Researchers have improved upon techniques that use thin films to compress infrared light, demonstrating three advantages that make the films more useful for practical applications. The researchers have proven that the "squeezed" infrared light can propagate at least four times further than previously shown; that the technology can "squeeze" a wider range of infrared wavelengths than previously demonstrated; and that the thin films can be integrated onto a variety of substrate materials and shapes.
These advances build on previous work that found a specific class of oxide membranes can compress infrared light - a useful characteristic for designing new infrared imaging technologies. The thin-film membranes confine infrared light far better than bulk crystals, which are the established technology for infrared light confinement.
"The thin film we use is a crystalline membrane of strontium titanate," says Yin Liu, co-corresponding author of a paper on the work and an assistant professor of materials science and engineering at North Carolina State University. "In our previous work, we did the characterization testing on a silicon substrate and found the material had fascinating properties but suffered from high loss. In other words, much of the light energy was lost as heat, which means the light cannot propagate very far."
"We suspected that this high loss was due to the silicon substrate, rather than the strontium titanate film itself," says Ruijuan Xu, co-corresponding author and an assistant professor of materials science and engineering at NC State. "To better understand the film's intrinsic characteristics, we suspended the thin film so that it was not in contact with a substrate and conducted a series of tests at the Advanced Light Source at Lawrence Berkeley National Laboratory. And there were two exciting results."
The first finding was that the strontium titanate had exceptionally low loss, meaning the light can propagate for a greater distance because it loses very little energy to heat.
"From an efficiency standpoint, this thin film is comparable to the most efficient polaritonic materials, which means that these films will be useful for practical applications," says Liu.
To explain polaritonic materials, we have to define phonons, photons and polaritons. Phonons and photons are both ways that energy travels through and between materials. Phonons are essentially waves of energy caused by how atoms vibrate. Photons are essentially waves of electromagnetic energy. You can think of phonons as units of sound energy, whereas photons are units of light energy. Phonon polaritons are quasi particles that occur when an infrared photon is coupled with an "optical" phonon - meaning a phonon that can emit or absorb light.
The second finding from the Advanced Light Source testing was that the thin film could confine far-infrared light, as well as mid-infrared light.
"When we tested the strontium titanate on a silicon substrate, it could only squeeze mid-infrared light," Xu says. "We knew that, theoretically, it could confine far-infrared light, but we now have the experimental evidence to prove it."
"The ability to confine far-infrared light is important from a practical standpoint," Liu says. "For example, it will be useful in engineering thermal management technologies to convert heat into infrared light. And being able to operate in a broader range of infrared wavelengths also expands the utility of these materials for developing molecular sensing technologies."
"We also think this work is important because we have demonstrated that you can take these thin films and apply them to substrates with various surface geometries - such as the one we used to suspend the thin film over empty space," Xu says. "You could also apply the thin film directly to a substrate other than silicon, such as materials that would allow the film to preserve its intrinsic 'low loss' properties.
"Our earlier work established a new class of optical materials for controlling light in infrared wavelengths, which has potential applications in photonics, sensors and thermal management," says Xu. "Our new work provides a deeper understanding of these materials and their properties, improving our ability to engineer these materials and incorporate them into practical applications."
"Another exciting aspect of these materials is that the technique we use to create these thin films is more scalable than the techniques used to create other polaritonic materials," says Liu. "Again, this helps to pave the way for practical use.
"We're open to working with industry partners who are interested in exploring ways to make use of these materials."
The paper, "Low-Loss Far-Infrared Surface Phonon Polaritons in Suspended SrTiO3 Nanomembranes," is published open access in the journal Advanced Functional Materials. First author of the paper is Konnor Koons, a Ph.D. student at NC State. The paper was co-authored by Reza Ghanbari and Yueyin Wang, Ph.D. students at NC State; Hans Bechtel and Stephanie Gilbert Corder of Lawrence Berkeley National Laboratory; and Javier Taboada-Gutiérrez and Alexey Kuzmenko of the University of Geneva.
The work was done with support from the U.S. National Science Foundation under grants 2340751 and 2442399; the American Chemical Society Petroleum Research Fund, under grant 68244-DNI10; the Advanced Light Source, a U.S. Department of Energy Office of Science User Facility, under contract DE-AC02-05CH11231; and the Swiss National Science Foundation, under grants TMPFP2_224378 and 200020_201096.
