For the practical use of femtosecond laser ablation, inputs of higher laser intensity are preferred to attain high-throughput material removal. However, the use of higher laser intensities for increasing ablation rates can have detrimental effects on ablation quality due to excess heat generation and air ionization. This paper employs ablation using BiBurst femtosecond laser pulses, which consist of multiple bursts (2 and 5 bursts) at a repetition rate of 64 MHz, each containing multiple intra-pulses (2–20 pulses) at an ultrafast repetition rate of 4.88 GHz, to overcome these conflicting conditions. Ablation of silicon substrates using the BiBurst mode with 5 burst pulses and 20 intra-pulses successfully prevents air breakdown at packet energies higher than the pulse energy inducing the air ionization by the conventional femtosecond laser pulse irradiation (single-pulse mode). As a result, ablation speed can be enhanced by a factor of 23 without deteriorating the ablation quality compared to that by the single-pulse mode ablation under the conditions where the air ionization is avoided.

Femtosecond laser pulses with GHz burst mode that consist of a series of trains of ultrashort laser pulses with a pulse interval of several hundred picoseconds offer distinct features in material processing that cannot be obtained by the conventional irradiation scheme of femtosecond laser pulses (single-pulse mode). However, most studies using the GHz burst mode femtosecond laser pulses focus on ablation of materials to achieve high-efficiency and high-quality material removal. In this study, we explore the ability of the GHz burst mode femtosecond laser processing to form laser-induced periodic surface structures (LIPSS) on silicon. It is well known that the direction of LIPSS formed by the single-pulse mode with linearly polarized laser pulses is typically perpendicular to the laser polarization direction. In contrast, we find that the GHz burst mode femtosecond laser (wavelength: 1030 nm, intra-pulse duration: 220 fs, intra-pulse interval time (intra-pulse repetition rate): 205 ps (4.88 GHz), burst pulse repetition rate: 200 kHz) creates unique two-dimensional (2D) LIPSS. We regard the formation mechanism of 2D LIPSS as the synergetic contribution of the electromagnetic mechanism and the hydrodynamic mechanism. Specifically, generation of hot spots with highly enhanced electric fields by the localized surface plasmon resonance of subsequent pulses in the bursts within the nanogrooves of one-dimensional LIPSS formed by the preceding pulses creates 2D LIPSS. Additionally, hydrodynamic instability including convection flow determines the final structure of 2D LIPSS.

Ultrashort laser pulses confine material processing to the laser-irradiated area by suppressing heat diffusion, resulting in precise ablation in diverse materials. However, challenges occur when high speed material removal and higher ablation efficiencies are required. Ultrafast burst mode laser ablation has been proposed as a successful method to overcome these limitations. Following this approach, we studied the influence of combining GHz bursts in MHz bursts, known as BiBurst mode, on ablation efficiency of silicon. BiBurst mode used in this study consists of multiple bursts happening at a repetition rate of 64 MHz, each of which contains multiple pulses with a repetition rate of 5 GHz. The obtained results show differences between BiBurst mode and conventional single pulse mode laser ablation, with a remarkable increase in ablation efficiency for the BiBurst mode, which under optimal conditions can ablate a volume 4.5 times larger than the single pulse mode ablation while delivering the same total energy in the process.

The GHz burst mode of femtosecond laser pulses can significantly improve ablation efficiency without deteriorating ablation quality. However, various parameters involved in GHz burst mode make it difficult to optimize the processing for practical implementation. In this Perspective, the author gives the history, current status, and future challenges and prospects of this new strategy to answer the question, ‘will GHz burst mode create a new path to femtosecond laser processing?’

