Abstract
Light trapping is a constant pursuit in photonics because of its importance in science and technology. Many mechanisms have been explored, including the use of mirrors made of materials or structures that forbid outgoing waves, and bound states in the continuum that are mirror-less but based on topology. Here we report a compound method, combining lateral mirrors and bound states in the continuum in a cooperative way, to achieve a class of on-chip optical cavities that have high quality factors and small modal volumes. Specifically, light is trapped in the transverse direction by the photonic band gap of the lateral hetero-structure and confined in the vertical direction by the constellation of multiple bound states in the continuum. As a result, unlike most bound states in the continuum found in photonic crystal slabs that are de-localized Bloch modes, we achieve light-trapping in all three dimensions and experimentally demonstrate quality factors as high as Q=1.09×106 and modal volumes as low as V=17.74(λ0/n)3 in the telecommunication regime. We further prove the robustness of our method through the statistical study of multiple fabricated devices. Our work provides a new method of light trapping, which can find potential applications in photonic integration, nonlinear optics and quantum computing.
Highlights
While most light-trapping methods rely on the use of mirrors to forbid radiation, it is recently realized that optical bound states in the continuum (BICs) provide an alternative approach
BICs can be understood as topological defects[36,37,38,39,40]: for example, they are fundamentally vortices in the far field polarization in photonic crystal (PhC) slabs, each carrying an integer topological charge[41]
Manipulations of these topological charges have led to interesting consequences, including resonances that become more robust to scattering losses[13] and unidirectional guided resonances that only radiate towards a single side without the use of mirrors on the other[42]
Summary
Numerical simulation All simulations are performed using the COMSOL Multiphysics in the frequency domain. J. et al Observation and differentiation of unique high-Q optical resonances near zero wave vector in macroscopic photonic crystal slabs. B. et al Ultralow-threshold electrically pumped quantum-dot photonic-crystal nanocavity laser. Resonant χ(2) nonlinear photonic crystal cavity based on a bound state in the continuum. F. Experimental observation of a polarization vortex at an optical bound state in the continuum. Topological nature of optical bound states in the continuum. Low-threshold lasing action in photonic crystal slabs enabled by fano resonances. Three-dimensional coupled-wave analysis for square-lattice photonic crystal surface emitting lasers with transverse-electric polarization: Finite-size effects. Analytical perspective for bound states in the continuum in photonic crystal slabs. Z. et al High-Q quasibound states in the continuum for nonlinear metasurfaces. B. et al Enabling enhanced emission and low-threshold lasing of organic molecules using special fano resonances of macroscopic photonic crystals.
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