Laser-induced Micro Research Articles

Numerical Simulation to Investigate Interactions of Generated Underwater Micro Shock Waves and Micro Bubbles by Focusing Femtosecond Pulse Laser

The purpose of this study is to elucidate the mechanism of propagation of the laser-induced micro shock waves under condition where the micro bubbles are generated. In this paper, effects of generated micro bubbles on propagation of the laser-induced micro shock waves were investigated by CFD (computational fluid dynamics). Firstly, the two models (1-D model and 1-D spherical symmetric model) were computed for comparison of the peak pressure variation of the shock waves with propagation. As for governing equations for the propagation of the shock waves, continuity equation, Euler’s momentum equation and Tait’s state equation are used. From the computation, it is confirmed that attenuation of pressure of the 1-D spherical symmetric model was earlier than the 1-D model. In addition, the attenuation of the 1-D spherical symmetric model agreed with the laser-induced shock waves obtained experimentally. However, the peak pressure and duration time of the shock wave was not the same as the experimental result. Then, the bubble behavior was included in the computation of the shock wave propagation. As for the bubble behavior, Rayleigh-Plesset equation is used. From this computation, pressure wave was obtained which superposed the pressure of the shock wave on the internal pressure of the bubble. Although the duration time of the pressure wave was close to the experimental result, the value of the pressure was almost the same as atmospheric pressure. It is suggested that there is a possibility that phenomenon other than the bubbles is generated such as plasma when the shock wave is generated by focusing the femtosecond pulse laser

Femtosecond Laser-Induced Anisotropic Structure and Nonlinear Optical Response of Yttria-Stabilized Zirconia Single Crystals with Different Planes.

Nonlinear optical properties have been extensively studied due to their promising nonlinear effects and various applications. With ultrashort duration and ultrahigh intensity, a femtosecond laser can fabricate various superior-quality micro-/nanostructures to improve the nonlinearity of materials, which are promising for stable and high-performance nonlinear devices. In this contribution, yttria-stabilized zirconia (YSZ) with fs laser-induced micro-/nanostructures is demonstrated to exhibit unique anisotropic light-material interaction and nonlinear optical response on [100], [110], and [111] planes. Time-resolved reflectivity of YSZ after fs laser excitation is investigated by a pump-probe experiment, which is consistent with simulations through the plasma model combined with a two-temperature model. These results indicate two early ablation mechanisms: Coulomb explosion and melting. Anisotropic crack structures are formed due to thermal stress, which is always ignored in fs laser fabrication and is verified by Raman mapping and analysis of slip systems on different crystal planes. Through the z-scan measurement, the nonlinear absorption (NLA) of crack structures is effectively improved, and a nearly 18 times enhancement of the NLA coefficient is acquired in [100] samples, while a 2 times enhancement in [110] and [111] samples. Such great enhancement of NLA is mainly due to the abundant presence of crack structures and the increase of fs laser-induced oxygen vacancies in [100] YSZ. These results provide a potential way of utilizing fs laser to improve the nonlinearity for the technological development in nonlinear devices.

Improving Adhesive Bonding of Al Alloy by Laser-Induced Micro–Nano Structures

In this paper, laser surface treatment of Al alloy was studied by comparison of sandblasting and laser ablation. The effects of laser spot distribution on surface roughness, contact angle, chemical composition and shear strength of Al alloy were analyzed. The experimental results showed that the bonding performance of Al alloy are promoted by controlling the laser spot spacing. When laser spot spacing was smaller than the size of the laser spot, micro–nano composite structures were induced by multiple laser pulses, which would improve shear strength by increasing the contact area between Al alloy and adhesive. Using laser surface treatment, the adhesion properties on Al alloy can be promoted.

