Separated nano jetting and micro jetting regimes by double-pulse irradiation of a metal film: towards multiscale printing.

The Double-Pulse (DP) version of the Laser-Induced Forward Transfer (LIFT) technique holds great potential to improve the resolution and flexibility of printing applications. In this study, we investigate the transfer of copper. A long laser pulse is first applied to melt thin copper films deposited on a transparent substrate, followed by an ultrashort laser pulse to initiate the transfer of the liquid material towards a receiver substrate. Time-resolved imaging experiments reveal that ejections from nanodrops to liquid jets with controllable diameters, from few micrometers down to the nanometers scale can be obtained with the control parameters of DP-LIFT. Comparing simulation and experiments we discuss how the ejection characteristics are governed by various factors including the shape, diameter and temperature of the melted pool created with the first long pulse. While the formation of microjets is due to the dynamical deformation of the melted film, as for the conventional LIFT process applied with liquid donors, the results indicate a different and distinct process for the formation of nanojets. We extrapolate from the observations a feature caused by the interaction of the shockwave, generated by the femtosecond laser irradiation, with the deformed surface of the pool. Ultimately, we establish the range of irradiation parameters leading to the observation of single separated microjets and nanojets. The latter are accompanied by nano printing demonstrations. Considering all accessible regimes together, a unique technological perspective is the possibility to achieve multi-scale printing from the same donor.

Internal structuring of gallium arsenide using short laser pulses.

Laser writing inside semiconductors attracts attention as a possible route for three-dimensional integration in advanced micro technologies. In this context, gallium arsenide (GaAs) is a material for which the best conditions for laser internal modification (LIM) have not been established yet. We address this question by using laser pulses at a fixed wavelength of 1550-nm. A large parameter space is investigated including the response to the applied pulse energy, pulse duration (from femtosecond to nanosecond) and the focusing conditions. We report that well-defined and reproducible internal modifications are achievable with tightly focused nanosecond pulses. The measured writing thresholds are systematically compared to those obtained in silicon (Si), a more extensively studied material. In comparison to Si, we also observe that GaAs is more prone to filamentation effects affecting the modification responses. The reported specific observations for LIM of GaAs should facilitate the future process developments for applications in electronics or photonics.

Ultrafast laser stabilization by nonlinear absorption for enhanced-precision material processing.

Using ultrafast lasers, sub-diffraction features can be produced thanks to the threshold-based response of materials to the local beam fluence. In practice, Gaussian beams with peak fluence near the modification threshold lead to high-resolution. However, this conflicts with reliability as the process becomes increasingly sensitive to pulse-to-pulse energy fluctuations. Using nonlinear absorption in a ZnS crystal, we demonstrate a passive extra-cavity energy stabilization method in a femtosecond laser material machining configuration. Processing precision and repeatability are enhanced as evidenced by highly reliable amorphous features produced on silicon with sizes ten times smaller than the spot size, becoming a practical solution for high-precision manufacturing applications.

Short-Pulse Laser-Assisted Fabrication of a Si-SiO2 Microcooling Device.

Thermal management is one of the main challenges in the most demanding detector technologies and for the future of microelectronics. Microfluidic cooling has been proposed as a fully integrated solution to the heat dissipation problem in modern high-power microelectronics. Traditional manufacturing of silicon-based microfluidic devices involves advanced, mask-based lithography techniques for surface patterning. The limited availability of such facilities prevents widespread development and use. We demonstrate the relevance of maskless laser writing to advantageously replace lithographic steps and provide a more prototype-friendly process flow. We use a 20 W infrared laser with a pulse duration of 50 ps to engrave and drill a 525 µm-thick silicon wafer. Anodic bonding to a SiO2 wafer is used to encapsulate the patterned surface. Mechanically clamped inlet/outlet connectors complete the fully operational microcooling device. The functionality of the device has been validated by thermofluidic measurements. Our approach constitutes a modular microfabrication solution that should facilitate prototyping studies of new concepts for co-designed electronics and microfluidics.

Short-Pulse Lasers: A Versatile Tool in Creating Novel Nano-/Micro-Structures and Compositional Analysis for Healthcare and Wellbeing Challenges.

