Light in Ballistic Resonance

Newly developed ballistic optical materials consist of a composite of two transparent materials, creating a plasmonic material. (Source: E. Simmons & K. Li)

Electronics are increa­singly being paired with optical systems, such as when accessing the internet on an electronically run computer through fiber optic cables. But meshing optics with electronics is challenging, due to their disparate scales. Electrons work at a much smaller scale than light does. The mismatch between electronic systems and optical systems means that every time a signal converts from one to the other, ineffi­ciency creeps into the system. Now, a team led by a Purdue University scientist has found a way to create more efficient metamaterials using semi­conductors and a novel aspect of physics that amplifies the activity of electrons.

This new class of materials has the potential to drama­tically increase the resolution in medical scanning and scientific imaging and dras­tically reduce the size of supercomputers, creating a future where scientists can see tiny things in far greater detail and devices are smaller and more powerful. Scientists have worked for decades to shrink photons down to a nanometer scale to make them more compatible with electrons. This can be achieved using rarefied materials and expensive production techniques to make hyper­bolic materials. Using hyper­bolic materials, scientists can shrink photons by compressing the light, making it easier to interface with electrical systems.

Evgenii Narimanov, a theoretical physicist and professor of electrical and computer engi­neering at Purdue, explained, “The most important thing about hyperbolic materials is that they can compress light to almost any scale. When you can make light small, you solve the problem of the disconnect between optics and electronics. Then you can make very efficient opto­electronics.” The problem lies in creating these hyper­bolic materials. They typically consist of interwoven layers of metals and dielectrics, and every surface must be as smooth and defect-free as possible at the atomic level, something that is difficult, time-consuming and expensive.

The solution, Narimanov believes, includes semiconductors. Not, he emphasized, because of anything special about the semi­conductors themselves. But because scientists and researchers have devoted the past 70 years or more to producing high-quality semi­conductors efficiently. Narimanov wondered if he could harness that proficiency and apply it to producing new and improved meta­materials. Unfor­tunately, semiconductors do not make inherently good optical meta­materials; they do not have enough electrons. They can work at relatively low frequencies, in the mid- to far-infrared scale. But to improve imaging and sensing techno­logies, scientists need meta­materials that work in the visible on near-infrared spectrum, at much shorter wavelengths than the mid- and far-infrared.

Narimanov and his colla­borators discovered and tested the optical pheno­menon ballistic resonance. In these new optical materials, which combine meta­material concepts with the atomic precision of single-crystal semiconductors, free, ballistic electrons interact with an oscillating optical field. Synchroni­zing the optical field with the frequency of the motion of the free electrons as they bounce within the confines of the thin con­ducting layers, forming the composite material, causes the electrons to resonate, enhancing the reaction of each electron and creating a meta­material that works at higher frequencies. While the researchers were not yet able to reach the wavelengths of the visible spectrum, they did get 60 % of the way there.

“We showed that there is a physics mechanism that makes this possible,” Narimanov said. “Before, people did not realize this was something that could be done. We have opened the way. We showed it is theo­retically possible, and then we experi­mentally demons­trated 60 % improvement in the operational frequency over existing materials.” “We will keep pushing this frontier,” Narimanov said. “Even if we are extremely successful, nobody is going to get semi­conductor meta­materials to the visible and near-infrared spectrum within a year or two. It may take about five years. But what we have done is provide the material platform. The bottleneck for photonics is in the material where electrons and photons can meet on the same length scale, and we have solved it.” (Source: Purdue U.)

Reference: K. Li et al.: Ballistic metamaterials, Optica 7, 1773 (2020); DOI: 10.1364/OPTICA.402891

Link: Electrical and Computer Engineering Dept., University of Texas, Austin, USA

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