Dry etching of monocrystalline silicon using a laser-induced reactive micro plasma

Dry etching is a prevalent technique for pattern transfer and material removal in microelectronics, optics and photonics due to its high precision material removal with low surface and subsurface damage. These processes, including reactive ion etching (RIE) and plasma etching (PE), are performed at vacuum conditions and provide high selectivity and vertical side wall etched patterns but create high costs and efforts in maintenance due to the required machinery.In contrast to electrically generated plasmas, laser-induced micro plasmas are controllable sources of reactive species in gases at atmospheric pressure that can be used for dry etching of materials.In the present study, we have demonstrated the laser-induced plasma etching of monocrystalline silicon. A Ti:Sapphire laser has been used for igniting an optically pumped plasma in a CF4/O2 gas mixture near atmospheric pressure. The influence of process parameters, like substrate temperature, O2 concentration, plasma-surface distance, etching duration, pulse energy and crystal orientation on etching rate and surface morphology has been investigated.Typical etching rates of 2–12 µm x min−1 can be achieved by varying mentioned parameters with a decreasing etching rate during the process. Different morphologies can be observed due to the parameters set, smooth as well as rough surfaces or even inverted pyramids.The presented etching method provides an approach for precise machining of silicon surfaces with good surface qualities near atmospheric pressure and sufficiently high material removal rates for ultraprecise surface machining.

Control of Generating Femtosecond Laser-Induced Micro Shock Wave and Bubbles for Development of Regenerative Medical System

Control of Generating Femtosecond Laser-Induced Micro Shock Wave and Bubbles for Development of Regenerative Medical System

Flexible control over optical reflection property of metallic surfaces via pulse laser

The control of fundamental optical properties, such as transmission and reflection, over metallic surfaces plays a significant role in multiple fields like solar cells and aerospace. The direct laser etching in air can perform a variety of flexible control abilities in optical functional metal surfaces. In this paper, the authors use the aluminum alloy as an example of widely metal materials and propose two kinds of controlling strategies: large-range controlling strategy and small-range precise controlling strategy. The former changes the process repeat number of femtosecond lasers or the scanning speed of nanosecond lasers. The latter combines the femtosecond laser and the nanosecond laser. The results show that when the process repeat number of femtosecond lasers is changed from 30 to 1, the height of the induced micropillars on the surface of Al alloy is changed from 80 to 6 μm. As a result, the reflectivity of samples will change from ∼16% to ∼87%. The nanosecond laser-induced micro-/nanostructures achieve the reflectivity changing from ∼18% to ∼79% when the scanning speed changes from 10 to 400 mm/s in a spectrum range of 250–2000 nm. What is more, after being fabricated in the way of two-step controlling strategy, the reflectivity raises from ∼30% to ∼40% when the scanning speed of nanosecond lasers changes from 10 to 200 mm/s, while the reflectivity of samples fabricated simply by nanosecond laser changes from ∼18% to ∼66% when the scanning speed changes in the same range and other parameters are kept the same. In addition, the authors demonstrated that the two-step precise controlling strategy is applicable to a variety of metals such as copper, stainless steel, and titanium.

Tailoring metallic surface properties induced by laser surface processing for industrial applications

As a simple, reproducible, and pollution-free technique with the potential of integration and automation, laser processing has attracted increasing attention. Laser processing, which includes laser polishing, laser cleaning, and fabrication of laser-induced micro-/nano-structures, has been demonstrated to yield smooth, clean, functional surfaces and effective joining. Laser polishing is an advanced, highly efficient, and ecofriendly polishing technology. This study demonstrated the laser polishing of a selective laser-melted Inconel 718 (IN718) superalloy and a titanium alloy sample. The surface roughnesses Ra and Rz of the IN718 superalloy were respectively reduced from 8 and 33 μm to 0.2 and 0.8 μm, and the Ra of the titanium alloy was reduced from 9.8 μm to 0.2 μm. Moreover, the wear resistance and corrosion resistance of the IN718 were apparently improved. As another surface-related processing method, laser cleaning was used to clean terminal blocks. Almost all the contaminants were removed, as verified by the absence of their chemical compositions and the decreased surface roughness. In addition, a superhydrophobic surface with a contact angle of over 160° and sliding angle of <8° on stainless steel was obtained by laser texturing treatment. These results demonstrate the high potential of laser processing in the scientific, technological, and industrial fields.

