As a convenient approach to achieve a high-quality factor, bound states in the continuum (BICs) have found increasing applications in diverse fields, including lasing, emission enhancement, nonlinear optics, and biochemical sensing. 

However, as the dielectric environment becomes inhomogeneous, which occurs when the refractive index of the superstrate deviates from that of the substrate, the quality factors of quasi-BICs decrease quickly. This greatly degrades the performance of many applications, especially biochemical sensing, which usually operates in aqueous media. 

Additionally, conventional quasi-BICs formed by localized Mie resonances have a large portion of energy confined within the high-index metasurface, limiting the light-matter interaction outside, where sensing analyte or emitting medium is located. 

An interdisciplinary team from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS) proposed and demonstrated a novel metasurface configuration supporting nonlocal quasi-BICs that can solve the above problems. 

The metasurface is composed of periodic four silicon nanodisks, forming two hybridized lattices with a small displacement. This measured bulk sensitivity reaches up to 407 nm/RIU and the limit of detection is down to 20 pg/mL for a protein biomarker for the early-stage breast cancer screening, improved by more than an order of magnitude over the state of the art.  

The study was published in Advanced Optical Materials on April 19th. 

Dr. LI Guangyuan, one of the corresponding authors of this study, said, "a quasi-BIC supported by the proposed metasurface configuration is especially suitable for sensing applications. The quality factor remains high even when the dielectric environment is asymmetric, making it very suitable for sensing application. Another striking feature is the homogeneous and significant near-field enhancement over large volumes outside the silicon nanodisks. This will greatly boost the light-analyte interactions and push down the limit of detection." 

Taking advantage of these unique characteristics, the research group explored the biosensing applications for the label-free detection of the human epidermal growth factor receptor 2 (ERBB2), which is a breast cancer biomarker for early-stage screening. Under different ERBB2 concentrations, they measured the resonance wavelength shifts induced by the antigen-antibody bonding. The results showed an ultralow limit of detection, as low as 20 pg/mL, more than an order of magnitude lower than the literature. 

"We anticipate that this metasurface platform will have broad applications in biosensing beyond the particular cancer biomarker studied in this research and outside the realm of antigen-antibody binding mechanisms," said Dr. LIU Yunhui, another corresponding authors of this study. 

The BIC offers substantial and uniform field enhancement outside the nanodisks, significantly improving biosensor detection limits. (Image by SIAT)