Optical Tamm states-induced tunable triple-band spin-dependent near-perfect absorption in a magnetic Weyl semimetal-Fibonacci heterostructure

In the world of light and matter, scientists have long sought a way to sort light based on its "spin," a fundamental property that makes light waves twist either to the left or to the right. This ability is crucial for the next generation of computing and communication, where information could be carried by the direction of this twist rather than just the intensity of the beam. Traditionally, achieving this sort of separation required bulky equipment and strong external magnets to force the light to behave in a specific way. However, a new class of materials known as magnetic Weyl semimetals offers a different path. These are solid crystals that, due to their unique internal structure, naturally create a magnetic-like effect for electrons without needing any outside help. This intrinsic property allows them to interact differently with left-twisting and right-twisting light, even in a completely magnet-free environment. The challenge has been that while these materials are excellent at distinguishing between the two types of light, they are not very good at capturing or absorbing it. To build useful devices, researchers need a way to trap the light inside the material long enough to be absorbed, and they need to do this for multiple colors of light at the same time, not just one.

A team of researchers has now proposed a solution that combines these magnetic crystals with a specially engineered stack of transparent layers to create a device capable of absorbing nearly all incoming light at three distinct frequencies simultaneously. The structure they designed places a thin film of the magnetic material on top of a sequence of dielectric layers arranged in a pattern known as a Fibonacci sequence. This pattern is not random, nor is it a simple repeating loop; instead, it follows a mathematical rule where the arrangement of layers grows in a way that creates a self-similar, fractal-like structure. When light hits this stack, it gets trapped in a specific zone between the magnetic film and the layered pattern, a phenomenon known as an optical Tamm state. Think of this like a musical instrument where the shape of the body allows it to resonate perfectly at three specific notes at once, rather than just one. In this case, the "notes" are frequencies of light in the mid- to far-infrared range, and the "instrument" is the engineered stack of layers.

The researchers used advanced computer simulations to model how this structure would behave when struck by light. They found that the device successfully absorbs nearly all of the light that hits it, but with a remarkable twist: it absorbs left-twisting light and right-twisting light at slightly different frequencies. This means that at any given moment, the device is acting as a perfect sponge for one type of spin while letting the other pass through or reflect. This separation happens naturally across three different bands of the spectrum, effectively creating three independent channels for processing information. The study shows that the position and strength of these absorption peaks can be finely tuned by adjusting the physical properties of the materials. For instance, changing the internal spacing of the magnetic atoms or the energy level of the electrons within the crystal shifts the frequencies where absorption occurs. Similarly, altering the thickness of the spacer layer or the refractive index of the materials in the stack allows scientists to dial the absorption up or down, or move the entire set of three channels to a different part of the spectrum.

One of the most significant findings is that the researchers can control the three absorption bands using distinct parameters that act on the system as a whole. By tuning the Weyl node separation, the Fermi level, or the dielectric layer thickness, they can independently adjust the resonance frequency and absorption intensity of the three bands. The high-index layers in the stack proved to be particularly sensitive, where tiny changes in thickness resulted in large shifts in the light frequencies, while other layers allowed for more gentle adjustments. This level of control suggests that the design could be used to build compact, integrated devices that handle multiple streams of spin-based information in parallel. Unlike older designs that could only manage a single channel of light, this approach breaks that limitation by using the complex, multi-layered architecture to support multiple resonances at once. The work demonstrates that by combining the intrinsic magnetic properties of Weyl semimetals with the unique light-trapping abilities of quasi-periodic structures, it is possible to create a system that is both highly efficient and highly tunable, paving the way for more sophisticated spin-based photonic technologies.