Optical biosensors with their sensitivity, compact design, and reliability stand out as versatile tools capable of detecting a wide range of analytes. Recently, nanophotonic structures supporting bound states in the continuum (BIC) modes have been actively studied, which is especially interesting for biosensing applications due to their high quality (Q) factor and strongly localized electric field, achieving favorable interaction between field and nanometer scale analyte on the sensing surface. Herein, we demonstrate an optical label-free sensing by accidental or Friedrich–Wintgen (FW) BIC supported on silicon nitride gratings. We compared the sensing performance in terms of bulk, and surface sensitivity, and figure of merit with FW-BIC in the leaky regime and with a symmetry-protected (SP) BIC, which are also supported by the studied platform. We exploit the fact that for FW-BIC a high-Q factor up to 498 comparable to that of SP-BIC (up to 425) retains for a much larger set of interrogation angles, providing excellent interrogation stability. We observed that FW-BIC has slightly higher bulk sensitivity than SP-BIC [186 and 158 nm/RIU (refractive index unit), respectively], but at the same time similar characteristics in terms of surface sensitivity and figure of merit. In addition, we show that both BIC resonances are significantly superior in all respects to the leaky regime due to better field confinement. Finally, the surface of sensing device was also functionalized to detect a cardiac biomarker, myoglobin, exhibiting the limit of detection of 49 ng/ml with clinically relevant level.

Optical bound states in the continuum (BIC) are highly localized light waves trapped in a structure regardless of the presence of leakage radiation channel that lies in a continuous spectrum of radiation.1 A BIC mode occurs at a certain special point of dispersion as a result of interference in the host structure that suppresses radiative mode and hence the light is trapped within the structure while otherwise it should leak out.2–5 BICs come in two different types—symmetry-protected (SP-BICs) and accidental or Friedrich–Wintgen bound states in the continuum (FW-BICs)—where the former occur only at the Γ point of the dispersion diagram of periodic systems as protected by symmetry’s incompatibility with far-field radiation, while the later can emerge off the Γ point whenever specific interference parameters between modes align.1 An ideal BIC has an infinite lifetime and quality-factor (Q-factor), which cannot be excited with radiation from the far-field. However, in reality, the ideal BIC turns to a quasi-BIC with finite Q-factor due to structural limitations, such as finite period, roughness, material absorption,1,6 and yet possessing a high Q-factor.7,8 Thus, quasi-BIC modes have been observed in various periodic systems, including an array of one-dimensional (1D) dielectric grating structures,3,6 photonic crystals,4,9 and different designs of metasurfaces,10–17 as well as aperiodic systems, such as single resonator18 and anisotropic waveguides.19 

Systems that support quasi-BIC have gained extensive attention nowadays due to the their high Q and highly confined electric fields,20 which is attractive for many potential applications, including lasing,21,22 upconversion photoluminescence,23 enhancement of second- and third-harmonic generation for wavelength conversion,10–15,24 light modulation,25 imaging,26 and biochemical sensing.18,27–38 In the later case for developing a biosensing platform, the overlap between analyte molecules and high-intensity electric fields can significantly enhance the device’s sensitivity and improve the limit of detection (LOD).39,40 In the vast majority of cases, SP-BICs are used for sensing applications.27–35 They, however, have one significant drawback that the Q-factor of such a structure varies significantly with the interrogation angle. For example, in Romano et al.,27 the authors observed a tenfold drop in the Q-factor when the interrogation angle changed by only 1 °. On the other hand, FW-BIC’s Q-factor remains relatively flat over a large range of interrogation angles,38 which is crucial for refractometric sensing applications where the wavelength shift of resonance induced by the analyte is typically very small and the narrow (high Q-factor) and robust resonance is needed.

