Abstract
Bound states in the continuum (BICs) and exceptional points (EPs) have been the subjects of recent intensive research as they exhibit exotic phenomena that are significant for both fundamental physics and practical applications. We investigated the emergence of the Friedrich–Wintgen (FW) type BIC and the EP in a dielectric waveguide comprising a metal grating, focusing on their dependence on the grating thickness. The BIC emerges at a branch near the anti-crossing formed of the two waveguide modes, for a grating of any thickness. With the grating-thickness change, the anti-crossing gap varies and the branch at which the BIC appears flips. We show that, when the slit is single mode, the BIC appears in the vicinity of the crossing between the two waveguide modes in the empty-lattice (zero slit-width) limit, while the results satisfy the criteria for the branch at which the BIC appears in the previous reports. In addition, we find that the EP appears near the BIC in the same device only on selecting the grating thickness. The BIC and EP in the dielectric waveguide comprising a metal grating, particularly with such tunability, are expected to result in the development of functional and high-performance photonic devices in addition to being a platform for the fundamental research of non-Hermitian systems.
Highlights
Resonant oscillations in open resonator systems normally decay with time because of the radiative dissipation of the energy
In our previous report[21], we presented that, in a dielectric waveguide connected with the far field through a metal grating, the branch at which the bound state in the continuum (BIC) appears depends on the polarization of the mode, which is explained by the difference in the parameter describing the coupling phase of the modes with the external radiation, in a consistent manner with the original theory of Friedrich and Wintgen[13]
We reported the emergence of the FW-BIC and exceptional points (EPs) in a dielectric waveguide comprising a metal grating, while focusing on their dependence on the grating thickness
Summary
Resonant oscillations in open resonator systems (the system that possesses the radiation port to the far region) normally decay with time because of the radiative dissipation of the energy. Two types of BIC, the symmetry-protected BIC and accidental or Friedrich-Wintgen (FW) BIC[13], have been mostly investigated The former originates from the incompatibility of the symmetry between the resonant mode and external radiation and appears at highly symmetric points such as the Γ point in the reciprocal space of the periodic structure. In our previous report[21], we presented that, in a dielectric waveguide connected with the far field through a metal grating, the branch at which the BIC appears depends on the polarization of the mode, which is explained by the difference in the parameter describing the coupling phase of the modes with the external radiation, in a consistent manner with the original theory of Friedrich and Wintgen[13]. The continuous controllability of the system with the use of such an additional parameter is expected to encourage experimental investigations and the application of the BIC- and EP-related phenomena in optics and photonics[37]
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