Driven by flexibility, precision, repeatability and eco-friendliness, laser-based technologies have attracted great interest to engineer or to analyze materials in various fields including energy, environment, biology and medicine. A major advantage of laser processing relies on the ability to directly structure matter at different scales and to prepare novel materials with unique physical and chemical properties. It is also a contact-free approach that makes it possible to work in inert or reactive liquid or gaseous environment. This leads today to a unique opportunity for designing, fabricating and even analyzing novel complex bio-systems. To illustrate this potential, in this paper, we gather our recent research on four types of laser-based methods relevant for nano-/micro-scale applications. First, we present and discuss pulsed laser ablation in liquid, exploited today for synthetizing ultraclean "bare" nanoparticles attractive for medicine and tissue engineering applications. Second, we discuss robust methods for rapid surface and bulk machining (subtractive manufacturing) at different scales by laser ablation. Among them, the microsphere-assisted laser surface engineering is detailed for its appropriateness to design structured substrates with hierarchically periodic patterns at nano-/micro-scale without chemical treatments. Third, we address the laser-induced forward transfer, a technology based on direct laser printing, to transfer and assemble a multitude of materials (additive structuring), including biological moiety without alteration of functionality. Finally, the fourth method is about chemical analysis: we present the potential of laser-induced breakdown spectroscopy, providing a unique tool for contact-free and space-resolved elemental analysis of organic materials. Overall, we present and discuss the prospect and complementarity of emerging reliable laser technologies, to address challenges in materials' preparation relevant for the development of innovative multi-scale and multi-material platforms for bio-applications.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu's approach to imperfect beams.

The so-called D-squared or Liu's method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal spatial Gaussian beam that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature.  It provides also an appropriate tool for new studies with the most extreme laser radiations.

Pulse-duration dependence of laser-induced modifications inside silicon.

The advent of ultrafast infrared lasers provides a unique opportunity for direct fabrication of three-dimensional silicon microdevices. However, strong nonlinearities prevent access to modification regimes in narrow gap materials with the shortest laser pulses. In contrary to surface experiments for which one can always define an energy threshold to initiate modifications, we establish that some other threshold conditions inevitably apply on the pulse duration and the numerical aperture for focusing. In an experiment where we can vary continuously the pulse duration from 4 to 21 ps, we show that a minimum duration of 5.4 ps and a focusing numerical aperture of 0.85 are required to successfully initiate modifications. Below and above thresholds, we investigate the pulse duration dependence of the conditions applied in matter. Despite a modest pulse duration dependence of the energy threshold in the tested range, we found that all pulse durations are not equally performing to achieve highly reproducible modifications. Taken together with previous reports in the femtosecond and nanosecond regimes, this provides important guidelines on the appropriate conditions for internal structuring of silicon.

Ultrafast Laser Writing Deep inside Silicon with THz-Repetition-Rate Trains of Pulses.

Three-dimensional laser writing inside silicon remains today inaccessible with the shortest infrared light pulses unless complex schemes are used to circumvent screening propagation nonlinearities. Here, we explore a new approach irradiating silicon with trains of femtosecond laser pulses at repetition rates up to 5.6 THz that is order of magnitude higher than any source used for laser processing so far. This extremely high repetition rate is faster than laser energy dissipation from microvolume inside silicon, thus enabling unique capabilities for pulse-to-pulse accumulation of free carriers generated by nonlinear ionization, as well as progressive thermal bandgap closure before any diffusion process comes into play. By space-resolved measurements of energy delivery inside silicon, we evidence changes in the interplay between detrimental nonlinearities and accumulation-based effects. This leads to a net increase on the level of space-time energy localization. The improvement is also supported by experiments demonstrating high performance for 3D laser writing inside silicon. In comparison to repeated single pulses, irradiation with trains of only four-picosecond pulses with the same total energy leads to an apparent decrease of the energy threshold for modification and drastic improvements on the repeatability, uniformity, and symmetricity of the produced features. The unique benefits of THz bursts can provide a new route to meet the challenge of 3D inscription inside narrow bandgap materials.

Phenomenological modelling of non-volatile memory threshold voltage shift induced by nonlinear ionization with a femtosecond laser.