Here, we realize a silicon nitride (SiN x) subwavelength grating structure supporting FW-BICs in liquid environment and demonstrate a novel label-free biosensor by taking the advantage of the BIC modes. This article is organized as follows. We start with the description of the methods used in this work, specifically numerical simulations, nanofabrication process, optical setups, and the sensing protocol. Then, we present the main results obtained using numerical calculations and optical measurements. This is followed by experiments to evaluate the bulk and surface sensitivity of the modes under study. Finally, we show myoglobin detection experiments.

Finite element method (FEM)-based simulations were carried out with COMSOL Multiphysics 6.1 (COMSOL AB, Sweden). The simulation was performed in two-dimensional (2D) geometry due to the translational symmetry of the grating structure. The simulated domain consists of a single unit cell with the Floquet boundary conditions. By adding a perfectly matched layer of 0.5  μm thickness, the substrate is simulated as a infinitely thick layer. The mesh has a step size of 2 nm inside the grating and increased outside, up to a value of 30 nm. The sketch of the proposed structure is shown in Fig. 1(a). It consists of periodic rectangular SiN x grating bars (n = 2.15) on a silicon dioxide (SiO 2) glass substrate (n = 1.45) with water as a background (n = 1.33). The grating period Λ, bars width w, and grating height t g were 400, 200, and 359 nm, respectively. The refractive index of SiO 2 and SiN x was earlier obtained from ellipsometry measurements.41 

FIG. 1.

(a) Schematic illustration of SiN x gratings with a thickness of t g = 359 nm, a pitch of Λ = 400 nm, and a width of grating w g = 200 nm. TM-polarized light has magnetic field parallel to grating bars. (b) Cross section and (c) bird’s-eye view of scanning electron microscope (SEM) images of the fabricated SiN x gratings.

FIG. 1.

(a) Schematic illustration of SiN x gratings with a thickness of t g = 359 nm, a pitch of Λ = 400 nm, and a width of grating w g = 200 nm. TM-polarized light has magnetic field parallel to grating bars. (b) Cross section and (c) bird’s-eye view of scanning electron microscope (SEM) images of the fabricated SiN x gratings.

Close modal

All the work was done in a class 10–100 cleanroom facility. First, 500  μm-thick fused silica wafers underwent a standard RCA cleaning procedure. Then, a t g = 359 nm thick Si-rich silicon nitride was deposited by low-pressure chemical vapor deposition (LPCVD) (furnace from Tempress) at 830  °C using dichlorosilane (SiH 2Cl 2) and ammonia (NH 3) gases.41 A 1D periodic lattice with a pitch of Λ = 400 nm and bars width of w = 200 nm was implemented by a conventional DUV lithography stepper (Canon FPA-3000 EX4, Canon). The procedure involves coating and baking of a 65 nm bottom anti-reflective coating layer (BARC) and 750 nm of a positive photoresist KRF M35G, exposure step with an optimal exposure dose of 430 J/m 2, and subsequent development. Then, SiN x grating bars were patterned using deep reactive ion etching (DRIE Pegasus, SPTS Technologies Ltd.). A three-step process was employed, including the BARC etch using O 2 plasma, C 4F 8 (75 SCCM) and SF 6 (38 SCCM) etching of SiN x, and the removal of the resist with O 2 plasma. Scanning electron microscopy (SEM Zeiss Supra 40VP, Zeiss) was used to examine the shape of the produced structures, as shown in Figs. 1(b) and 1(c).

We conducted free-space angle-resolved reflectivity measurements using the setup schematically depicted in Fig. S1 in the supplementary material. Our light source was a broadband supercontinuum laser (SuperK from NKT Photonics A/S, radiation wavelength λ = 350–2400 nm), and a spectrometer (CCS175/M, Thorlabs, detection range λ = 500–1000 nm) was used as a detector. To avoid parasitic heating, the beam from the light source passes through a 900 nm shortpass filter (FESH0900, Thorlabs). Thereafter, the light is polarized by a film polarizer [TM polarization (LPVIS100-MP-2, Thorlabs)] and directed to the sample. Through manual rotation of the mechanical stage (CR1/M, Thorlabs), the angle of incidence was varied with a step of 1 °( ± 0.5 °) from 2 ° up to 45 °. The beam reflected from the sample is focused into a multi-mode fiber using a parabolic mirror and fed into the spectrometer. The TM reflectance spectrum from a gold mirror was taken as a reference. For the measurements under normal incident angle, another setup described in Ref. 42 was used (see Fig. S2 in the supplementary material) with the aforementioned light source and detector.

