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Femtosecond laser machining of materials has transformed into a mature high‐precision technology, giving access to a wide range of applications. Yet, for a complete exploitation of its full potential, a detailed knowledge of the complex laser–matter interaction processes is required. For the case of the application‐relevant material steel, a complete study of the electron dynamics and material transformation upon single laser pulse irradiation (800 nm, 120 fs) is reported. Detailed analysis of the obtained time‐ and fluence‐dependent reflectivity map reveals that ultrafast electron heating during the pulse is directly followed by energy transfer to the lattice within a few picoseconds, reaching fluence‐dependent peak temperatures from below melting threshold to above the boiling point. Moreover, an innovative approach to obtain the ultrafast‐evolving 3D structure of a femtosecond laser excited material is reported, unraveling the dynamics of complex processes as melting, ablation, and solidification. Combined with modeling, the evolving optical properties, multilayer structure, and expansion velocities can be precisely determined. The information obtained from this study will contribute to further increase the achievable precision and wealth of structures that can be produced by designing efficient pulsed energy deposition schemes for selective re‐excitation of the material. [ABSTRACT FROM AUTHOR]

Laser direct writing is a widely employed technique for 3D, contactless, and fast functionalization of dielectrics. Its success mainly originates from the utilization of ultrashort laser pulses, offering an incomparable degree of control on the produced material modifications. However, challenges remain for devising an equivalent technique in crystalline silicon which is the backbone material of the semiconductor industry. The physical mechanisms inhibiting sufficient energy deposition inside silicon with femtosecond laser pulses are reviewed in this article as well as the strategies established so far for bypassing these limitations. These solutions consisting of employing longer pulses (in the picosecond and nanosecond regime), femtosecond‐pulse trains, and surface‐seeded bulk modifications have allowed addressing numerous applications. [ABSTRACT FROM AUTHOR]

B Mid-Infrared Femtosecond Pulse Generation in Hollow-Core Fibers b In article number 2200882, Ang Deng, Wonkeun Chang, and colleagues present a novel approach to generate high-energy femtosecond pulses in mid-infrared from a gas-filled hollow-core fiber. Microjoule-Level Mid-Infrared Femtosecond Pulse Generation in Hollow-Core Fibers (Laser Photonics Rev. 17(6)/2023) The efficient conversion of near-infrared pump to mid-infrared is facilitated by engineering the cross-sectional geometry of cladding elements in the hollow-core fiber. [Extracted from the article]