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Photonic alloys, alloy-like materials combining two or more photonic crystals, are promising candidates for the development of structures that control the propagation of electromagnetic waves, also known as waveguides. Despite their potential, these materials typically reflect light back in the direction from which it originated.

This phenomenon, known as backscattering of light, limits the transmission of data and energy, adversely affecting the performance of materials as waveguides. Thus, reliably reducing or preventing backscattering of light in photonic alloys will be a key cornerstone towards the practical use of these materials.

Researchers from Shanxi University and the Hong Kong University of Science and Technology recently developed a new photonic alloy with topological properties that allows microwaves to propagate without backscattering light. This material introduced in Physical examination lettersmay pave the way for the development of new topological photonic crystals.

“Our paper introduces a new concept: the topological photonic alloy as a non-periodic topological material,” Lei Zhang, co-author of the paper, told Phys.org. “We achieved this by combining unmagnetized and magnetized rods in a non-periodic 2D photonic crystal configuration. This created photonic alloys that support chiral end states in the microwave regime.”

The main goal of the recent study by Zhang and his colleagues was to develop a new photonic alloy exhibiting a topological end state, drawing inspiration from the unique physical properties of alloys. The researchers created their material by randomly mixing yttrium iron garnet (YIG) rods and magnetized YIG rods composed of substitutional or interstitial alloys.

“In our experimental setup, a vector network analyzer is used to establish connections between the source and probe antennas,” Zhang explained. “The output antenna is fixed at a certain position within the sample, while the position of the probe antenna is changed to collect valuable information about the intensity and phase of the electromagnetic waves. To facilitate this process, there are circular holes in a metal plate, through which both antennas are placed.”

Zhang and his colleagues used a metal cladding that serves as a “topologically trivial material,” with a Chern number of zero. When this shell covers a photonic topological insulator with a Chern number of 1, a topological end state appears at their boundary, in accordance with the volume edge correspondence principle.

“The microwave absorber in this setup is to suppress the transmission of boundary states,” Zhang said. “By using the absorber, we prevent the formation of a closed loop throughout the boundary state, which can prevent accurate characterization of nonreciprocal phenomena.”

Experiments conducted by this team of researchers showed that their topological photonic alloy even exhibits topological properties with low doping concentration of magnetized rods without requiring order. This remarkable discovery may open up new possibilities for the experimental realization of topological end states, as it suggests that chiral end states can be produced without breaking time-reversal symmetry throughout the crystal.

“In our next studies, we plan to investigate multi-component topological photonic alloy systems,” Zhang added. “Multicomponent systems possess a greater number of degrees of freedom, which allows the manipulation of different parameters and leads to a wider range of intriguing effects. Furthermore, we soon plan to explore the possibility of realizing similar phenomena at optical frequencies, and establishing the relevance of these results for photonics applications would be very intriguing.”

Zhang and his colleagues hope to soon extend their recent findings to the optical domain. This would potentially open up new opportunities for light manipulation and the development of innovative photonic devices.