Direct observation of exceptional points in coupled photonic-crystal lasers with asymmetric optical gains

• Published in: Nature communications Vol. 7; no. 1; p. 13893
• Main Authors: Kim, Kyoung-Ho, Hwang, Min-Soo, Kim, Ha-Reem, Choi, Jae-Hyuck, No, You-Shin, Park, Hong-Gyu
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 21.12.2016; Nature Publishing Group; Nature Portfolio
• Subjects: 639/624/399/1022; 639/624/400; 639/624/400/1113; Graphene; Humanities and Social Sciences; multidisciplinary; Phase transitions; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms13893

Although counter-intuitive features have been observed in non-Hermitian optical systems based on micrometre-sized cavities, the achievement of a simplified but unambiguous approach to enable the efficient access of exceptional points (EPs) and the phase transition to desired lasing modes remains a challenge, particularly in wavelength-scale coupled cavities. Here, we demonstrate coupled photonic-crystal (PhC) nanolasers with asymmetric optical gains, and observe the phase transition of lasing modes at EPs through tuning of the area of graphene cover on one PhC cavity and systematic scanning photoluminescence measurements. As the gain contrast between the two identical PhC cavities exceeds the intercavity coupling, the phase transition occurs from the bonding/anti-bonding lasing modes to the single-amplifying lasing mode, which is confirmed by the experimental measurement of the mode images and the theoretical modelling of coupled cavities with asymmetric gains. In addition, we demonstrate active tuning of EPs by controlling the optical loss of graphene through electrical gating. Non-Hermitian optical systems have been shown to sustain lasing when they go from a PT-symmetric to a PT-symmetry-broken phase. Here, Kim et al . study this phase transition of lasing modes in partially graphene-covered coupled microcavities and show tuning of an exceptional point.