Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays

Experimental investigation on enhanced ablation of silicon with the combined pulse laser

The combined pulse laser (CPL) based on long-pulsed lasers has been proven to be an effective way of improving laser processing efficiency by combining their processing advantages. In this Letter, a CPL with a 500 mJ millisecond pulse and a 150 mJ nanosecond pulse is utilized to study the interaction process between laser and silicon. Based on high-speed images of plasma distribution and laser supported detonation wave (LSDW), as well as the height characteristics of ablation morphology, the energy coupling of plasma with variable delay time is analyzed. The irradiation of the material by millisecond pulse within the delay time enhances the strong coupling process between nanosecond pulse and the target.

Advances in Laser Drilling of Structural Ceramics

The high-quality, high-efficiency micro-hole drilling of structural ceramics to improve the thermal conductivity of hot-end parts or achieve high-density electronic packaging is still a technical challenge for conventional processing techniques. Recently, the laser drilling method (LDM) has become the preferred processing tool for structural ceramics, and it plays an irreplaceable role in the industrialized processing of group holes on structural ceramic surfaces. A variety of LDMs such as long pulsed laser drilling, short pulsed laser drilling, ultrafast pulsed laser drilling, liquid-assisted laser drilling, combined pulse laser drilling have been developed to achieved high-quality and high-efficiency micro-hole drilling through controlling the laser–matter interaction. This article reviews the characteristics of different LDMs and systematically compares the morphology, diameter, circularity, taper angle, cross-section, heat affect zone, recast layer, cracks, roughness, micro–nano structure, photothermal effect and photochemical reaction of the drilling. Additionally, exactly what processing parameters and ambient environments are optimal for precise and efficient laser drilling and their recent advancements were analyzed. Finally, a summary and outlook of the LDM technology are also highlighted.

Fabrication of Planar Microelectrode Array Using Laser-Patterned ITO and SU-8

For several decades, microelectrode array (MEA) has been a powerful tool for in vitro neural electrophysiology because it provides a unique approach for monitoring the activity of a number of neurons over time. Due to the various applications of MEAs with different types of cells and tissues, there is an increasing need to customize the electrode designs. However, the fabrication of conventional MEAs requires several microfabrication procedures of deposition, etching, and photolithography. In this study, we proposed a simple fabrication method with a laser-patterned indium tin oxide (ITO) conductor and SU-8 photoresist insulation. Unlike in a conventional metal patterning process, only the outlines of ITO conductors are ablated by laser without removing background ITO. Insulation is achieved simply via SU-8 photolithography. The electrode sites are electroplated with iridium oxide (IrO_X) to improve the electrochemical properties. The fabricated MEAs are electrochemically characterized and the stability of insulation is also confirmed by impedance monitoring for three weeks. Dissociated neurons of rat hippocampi are cultured on MEAs to verify the biocompatibility and the capacity for extracellular neural recording. The electrochemical and electrophysiological results with the fabricated MEAs are similar to those from conventional SiN_X-insulated MEAs. Therefore, the proposed MEA with laser-patterned ITO and SU-8 is cost-effective and equivalently feasible compared with the conventional MEAs fabricated using thin-film microfabrication techniques.

Hybrid laser precision engineering of transparent hard materials: challenges, solutions and applications

Laser has been demonstrated to be a mature and versatile tool that presents great flexibility and applicability for the precision engineering of a wide range of materials over other established micromachining techniques. Past decades have witnessed its rapid development and extensive applications ranging from scientific researches to industrial manufacturing. Transparent hard materials remain several major technical challenges for conventional laser processing techniques due to their high hardness, great brittleness, and low optical absorption. A variety of hybrid laser processing technologies, such as laser-induced plasma-assisted ablation, laser-induced backside wet etching, and etching assisted laser micromachining, have been developed to overcome these barriers by introducing additional medium assistance or combining different process steps. This article reviews the basic principles and characteristics of these hybrid technologies. How these technologies are used to precisely process transparent hard materials and their recent advancements are introduced. These hybrid technologies show remarkable benefits in terms of efficiency, accuracy, and quality for the fabrication of microstructures and functional devices on the surface of or inside the transparent hard substrates, thus enabling widespread applications in the fields of microelectronics, bio-medicine, photonics, and microfluidics. A summary and outlook of the hybrid laser technologies are also highlighted.

Laser processing of alumina ceramic by a spatially superposing millisecond laser and a nanosecond laser with different beam shapes

Advanced combined pulse laser (CPL) processing technology with high processing efficiency is of interest for both academic and engineering prospects. However, the influence of the spatial superposition of the CPL on the processing quality is unclear. Here, we use a CPL composed of a nanosecond and millisecond laser with different beam shapes to drill alumina ceramic. Experimental and simulation results suggest that the CPL drilling process actively homogenizes the laser in the hole through multi-reflection of the laser, and thus holes with high circularity are obtained without the influence of the beam shape of the nanosecond laser. The research shows this to be a novel processing method, and that the processing quality is independent of the laser beam shape.