The behaviour of semiconductor materials and devices subjected to femtosecond laser irradiation has been under scrutiny, for many reasons, during the last decade. In particular, recent works have shown that the specific functionality and/or geometry of semiconductor devices, among which non-volatile memory (NVM) devices hold a special place, could be used to improve the knowledge about ultrafast laser-semiconductor interactions. So far, such an approach has been applied to draw conclusions about the spatio-temporal properties of laser propagation in bulk materials. Here, by comparing the evolution of the electrical characteristics of Flash cells under the cumulative effect of repeated femtosecond laser pulses with first-order physical considerations and TCAD (Technology Computer Aided Design) simulations, we clearly establish the role of the carriers created by nonlinear ionization on the functionality of the structures. The complete electrical analysis informs indirectly on the energy of the laser-produced free-carriers which, to date, was almost inaccessible by an experimental method applicable to the bulk of a material. Establishing the link between the carrier energy and laser parameters is of major importance to improve the comprehension of the nonlinear ionization mechanisms associated to intense laser-semiconductor interactions and applied in various fields from microelectronics to laser micromachining.

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.Ultrafast laser processing is a versatile three-dimensional photonic structuring method but it has been limited to wide band gap materials like glasses. Here, Chanal et al. demonstrate direct refractive-index modification in the bulk of silicon by extreme localization of the energy deposition.

Generating liquid nanojets from copper by dual laser irradiation for ultra-high resolution printing.

When the energy of a short laser pulse is localized in a fluid material, a flow motion is induced that can lead to the generation of free-surface jets. This nozzle-free jetting process is exploited to print conductive materials, typically metal nanoparticle inks, but this approach remains limited to the transfer of low viscosity fluids with a minimum feature size of few micrometers. We introduce a dual-laser method to achieve reproducible high-aspect-ratio jets from thin solid films. A first laser irradiation induces the melting of copper thin films and a second synchronized short pulse irradiation initiates the jetting process. Using time-resolved microscopy, we investigate the influence of the film thickness on the flow motion mechanisms and the ejection dynamics. For a wide range of laser fluences, we present observations similar to those obtained when the jets are generated by a single laser pulse from liquid donor films. The use of a solid film allows reducing the film thickness and then the volume of transferred material. Finally, we analyze these results in the perspective of using this double pulse LIFT technique for additive manufacturing of nano-micro-structures. Stable jets are formed from the copper films over distances exceeding 50-µm and are exploited to demonstrate periodic printing of 1.5-µm diameter droplets.

Ideal radiation source for plasma spectroscopy generated by laser ablation.

Laboratory plasmas inherently exhibit temperature and density gradients leading to complex investigations. We show that plasmas generated by laser ablation can constitute a robust exception to this. Supported by emission features not observed with other sources, we achieve plasmas of various compositions which are both uniform and in local thermodynamic equilibrium. These properties characterize an ideal radiation source opening multiple perspectives in plasma spectroscopy. The finding also constitutes a breakthrough in the analytical field as fast analyses of complex materials become possible.

Millijoule femtosecond micro-Bessel beams for ultra-high aspect ratio machining.

We report on a functional experimental design for Bessel beam generation capable of handling high-energy ultrashort pulses (up to 1.2 mJ per pulse of 50 fs duration). This allows us to deliver intensities exceeding the breakdown threshold for air or any dielectric along controlled micro-filaments with lengths exceeding 4 mm. It represents an unprecedented upscaling in comparison to recent femtosecond Bessel beam micromachining experiments. We produce void microchannels through glass substrates to demonstrate that aspect ratios exceeding 1200∶1 can be achieved by using single high-intensity pulses. This demonstration must lead to new methodologies for deep-drilling and high-speed cutting applications.

Bessel-like photonic nanojets from core-shell sub-wavelength spheres.

It is accepted so far that the formation of photonic nanojets requires the use of large dielectric spheres (several wavelengths in diameter). Here we show both numerically and experimentally that similar effects can be obtained with properly engineered sub-wavelength core-shell colloids. The design of the spheres is strongly inspired by a far-field approach for the generation of Bessel beams.