This paper presents a new technique, termed femtosecond laser shock peening ablation in liquids (fs-LSPAL), which can realize simultaneous crack micro/nanomanufacturing and hierarchical micro/nanolaser ablation, giving rise to the formation of diverse multiscale hierarchical structures, such as macroporous ratcheted structures and en échelon microfringes decorated with parabolic nanoripples. Through analysis of surface morphologies, many phenomena have been confirmed to take place during fs-LSPAL, including en échelon cracks, nanostriation, ripple densification, crack branching, and selective formation of high spatial frequency laser-induced periodic surface structures of 100–200 nm in period. At a high laser power of 700 mW, fs-LSPAL at scanning speeds of 0.2 mm s⁻¹ and 1 mm s⁻¹ enables the generation of height-fluctuated and height-homogeneous hierarchical structures, respectively. The height-fluctuated structures can be used to induce 'colony' aggregates of embryonic EB3 stem cells. At 200 mW, fs-LSPAL at 1 mm s⁻¹ is capable of producing homogeneous tilt macroporous structures with cracked structures interleaved among them, which are the synergistic effects of bubble-induced light refraction/reflection ablation and cracks. As shown in this paper, the conventional laser ablation technique integrated with its self-driven unconventional cracking under extreme conditions expands the horizons of extreme manufacturing and offers more opportunities for complex surface structuring, which can potentially be used for biological applications.

In this study, we demonstrate a technique termed underwater persistent bubble assisted femtosecond laser ablation in liquids (UPB-fs-LAL) that can greatly expand the boundaries of surface micro/nanostructuring through laser ablation because of its capability to create concentric circular macrostructures with millimeter-scale tails on silicon substrates. Long-tailed macrostructures are composed of layered fan-shaped (central angles of 45°–141°) hierarchical micro/nanostructures, which are produced by fan-shaped beams refracted at the mobile bubble interface (≥50° light tilt, referred to as the vertical incident direction) during UPB-fs-LAL line-by-line scanning. Marangoni flow generated during UPB-fs-LAL induces bubble movements. Fast scanning (e.g. 1 mm s⁻¹) allows a long bubble movement (as long as 2 mm), while slow scanning (e.g. 0.1 mm s⁻¹) prevents bubble movements. When persistent bubbles grow considerably (e.g. hundreds of microns in diameter) due to incubation effects, they become sticky and can cause both gas-phase and liquid-phase laser ablation in the central and peripheral regions of the persistent bubbles. This generates low/high/ultrahigh spatial frequency laser-induced periodic surface structures (LSFLs/HSFLs/UHSFLs) with periods of 550–900, 100–200, 40–100 nm, which produce complex hierarchical surface structures. A period of 40 nm, less than 1/25th of the laser wavelength (1030 nm), is the finest laser-induced periodic surface structures (LIPSS) ever created on silicon. The NIR-MIR reflectance/transmittance of fan-shaped hierarchical structures obtained by UPB-fs-LAL at a small line interval (5 μm versus 10 μm) is extremely low, due to both their extremely high light trapping capacity and absorbance characteristics, which are results of the structures’ additional layers and much finer HSFLs. In the absence of persistent bubbles, only grooves covered with HSFLs with periods larger than 100 nm are produced, illustrating the unique attenuation abilities of laser properties (e.g. repetition rate, energy, incident angle, etc) by persistent bubbles with different curvatures. This research represents a straightforward and cost-effective approach to diversifying the achievable hierarchical micro/nanostructures for a multitude of applications.

The extremely high peak intensity associated with ultrashort pulse width of femtosecond (fs) lasers enabled inducing nonlinear multiphoton absorption in materials that are transparent to the laser wavelength. More importantly, focusing the fs laser beam inside the transparent materials confined the nonlinear interaction to within the focal volume only, realizing three-dimensional (3D) micro/nanofabrication. This 3D capability offers three different processing schemes for use in fabrication: undeformative, subtractive, and additive. Furthermore, a hybrid approach of different schemes can create much more complex 3D structures and thereby promises to enhance the functionality of the structures created. Thus, hybrid fs laser 3D microprocessing opens a new door for material processing. This paper comprehensively reviews different types of hybrid fs laser 3D micro/nanoprocessing for diverse applications including fabrication of functional micro/nanodevices.