Experimental study of different pulse delays on the phenomenon of double shock waves induced by a millisecond-nanosecond combined-pulse laser

We study the motion morphology, distance, and velocity of plasma and laser-induced shock waves induced by a millisecond-nanosecond (ms-ns) combined-pulse laser with different pulse delays on silicon. The laser shadowgraph method is used, and the phenomenon of double laser-induced shock waves has been found while the pulse delay is 1.2-1.8 ms. The controlling variable method is used to study this phenomenon, and it is found that it is mainly related to the ignition of the laser-supported absorption wave induced by the ms laser. Moreover, the plasma expansion velocity increases with the increase of pulse delay, the axial propagation distance of laser-induced shock waves increases monotonically with pulse delay, and the velocity of laser-induced shock waves decreases with the increase of pulse delay.

Nanosecond-millisecond combined pulse laser drilling of alumina ceramic

A nanosecond-millisecond combined pulse laser (CPL) drilling method was proposed for drilling alumina ceramic. The total energy consumption of the CPL drilling was 1/7 of that of a conventional millisecond laser, and the drilling quality was better. The simulation results demonstrated that, due to the nonuniform reflection of the millisecond laser in the keyhole, the ellipse keyhole ablated by the off-axis incident nanosecond pulses had no effect on the circularity of the through hole. In addition, the multireflection of the laser in the keyhole enhanced the absorption, so the keyhole ablated by the nanosecond pulses could be used as a target for limiting the absorption of the subsequent millisecond pulses. In this context, the keyhole could be used to reduce the hole diameter if the subsequent millisecond laser had a bigger spot size, and this CPL drilling method could be used as an effective group hole drilling method.

Laser processing of alumina ceramic by spatially and temporally superposing the millisecond pulse and nanosecond pulse train

A novel combined laser pulses (CLPs) consisting of a millisecond (ms) pulse and an assisted nanosecond (ns) pulse train was proposed for drilling alumina ceramic. The processing efficiency and quality were well improved by spatially and temporally superposing the ms and ns laser beams. As a result, due to the multi-reflection of keyhole and ejection of melt, the temporally superposed CLPs could decrease the energy consumption of the drilling by an order of magnitude compared with the conventional ms pulse. On the other hand, the spatial distribution of the ns laser on the focal plane was elliptical due to the off-axis distortion of the optical system. However, since the reflection of the laser in the keyhole was non-uniform, the spatially superposed CLPs showed no dependence on the shape of the focused elliptical ns laser spot in terms of the drilling quality. The research results have an important guiding for improving the efficiency and quality of laser processing, especially for the alumina ceramic laser processing.

Experimental study of the morphological evolution of the millisecond-nanosecond combined-pulse laser ablation of aluminum alloy

A comparative study of the interactions of the millisecond-nanosecond combined-pulse laser and millisecond pulse laser with aluminum alloy is presented. During the interaction between the laser and aluminum alloy, the coupling effect of the nanosecond pulse laser on the molten pool formed by the millisecond laser is analyzed. During the nanosecond laser irradiation in the combined approach, the expansion velocity of the plasma plume reaches 600 m/s, while the laser ablation depth increased by approximately 9 times. Based on experiments, the high-speed shadow imaging method, correlation of the temperature evolution characteristics, and ablation morphology are considered for a comprehensive analysis. The coupling effect between the millisecond-nanosecond combined-pulse laser and aluminum-alloy molten pool phase is explained.

Surface damage induced by a combined millisecond and nanosecond laser

The surface damage morphologies of single-crystal silicon induced by a combined pulse laser (CPL) and a single millisecond laser are investigated, respectively. The CPL includes a millisecond (ms) laser superposed by a nanosecond (ns) laser. Inspected by an optical microscope, it was found that the surface damage was more serious when the sample was irradiated by the CPL than by a single ms laser with the same incident laser energy. Besides surface cleavage, obvious ablation and fold areas were discovered by CPL irradiation. A two-dimensional spatial axisymmetric model was established to assess the difference between single ms laser and CPL irradiation and explain the generation mechanism of the different surface damage. This was attributed to the preheating effect by the ms laser and the surface damage caused by the ns laser.

Chair-like pulses in an all-normal dispersion Ytterbium-doped mode-locked fiber laser

We report, for what we believe is the first time, generation of stable chair-like pulses (a pulse shape with an initial long flat portion followed by a short high peak power portion resembling the shape of a chair) by mode locking of a Ytterbium (Yb)-doped fiber laser. Chair-like pulse shapes are achieved by implementing dual saturable absorbers, one based on a nonlinear optical loop mirror (NOLM) and the other based on nonlinear polarization rotation (NPR) inside the cavity. The transmission characteristics of the NOLM-NPR pair leading to the formation of chair-like pulses are numerically investigated. We also report the amplification characteristics of chair-like pulses in an external multistage Yb-doped fiber amplifier setup at different repetition rates of the pulse train. It was found that the chair-like pulses are suitable for amplification, and more than 10 W of average power at 460 kHz repetition rate have been obtained at total pump power of ∼20  W coupled to the power amplifier. At a lower repetition rate (115 kHz), ∼8  W of average power were obtained corresponding to ∼70  μJ of pulse energy with negligible contribution from amplified spontaneous emission or stimulated Raman scattering. We believe that such an oscillator-amplifier system could serve as an attractive tool for micromachining applications.