In 2016, David J. Thouless, F. Duncan M. Haldane, and J. Michael Kosterlitz received the Nobel Prize in Physics for their theoretical discovery of topological phase transitions and topological phases of matter.

Their work established the foundation for a new class of matter: topological materials, which exhibit unique properties, stemming from the topological non-triviality of their band structures. A key example is the topological edge states found in topological insulators.

Topological properties are not just confined to electronic systems. They can also manifest in light waves in periodic systems, such as photonic crystals (PCs), where light propagation is governed by the photonic band structure. Indeed, topologically non-trivial band gaps and topological edge states of photons have been predicted and experimentally confirmed. This has given rise to the field of topological photonics, which is rapidly gaining interest due to its potential applications such as topological photonic-integrated circuits.

Compared to the electronic case, topological photonics has several distinct features. Topological properties in PCs can be realized over a wide range of frequencies, from microwaves to visible light. Testing new ideas is straightforward, often requiring a simple modification to computer aided design data. In addition, it is possible to directly measure photon band structure and polarization state. From an engineering perspective, topological photonics can enable one-way waveguides and one-way ring resonators.

To highlight recent progress, a new special topics issue published in the Journal of the Physical Society of Japan presents five cutting-edge review articles.

Prof. Satoshi Iwamoto presents a comprehensive review covering the recent advances in valley PC (VPC) waveguides, with a special focus on the experimental realization of semiconductor-based waveguides. He also investigated the recently developed heterostructure VPC waveguides.

Isobe et al. explored photonic systems beyond the conventional topological band theory, particularly those described by generalized eigenvalue equations and analysed the symmetry-protected points in the PC.

A study by Takahashi et al. focused on topological surface states and Weyl points in PCs, discussing their theoretical foundations and experimental demonstrations, including their recent experimental results on a simple three-dimensional PC in a microwave regime.

A detailed review by Yao et al. explored the design and fabrication of topological PCs on silicon-on-insulator wafers, focusing on experimental verification of double Dirac cones and topologically protected edge modes in the mid-infrared range.

Finally, Ameniya et al. discussed the application of topological waveguides as part of topological photonic-integrated circuits for controlling internal degrees of freedom of light.

As a novel route to controlling light, topological photonics has applications in a variety of fields, from optical communications and signal processing to sensing and quantum technology. Looking forward, continued advances can yield more compact and integrated topological photonic devices, opening new avenues in both physics and engineering.

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