Alchemists once claimed they could summon profound powers and transmute the characteristics of materials through the application of science. From seeking the philosopher's stone – the Magnum Opus – to transforming base metals into noble metals like gold or silver, they labored to manipulate the structures of the physical world and sought perfection.
Now a group of researchers are creating their own kind of "alchemy" in a femtosecond – one quadrillionth of a second – which transforms humble silica material into all manner of Lilliputian medical and optical devices.
Yves Bellouard is an Associate Professor in Micro and Nano Scale Engineering at the Mechanical Engineering Department of Eindhoven University of Technology, and his work with ultrafast lasers and materials processing is set to revolutionize fields from medicine to computer storage to micromanufacturing.
Some of Bellouard's work is focused on using a femtoprinter to apply 3D patterns to glass. The properties of the glass are manipulated in areas exposed to the laser light, and depending on the intensity of that light, the refractive index of the glass material, an important characteristic of those optical properties, can be adjusted with great precision. Bellouard's technique creates a pattern or a network through which light can be channeled. He says this approach will one day be applied to optical computer chip design and optical motion sensors.
"To make microsystems easier to fabricate and to increase their performance and reliability, we have focused our research on monolithic integration based on a concept of 'system material.' Rather than building up a device by combining materials and fitting them together, this approach turns a single piece of material into a system by tailoring its properties in selected locations," Bellouard says. "The material is no longer just an element of a device, but becomes a device on its own. This method has many advantages. It greatly simplifies processing, reduces microsystem-assembly steps – a common source of expense, inaccuracy, and failure – and presents new design opportunities."
So what is it about pulsing light in the femtosecond realm that could be a game changer? It takes 300,000,000,000,000 femtoseconds for the human eye to blink. Put yet another way, light itself can only travel .3 micrometers in one femtosecond.
It's all about duration and what a little fraction of time can do to change the various properties of materials as they're constructed on a minute scale. Within the relative timeline of laser operations, a picosecond is essentially a snail's pace. If laser interactions meant to change or manipulate materials take more than a picosecond, those materials may react in unwanted ways.
It takes time for heat to spread and diffuse away from a hot spot, and in that time, damage can occur to the surrounding structures of a given object as its intended features are created. Though it may not seem like much, it does take a finite amount of time for heat to diffuse across the atomic lattice of solid materials such as metals or glass, and if a laser pulse is longer than a picosecond, heat damage occurs to surrounding substrate material and the laser's energy is largely wasted. Once an operation can be completed within the duration of a few femtoseconds, those pulses are absorbed before significant heat diffusion can occur. That means nearly all of a laser's energy can be used on the task at hand before a given material reacts to heat.
The current technology is hampered by the fact that most microsystem manufacturing processes are done with large, expensive and energy-inefficient machines which also require clean rooms to function correctly. Bellouard also heads up Femtoprint.eu, a project aimed at reducing the size of the required laser to fit in an area the size of a shoebox, and unlike current laser manufacturing processes, the Femtoprint process makes use of a very low power laser of not more than 220 mW. Amazingly, that's about the same power output as that of a bright LED.
But the ultimate promise of ultrafast laser technologies is the idea that they offer a departure from traditional microsystems manufacturing techniques. Bellouard says the process is capable of making submicron-sized features and integrating them into structures with multifunctionality. Another benefit, he says, is that those features can be simultaneously integrated within mechanical, electrical, fluidic, and optical components.
Using this sort of technique, devices aren't built as much as they're unbound. Unlike the process of creating a device by combining and assembling various materials and their functions, the ultrafast laser process manipulates a single piece of material into a "microsystem" by modifying properties of the material itself. Using glass or silica as a main component – not silicon – fused silica microdevices are made in a two-step process.
If that sounds a bit like alchemy, then so be it.
In the first step, bulk silica is exposed to low-level laser bombardment that, while it creates no observable change in the material's makeup, produces just enough change to the density of the fused silica in the desired locations.
The second step etches that target substrate with a hydrofluoric acid solution, and as the rate of etching occurs faster in the laser-targeted portions of the material, the design and manufacture of mechanical and fluidic components is accomplished by precisely controlling the shape of the areas exposed to the laser treatment.
It's the vastly reduced energy requirements for such speedy laser exposures of the silica which allows for the creation of exceptionally compact and air-cooled ultrafast lasers which can then be driven by a 3D printing system and 3D design software.