Jon P. Longtin

Inert gas beam delivery for ultrafast laser micromachining at ambient pressure

Ultrafast laser micromachining is realized by focusing a femtosecond laser beam to a small spot, where very high optical intensity is achieved at the workpiece. Often, however, the beam must pass through a gas, e.g., air, before reaching the workpiece. At the very high laser intensities associated with ultrafast lasers, the gas can ionize, resulting in a rapid increase in free electron (plasma) density, which decreases the gas refractive index, resulting in plasma defocusing and self-phase modulation. Plasma-induced effects distort the temporal and spatial profile of the laser beam, which degrade feature quality and repeatability for ultrafast laser micromachining. In addition, plasma absorption reduces the energy available for materials processing, resulting in a decreased material removal rate. To avoid these effects, processing has traditionally been performed in a vacuum chamber, however this makes real-time processing on a large scale impractical. This article presents a beam delivery technique that ...

Plasma Absorption of Femtosecond Laser Pulses in Dielectrics

Dielectric (high bandgap) materials represent an important and diverse class of materials in micro and nanotechnology, including MEMS devices, biomedical and bioengineering systems, multilayer thin film coatings, fiber optics, etc. Micromachining dielectrics using ultrafast lasers is an exciting and promising new research area with many significant advantages, including precision material removal, negligible heating of the workpiece, micron and sub-micron-size feature fabrication, and high aspect ratio features. During ultrafast laser processing of dielectrics, the intense laser pulse ionizes the irradiated material and produces an optical breakdown region, or plasma, that is characterized by a high density of free electrons. These high-density electrons can efficiently absorb a large fraction of the laser irradiance energy, part of which will then be coupled into the bulk material, resulting in material removal through direct vaporization. The energy deposited into the material depends on the time and space-dependent breakdown region, the plasma rise time, and the plasma absorption coefficient

Modeling optical breakdown in dielectrics during ultrafast laser processing

Laser ablation is widely used in micromachining, manufacturing, thin-film formation, and bioengineering applications. During laser ablation the removal of material and quality of the features depend strongly on the optical breakdown region induced by the laser irradiance. The recent advent of amplified ultrafast lasers with pulse durations of less than 1 ps has generated considerable interest because of the ability of the lasers to process virtually all materials with high precision and minimal thermal damage. With ultrashort pulse widths, however, traditional breakdown models no longer accurately capture the laser-material interaction that leads to breakdown. A femtosecond breakdown model for dielectric solids and liquids is presented that characterizes the pulse behavior and predicts the time- and position-dependent breakdown region. The model includes the pulse propagation and small spatial extent of ultrashort laser pulses. Model results are presented and compared with classical breakdown models for 1-ns, 1-ps, and 150-fs pulses. The results show that the revised model is able to model breakdown accurately in the focal region for pulse durations of less than 10 ps. The model can also be of use in estimating the time- and position-resolved electron density in the interaction volume, the breakdown threshold of the material, shielding effects, and temperature distributions during ultrafast processing.

Breakdown threshold and localized electron density in water induced by ultrashort laser pulses

Optical breakdown by ultrashort laser pulses in dielectrics presents an efficient method to deposit laser energy into materials that otherwise exhibit minimal absorption at low laser intensities. During optical breakdown, a high density of free electrons is formed in the material, which dominates energy absorption, and, in turn, the material removal rate during ultrafast laser-material processing. Classical models assume a spatially uniform electron population and constant laser intensity in the focal region, which results in time-dependent expressions only, i.e., the rate equations, to predict electron evolution induced by nanosecond and picosecond pulses. For femtosecond pulses, however, the small spatial extent of the pulse requires that the pulse propagation be considered, which results in an inhomogeneous plasma and localized electron formation during optical breakdown. In this work, a femtosecond breakdown model is combined with the classical rate equations to determine both time- and position-depen...

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Sensors for harsh environments by direct-write thermal spray

High-temperature thermocouple sensors for harsh environments have been fabricated using thermal spray technology with excellent performance demonstrated. Concepts for strain sensors fabricated with thermal spray technology are also being developed. This work reports on functional high-temperature thermocouples and strain gauge concepts fabricated using thermal spray processing.

Ultrafast laser micromachining of silica aerogels

Silica aerogels are unique nanostructured materials that possess many distinctive qualities, including extremely low densities and thermal conductivities, very high surface-area-to-volume ratios, and large strength-to-weight ratios. Aerogels, however, are very brittle, and are not readily shaped using traditional machining operations. Ultrafast laser processing may provide an alternative for precision shaping and machining of these materials. This paper discusses investigations of ultrafast laser machining of aerogels for material removal and micromachining. The advantages of ultrafast laser processing include a minimal thermal penetration region and low processing temperatures, precision removal of material, and good-quality feature definition. In this work, an amplified femtosecond Ti:sapphire laser system is used to investigate the breakdown threshold, material removal rate per pulse, and specific issues associated with laser processing of aerogels, as well as recommendations for further investigations for these unique materials.

Modeling thermal conductivity of thermal spray coatings: comparing predictions to experiments

Thermal conductivity plays a critical role in the thermal transport of thermal-sprayed coatings. In this article, a combined image analysis and finite-element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy images of the coating microstructure. Images are analyzed with a collection of image-processing algorithms to reveal the microscopic coating morphology. The processed digital image is used to generate a two-dimensional finite-element mesh in which pores, cracks, and the bulk coating material are identified. The effective thermal conductivity is then simulated using a commercial finite-element code. Results are presented for three coating material systems [yttriastabilized zirconia (YSZ), molybdenum, and NiAl], and the results are found to be in good agreement with the experimental values obtained using the laser flash method. The YSZ coatings are also annealed, and the analysis procedure was repeated to determine whether the technique can accurately assess changes in coating morphology.

Real-time control of ultrafast laser micromachining by laser-induced breakdown spectroscopy.

Ultrafast laser micromachining provides many advantages for precision micromachining. One challenging problem, however, particularly for multilayer and heterogeneous materials, is how to prevent a given material from being ablated, as ultrafast laser micromachining is generally material insensitive. We present a real-time feedback control system for an ultrafast laser micromachining system based on laser-induced breakdown spectroscopy (LIBS). The characteristics of ultrafast LIBS are reviewed and discussed so as to demonstrate the feasibility of the technique. Comparison methods to identify the material emission patterns are developed, and several of the resulting algorithms were implemented into a real-time computer control system. LIBS-controlled micromachining is demonstrated for the fabrication of microheater structures on thermal sprayed materials. Compared with a strictly passive machining process without any such feedback control, the LIBS-based system provides several advantages including less damage to the substrate layer, reduced machining time, and more-uniform machining features.

Laser-induced surface-tension-driven flows in liquids

Abstract High-intensity, short-pulse laser radiation incident on the free surface of an absorbing dielectric liquid results in heating that can alter the liquid surface tension, causing Marangoni convection. This flow can dominate the transport of thermal energy in the liquid. In this work, both a scaling analysis and a full numerical simulation of the governing equations are performed. A thermal mechanism is proposed as the driving force for these flows. The dependence on beam size and temperature increase in the liquid is investigated, with good agreement found among the scaling analysis, numerical simulations and experimental data obtained from a previous study. The importance of natural convection and thermal conduction on the fluid-thermal transport was assessed numerically, with both found to be negligible for this liquid–laser system. Velocity and temperature profiles at the liquid surface are also discussed.