The optical mode coupling of a twin nano-cavity is theoretically and experimentally studied. The twin nano-cavity has a structure of Metal-Dielectric-Metal-Dielectric-Metal, and it is a non-Hermitian system. Linear and non-linear optical mode coupling are obtained when the nano-cavity is optical excited. When one of the dielectric layers is optically active, non-linear mode coupling is obtained and it is a function of the excitation power; when the excitation power does not generate significant 𝜒(3) , then the mode coupling is in linear regime. However, when the excitation power is at a value that 𝜒(3) contribution becomes significant, the mode coupling is then in the nonlinear regime. Furthermore, through varying the thickness of metal partition layer between two cavities, regimes where strong and weak coupling between the two nano-cavities were identified. In strong coupling, there is a clear mode splitting. However, in weak coupling regime the mode splitting becomes too close to be distinguished. Moreover, the vanishing reflection leads to unidirectional reflectionless propagation, which is also known as unidirectional invisibility [1]. If an optically active material is introduced replacing one of the dielectric layers. This produces additional novel optical characteristics; under high incident power nonlinear effect becomes significant, leading to reciprocity broken and optical bistability observed [2]. However, at low incident power when non-linear optical effect is not significant, double exceptional points are obtained similar to the result obtained with the Metal-Dielectric-Metal-Dielectric-Metal heterostructure described above. Initial experimental work agrees well with the theoretical results. This work lays the basis for designing novel non-Hermitian photonic devices with characteristics such as double exceptional points and bistable optical transmission/reflection. The proposed heterostructure has potential applications in optical communications, optical sensing, photo-detection, power regulator and nano-photonic devices.
 

1. Jianming Mai, Yu Chen, Guixin Li and Kok Wai Cheah, Optics Express, 30(22), 40053-40062 (2022).
2. Jianming Mai and Kok Wai Cheah, Optics Express, 30(26), 46357-46265 (2022).

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