Relative spectral response calibration using Ti plasma lines

This work introduces the branching ratio (BR) method for determining relative spectral responses, which are needed routinely in laser induced breakdown spectroscopy (LIBS). Neutral and singly ionized Ti lines in the 250–498 nm spectral range are investigated by measuring laser-induced micro plasma near a Ti plate and used to calculate the relative spectral response of an entire LIBS detection system. The results are compared with those of the conventional relative spectral response calibration method using a tungsten halogen lamp, and certain lines available for the BR method are selected. The study supports the common manner of using BRs to calibrate the detection system in LIBS setups.

Laser-induced surface modification of biopolymers – micro/nanostructuring and functionalization

The medical-grade polydimethylsiloxane (PDMS) elastomer is a widely used biomaterial in medicine for preparation of high-tech devices because of its remarkable properties. In this paper, we present experimental results on surface modification of PDMS elastomer by using ultraviolet, visible, and near-infrared ns-laser system and investigation of the chemical composition and the morphological structure inside the treated area in dependence on the processing parameters – wavelength, laser fluence and number of pulses. Remarkable chemical transformations and changes of the morphological structure were observed, resulting in the formation of a highly catalytically active surface, which was successfully functionalized via electroless Ni and Pt deposition by a sensitizing-activation free process.The results obtained are very promising in view of applying the methods of laser-induced micro- and nano-structuring and activation of biopolymers’ surface and further electroless metal plating to the preparation of, e.g., multielectrode arrays (MEAs) devices in neural and muscular surface interfacing implantable systems.

Laser-plasma induced shock waves in micro shock tubes

Shock wave generation with help of lasers or shock tubes, for example, is a common subject at macroscopic scale. On the other hand, the current tendency towards smaller scales becomes more and more important. In particular, shock waves in small channels or tubes with sub-mm or micron-sized diameter have attracted much interest within the last years. But downscaling of shock wave effects such as pressure decrease and Mach number attenuation during propagation, viscous and heat effects, laminar and turbulent flow is not necessarily straightforward from macro to micro or even nano range. Although several theoretical investigations are available in this new field, there is a strong demand on experiments, which are mostly missing due to the lack of suitable methods. The present work introduces a novel method for the generation of shock waves at microscale, namely laser-induced micro shock waves (LIMS). The LIMS method applies a femtosecond laser to induce an optical breakdown in a thin aluminum target located at the entrance of a micro tube. Subsequently, a shock wave is launched by the high pressure aluminum plasma and starts propagating into the tube. The topical work presents, for the first time, experimental investigations on direct micro shock wave generation and propagation at well-defined conditions in micron-sized tubes. They are performed for different conditions and tubes down to 50 μm diameter. Different from previous shock wave investigations in the mm-diameter range that involve pressure transducers, the present work applies non-contact measurements by optical methods. The experiments are supported by additional simulations. A one-dimensional numerical hydrocode is applied to simulate the shock wave generation process. Further propagation of the shock in a micro tube is analyzed by solving two-dimensional Navier–Stokes equations. Both simulations agree well with the experimental results.

Formation Dynamics of Ultra-Short Laser Induced Micro-Dots in the Bulk of Transparent Materials

In this paper, we study the formation dynamics of ultra-short laser-induced micro dots under the surface of transparent materials. Laser-induced micro dots find their application in direct part marking, to address full life cycle traceability. We first demonstrate the possibility of direct laser part marking into the cladding of an optical fiber. Then, we monitor the laser affected zone with the help of a time-resolved phase contrast microscopy setup in a fused silica substrate. We show that the transient energy relaxation processes affect the host material over a region that exceeds the micro dot size by several micrometers.

Development of an opto-hydrodynamic process to remove nanoparticles from solid surfaces

We developed a surface cleaning process to remove nanoscale contaminants as small as 10nm from solid surfaces using a laser-induced micro liquid jet. In the process, laser-induced breakdown of a micron-sized water droplet (∼10nl) produces a high-speed jet with speeds up to 1600m/s liquid jet via an explosive vaporization process. Impingement of the liquid jet with atomized droplets on the contaminated substrate removes the nanoparticles under the hydrodynamic drag forces. The process parameters, including the incident laser energy and the position of the droplet relative to the laser focus, were optimized to maximize the cleaning power. Polystyrene latex particles 20, 30, or 90nm in diameter and Al2O3 particles 10–50nm in diameter were used to demonstrate the cleaning performance of the opto-hydrodynamic cleaning technique. The proposed cleaning process is expected to be useful for selectively cleaning local areas with minimal exposure to water.