The biofunctionalization protocol for myoglobin is schematically illustrated in Fig. 2. Prior to surface functionalization, the sensor chips were sonicated for 5 min in acetone, ethanol, deionized water, respectively, and 10 min in methanol (99.9%, VWR chemicals)/HCl (37%, Sigma-Aldrich) solution with 1:1 volume ratio in order to remove organic contamination. The chips were rinsed with water and dried with a nitrogen gun afterward. For the next step, it is necessary to generate active hydroxyl groups on the surface. For that purpose, we used oxygen treatment for 5 min (45 SCCM, 0.8 mbar), followed by immersion in a 15% HNO 3 (65%, Sigma-Aldrich) solution at 75  °C for 25 min.43 Thereafter, the sample was rinsed gently with DI water and dried with a nitrogen gun. Subsequently, we incubated the samples in a laminar flow bench with a 3 mM solution of APTMS (97%, Sigma-Aldrich) in anhydrous toluene (99.8%, Sigma-Aldrich) (150 ml toluene and 75 ml aminosilane) for 30 min at room temperature.44,45 It was followed by rinsing with toluene and methanol twice each, and blow-drying with nitrogen. Then, the structure surface was biofunctionalized using the protocol from Beliaev et al.42 Briefly, the samples were incubated for 60 min in a solution containing 0.1 mg/ml myoglobin antibody (Thermo Fisher Scientific, Waltham, MA, USA), 200 mM EDC (Sigma Aldrich; Merck KGaA, Darmstadt, Germany), and 50 mM NHS (Sigma Aldrich; Merck KGaA, Darmstadt, Germany) in phosphate-buffered saline (PBS) solution. After that, the sample was rinsed with DI water three times and dried with nitrogen. Finally, myoglobin from equine skeletal muscle (Sigma Aldrich; Merck KGaA, Darmstadt, Germany) was dissolved in PBS at concentrations of 1, 25, 50, 100, 250, 500, and 1000 ng/ml. Then, 35  μl of the solution are added to each sample and allowed to react for 30 min before rinsing thrice in DI water and drying under a stream of nitrogen. This was followed by adding a PBS solution on top of the sample, as well as placing a BK-7 coverslip glass (BRAND, Wertheim, Germany) with a thickness of 130  μm above to contain the analyte liquid and to maintain the surface of liquid flat.

FIG. 2.

Schematic of biosensing protocol for myoglobin detection. Representation of the four main steps: surface hydroxylation; surface silanization with APTMS; immobilization of the myoglobin antibody, and binding of myoglobin to the surface.

FIG. 2.

Schematic of biosensing protocol for myoglobin detection. Representation of the four main steps: surface hydroxylation; surface silanization with APTMS; immobilization of the myoglobin antibody, and binding of myoglobin to the surface.

Close modal

Figure 3(a) shows the dispersion of TM eigenmodes of the SiN x gratings in water on top of a fused silica substrate where the bold red curve (mode 2) represents the dispersion of the eigenmode under consideration. We can observe that the structure supports two types of BICs: SP-BIC at the Γ point of “mode 1” and FW-BIC off the Γ point of “mode 2.” The BICs are evident from the diverging singularities of the radiative Q-factor in Fig. 3(b), at which the Q-factor becomes highest. In addition, for “mode 1,” Q-factor drops sharply by several orders of magnitude even with a small deviation of in-plane wavevector from the Γ point. On the other hand, for “mode 2” in the FW-BIC region, the Q-factor decreases gradually and remains high over a larger interval. The electric field distribution of modes is depicted in Fig. 3(c). One can see that at the Γ point of “mode 2” at leaky regime the maximum electric field is confined at both edges of the bar and that there is a small portion of field between the grating bars, which does not provide significant interaction between fields and analyte. However, at the point where the transition to FW-BIC occurs, the situation changes: the maximum electric field is confined in the region between two grating bars with much better optical confinement than leaky mode. This situation is particularly promising for biosensing applications due to the high localization of the field with which a perspective analyte can interact. Furthermore, a similar field profile is seen for SP-BIC for “mode 1,” which could be used for sensing at a normal incidence angle,32 as well as guided mode resonance in similar grating structures.39,42,44,46

