Ultrafast Laser Processing Research Articles

Nanostructured electrically conductive hydrogels obtained via ultrafast laser processing and self-assembly.

Electrically conductive polymers have emerged as functional materials for future electronics due to their high electrical conductivity, real-time responsiveness, easy film-formation ability and desirable stretchability. However, the previously developed conductive polymer electronics are still limited to macroscopic hydrogels or films without complicated designs of fine features. Herein, a carbon nanotube-doped hydrophilic photoresist was ultrafast laser processed as an absorbent 3D scaffold to fabricate nanostructured electrically conductive hydrogels (NECHs) for the first time. Taking advantage of the intermolecular forces, we in situ interpenetrated π-conjugated poly(3,4-ethylenedioxythiophene) into NECHs by self-assembly to combine fine features (resolution down to 500 nm, at least two-order accuracy improvement than that in the case of standard 3D-printed electronics) and achieve a high electrical conductivity (0.1-42.5 S m-1), device-level mechanical properties and desirable tolerance to humid/acid environments. Consequently, several reliable, nanostructured, metal-free electrical circuits, alcohol micro-sensors, interdigital capacitors, and loop inductors have been experimentally identified and characterized. The NECHs successfully break current limitations by making better use of the two photon hydrogelation and highly conductive polymer. Optical clarity, conductivity, and extensibility of the NECHs promise their applications in micro energy storage devices, epidermal electronics, nanorobotics and electrical circuit boards for challenging conditions.

Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

This review is focused on the realization of liquid-repellent surfaces, inspired by two biological models: “dry” superhydrophobic leaves and “slippery” liquid-repellent carnivorous plants using ultrafast laser processing. After a short introduction to a biomimetic development process, an overview of the laser-fabricated structures, which were intensively used for the realization of biomimetic “dry” and “slippery” liquid-repellent surfaces, is given. The influence of process parameters on the structure morphology is discussed. A summary of superhydrophobic and liquid-repellent modifications of different materials (metals, semiconductors, and polymers), including wettability characteristics and processing details, is provided. The technological applications of laser-structured liquid-repellent surfaces are discussed.

Sub-wavelength self-organization of chalcogenide glass by direct laser writing

Chalcogenide glasses (GhGs) are quoted as a key material for the development of infrared technologies due to its unique optical properties. The right choice of processing technique is crucial to reach progress in this field, especially when achieving the nanoscale, to further comprise new functionalities to ChGs. Herein, we demonstrated the sub-wavelength self-organization of ChG with hierarchical nano/micro control by using ultrafast laser processing, representing an important step towards the fabrication of photonic devices of tailored architectures. Particularly, those findings are demonstrated through laser induced forward transfer with femtosecond pulses applied to As2S3 thin films, which enabled the controlled deposition of nanostructures within a micrometric region (with resolution on the order of 3 μm). The self-organized patterns exhibits periodicity with subwavelength dimensions on the order of λ/4, with λ ∼800 nm. In the best of our knowledge, the investigation addressed herein correspond to the first demonstration of self- organization of ChG using direct laser writing, extending the applicability of the method for materials nanostructuring.

Antireflection Surfaces for Biological Analysis Using Laser Desorption Ionization Mass Spectrometry.

Laser desorption ionization mass spectrometry (LDI-MS) is a primary tool for biological analysis. Its success relies on the use of chemical matrices that facilitate soft desorption and ionization of the biomolecules, which, however, also limits its application for metabolomics study due to the chemical interference by the matrix compounds. The requirement for sample pretreatment is also undesirable for direct sampling analysis or tissue imaging. In this study, antireflection (AR) metal surfaces were investigated as sample substrates for matrix-free LDI-MS. They were prepared through ultrafast laser processing, with high light-to-heat energy conversion efficiency. The morphology and micro/nanostructures on the metal surfaces could be adjusted and optimized by tuning the laser fabrication process. The super-high UV absorption at 97% enabled highly efficient thermal desorption and ionization of analytes. The analytical performance for the matrix-free LDI was explored by analyzing a variety of biological compounds, including carbohydrates, drugs, metabolites, and amino acids. Its applicability for direct analysis of complex biological samples was also demonstrated by direct analysis of metabolites in yeast cells.

