Microfluidic Refractive Index Sensor Based on All-Dielectric Metasurfaces

Objective This paper proposes a microfluidic sensing device with a metasurface of a single silicon-notched disk resonator based on the electromagnetic properties of all-dielectric metamaterials. The simulation of the finite -difference time-domain (FDTD) method shows that the structure can generate triple Fano resonances, which include a bright dipole resonance directly excited by incident light, a high-order hybrid resonance, and a magnetic resonance generated by the interference of bright and dark modes under an asymmetric structure. In addition, the impact of structural parameters on Fano resonances and the thickness of the analyte covered in the microfluidic device on sensing characteristics are investigated. The sensitivity of the structure after optimizing parameters can reach a maximum of 400 .36 nm/RIU and the quality factor Q can reach a maximum of 1252 .3, and it is proven that increasing the thickness of the solution within a certain limit can improve the performance of sensor detection. Methods Under the influence of a light field, the dielectric material can produce magnetic (first-order Mie resonance) and electrical resonances (second-order Mie resonance ), which can effectively avoid the ohmic loss caused by metal plasma resonance and improve the sensing detection performance. Fano resonance originates from the destructive interference between the bright mode (or superradiation mode) and dark mode (or subradiant mode) in the near field, and the light field is enhanced by forming an ultra-narrowband spectral response. Based on the basic theory of Mie resonance and the property of Fano resonance, a metasurface of notched silicon disk is proposed, which is combined with a microfluidic device for liquid sensing detection. The spectral response can be observed using the FDTD method for simulation. When the structure is symmetrical, the incident light can directly excite the electric dipole resonance, but when the symmetry of the structure is broken, the electromagnetic phenomenon changes completely. The dark mode in the structure is excited, and two asymmetric Fano resonances I and III with narrower line widths are generated based on the original resonance, producing electric quadrupole hybrid mode resonance coupled with electric and magnetic dipole resonances, respectively. To promote the performance of sensing, the influence of the structural parameters on the transmission characteristics is discussed. Choosing the optimized structural parameters, the influence of the thickness of the analyte in the microfluidic channel on the sensing performance is analyzed. Results and Discussions The metasurface can induce Fano resonance by breaking the symmetry of the structure. Three asymmetric Fano resonance lines can be generated in the structure proposed in this paper by introducing a gap in the structure to excite the dark mode. The electromagnetic field distribution of each section and the characteristics of the basic electromagnetic source show that an electric quadrupole mode exists in the silicon-notched disk at the resonance I (lambda 1=1219.95 nm ). This subradiant mode hybridizes the dipole superradiant mode in the notched disk to produce a sharply abrupt Fano resonance spectrum. It exhibits a clear broad resonance spectrum at the resonance II (lambda 2=1311.70 nm ), indicating an electric dipole resonance in a super radiative state. A circular displacement current is formed in the xoy plane, and a net induced magnetic dipole moment is generated along the z-axis, resulting in a magnetic Fano resonance with an extremely narrow linewidth at the resonance III (lambda 3=1527.55 nm). By analyzing the influence of structural parameters, it can be found that Fano I and II are not sensitive to changes in the length l and the width g of the rectangular gap, Fano III has a considerable blue shift as it increases, and as the structural period P, silicon disk's radius R, and silicon disk's thickness t increase, three Fano peaks have a certain redshift. In addition, by fixing the optimized structural parameters, the influence of different thicknesses h of the analyte with a refractive index of 1.33 in the microfluidic channel on the Fano resonance peak is investigated. The results show that the thickness h of the analyte increases, the resonance peak appears red-shifted, and when the analyte thickness reaches 332 nm, the detection of the sensor reaches a saturated state. This finding indicates that the metasurface structure is extremely sensitive to changes in the thickness of the thin analyte. To improve sensor stability, the analyte thickness should be set above 322 nm. The optimized structure parameters are chosen, and the sensing properties of the 332 nm thickness solution are investigated. The corresponding sensitivities at each resonance are 400 .36, 203 .32, and 160.47 nm /RIU, and the corresponding Q values are 889 .6, 17 .3, and 1252.3. Furthermore, by choosing Fano I, the sensitivities of the sensor for detecting thin analytes under 205 nm and 332 nm solution thicknesses are compared. It can be observed that when the thickness of the solution is less than that of the saturation region, the detection sensitivity is lower. Conclusions Owing to the low-loss characteristics of the all-dielectric materials, this paper proposes a metasurface with a silicon gap disk structure as a platform for microfluidic sensing and detection. The structure can be directly excited by incident light to produce a broad-spectrum electric dipole bright mode in a symmetrical state. When a rectangular nano-notch is introduced into the structure, the dark mode is excited, resulting in the formation of two new Fano resonances. The hybrid resonance is produced by the coupling of the electric quadrupole mode and electric dipole, as well as the magnetic Fano resonance is dominated by the magnetic dipole. The transmission spectrum characteristics are examined using the 3D FDTD method, and the influence of various structural parameters is investigated. It is observed that the structure can simultaneously achieve independent and nonindependent parameter tunings. Furthermore, the effect of the analytes with varying thicknesses in the microfluidic device on sensing performance is investigated. By selecting the optimized structural parameters, the sensor has a maximum sensitivity of 400.36 nm /RIU and a maximum Q value of 1252.3. The proposed structure provides a theoretical reference for the design of biosensor detection devices.