FIG. 3.

(a) Simulated band structure of the TM modes of the proposed SiN x gratings in water on SiO 2 substrates. The dashed line is a light line in water. There is symmetry-protected (SP-BIC) at the Γ point for “mode 1” and leaky regime for “mode 2.” Also, there is Friedrich–Wintgen BICs (FW-BICs) off the Γ point of “mode 2.” “mode 3” here corresponds to the waveguide regime below the light line. (b) Simulated Q factor of “mode 1” and “mode 2” in terms of parallel wavevector, k x. (c) Electric field distributions at points 1–3.

FIG. 3.

(a) Simulated band structure of the TM modes of the proposed SiN x gratings in water on SiO 2 substrates. The dashed line is a light line in water. There is symmetry-protected (SP-BIC) at the Γ point for “mode 1” and leaky regime for “mode 2.” Also, there is Friedrich–Wintgen BICs (FW-BICs) off the Γ point of “mode 2.” “mode 3” here corresponds to the waveguide regime below the light line. (b) Simulated Q factor of “mode 1” and “mode 2” in terms of parallel wavevector, k x. (c) Electric field distributions at points 1–3.

Close modal

Figure 4(a) shows the simulated reflectance spectra for variable angle of incidence in water. It is evident that around 856 nm for an angle of incidence of 34 ° the resonance becomes so narrow that the numerical resolution (0.05 nm) is no longer sufficient enough and the resonance “disappears,” which is a characteristic feature of BIC’s behavior. Then, to experimentally characterize the sample and reconstruct the band diagram, we performed angle-resolved reflectivity measurements (details of the measurement are presented in Sec. II). Figure 4(b) presents the angle-resolved reflectance spectra of the fabricated device. Results demonstrate that, in general, the measured reflectance spectra qualitatively agree well with the simulated ones.

FIG. 4.

Calculated (a) and measured (b) reflectance spectra for TM polarization for different incident angle between 0 ° and 45 ° in water.

FIG. 4.

Calculated (a) and measured (b) reflectance spectra for TM polarization for different incident angle between 0 ° and 45 ° in water.

