In Oct 2023, Dr Sailing He, Professor of Zhejiang University and Adjunct Professor of Shanghai Institute for Advanced Study of Zhejiang University (SIAS), and his collaborators published a research article ‘Phase Interrogation Sensor Based on All-Dielectric BIC Metasurface’ as cover paper in Nano Letters, presenting for the first time the concept of a phase-interrogating all-media Bound states In the Continuum(BIC) refractive index sensor.
All-dielectric metasurfaces (ACMS) had received increasing attention due to low material absorption, compatibility with CMOS processes, low cost, and excellent biophilicity in recent years, especially BIC-based ACMS with ultra-high Q resonance. Originally used to describe electrons confined in potential barriers and then extended to the study of light and electromagnetic waves, BIC refers to discrete states in the continuum spectrum that are bound within the continuum mode and do not radiate outward, with infinite Q value in theory, thus potentially useful in lasers, nonlinear optics, chiral optics, spectral imaging and sensing.
In refractive index sensing, BIC are crucial for the enhancement of sensing performance. Recent studies tend to focus on the design, construction, and optimization for BIC resonant structures with high Q values to improve sensing. However, their refractive index sensitivities are still limited to the range of tens to hundreds of nanometers per RIU (refractive index unit), significantly lower than that of metal substrate-based sensors (e.g., plasma sensors and hyperbolic metamaterials sensors). Some works achieve a quality factor of merit (FoM) comparable to that of metal substrates, the refractive index resolution is usually only on the order of 10-4 to 10-5 RIU due to lower sensitivity, much lower than that of the refractive index resolution of metal substrate-based sensors, keeping all-media BIC hypersurfaces out of ultra-high sensitivity sensing.
The low performance of all-dielectric hypersurface-based sensors compared to metals limits their application and development. In this work, an all-dielectric BIC hypersurface was used for phase-interrogation refractive index sensing with high sensitivity. A high-performance microfluidic BIC sensing chip with Q up to 1200 was realized. Notably, a record BIC phase sensing sensitivity of 2.7 × 104 degree/RIU was recorded, with refractive index resolution 2-3 orders of magnitude higher than that of similar sensors.
Figure 1. Design of BIC sensing hypersurfaces
Dual rectangular column BIC surface sensors are arrays of axially symmetrically distributed dual rectangular silicon columns periodically arranged on a quartz substrate, as shown in Figure 1. The rectangular silicon columns were each rotated around the center in the opposite direction by a certain angle, and due to the destruction of symmetry within the surface, radiation leakage occurred in the original symmetry-protected BIC mode, which was called the quasi-BIC mode (Quasi-BIC). This quasi-BIC mode still had a large Q value. The electric field distribution localized at the outer edges of the metastructure was advantageous for exploiting it for highly sensitive refractive index sensing. The reflection behavior exhibited great constraints on the rotation angle. The Fano scheme and the resonance linewidths decreased with decreasing rotation angle (symmetry restoration), and the Q-value was linearly constrained to the negative quadratic of the rotation angle θ. The BIC hypersurfaces were prepared by an electron beam exposure process with Electron Beam Lithography(EBL)as core equipment, with metal mask prepared by vapor deposition process, compatible with semiconductor micro-nanofabrication processes (lithography, etching, etc).
Figure 2. Preparation and characterization of BIC hypersurface structure arrays
The resonance wavelength of BIC arrays shown in Figure 2 was near 1550 nm, which laid the foundation for future compatible and standardized processes. The BIC arrays with different rotation angles showed significant differences in scattering intensity, and five BIC arrays with different rotation angles were processed, prepared and characterized with fine perpendicularity. The BIC microfluidic chip was prepared using a glass substrate, which can be conveniently plasma covalently bonded with PDMS and other polymers, and the BIC microfluidic chip was prepared (Figure 3), which could to be applied in liquid phase detection at the microliter scale.
Figure 3. High-performance BIC microfluidic sensing chip
Figure 4. Spectral characterization test of BIC structure array
The constructed hypersurface optical test system was used for transmission spectral measurements and reflection phase extraction. The transmission spectra showed significant bound-state properties (Figure 4), and the resonance linewidths kept shrinking with decreasing rotation angle (restoration of symmetry within the metastructure plane). Q values went up to 1200 corresponding to BIC arrays with a 5° rotation angle. The BIC structure was characterized by insensitivity to the incident angle and possesses polarization selectivity, providing a theoretical basis for phase extraction (phase difference of orthogonally polarized components) and phase sensing.
The BIC structure proposed showed refractive index sensitivity, and its sensing sensitivity for resonant wavelength drift reached 501 nm/RIU, slightly higher than that of the same type of BIC refractive index sensor. In Figure 5, there was a significant constraint between the drastic change in phase spectra (phase versus wavelength) and the rotation angle (symmetry breaking amount). The phase refractive index sensing displayed excellent sensing performance, with a sensitivity of 2.7 × 104 deg/RIU. Based on this estimation, the refractive index resolving ability was up to the order of 10-7 RIU. This performance significantly exceeded that of the vast majority of BIC hypersurfaces and all-media hypersurfaces, where the refractive index resolution is typically in the order of 10-4 to 10-5 RIU.
Figure 5. Refractive index sensing performance of phase and wavelength parameters
In this work, the experimentally obtained refractive index resolution is more than two orders of magnitude higher than that of most all-media hypersurface sensors based on silicon rectangular column dimer structure with the rotation angle as the symmetry breaking parameter for high-sensitivity phase sensing. The phase-interrogating all-media BIC refractive index sensing chip proposed raise the possibility of ultrasensitive analytical sensing for all-media hypersurfaces for very trace detection in biochemical analysis and medical diagnostics. To access the article, please visit https://pubs.acs.org/doi/10.1021/acs.nanolett.3c03089.