Ultrafast laser processing of silicon for photovoltaics

ABSTRACTThe photovoltaics market has been growing rapidly in the past decade or so, driven by policy support, growing economies of scale, and technological improvements. Continued advances in photovoltaics manufacturing and technologies may drive further cost reductions and facilitate market growth going forward. Here, we review one such potential advance: the use of ultrafast laser processing in silicon photovoltaic production. We provide an overview of the current major capabilities of ultrafast laser processing of silicon, including texturing, hyperdoping, and combined texturing and hyperdoping. We describe each process, survey recent advances, compare to alternative methods, and report the state-of-the-art of each process in relation to photovoltaic devices. We also discuss the major challenges facing each process. We close with a prospectus for research and applications. We conclude that there are no major technical obstacles to the application of ultrafast laser texturing to photovoltaics manufacturing currently, while ultrafast laser hyperdoping requires further research and development before adoption. We also show that the use of hyperdoping for intermediate band silicon photovoltaics likely requires concurrent surface texturing or other absorption-enhancement techniques to yield photoconversion efficiency improvements.

Crossing the threshold of ultrafast laser writing in bulk silicon

An important challenge in the field of three-dimensional ultrafast laser processing is to achieve permanent modifications in the bulk of silicon and narrow-gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects in the pre-focal region. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating the conditions for an interaction with an extreme numerical aperture near 3 in a perfect spherical sample, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved inside silicon. This opens the door to the direct writing of three-dimensional monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.

In-Situ Real-Time Focus Detection during Laser Processing Using Double-Hole Masks and Advanced Image Sensor Software.

In modern high-intensity ultrafast laser processing, detecting the focal position of the working laser beam, at which the intensity is the highest and the beam diameter is the lowest, and immediately locating the target sample at that point are challenging tasks. A system that allows in-situ real-time focus determination and fabrication using a high-power laser has been in high demand among both engineers and scientists. Conventional techniques require the complicated mathematical theory of wave optics, employing interference as well as diffraction phenomena to detect the focal position; however, these methods are ineffective and expensive for industrial application. Moreover, these techniques could not perform detection and fabrication simultaneously. In this paper, we propose an optical design capable of detecting the focal point and fabricating complex patterns on a planar sample surface simultaneously. In-situ real-time focus detection is performed using a bandpass filter, which only allows for the detection of laser transmission. The technique enables rapid, non-destructive, and precise detection of the focal point. Furthermore, it is sufficiently simple for application in both science and industry for mass production, and it is expected to contribute to the next generation of laser equipment, which can be used to fabricate micro-patterns with high complexity.

Large-Scale Tunable 3D Self-Supporting WO3 Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting.

Hydrogen production from water based on photoelectrochemical (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO3 micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidation. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO3 nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent density of 1.2 mA cm-2 at 1.0 V vs Ag/AgCl (1.23 V vs RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidation conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective, and controllable for large-scale fabrication, which should provide a new and general route that how the properties of conventional metal oxides can be improved via hierarchical 3D micro-nano configurations.

Ultrafast laser processing of copper: A comparative study of experimental and simulated transient optical properties

In this paper, we present ultrafast measurements of the complex refractive index for copper up to a time delay of 20ps with an accuracy <1% at laser fluences in the vicinity of the ablation threshold. The measured refractive index n and extinction coefficient k are supported by a simulation including the two-temperature model with an accurate description of thermal and optical properties and a thermomechanical model. Comparison of the measured time resolved optical properties with results of the simulation reveals underlying physical mechanisms in three distinct time delay regimes. It is found that in the early stage (−5ps to 0ps) the thermally excited d-band electrons make a major contribution to the laser pulse absorption and create a steep increase in transient optical properties n and k. In the second time regime (0–10ps) the material expansion influences the plasma frequency, which is also reflected in the transient extinction coefficient. In contrast, the refractive index n follows the total collision frequency. Additionally, the electron-ion thermalization time can be attributed to a minimum of the extinction coefficient at ∼10ps. In the third time regime (10–20ps) the transient extinction coefficient k indicates the surface cooling-down process.