Close modal

Figure 5(a) depicts several reflectance spectra at different angles of incidence in the range of θ = 8 °–44 °. A clear transition from the Fano resonance to the Lorentzian one around θ = 33 ° ( λ 840.9 nm) and back to the Fano-type resonance was experimentally observed. This behavior is typical for BIC resonances and has been observed previously.47,48 The evolution from a Fano resonance to a Lorentzian resonance and back to a Fano resonance reflects the sensitivity of the spectral response to changes in the phase relationship between incident light and resonant modes. At specific angles, constructive or destructive interference leads to symmetric resonance shapes resembling Lorentzians, while deviations result in the reemergence of Fano-type asymmetry.48 Next, we analyze the relation between the incidence angle and Q-factor for numerically simulated and measured ones [Figs. 5(b) and 5(c)]. From the experimental data presented in Fig. 5(c), one can see that as the angle of incidence approaches 33 °, the resonance becomes narrower and, accordingly, Q-factor increases as well up to 498 [full width at half maximum (FWHM) = 1.68 nm]. However, in comparison with the numerical data in Fig. 3(b), the observed maxima of Q-factor was smaller by orders of magnitude. We attribute such results to imperfections of nanofabricated samples, such as roughness on the surface, possible slightly tilted sidewalls, dimensional deviations, and intrinsic losses in SiN x. To take these into account in our simulations, we added an additional imaginary part of the SiN x refractive index δn: 0.002i and 0.004i.49 The results are shown in Figs. 5(b) and 5(c), giving good agreement with the experiment observed by considering losses δ n = 0.004 i. We note that the decrease of SP-BIC’s Q-factor by loss (Sim. 2 with δ n = 0.004 i) is by far more significant ( 10 8 →∼ 10 2) than that of FW-BIC ( 10 5 →∼ 10 2) compared with ideal case of Figs. 3(b) and 3(c) and eventually Q-factors become the same order of magnitude for both BICs. This implies that the SP-BIC on grating structures is more vulnerable to material loss and imperfection of nanofabrication, which occurs in experimental situation, than FW-BIC. Furthermore, we measured the behavior of Q-factor for “mode 1” for angles of incidence in the range of θ = 0 °–7 °. From the results in Fig. 5(b), one can see a rapid increase of Q-factor when approaching the normal angle of incidence, which is the characteristic sign of SP-BIC. Again, in an ideal system, Q-factor should be infinitely high, but due to the imperfections of the system we see a finite Q-factor reaching 425 (FWHM of 1.49 nm), which agrees well with the numerical data when losses are included. However, when the angle of incidence deviates from normal to 2 °, we see a more than twofold drop in Q-factor to 187. Meanwhile, for “mode 2” around FW-BIC point moving away from an angle of θ = 33 °, Q-factor does not drop significantly, maintaining relatively high Q-factor ( > 350) within the angular window of θ = 33 ± 5 °. In other words, the system is robust against imperfections in the optical system, which is advantageous for practical applications.

FIG. 5.

(a) Measured reflectance spectra of mode 2 supported on the fabricated SiN x grating for several incidence angles. (b) and (c) Dependence of the observed quasi-BIC Q-factor extracted from the measurements (red) and simulations (blue, green) on the incidence angle for “mode 1” (circles) (b) and “mode 2” (triangles) (c) with weighted imaginary part of SiN x. Sim.1 (blue): no losses, i.e., δ n = 0 i, Sim.2 (green): losses, i.e., δ n = 0.004 i.

FIG. 5.

(a) Measured reflectance spectra of mode 2 supported on the fabricated SiN x grating for several incidence angles. (b) and (c) Dependence of the observed quasi-BIC Q-factor extracted from the measurements (red) and simulations (blue, green) on the incidence angle for “mode 1” (circles) (b) and “mode 2” (triangles) (c) with weighted imaginary part of SiN x. Sim.1 (blue): no losses, i.e., δ n = 0 i, Sim.2 (green): losses, i.e., δ n = 0.004 i.

Close modal
We performed bulk-refractive index sensitivity measurements ( S B) to demonstrate the potential of designed structures for sensing purposes. S B can be defined as40,
S B = Δ λ Δ n ( n m / R I U ) ,
(1)
where Δ λ is the resonance wavelength shift as opposed to a change in the refractive index of the background (bulk) and RIU stands for the refractive index unit. For this intent, the sensor chips were immersed in a solution with glycerol diluted in de-ionized water (0–50% volume fraction). This is equivalent to the variations in refractive index between 1.33 and 1.40 RIU, which is dielectric environment relevant to label-free assays.50–52 The refractive index values of each solution were calculated considering the weight fraction of water and glycerol in the mixture. Due to the viscous nature of glycerol, each measurement was followed by an intensive rinse.

We studied three cases here: SP-BIC of “mode 1” at normal angle of incidence, leaky regime of “mode 2” at normal angle, and FW-BIC of “mode 2” at θ = 33 °. These resonances red-shift as RI changes, as shown in Fig. 6(a). Additionally, while resonances associated with leaky regime and FW-BIC of “mode 2” are pronounced, the SP-BIC of “mode 1” resonance is not so distinct. We attribute this to limitations in the optical setup and imperfections in the nanofabrication process. The linear response curves allowed us to calculate S B of 158, 106, and 186 nm/RIU [see Fig. 6(b)], for SP-BIC of “mode 1,” leaky regime of “mode 2,” and FW-BIC of “mode 2,” respectively. The highest S B for FW-BIC of “mode 2” can be attributed to the field profile, which favors the larger light–matter interaction. Moreover, obtained S B values are comparable to and slightly higher than other silicon nitride-based nanostructure platforms with BICs (see Table S1 in the supplementary material).

