Out of many possible impacts of topology in optics, the one-way edge states existing on an interface between two topologically distinct media has drawn a lot of attention. These guiding modes flow in only one direction and can be scattering-immune to defects, allowing very recent demonstrations such as topological lasers with robust operation [1–3]. We expect this to be just the beginning of topological optical devices in giving either unconventional or robust configuration. To achieve these exciting possibilities, current works seek to realize the photonic analogs of different exotic topological states of matter in condensed matter physics and to explore unique connections of topological invariants, e.g. Chern numbers, to optical properties at the same time. It is worth mentioning that one huge advantage in employing topology in optics is that the required complex structures and material responses can be easily tailor-made by artificial materials such as photonic crystals and metamaterials. There also exist many open questions whether photons which are integer spin bosonic particles can show fundamental topological behavior similar to electrons which are half-integer spin fermionic particles.

It is the aim of this Feature Issue of Optics Express to present some of the research lines that have been recently devoted to topological photonics and materials.

An optical system can have symmetry-protected states, e.g. a Dirac cone in the band structure, and become gapped by breaking that symmetry while the bands above and below become topologically non-trivial. One-way edge states can exist within such a photonic band gap. On the other hand, some topologically gapless systems, such as those contain Weyl points, can support other topological states. Lu et.al [4]. systematically reviews these approaches to generate one-way edge states. Different kinds of symmetry considerations are also discussed in order to give topological non-trivialness. The edge states can then be used to construct useful devices. Hang et.al. demonstrates a resonating cavity as an example [5].

One usual way to generate a topologically non-trivial system is by breaking time-reversal symmetry. It generates a pseudo magnetic field, which is intimately connected to nontrivial topology. Wang et. al. [6] applies dynamic modulation to break such symmetry with controllable parameter. It allows a phase transition from type-I to type-II Weyl points. In practice, dynamic modulation in time can be mimicked by a spatial modulation of waveguides in the propagation direction [7].

There is also an intuitive way to generate topological systems. We can build up two dimensional systems from the Su-Schrieffer–Heeger model, a chain of alternating structures or couplings, which has non-trivial topology in one dimension already. Lieb, Kagome or honeycomb lattices are some of the candidates, which can be used also to generate flat bands for slow light or localized orbital manipulations. Lu et.al. investigates the coexistence of edge states and flat bands [8]. Schomerus extends the discussion of topology of these lattices to the nonlinear regime, in which saturable gain and loss can be added and is useful in considering lasering action [9]. Non-hermiticity can also be exploited in these waveguides to pick up the desired mode for lasing [10]

A pseudo magnetic field in many topological systems can be generated by breaking time-reversal symmetry. It can also be generated by spin-orbit coupling. In this issue, we also incorporate several articles in using spin-orbit coupling for applications. These can give some insight for future topological applications. Jacob et. al. uses it to generate spin-dependent optical force for a particle on a non-reciprocal waveguide [11]. Arakawa and Fong et.al., couple the orbital angular momentum of thelight to the spin angular momentum in the electronic degrees of freedom [12]. Cai et.al. investigates how orbital angular momentum of light can be transmitted through a random medium with turbulence [13].

Last but not least, we are looking forward to more optical properties that can be regarded as direct consequences of topology. An actual topological insulator, e.g. Bi2Te3. can be used as an efficient saturable absorber for Q-switching [14]. A jump in Goos-Hänchen shift can be used to reveal a corresponding transition of topological invariant [15].

In summary, topology is emerging as a powerful way to understand optics or to design optical components. The articles presented in this Feature Issue may serve not only a current summary of different approaches in this research field, but may also encourages future works on both theory and optical applications.

We also would like to thank all the authors who have contributed to this issue and would like to thank Andrew Weiner, John Long and Carmelita Washington in Optical Express for putting up this Feature Issue.