Ultrafast Engineering


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An illustration of atoms forming a tentative bond. Image: SLAC National Accelerator Laboratory

The fast-paced field of micro/nanofabrication is moving even more rapidly than you might think. How fast? Femtosecond fast – as in one quadrillionth of a second. That’s the time scale of the ultrafast laser systems used to measure and manipulate the molecular building blocks of today’s smaller, smarter machines.

Ultrafast laser systems play multiple roles in demanding applications like MEMS/NEMS engineering, biomedicine, and battery research. The controlled, high-power femtosecond pulses of energy they produce is useful in high-precision materials processing tasks such as drilling, soldering, cutting, and ablating. And as radiation sources for spectroscopic or microscopic studies of chemical reactions, they elucidate the complex atomic and molecular processes as they occur.

Femtosecond Flashback

By applying ultrafast lasers to the real-time study of reactions, California Institute of Technology chemist Ahmed Zewail earned a 1999 Nobel Prize and capped off a century of painstaking inquiry into the nature of the chemical bond. His work built on efforts in the 1960s and 1970s to speed up the available dye lasers, which were generally too complex and inefficient for use beyond the most elite chemical physics laboratories. The first coherent femtosecond laser pulses were achieved at Bell Labs in the early 1980s, at last providing experimentalists with a way to explore age-old questions about the nature of chemical bonds, molecular vibrations, and reaction dynamics. The advent of the titanium-doped sapphire laser in 1986, followed by Kerr-lens mode-locked (KLM) Ti:sapphire systems five years later, was a turning point in the field, paving the way for today’s wide range of fs and sub-fs solid-state, semiconductor, and fiber laser systems.

The Linac Coherent Light Source (LCLS) takes X-ray snapshots of atoms and molecules at work. Image: SLAC National Accelerator Laboratory

For example, in February, researchers at the Department of Energy’s SLAC National Accelerator Laboratory (Menlo Park, CA) reported success in what has always been considered impossible: visualizing the very first step in the birth of a molecule, the so-called transition state when two atoms begin to bond together. “Because so few molecules inhabit the transition state at any given moment, no one thought we’d ever be able to see it,” said SLAC project leader Anders Nilsson. The key experiments were conducted on SLAC’s Linac Coherent Light Source, which uses ultrafast X-ray laser pulses to reveal chemical reactions in a new light, revealing subtle changes in the electrons in a sample’s constituent atoms. For engineers, basic breakthroughs like this will unleash a torrent of new practical possibilities, just as past achievements have cleared a path for recent technologies and applications like the ones highlighted below.

Biomedicine

Some of the most immediate consumer benefits from femtosecond lasers have fallen in the health care sector. Ultrafast applications span the spectrum of health care, from biomolecular imaging to clinical diagnostics to surgery to medical device fabrication. Most notable from the commercial standpoint is an ultrafast variation of LASIK surgery for vision correction. Known by various names, bladeless or all-laser LASIK was first performed in 2000 and approved by the FDA in 2001. By substituting an ultrafast laser for traditional steel-bladed mechanical microkeratome, bladeless LASIK makes it easier for surgeons to create very precise “flaps” in the cornea for better patient safety and faster recovery times. Ultrashort laser surgery is also under investigation for bone and cartilage repair, tissue engineering, dermatology, tumor resection, and intra-operative diagnostics.

This 3-mm figure skater was printed using high-resolution 3D laser lithography system. Image: Nanoscribe

Advanced Manufacturing

The nanotech world is embracing ultrafast lasers in direct-writing 3-D lithography, metrology, micromachining, and deposition techniques. The two-photon approach is well-established in traditional photolithography. Ultrafast lasers dramatically increase the technique’s speed and precision while eliminating the need for reticles or photomasks. It’s based on the use of photosensitive polymeric resins that cure by absorbing focused laser light at known wavelengths.

Nanoscribe, a spin-off of Germany’s Karlsruhe Institute of Technology, has put the method to work in what it claims to be the highest-resolution 3-D printer on the market. The Photonic Professional and Photonic Professional GT systems combine the advantages of 3D printing and maskless laser lithography for applications in 3-D micro- and nanofabrication, said Nanoscribe spokesperson Anke Werner. The system’s sub-micrometer resolution performance is a function of its turnkey femtosecond near-IR fiber laser system, which combines high laser power with high-numerical-aperture focusing optics that produce a laser spot size small enough to create submillimeter features.

The direct-writing system operates in either fixed-beam/moving-sample (FBMS) mode or a higher speed moving-beam/fixed-sample (MBFS) layer-by-layer mode. In FBMS building, the laser beam is focused in a stationary position while piezo actuators move the substrate in three dimensions to cure the resin at precise trajectories. The MBFS approach uses galvanometric mirrors similar to those used in light shows and DVD players to scan the laser beam laterally, achieving precise beam contact on the substrate. Werner said the company’s technology is positioned for scientific, industrial, and medical applications.

Next-Generation Batteries

Between the movement toward all-electric vehicles and the world’s surging number of battery-powered personal devices, there’s an urgent demand for greener, higher-capacity batteries. Before engineers can solve the cost, performance, and capacity issues that limit progress, they have to know exactly what’s happening, electrochemically, in a battery.

Seeking a better understanding what limits the performance of lithium-based batteries, Richard E. Russo, a senior scientist in LBNL’s Laser Materials Interaction Group, uses a method called femtosecond laser-induced breakdown spectroscopy (LIBS) to analyze battery component samples produced through laser ablation (LA). Russo’s group probed compositional variations in the Solid Electrolyte Interphase (SEI) layer of experimental Li batteries by measuring spectral emissions from different elements and molecules to a surface depth of 7 nm. His group has also taken advantage of fs LIBS’s high resolution, reduced thermal effects, and light-element sensitivity to develop 3-D maps of multiple elements in Li-ion anodes, cathodes, solid-state electrolytes, and other components. Images calculated on the principles of computed tomography (CT) scanning resulted in 2-day layer-by-layer maps, 2-D cross-sectional images, and 3-D volume renderings of various elements of interest. “The interaction of femtosecond lasers with materials provides unique insight into nonlinear phenomena and ultrafast processes that continue to push the boundaries.”

Michael MacRae is an independent writer.

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The interaction of femtosecond lasers with materials provides unique insight into nonlinear phenomena and ultrafast processes that continue to push the boundaries.

Richard Russo, Lawrence Berkeley National Laboratory

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March 2015

by Michael MacRae, ASME.org