FIG. 6.

(a) Reflectance spectra of the fabricated structure in glycerol-aqueous solutions. Spectra are normalized using the reflectance spectrum from a gold mirror that was taken as a reference. (b) Resonance wavelength of the proposed device vs the change in RI ( ΔRI) associated with different glycerol concentrations in the solution. (c) Resonance wavelength as a function of deposited thicknesses of Al 2O 3.

FIG. 6.

(a) Reflectance spectra of the fabricated structure in glycerol-aqueous solutions. Spectra are normalized using the reflectance spectrum from a gold mirror that was taken as a reference. (b) Resonance wavelength of the proposed device vs the change in RI ( ΔRI) associated with different glycerol concentrations in the solution. (c) Resonance wavelength as a function of deposited thicknesses of Al 2O 3.

Close modal
Apart from bulk-refractive index sensitivity, another important parameter is usually used to evaluate a sensing performance, called figure of merit (FOM), which represents overall detection capability in terms of wavelength shift and FWHM of the resonance,40 
F O M = S B F W H M ( 1 / R I U ) .
(2)
The obtained values of FOM based on measurements were found to be 104.7, 17.2, and 110.7 RIU 1 for SP-BIC of “mode 1,” leaky regime of “mode 2,” and FW-BIC of “mode 2,” respectively. FOM values for both BIC resonances are very close to each other. At the same time, these are significantly higher than those for the leaky regime of “mode 2,” which is explained by better field confinement of BIC resonaces.

As for the next step, we performed measurements to estimate the surface sensitivity of our device. For that, we deposited 2, 4, and 6 nm thick layers of aluminum oxide (Al 2O 3) by atomic-layer deposition on top of the sample surface as a dummy analyte. The method allows layers to be deposited very conformally with excellent thickness control.53 The measurement results are presented in Fig. 6(c). A consistent red shift was observed for all modes with increasing thickness of the deposited oxide. Moreover, from the slope of the linear fit, one can observe that SP-BIC of “mode 1” and FW-BIC of “mode 2” have very similar performance in terms of surface sensitivity, and both are better than leaky regime of “mode 2.” It is associated with enhanced interaction between the field of BIC modes and the thin film, mimicking the analyte.

Next, we conducted sensing performance characterization, using myoglobin as an analyte and its antibody as the antibody–antigen pair. For this purpose, we chose FW-BIC of “mode 2” with the highest S B and comparable with SP-BIC FOM and surface sensitivity. Myoglobin is a small (17 kDa) protein, which is used as a biomarker for the acute myocardial infarction diagnosis.54 

To validate the fabricated structure as a potential myoglobin sensor, the resonance position of sensor chips in PBS solution was measured in the following four steps (see Fig. S3 in the supplementary material). Step 1: After treatment with O 2 plasma and NHO 3 solution, i.e., “hydroxyl groups activation.” Step 2: coating the sensor surface with APTMS. Step 3: addition of antibody. Step 4: binding of myoglobin. Then, we performed quantitative measurements of wavelength shift as opposed to myoglobin concentration. The resonance position following the addition of an antibody was used as a blank or baseline, and seven different concentrations from 1 ng/ml to 1  μg/ml were applied to the samples. The resulting detection curve exhibited a standard sigmoidal-shape, as demonstrated in Fig. 7. Here, the error bars represent the standard deviation of measurements (three chips were used per concentration, and each chip was measured four times). In order to estimate the LOD, i.e., a minimum quantity of an analyte that can be detected,55,56 the response curve was fitted using the Logistic function,57,58
Δ λ = 1 + ( C / .5 ) ( n m ) ,
(3)
where C is the concentration of the analyte and the R 2 value was 0.987 94. The LOD was found to be 49 ng/ml, which is a relevant value for potential clinical applications.54 The normal level of myoglobin concentration in human blood is in the range of 6–85 ng/ml.54 Upon the heart muscle destruction, this level is rapidly increase to 250 ng/ml and above.54 Myoglobin is present in both skeletal muscles (with low specificity) and cardiac muscles, and its levels in the bloodstream increase quickly following damage to the myocardial muscle cells,59 and importantly is the first biomarker to be ejected into the bloodstream.60 Therefore, even though myoglobin has lower specificity, it is often used in AMI diagnosis along with other biomarkers, such as troponin I,61,62 since its level can increase due to both skeletal muscle damage and renal failure.61 
FIG. 7.

