Infrared light and crystal lattice vibrations in polar material multilayers

What is it about?

We studied here the synergistic interaction between infrared (IR) light, within a specific spectral range, and the crystal lattice vibrations inside a polar material, in the form of repeated ultrathin layers with a dielectric spacer in between them. This research aimed to harness IR beams for advancing the capabilities of IR photonic devices. Specifically, we showed how judicious control of this interaction enabled extreme beam steering and thermal images with pronounced edges. Polar materials behave like crystals made of oppositely charged ions. Many semiconductors, like Silicon Carbide, Gallium Nitride, Indium Phosphide, and others are polar. The vibrational motion of the oppositely charged ions is driven strongly by the electric field of the input IR light, and gets maximized within a particular spectral band known as the reststrahlen band. As a consequence, such a polar material, also known as a phonon-polariton material, becomes extremely highly polarized, comparably to metals where electrons can freely move within the crystal lattice. As a result, IR light in the reststrahlen band gets completely reflected when it hits the surface of a phonon-polariton materials, just like light gets reflected at the surface of a metallic mirror. This is the reason why phonon-polariton material have been largely overlooked in the past for their use in devices where strong IR beam control is required. We circumvented the inherent high reflectivity of phonon-polariton materials by taking ultrathin layers in a repeated pattern. In this way, instead of IR light completely reflecting off the interface of the phonon-polariton material, it can tunnel through it, provided each layer is extremely thin. Actually, each layer needs to be about 30 times or more smaller than the IR light wavelength in free space. Practically, since reststrahlen band wavelengths are close to or larger than ten microns, the phonon-polariton material layers must be just a few hundred nanometers thick or less; this is about 500 times smaller than the size of a human hair. Additionally, the dielectric layer spacers between the ultrathin phonon-polariton layers serve as a tuning knob to engineer the interference of IR light reflecting back and forth from all the layers. We found that such an IR light interference, is highly sensitive not only to the thickness of the dielectric layers but also to the angle at which the IR beam hits the multilayer structure. In fact, we found it is possible for the IR interference to be almost completely destructive for all incident angles, except for a narrow range of large incident angles, close to grazing incidence. Furthermore, our findings showed that this IR light interference controlled the effective wavevector inside the multilayer in a powerful way, so that this effective wave vector can even be opposite to the input wavevector and opposite to the IR light wave energy flow inside the multilayer. Think of the incident IR light as a marching band moving forward towards the multilayer structure. When the marching band crosses the interface of the multilayer structure, its members slow down for a bit, while turning their backs 180 degrees to still face the interface. Subsequently, they continue on with a backward march. In this way, the marching band continues to move in the forward direction while at the same time its members are facing the opposite direction.

Featured Image

Photo by Thom Milkovic on Unsplash

Photo by Thom Milkovic on Unsplash

Why is it important?

IR light is extremely important to modern applications, even if it cannot be seen by the naked human eye. IR light represents light emitted by warm objects and can be captured by thermal cameras. IR light can penetrate and see deeper than the surface and is used to diagnose certain cancers, in dentistry, to evaluate building structural stability as well as in art conservation. On the other hand, naturally occurring materials that can provide a strong control over IR light flow, which are needed to make devices for these applications, are hard to come by. In this article, we have provided a design route to make transparent systems with phonon- polariton material in their reststrahlen band. These materials were thought of as inappropriate for light manipulation in this spectrum. We showed how judicious design harnesses the IR light interference, coming from IR light reflecting back and forth from the individual layers of the multilayer structure. We demonstrated interference which is completely destructive for all incident angles, except for some specific range of large incident angles, close to grazing incidence. That is to say, such a system would totally reflect IR light in the reststrahlen band except for light coming almost parallel to the multilayer structure. This property is highly relevant to thermal image processing, as it would lead to pronounced edges of the thermal image, thus facilitating edge detection. Furthermore, we unveiled interference with an effective wavevector opposite to the input wavevector and opposite to the IR light energy flow inside the multilayer. We showed how to utilize this effect to make IR light superprisms, which deflect IR light nearly by 90 deg. from its original path. This is a key property for the advancement of IR photonic components.

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