Ultrafast Laser Pulses for Structuring Materials at Micro/Nano Scale: From Waveguides to Superhydrophobic Surfaces

The current demand for fabricating optical and photonic devices displaying high performance, using low-cost and time-saving methods, prompts femtosecond (fs)-laser processing as a promising methodology. High and low repetition femtosecond lasers enable surface and/or bulk modification of distinct materials, which can be used for applications ranging from optical waveguides to superhydrophobic surfaces. Herein, some fundamental aspects of fs-laser processing of materials, as well as the basics of their most common experimental apparatuses, are introduced. A survey of results on polymer fs-laser processing, resulting in 3D waveguides, electroluminescent structures and active hybrid-microstructures for luminescence or biological microenvironments is presented. Similarly, results of fs-laser processing on glasses, gold and silicon to produce waveguides containing metallic nanoparticles, analytical chemical sensors and surface with modified features, respectively, are also described. The complexity of fs-laser micromachining involves precise control of material properties, pushing ultrafast laser processing as an advanced technique for micro/nano devices.

Ultrafast Laser Fabrication of Functional Biochips: New Avenues for Exploring 3D Micro- and Nano-Environments

Lab-on-a-chip biological platforms have been intensively developed during the last decade since emerging technologies have offered possibilities to manufacture reliable devices with increased spatial resolution and 3D configurations. These biochips permit testing chemical reactions with nanoliter volumes, enhanced sensitivity in analysis and reduced consumption of reagents. Due to the high peak intensity that allows multiphoton absorption, ultrafast lasers can induce local modifications inside transparent materials with high precision at micro- and nanoscale. Subtractive manufacturing based on laser internal modification followed by wet chemical etching can directly fabricate 3D micro-channels in glass materials. On the other hand, additive laser manufacturing by two-photon polymerization of photoresists can grow 3D polymeric micro- and nanostructures with specific properties for biomedical use. Both transparent materials are ideal candidates for biochips that allow exploring phenomena at cellular levels while their processing with a nanoscale resolution represents an excellent opportunity to get more insights on biological aspects. We will review herein the laser fabrication of transparent microfluidic and optofluidic devices for biochip applications and will address challenges associated with their potential. In particular, integrated micro- and optofluidic systems will be presented with emphasis on the functionality for biological applications. It will be shown that ultrafast laser processing is not only an instrument that can tailor appropriate 3D environments to study living microorganisms and to improve cell detection or sorting but also a tool to fabricate appropriate biomimetic structures for complex cellular analyses. New advances open now the avenue to construct miniaturized organs of desired shapes and configurations with the goal to reproduce life processes and bypass in vivo animal or human testing.

Advances and opportunities of ultrafast laser synthesis and processing

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Ultrafast laser-induced morphological transformations

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Industrial applications of ultrafast laser processing

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Sapphire-Based Fresnel Zone Plate Fabricated by Femtosecond Laser Direct Writing and Wet Etching

Here, we report a sapphire-based Fresnel zone plate (FZP), which is fabricated by femtosecond laser direct writing assisted with subsequent wet etching. With this method, we solved the problem of high surface roughness caused by ultrafast femtosecond laser processing. We have obtained $\sim 12$ -nm average surface roughness smaller than 1/25 of the optical working wavelength. As-formed sapphire FZP also exhibited a well-defined geometry. More importantly, ultraviolet (UV) light focusing and imaging can be easily achieved. Due to the high material hardness, thermal and chemical stabilities of sapphire, such sapphire FZP, may have great potential in UV imaging and focusing under some harsh environments.