Measured resonance shifts (FW-BIC of “mode 2”) between myoglobin antibody as a blank and myoglobin molecules as an antigen in PBS solution. The concentration axis is on a logarithmic scale.

FIG. 7.

Measured resonance shifts (FW-BIC of “mode 2”) between myoglobin antibody as a blank and myoglobin molecules as an antigen in PBS solution. The concentration axis is on a logarithmic scale.

Close modal

It is worth mentioning that the imperfections of the optical setup, such as a limited resolution and coarse angular step, made a negative contribution to the assessment of the biosensing performance of the structure. Moreover, after each functionalization step, the sample had to be removed from the optical setup and then installed back again, which was a significant source of errors, even with fairly careful experiments.

In this work, we demonstrated a numerical analysis, nanofabrication, and optical characterization for the realization of the optical sensor device based on accidental or Friedrich–Wintgen bound states in the continuum phenomena and compared it to symmetry-protected and leaky cases. The system consists of a silicon nitride subwavelength grating on top of a fused silica substrate immersed in liquid environment. FW-BIC resonance exhibits a Q-factor of 498, which remains > 350 even with a deviation of ± 5 ° and is more robust than SP-BIC resonance, where there is a more than twofold drop in the Q-factor (from 425 to 187) with a deviation of even 2 °. Also, FW-BIC resonance shows slightly better performance in terms of bulk sensitivity, but very similar in terms of surface sensitivity and FOM with SP-BIC case. Additionally, due to better field confinement, both BIC resonances are significantly superior to the leaky regime in all respects. Furthermore, the device was tested for the cardiac biomarker (myoglobin) detection using FW-BIC resonance. The LOD was found to be 49 ng/ml, which is within normal myoglobin level in humans, demonstrating potential for a myoglobin sensor platform for clinical studies. Additional biosensing experiments are planned to be done for the three abovementioned resonances in order to really evaluate surface sensitivity and provided LOD. Moreover, the proposed sensing architecture can be extended to the detection of other biomarkers like viruses by using appropriate functionalization procedures, showing the potential of versatile biosensing platform with the prospect mass production enabled by deep UV lithography, integration, and multiplexed sensing nodes for point-of-care diagnostics.

See the supplementary material for more details on optical setups that were used in these works, for measured spectra after each of the functionalization steps, and for comparison of the overall performance of our device, as well as current state-of-the-art for BIC-based optical biosensors on silicon nitride platforms in visible to near-infrared wavelength range.

This work was supported by Novo Nordisk Foundation, Exploratory Interdisciplinary Synergy Programme, Grant No. NNF21OC0070706 and by the Danish National Research Foundation through NanoPhoton—Center for Nanophotonics, Grant No. DNRF147.

The authors have no conflicts to disclose.

Leonid Yu. Beliaev: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Visualization (equal); Writing – original draft (equal). Osamu Takayama: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Visualization (equal); Writing – original draft (equal). Sanshui Xiao: Funding acquisition (lead); Investigation (equal); Supervision (lead); Visualization (equal); Writing – original draft (equal); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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