Important progress has been made in the research of backhand optical transmission

Recently, the team of Professor Dong Jianwen of the School of Physics of Sun Yat-sen University successfully observed the backhand surface state of topological protection in a time-inverted photonic crystal system. The research paper was published in Nature Communications. Liu Jianwei is the first author of the paper, Chen Wenjie and Dong Jianwen are co-corresponding authors, and Chen Xiaodong and Chen Ke also make important contributions to the work.

With the fusion of topological photonics and integrated photonics, a large number of non-scattering loss photonic devices based on chiral edge states have been designed. In recent years, a new topological optical transmission mode different from the traditional chiral edge state, the backhand edge state, has been proposed and observed in magneto-optical photonic crystals.

However, the implementation of the backhand edge state usually needs to break the system time inversion symmetry, and the use of magneto-optical materials is difficult to generalize to the visible band. Therefore, the realization of backhand edge state in the time inversion invariant system will help to generalize the backhand transmission phenomenon to micro-nano optical systems, and provide a new scheme for light field control in integrated photonic systems.

Photonic crystal protocell structure (a) and top view (b); (c) photographs of experimental samples; (d-f) upper and lower surface backhand surface dispersion curves; (g-h) Photorobust transmission in backhand mode. Photo courtesy of the research team

In response to the above problems, the researchers cleverly use the longitudinal interlayer coupling equivalent to achieve non-reciprocal sub-adjacent intralayer coupling in two-dimensional systems (equivalent to introducing a canonical magnetic field in two-dimensional systems). In order to experimentally observe the phenomenon of backhand light transfer, they designed the optical structure required to introduce a canonical magnetic field and prepared experimental samples by stacking printed circuit boards. Using microwave near-field scanning technology combined with Fourier analysis, the research team successfully measured the optical dispersion curve and optical transmission unidirectional spectral line of the backhand transmission light mode.

Since the system canonical magnetic field can be controlled by the light field momentum kz, the researchers successfully observed the momentum dependent backhand light transmission phenomenon unique to the time inversion symmetry system, that is, the transmission direction of the backhand transmission optical mode is regulated by controlling the size of the momentum kz. In addition, by introducing boundary defects and turning structures, it is found that when the backhand surface light field encounters surface defects, the light field can continue to propagate around the defects without backscattering. Thus, the researchers demonstrated that the backchiral optical transport mode has a unidirectional, anti-scattering robust transmission property.

This work is the first to realize the backhand surface state in a time-inversion symmetric system, which is expected to introduce the backhand transport characteristic into the integrated photonics system and enrich the design freedom of integrated photonic chips. (Source: China Science News Zhu Hanbin)

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