Non-Hermitian physical systems have attracted considerable attention lately for their unconventional behaviour around exceptional points (EPs)—spectral singularities at which eigenvalues and eigenvectors coalesce. In particular, many new EP-related concepts such as unidirectional lasing and invisibility, as well as chiral transmission, have been realized. Given the progress in understanding the physics of EPs in various photonic structures, it is surprising that one of the oldest theoretical predictions associated with them, a remarkable broadening of the laser linewidth at an EP, has been probed only indirectly so far. Here, we fill this gap by steering a phonon laser through an EP in a compound optomechanical system formed by two coupled resonators. We observe a pronounced linewidth broadening of the mechanical lasing mode generated in one of the resonators when the system approaches the EP.
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This work was supported by NSF grant no. EFMA1641109, ARO grant no. W911NF1210026, ARO grant no. W911NF1710189 and the European Commission under project NHQWAVE (MSCA-RISE 691209). S.K.O. was supported by ARO grant no. W911NF-16-1-0339. S.K.O thanks J. Mateo for his continuous support. J.Z. is supported by the NSFC under grant nos. 61622306, 11674194. Y.-X.L. is supported by the NSFC under grant no. 61025022. Y.-X.L. and J.Z. are supported by the National Basic Research Program of China (973 Program) under grant no. 2014CB921401, the Tsinghua University Initiative Scientific Research Program and the Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation. F.N. is partially supported by the MURI Center for Dynamic Magneto-Optics via AFOSR Award no. FA9550-14-1-0040, Asian Office of Aerospace Research and Development (AOARD) (grant no. FA2386-18-1-4045), the IMPACT program of JST, JSPS-RFBR grant no. 17-52-50023, CREST grant no. JPMJCR1676, RIKEN-AIST Joint Research Fund and the Sir John Templeton Foundation. K.P., D.O.K. and S.R. are supported by the Austrian Science Fund (FWF) through project no. SFB NextLite F49-P10. H. Yilmaz prepared the chromium-coated silica nanofibre tip for the experiments.
Supplementary discussion and experimental details; Supplementary Figures 1–9; Supplementary References 1–15.
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