Optimization of CMP Pad Surface by Laser Induced Micro Hole

In this paper, the optimization of the chemical mechanical planarization (CMP) pad design is investigated to reduce defect generation during the CMP process. Recently, obtaining the optimum performance from the viewpoint of scratch generation has become the most challenging area in the CMP process. To achieve uniform performance of the process at the defect level, the surface texture and/or groove of the CMP pads should be designed effectively, and their original surface should be maintained throughout their lifetime. Because the transport of slurry and wear debris, which is the source of CMP scratches, is highly dependent on the surface geometry of the CMP pad, maintaining suitable process conditions is considered the most important design factor. In this work, the surface geometries of two CMP pads with different body structures are modified by a laser-induced micro hole; one is a porous-type pad and the other is a nonporous type. Based on the experimental and numerical simulation results, the micro-hole induced pads had a lower defect level than the conventional porous pads without holes because the micro holes acted as a defect source or coarse particle reservoir to prevent micro scratch during the process. In addition, the micro holes of the porous pad showed a lower defect generation than the holes of the nonporous pad as the process time increased.

Bone response to laser-induced micro- and nano-size titanium surface features

This study explored whether laser-induced, site-specific implant surface modifications with micro- and nano-scale topography were able to promote bone formation. The aim was to evaluate the biomechanical and histological response to partly laser-modified titanium implants in comparison with machined implants. After an early 8-week healing period in rabbit tibia and femur, a 250% increase in removal torque was demonstrated for the partly laser-modified surface. Further, different fracture mechanisms were demonstrated for the two surfaces. Histologically, significantly more bone was found in direct contact with the laser-modified surface for the implants in the tibia sites, and a similar amount of bone tissue was observed in contact with the implant in the femoral sites. In conclusion, an improved bone-implant interface anchorage was promoted by an increase in micro- and nano-scale implant surface topography and surface oxide induced by topological laser treatment. From the Clinical Editor Nanosized grooves in titanium implants markedly improve bone-implant anchorage by increasing the amount of bone formed in direct contact with the metal prosthesis.

Microvoid channel polymer photonic crystals with large infrared stop gaps and a multitude of higher-order bandgaps fabricated by femtosecond laser drilling in solid resin

Photosensitive polymer materials are ideally suited for laser-induced micro- and nanostructuring, as structural and compositional changes are achieved already under exposure to moderate intensities of high-repetition rate ultrashort-pulsed light. Photonic crystals with bandgaps in the infrared or the visible spectral region are a particularly interesting application, because highly correlated structural elements at a size of only a few hundred nanometers are required. We fabricated infrared photonic crystals based on microvoid channels inside solid polymer material. Femtosecond-pulsed visible light was focused into UV-cured Norland NOA63 resin by a high numerical aperture objective. In the focal spot microexplosions drive the material out of the center of the focus. Void channels of 0.7–1.3 μm diameter are generated by translating the sample along a preprogrammed pathway. Woodpile structures of void channels at layer spacings of 1.6–2.6 μm and in-plane channel spacings of 1.2–1.3 μm allowed for bandgap-induced suppression of infrared transmission in the stacking direction of as much as 86% by only 20 layers. As these structures are highly correlated and do not contain many imperfections, up to three higher-order stop gaps are observed. Consistent with theory, the number and gapwidth of higher-order gaps strongly increases with the ratio between layer- and in-plane spacing. Due to their low refractive index contrast and the missing interconnectivity of voids our structures do not provide complete photonic bandgaps. However, their manifold of sizable higher-order gaps allows for the engineering of photonic stop gaps down to the near-infrared wavelength region using comparatively large structural dimensions.