Abstract
High index dielectric nanostructure supports different types of resonant modes. However, it is very challenging to achieve high-Q factor in a single subwavelength dielectric nanoresonator due to non-hermtian property of the open system. Here, we present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric nanocavity by exploring quasi-bound state in the continuum. Unlike conventional method relying on heavy computation (ie, frequency scanning by FDTD), our approach is built upon leaky mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair leaky modes. The Q-factor can be up to 2.3*10^4 for square subwavelength nanowire (NW) (n=4), which is 64 times larger than the highest Q-factor (Q=360) reported so far in single subwavelength nanodisk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric(magnetic) at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes (Q=380) in the scattering spectrum of a single silicon subwavelength nanowire.
Highlights
The Q-factor of a cavity is defined as the energy dissipation per unit circle versus the energy stored in the resonator
We demonstrated that from the leaky mode perspective, many QBIC can be found in a single dielectric nanocavity with a rectangular cross section by constructing avoid-crossing of pair modes
In previous work,[11,26,27] we have demonstrated that the leaky modes supported by the single dielectric nanostructure play the dominant role in describing its optical properties
Summary
The Q-factor of a cavity is defined as the energy dissipation per unit circle versus the energy stored in the resonator. It turns out that the high-Q modes can be treated as a superposition of TEðm; lÞ and TEðm − 2; l þ 2Þ or TEðm; lÞ and TEðm þ 2; l − 2Þ modes accompanied by the avoid-crossing features of the real part of the eigenvalues at a given size ratio R. The strong confinement of the electric field corresponds to the suppression of the radiation in limited leaky channels or radiation quenching to a minimum in the momentum space. This conclusion can be generalized to other geometries, such as rectangular NW with transverse magnetic polarization, single cylinder with finite thickness, cuboid, etc. Our results may find applications in boosting light– matter interaction, such as the nonlinear optics effect, strong coupling, and lasers
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