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Proceedings ArticleDOI

Ultra-high sensitivity near multiple bound states in the continuum in microcavity resonators

TL;DR: In this paper, the topological properties of multiple optical bound states in the continuum using continuous parameter tuning were studied using the formation of ultra-high-Q resonances to investigate the potential of quality-sensing in a speciality optical microcavity.
Abstract: We study the topological properties of multiple optical bound states in the continuum using continuous parameter tuning. We utilize the formation of ultra-high- Q resonances to investigate the potential of quality-sensing in a speciality optical microcavity.
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Journal ArticleDOI
TL;DR: Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away.
Abstract: Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. The fascinating wave phenomenon of ‘bound states in the continuum’ spans different material and wave systems, including electron, electromagnetic and mechanical waves. In this Review, we focus on the common physical mechanisms underlying these bound states, whilst also discussing recent experimental realizations, current applications and future opportunities for research.

1,612 citations

Journal ArticleDOI
TL;DR: In this article, the authors use Feshbach's theory of resonances to demonstrate that bound states in the continuum (BIC's) can occur due to the interference of the resonances belonging to different channels.
Abstract: We use Feshbach's theory of resonances to demonstrate that bound states in the continuum (BIC's) can occur due to the interference of resonances belonging to different channels. If two resonances pass each other as a function of a continuous parameter, then interference causes an avoided crossing of the resonance positions and for a given value of the continuous parameter one resonance has exactly vanishing width and hence becomes a BIC. The condition for a BIC relates the positions of the noninterfering resonances with the coupling matrix elements between the various channels. In the neighborhood of the BIC point one resonance remains anomalously narrow for a finite range of values of the separation of the noninterfering resonances. Whether or not two resonances interfere is not directly related to whether or not they overlap. All these results, including the occurrence of exactly bound states in the continuum, are not consequences of approximations inherent in Feshbach's theory but are general features of a coupled-channel Schr\"odinger equation with only one open channel. We illustrate the results in a simple but realistic model, where all matrix elements involved can be calculated analytically. We also discuss the case of coupled Coulombic channels where BIC's are caused by perturbations interfering with a Rydberg series of autoionizing resonances. Below the continuum threshold the analogy to a BIC is an infinitely narrow perturbation of the bound-state spectrum. Near such an infinitely narrow perturbation we may observe approximate level crossings.

525 citations

Journal ArticleDOI
TL;DR: The presence of these high-quality modes at both fundamental and second-harmonic wavelengths leads to an extremely high enhancement in second harmonic generation, thus preluding a framework to fabricate composite media with high effective nonlinearity.
Abstract: We reveal the potential of bound states in the continuum (BIC) to enhance the nonlinear response in specialty optical resonators in the presence of gain and loss. We demonstrate this phenomenon in a square core–shell AlGaAs nanowire having a proper engineered spatial variation of gain and loss to sustain quasi–BICs. The presence of these high-quality modes at both fundamental and second-harmonic wavelengths leads to an extremely high enhancement in second harmonic generation, thus preluding a framework to fabricate composite media with high effective nonlinearity.

21 citations