Robust bound states in the continuum in a dual waveguide system

Bound states in the continuum(BICs)provide a fascinating platform to route/manipulate waves with ultralow loss by patterning low-refractive-index materials on a high-refractive-index substrate.Principally,the phase of leaking channels can be manipulated via tuning the structural parameters to achieve destructive interference(i.e.,the BIC condition),surprisingly leading to the total elimination of dissipation to the continuum of the substrate.Despite recent developments in BIC photonics,the BIC conditions can only be satisfied at specified geometric sizes for waveguides that dim their application prospects.Here,we propose a dual waveguide system that support BICs under arbitrary waveguide sizes by solely changing the intervals between the two waveguides.Our calculation results show that robust BICs in such architectures stem from the interaction(destructive interference)between leaking waves from the two waveguides.Furthermore,a cladding layer is introduced to improve the fabrication tolerance and reduce the sensitivity of the low-loss condition on the waveguide intervals of the presented dual waveguide system.The proposed approach offers an intriguing solution to establish a BIC concept and may be helpful to improve the potential of BIC photonic devices and circuits.

Indefinite metacavities coupled to a mirror:bound states in the continuum with anomalous resonance scaling

Indefinite metacavities(IMCs)made of hyperbolic metamaterials show great advantages in terms of extremely small mode volume due to large wave vectors endowed by the unique hyperbolic dispersion.However,quality(Q)factors of IMCs are limited by Ohmic loss of metals and radiative loss of leaked waves.Despite the fact that Ohmic loss of metals is inevitable in IMCs,the radiative loss can be further suppressed by leakage engineering.Here we propose a mirror coupled IMC structure which is able to operate at Fabry–Pérot bound states in the continuum(BICs)while the hyperbolic nature of IMCs is retained.At the BIC point,the radiative loss of magnetic dipolar cavity modes in IMCs is completely absent,resulting in a considerably increased Q factor(>90).Deviating from the BIC point,perfect absorption bands(>0.99)along with a strong near-field intensity enhancement(>1.8×10^(4))appear when the condition of critical coupling is almost fulfilled.The proposed BICs are robust to the geometry and material composition of IMCs and anomalous scaling law of resonance is verified during the tuning of optical responses.We also demonstrate that the Purcell effect of the structure can be significantly improved under BIC and quasi-BIC regimes due to the further enhanced Q factor to mode volume ratio.Our results provide a new train of thought to design ultra-small optical nanocavities that may find many applications benefitting from strong light–matter interactions.

Sidelobe suppression and axial resolution enhancement in 4pi microscopy with higher-order radially polarized Laguerre–Gaussian beams using subtractive imaging

Subtractive imaging is used to suppress the axial sidelobes and improve the axial resolution of 4 pi microscopy with a higher-order radially polarized(RP)Laguerre–Gaussian(LG)beam.A solid-shaped point spread function(PSF)and a doughnut-shaped PSF with a dark spot along the optical axis are generated by tightly focusing a higher-order RP-LG beam and a modulated circularly polarized beam,respectively.By subtracting the two images obtained with those two different PSFs,the axial sidelobes of the subtracted PSF are reduced from 37% to about 10% of the main lobe,and the axial resolution is increased from 0.21λ to 0.15λ.

Solid-state nanopores for ion and small molecule analysis

Solid-state nanopore in analytical chemistry has developed rapidly in the 1990s and it is proved to be a versatile new tool for bioanalytical chemistry. The research field of solid-state nanopore starts from mimicking the biological nanopore in living cells. Understanding the transport mechanism of biological nanopore in vivo is a big challenge because of the experimental difficulty, so it is essential to establish the basic research of artificial nanopores in vitro especially for the analysis of ions and small molecules. The performance of solid-state nanopores could be evaluated by monitoring currents when ions and molecules passed through. The comparison of the two types of nanopores based on current-derived information can reveal the principle of biological nanopores, while the solid-state nanopores are applied into practical bioanalysis. In this review, we focus on the researches of the solid-state nanopores in the fabrication process and in the analysis of ions and small molecules. Fabrication methods of nanopores,ion transport mechanism, small molecule analysis and theoretical studies are discussed in detail.