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Study Sheds Light on Optical Refractive Index Sensing of High Sensitivity and Ultra-High FOM

An Optics Communications pre-proof study proposed a novel optical refractive index sensor design based on electromagnetic multipolar quasi-bound states in the continuum (BICs) to enhance its figure of merit (FOM). This creates opportunities for ultra-high FOM optical refractive index sensing with high sensitivity.

Study: Electromagnetic multipolar quasi-bound states in the continuum for optical sensing. Image Credit: MOLPIX/

An optical sensor with a high figure of merit (FOM) is essential for a wide range of sensing applications. The FOM of an optical sensor can be raised significantly by employing bound states in the continuum (BIC) due to its theoretically infinite Q factor (a measure of resonator modes' damping).

This study reveals magnetic dipole (MD) quasi-BIC is the most suitable option among electromagnetic quasi-BICs for effective optical refractive index sensing.

Significance of Optical Refractive Index Sensors

Optical refractive index sensors based on metasurface, dielectric grating, surface plasmon resonance (SPR), PhC, and fiber have been extensively employed in biomedical and environmental monitoring because of their compact size and high accuracy. The FOM and sensitivity can adequately describe a sensor's performance.

The introduction of bound states in the continuum (BIC) concept in optical imaging has enhanced the performance of all-optical refractive index sensors because the frequency of BIC has an ideally infinite Q factor while being part of the radiation continuum's spectrum.

BICs are categorized based on their primary electromagnetic multipolar features, such as magnetic dipole (MD) BIC, electric dipole (ED) BIC, and toroidal dipole (TD) BIC.

BICs demonstrate several novel possibilities and capabilities, such as high Q factor lasers, micro-cavities, optical sensors, and severe diffraction control.

The ultrahigh Q factor is achieved by shifting its BIC to quasi-BIC, which significantly increases optical sensing performance. The electromagnetic multi-pole expansion examines the distribution of magnetic and electric fields in quasi-BICs.

Although the far-field quasi-BIC of all-dielectric metasurfaces exhibit Fano resonances with sharp and asymmetric profiles, their near-field electromagnetic patterns are radically different.

BIC-based optical refractive sensors have been experimentally validated and designed on an industrial level. However, no significant research has thoroughly examined the impact of various electromagnetic multipolar BICs on optical refractive index sensing.

Using Electromagnetic Multipolar Quasi-Bound States in the Continuum for Optical Sensing

In this study, the performance of electromagnetic multipolar quasi-BICs-mediated optical refractive index sensors on a silicon (Si) metasurface was numerically investigated and analyzed.

The optical refractive index sensors were examined using Eigen-solution and excitation simulation. The parameters of eigenmodes for the all-dielectric metasurface were calculated by the finite element technique (FEM).

The electric field intensity filling ratio (IFR) helped validate the differentiation of sensitivity of various electromagnetic multipolar quasi-BIC modes. IFR was determined as the ratio of the whole resonant near-electric field's field intensity to the sensing region's electric field intensity.

The performance of the optical refractive index sensor based on specific electromagnetic multipolar quasi-BICs of the proposed metasurface was validated by utilizing the FEM to compute the dependency of reflection on refractive index change for quasi-BICs modes.

The effect of the refractive index of the surroundings on the resonant wavelengths and accompanying Q factors of electromagnetic multipolar quasi-BICs was investigated to obtain the required sensitivity.

Important Findings of the Study

H. Chen et al. designed an optical refractive index sensor in an all-dielectric metasurface based on electromagnetic multipolar quasi-BICs. The proposed magnetic dipole (MD) quasi-BIC was the best option for high FOM optical sensing due to its unique electromagnetic near-field distribution.

Sensitivity and figure of merit are two crucial variables to consider when assessing the performance of an optical refractive index sensor. High sensitivity does not guarantee a low detective if the resonant peak is too wide.

On the other hand, the optical index sensor with a high FOM can detect more precisely in the case of small volume structures and weak ambient refractive index change because FOM is connected to sensitivity and the Q factor of resonance.

BICs with infinite Q factors are turned into quasi-BICs with finite Q factors by violating the symmetry of their unit cell. This allows them to interact effectively with the incident light.

The proposed all-dielectric metasurface had the highest sensitivity (225.27 nm/RIU) and maximum FOM (441762.93). Moreover, it can be fabricated by engineering the symmetry characteristics and the number of unit cells on all-dielectric metasurface platforms.

The results suggest that magnetic dipole quasi-BIC is well suited for optical sensing applications, where high sensitivity and ultra-high FOM can be attained.


H. Chen, T. Pan and Z. Zhang. (2022). Electromagnetic multipolar quasi-bound states in the continuum for optical sensing, Optics Communications.

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Owais Ali

Written by

Owais Ali

NEBOSH certified Mechanical Engineer with 3 years of experience as a technical writer and editor. Owais is interested in occupational health and safety, computer hardware, industrial and mobile robotics. During his academic career, Owais worked on several research projects regarding mobile robots, notably the Autonomous Fire Fighting Mobile Robot. The designed mobile robot could navigate, detect and extinguish fire autonomously. Arduino Uno was used as the microcontroller to control the flame sensors' input and output of the flame extinguisher. Apart from his professional life, Owais is an avid book reader and a huge computer technology enthusiast and likes to keep himself updated regarding developments in the computer industry.


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