Progress in ultrafast laser processing and future prospects

AbstractThe unique characteristics of ultrafast lasers have rapidly revolutionized materials processing after their first demonstration in 1987. The ultrashort pulse width of the laser suppresses heat diffusion to the surroundings of the processed region, which minimizes the formation of a heat-affected zone and thereby enables ultrahigh precision micro- and nanofabrication of various materials. In addition, the extremely high peak intensity can induce nonlinear multiphoton absorption, which extends the diversity of materials that can be processed to transparent materials such as glass. Nonlinear multiphoton absorption enables three-dimensional (3D) micro- and nanofabrication by irradiation with tightly focused femtosecond laser pulses inside transparent materials. Thus, ultrafast lasers are currently widely used for both fundamental research and practical applications. This review presents progress in ultrafast laser processing, including micromachining, surface micro- and nanostructuring, nanoablation, and 3D and volume processing. Advanced technologies that promise to enhance the performance of ultrafast laser processing, such as hybrid additive and subtractive processing, and shaped beam processing are discussed. Commercial and industrial applications of ultrafast laser processing are also introduced. Finally, future prospects of the technology are given with a summary.

Ultrafast laser ablation of soda-lime glass for fabricating microfluidic pillar array channels

Microfluidic devices constitute the key technology enabling the development of more functional lab-on-a-chip systems for medical applications. This study presents an alternative method for precisely fabricating microfluidic pillar array channels by using irradiation by an ultrashort pulse from an ultrafast laser. Unlike conventional photolithography, which requires complex procedures and techniques, the ultrafast laser process is a straightforward approach for fabricating a functional microfluidic device with multiple pulses of an ultraviolet (UV) wavelength. To satisfy the requirements of the industrial process for mass production of microfluidic devices, a picosecond (PS) laser is used as a light source. Under the optimal energy fluence of 15.28J/cm2 with a pulse overlap of 95%, the scanning curve process can be executed in the clockwise direction for forming microfluidic pillar array structures. Simultaneously, the experimental evidence shows that the ultrafast laser process produces ablated soda-lime glass channels with hydrophilic surfaces on their inner walls. Based on the processing parameters of pillar structure with the number of pulses, the value of ablation rate can be 0.04μm/pulse. This work provides a direct patterning process through which a pillar array with functional structures can be constructed within the microfluidic device.

Ultrafast laser processing of materials: from science to industry.

Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific, technological and industrial potential. In ultrafast laser manufacturing, optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions. Control of photo-ionization and thermal processes with the highest precision, inducing local photomodification in sub-100-nm-sized regions has been achieved. State-of-the-art ultrashort laser processing techniques exploit high 0.1–1 μm spatial resolution and almost unrestricted three-dimensional structuring capability. Adjustable pulse duration, spatiotemporal chirp, phase front tilt and polarization allow control of photomodification via uniquely wide parameter space. Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer. The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput. Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.

Femtosecond laser rapid fabrication of large-area rose-like micropatterns on freestanding flexible graphene films.

We developed a simple, scalable and high-throughput method for fabrication of large-area three-dimensional rose-like microflowers with controlled size, shape and density on graphene films by femtosecond laser micromachining. The novel biomimetic microflower that composed of numerous turnup graphene nanoflakes can be fabricated by only a single femtosecond laser pulse, which is efficient enough for large-area patterning. The graphene films were composed of layer-by-layer graphene nanosheets separated by nanogaps (~10–50 nm), and graphene monolayers with an interlayer spacing of ~0.37 nm constituted each of the graphene nanosheets. This unique hierarchical layering structure of graphene films provides great possibilities for generation of tensile stress during femtosecond laser ablation to roll up the nanoflakes, which contributes to the formation of microflowers. By a simple scanning technique, patterned surfaces with controllable densities of flower patterns were obtained, which can exhibit adhesive superhydrophobicity. More importantly, this technique enables fabrication of the large-area patterned surfaces at centimeter scales in a simple and efficient way. This study not only presents new insights of ultrafast laser processing of novel graphene-based materials but also shows great promise of designing new materials combined with ultrafast laser surface patterning for future applications in functional coatings, sensors, actuators and microfluidics.

Ultrafast laser processing of materials: a review

We present an overview of the different processes that can result from focusing an ultrafast laser light in the femtosecond–nanosecond time regime on a host of materials, e.g., metals, semiconductors, and insulators. We summarize the physical processes and surface and bulk applications and highlight how femtosecond lasers can be used to process various materials. Throughout this paper, we will show the advantages and disadvantages of using ultrafast lasers compared with lasers that operate in other regimes and demonstrate their potential for the ultrafast processing of materials and structures.