Ultrasensitive THz All-Dielectric Metasurface Biosensor Based on Bound States in the Continuum

THz radiation’s unique characteristics have drawn significant attention in the medical research community. Of particular interest in biosensing is the fact that a lot of biomolecules have vibrational and rotational energy levels that lie in the THz region. Thus, molecules like proteins and DNA display a characteristic frequency response in that part of the electromagnetic spectrum which can be used for label-free, non-destructive detection of biomolecules [1]. The main obstacle in current solutions is the attenuation of the signal through water absorption which reduces the sensitivity and specificity. Additionally, the bulkiness and cost of existing THz instrumentation also hinder commercialization of diagnostic platforms, thus limiting their use to research laboratories.<br/><br/>A potential avenue for tackling these problems is through the use of THz dielectric metasurfaces. By incorporating the benefits of THz radiation and the unique abilities of metasurfaces, they have the potential to cater to the demands of biosensing.<br/><br/>Currently, THz sensors rely on testing dry samples which is unrepresentative compared to the in-vivo environments. To overcome this, it is necessary to examine small volume samples with reduced hydration. This can be a challenge, however, due to the large-scale difference between sample and wavelength. A potential solution to this is to confine the THz radiation using metasurfaces and microfluidic channels [2]. Most of the research has been focused on metallic metasurfaces, but more recently dielectric-based metasurfaces have been demonstrated which exhibit the same capabilities of manipulating light as metallic ones, without the inherent absorption losses related to them.<br/><br/>When it comes to the design of biosensors, dielectric metasurfaces can be made to exhibit a strong electric field enhancement and energy localization. This confinement enhances light-matter interaction. When combined with microfluidics which will allow testing of small volume samples with precise liquid control, the metasurface can be made highly sensitive to small changes in the analyte concentrations.<br/><br/>A noteworthy approach that has been gaining a lot of research interest, regarding the further enhancement of the localization of energy, and thus the sensitivity of sensors, is based on the quantum mechanical phenomenon of bound states in the continuum (BIC) [3]. By introducing asymmetry in the unit-cell structure of dielectric metasurfaces in the form of supercavity modes, it is possible to realize extremely high Q-factors as well as a significant enhancement in the electric field confinement, thus providing a new avenue for achieving ultrasensitive biosensors.<br/><br/>In that context, here we present numerical simulations of a dielectric THz metasurface biosensor based on BICs. The periodic unit-cell structure consists of an array of silicon cuboids, positioned on top of a quartz substrate. By modifying the geometrical parameters of the structure, we were able to leverage the physics of BICs to tune both the resonance and the Q-factor of the metasurface as well as obtain a strong field enhancement. We also demonstrate sensitivity to local permittivity by simulating the resonance shift in the presence of an analyte model.<br/><br/><br/><br/>References:<br/>[1] http://dx.doi.org/10.1016/j.tibtech.2016.04.008<br/>[2] https://doi.org/10.1007/s10762-021-00792-9<br/>[3] https://onlinelibrary.wiley.com/doi/10.1002/adma.201901921