Parity–time symmetry and exceptional points in photonics

Exploiting the interplay between gain, loss and the coupling strength between different optical components creates a variety of new opportunities in photonics to generate, control and transmit light. Inspired by the discovery of real eigenfrequencies for non-Hermitian Hamiltonians obeying parity–time (PT) symmetry, many counterintuitive aspects are being explored, particularly close to the associated degeneracies also known as ‘exceptional points’. This Review explains the underlying physical principles and discusses the progress in the experimental investigation of PT-symmetric photonic systems. We highlight the role of PT symmetry and non-Hermitian dynamics for synthesizing and controlling the flow of light in optical structures and provide a roadmap for future studies and potential applications.

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Fig. 1: PT symmetry.
Fig. 2: PT symmetry and EPs in optical systems.
Fig. 3: EPs and PT symmetry in lasers.
Fig. 4: Topological photonics in non-Hermitian systems.

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Acknowledgements

The authors thank A. Brandstötter, M. Horodynski, M. Kühmayer, K. Pichler, A. Schumer and H. Yilmaz, for their help with the figures and illustrations. Ş.K.Ö. is supported by Army Research Office (ARO) grant no. W911NF-16-1-0339, W911NF-18-1-0043, Air Force Office of Scientific Research (AFOSR) award no. FA9550-18-1-0235, National Science Foundation (NSF) (1807485), and by The Pennsylvania State University, Materials Research Institute (MRI). Ş.K.Ö. thanks Jan Mateo for his continuous support. S.R. is supported by the Austrian Science Fund (FWF) through projects no. SFB-NextLite F49-P10 and I 1142-N27, and by the Horizon 2020 programme of the European Union (RISE project NHQWAVE 691209). F.N. is supported in part by the MURI Center for Dynamic Magneto-Optics via AFOSR award no. FA9550-14-1-0040, ARO grant no. W911NF-18-1-0358), Asian Office of Aerospace Research and Development (AOARD) grant no. FA2386-18-1-4045, Japan Science and Technology Agency (JST) (Q-LEAP program, ImPACT program, and CREST grant no. JPMJCR1676), Japan Society for the Promotion of Science (JSPS) (JSPS-RFBR grant no. 17-52-50023, and JSPS-FWO grant no.VS.059.18N), RIKEN-AIST Challenge Research Fund, and the John Templeton Foundation.

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Ş.K.Ö. and S.R. wrote the manuscript with contributions from F.N. All authors read and agreed with the content and discussions in the manuscript.

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Özdemir, Ş.K., Rotter, S., Nori, F. et al. Parity–time symmetry and exceptional points in photonics. Nat. Mater. 18, 783–798 (2019). https://doi.org/10.1038/s41563-019-0304-9

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