ADELPHI, Md. -- For centuries, alchemists sought unsuccessfully to transmute lead into gold. But what if, instead of transforming one material into another, we could trick lead into behaving like gold? Controlling the spectral properties of materials like this could enable a new class of high speed optical computers with faster response times than electronics based systems.
Scientists at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory worked alongside researchers from Tulane University and King’s College London to demonstrate how to make one substance mimic the behavior of another when it is subjected to laser light. The Army Research Office, an element of laboratory, funded the research.
“This technique represents an opportunity in both materials science and chemistry to substitute simpler and cheaper compounds that can mimic the desired properties of more expensive materials,” said Prof. Denys Bondar of Tulane University.
In particular, the researchers homed in on material properties that determine how a substance responds to a stimulus.
For example, different materials behave differently when they have a voltage applied to them. Their response to the voltage determines whether they classify as conductors, insulator or semi-conductors. In conductors, the voltage causes an electric current in which electrons flow from high voltage to low voltage. In insulators, no current flows.
In the case of voltage and current, the stimulus and the response usually share a linear relationship. In most conductors and for a large range of voltages, the current is simply proportional to the voltage.
Some materials can produce a much more complicated relationship between stimuli and response however. If a laser illuminates two different materials with a fixed frequency and steady intensity, they can respond quite differently. By that logic, researchers can also change the laser in such a manner that both materials produce the same response.
“If I have two different materials and I illuminated them both with the same laser, they would each have a certain response,” said Dr. Kurt Jacobs, a scientist at the laboratory who collaborated on the research. “We modulated the laser so that we could vary its amplitude and have the second material respond like the first material.”
The researchers showed that they could control the frequency and intensity of the laser illumination to induce one material to demonstrate the same current as a completely different material under steady illumination.
According to Jacobs, this is possible because the response to laser light is sufficiently complex. The fact that even slight changes to the laser’s properties can greatly alter how the material behaves opens the door to many different stimulus-response combinations. In regard to current, specific settings for the laser translate to various amounts of energy the laser delivers to the material.
“The current is induced by the laser light (i.e. the laser’s energy) which shines on the material and stimulates the electrons,” Jacobs said. “Some of that energy is then converted into an electrical force that moves the electrons through the material. So this new current is generated by the laser light.”
However, current is simply one example of a property that the researchers can control with this method. The team also showed that any bulk response induced in a material by a laser can be tailored in the same way.
“The fact that it is possible to control the bulk response of materials in this way may mean that it is possible to control more complex aspects of material behavior,” Bondar said. “It would be tremendously useful, for example, if one could use this kind of control to induce responses that mimic exotic properties such as superconductivity at high temperatures.”
In short, the researchers found a way to construct a recipe for essentially any desired response as long as a mathematical model of material can be constructed. This capability was previously unprecedented for many-bodied systems.
“This ideal of control pertains to how you have some desired result, whether that’s an end state of a system or an output spectrum,” McCaul said. “Then the question becomes ‘What do I have to do to the system to get that desired output?’ If we’re using electromagnetic waves, like with a laser, to manipulate the system, then we want to know the shape of the pulse that we need to apply.”
A demonstration of this much control over a material’s properties presents many advantages to researchers, Jacobs said. In addition to cost-saving measures where more expensive materials can be substituted with cheaper alternatives with just a change in the laser drive, this technique allows for tailor-made responses that may not otherwise exist.
“The work is not necessarily about having one material mimic another material, but in providing a tailored response,” Jacobs said. “By applying the right control, you can generate a response that you can’t easily find in any material.”
For instance, high harmonic generation refers to a process where an intense laser pulse aimed at a gas or a solid produces light at various higher frequencies. The same method that allows researchers to adjust the laser settings to obtain the desired material properties may prove useful in reliably generating higher frequencies of light than is currently possible.
Other examples of applications include reservoir and quantum computing, where the mimicry of material properties with a laser can add another layer of depth to the input-output relationship within both computer systems.
"ARO invested in Dr. Bondar's work to determine if extreme non-linear optical responses of materials can provide a new approach for optical computing,” said Dr. Marc Ulrich, electronics division chief at ARO. “Dr. Bondar's first step in the studies on material responses resulted in the fantastic surprise that a material's properties can be driven. It suggests that our future technologies might not have to be limited by a material's intrinsic properties.”
Such endeavors open the path for new ways of thinking about information processing units as well as new physical paradigms for computing beyond digital electronics, Bondar said.
“The intrinsic properties of any material are defined by its behavior at equilibrium,” McCaul said. “Indeed, the different equilibrium properties are how we distinguish between materials: lead is heavier than gold and has a different color of optical response. But once you drive things so that they’re no longer resting in their natural state, their properties depend on the way they are being driven.”
In this case, the properties that a material can exhibit are no longer limited by their intrinsic nature. Instead, only the sort of driving performed on the material can determine its properties.
“If you understand how to design this driving process, you’re getting to the stage where you can control the properties of any material to suit your needs,” McCaul said. “That’s why the application of this research is so diverse.”
The team published a paper about this research in the journal Physical Review Letters. The journal Physics also published a Viewpoint article about the research.
CCDC Army Research Laboratory is an element of the U.S. Army Combat Capabilities Development Command. As the Army’s corporate research laboratory, ARL discovers, innovates and transitions science and technology to ensure dominant strategic land power. Through collaboration across the command’s core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win the nation’s wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.
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