lasers FOCUS a dense free electron plasma within the focal volume. Achieving the critical plasma density(~ 10 21 cm-3) necessary for optical breakdown typically requires intensities on the order of 10 TW · cm-3, corresponding to a 100 nJ, 100 fs near infrared pulse tightly focused to a micrometer-scale volume. The ultrafast timescale of the laser pulse decouples the initial energy deposition from subsequent lattice heating, as electron-phonon coupling transfers energy to the lattice at the picosecond timescale. As a result, the energy is rapidly confined to a highly localized region, leading to extreme temperature increase( up to several 1000 ° C). The relaxation of thermal stress within 0.1 – 1 ns generates shock waves with pressures ranging from 0.1 to 10 GPa, while thermal diffusion and re-solidification of the molten glass occur on microsecond timescales [ 2 ].
In the multiple-pulse regime, the repetition rate plays a critical role in determining the material response. At low repetition rates( typically < 100 kHz), where the pulse separation exceeds the thermal diffusion time, each pulse interacts with a cooled modified material, potentially leading to incubation effects that might lower the modification threshold. Conversely, at high repetition rates(> 100 kHz), cumulative heating induces out-of-focus thermal modifications, triggering complex and rich thermodynamical and viscoelastic processes. These include melting, temperature gradient-driven material redistribution, and rapid quenching, which can result in densification, mechanical stresses, the formation of defects and colored centers, elemental migration and local phase transition.
The resulting bulk modifications in glasses span a wide spectrum of 3D-localized microstructural changes leading to a broad variety of optical contrasts, depending on the irradiation conditions and glass compositions. These range from low and smooth refractive index modifications to anisotropic birefringent nanograting structures, and highly scattering micro-voids.
Laser-induced formation of new chemical species, space-selective crystallization, nanopores or dielectric / metallic nanoparticles growth further diversify the achievable optical properties that extend beyond linear and passive responses, including, for example, enhanced luminescence, second harmonic generation, magneto-optical responses or giant optical rotation, to cite a few.
ADVANCED PHOTONIC ARCHITECTURES Glass nano-micro-modifications can be distributed in 3D either by moving the sample or by scanning the beam focus position allowing for the versatile prototyping of embedded multi-scale and multi-properties photonic architectures for innovative bulk optics elements, on-chip photonic integrated circuits( PICs) or fibered components( see Fig. 2).
For example, laser structuring of homogeneous smooth refractive index changes has been widely
Figure 1. Illustration of the laser-glass interaction mechanisms with typical timescales.
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