Active topological photonics (1912.05126v2)

Abstract: Topological photonics has emerged as a novel route to engineer the flow of light. Topologically-protected photonic edge modes, which are supported at the perimeters of topologically-nontrivial insulating bulk structures, have been of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there is a sharp rise of interest in introducing gain materials into such topological photonic structures, primarily aiming at revolu-tionizing semiconductor lasers with the aid of physical mechanisms existing in topological physics. Examples of re-markable realizations are topological lasers with unidirectional light output under time-reversal symmetry breaking and topologically-protected polariton and micro/nano-cavity lasers. Moreover, the introduction of gain and loss provides a fascinating playground to explore novel topological phases, which are in close relevance to non-Hermitian and parity-time symmetric quantum physics and are in general difficult to access using fermionic condensed matter systems. Here, we review the cutting-edge research on active topological photonics, in which optical gain plays a pivotal role. We discuss recent realizations of topological lasers of various kinds, together with the underlying physics explaining the emergence of topological edge modes. In such demonstrations, the optical modes of the topological lasers are deter-mined by the dielectric structures and support lasing oscillation with the help of optical gain. We also address recent researches on topological photonic systems in which gain and loss themselves essentially influence on topological prop-erties of the bulk systems. We believe that active topological photonics provides powerful means to advance mi-cro/nanophotonics systems for diverse applications and topological physics itself as well.

Overview of Active Topological Photonics

The paper, "Active Topological Photonics," presents a comprehensive review of the emerging field of topological photonics with a particular emphasis on the role of optical gain in such systems. Utilizing the robust and exotic properties of topological insulators, this research explores novel avenues to modify and control the flow of light by harnessing topologically protected edge modes. These modes, residing at the boundary of topologically nontrivial systems, offer resilient pathways for light, impervious to conventional scattering mechanisms usually invoked by material imperfections and fabrication defects.

Key Contributions and Findings

1. Topological Lasers:
   • The paper explores the recent advancements in semiconductor lasers united with topological photonics, leading to innovations like topological microcavity lasers and polariton lasers. These devices exploit the benefits of topological edge states to achieve robust lasing, which is resistant to common disruptions inherent in traditional laser structures.
   • Lasing from chiral edge modes introduced by broken time-reversal symmetry (TRS) demonstrates unidirectional light emissions which are critical for ensuring immunity to backscatter, proving advantageous for various photonic circuits.
2. Nanocavity Lasers:
   • The paper articulates the transition from micro to nanoscale photonic topological devices. By scaling down to nanocavities, the ability to attain substantial enhancements in light-matter interactions and to reduce energy consumption is achieved. This miniaturization is vital for future applications in densely integrated photonic devices.
3. Non-Hermitian Topological Phases:
   • The introduction and exploration of non-Hermitian photonics offer a valuable contribution by investigating systems with gain and loss, which present new phases analogous to those in quantum systems but are typically harder to observe in condensed matter scenarios. These phases underline the relationship between non-Hermitian physics and emergent topological phenomena.
4. Impact of Non-Hermiticity:
   • The paper identifies how non-Hermitian features enhance the traditional understanding of topological insulators. Applying alternating gain and loss can induce distinct phases, presenting opportunities for reconfigurable photonic systems, which could lead to novel device functionalities and the exploration of dynamical topology.
5. Symmetry in Non-Hermitian Topology:
   • It explores how classification frameworks in non-Hermitian topology expand beyond the usual topological insulator paradigm, introducing new symmetry classes with implications for designing systems that exploit these symmetries.

Implications and Future Directions

The implications of this research are profound for both theoretical physics and practical photonics engineering. The potential to manipulate and harness topological properties and the robust nature of topological lasers suggest that we might soon achieve more efficient and stable laser systems adept for challenging environments or compact circuit designs. Moreover, the unique properties of active topological photonics could spur advancements not only in fiber-optic communications but also in quantum computing and advanced signal processing.

Looking ahead, exploring gain- and loss-driven topological phases could lead to dynamic photonic devices that adapt their topology based on operational conditions. This tunability and the low energy requirements of nanocavities are likely to become a keystone of future optoelectronic systems.

In summary, the intersection of topology, gain, and non-Hermiticity in photonics broadens the horizon for controlling light while opening novel pathways for fundamental and application-driven explorations in the field. The evolution of such technologies promises significant strides in both developing next-gen optical devices and understanding new physical principles.