Ultra‑Sensitive Biosensing Using 2‑D Hole‑Array Grating‑Coupled Bloch Surface Waves

Abstract

We present a compact, label‑free biosensor that couples Bloch surface waves (BSWs) through a two‑dimensional (2‑D) sub‑wavelength hole‑array grating deposited on the top layer of a distributed Bragg reflector (DBR). By exciting BSWs either at the grating surface (DG‑BSW) or at the DBR–solution interface (GC‑BSW), we achieve angular sensitivities of 1,190°/RIU and 2,255°/RIU, respectively. Numerical modeling confirms that material losses in the multilayer stack have minimal impact on resonance quality, enabling robust, high‑figure‑of‑merit sensing across a broad refractive index range.

Background

Photonic devices that support surface‑mode resonances—such as surface plasmon polaritons (SPPs), guided‑mode resonances, and Bloch surface waves (BSWs)—are at the forefront of label‑free, real‑time biosensing for medical diagnostics and environmental monitoring. While SPR remains the industry standard, its reliance on metallic films limits sensitivity (typically a few hundred nm/RIU) and introduces absorption losses. BSWs, generated in all‑dielectric photonic crystals, offer markedly higher sensitivity, tunable field enhancement, and negligible absorption, making them attractive for next‑generation biosensors.

Existing BSW sensors often employ bulky prism‑coupling (Kretschmann) geometries. Few studies have explored grating‑coupled configurations that simplify the optical setup. Here, we propose two 2‑D grating‑coupled designs that excite BSWs at distinct locations: the surface of the grating (DG‑BSW) and the DBR–solution interface (GC‑BSW). By comparing these configurations, we identify optimal parameters for maximizing angular sensitivity while minimizing fabrication complexity.

Methods

Case 1: Surface Diffraction‑Grating BSW Configuration (DG‑BSW)

The DG‑BSW structure (Fig. 1) consists of a five‑period DBR (SiO₂/TiO₂) followed by a 60‑nm SiO₂ buffer and a 116‑nm Si₃N₄ grating patterned with circular air holes. The grating period, hole radius, and thickness are Λ = 510 nm, r = 145 nm, and h = 116 nm, respectively. Light incident at polar angle θ and azimuthal angle φ is coupled into the BSW mode through the grating. Material loss is incorporated via the imaginary refractive indices: γ_SiO₂ = 0 and γ_TiO₂ = 10⁻⁴. The DBR layers are quarter‑wavelength thick at the operating wavelength (λ₀ = 657 nm), yielding d_L = 100 nm and d_H = 70 nm.

Ultra‑Sensitive Biosensing Using 2‑D Hole‑Array Grating‑Coupled Bloch Surface Waves

Rigorous coupled‑wave analysis (RCWA) implemented in Diffract MOD (RSoft Photonics Suite) was used to compute reflection spectra and field distributions. Simulations reveal a sharp Fano resonance at θ = 4.3° for λ ≈ 660 nm, with the BSW field penetrating ~200 nm into air and reaching an intensity 42× the incident field.

Because the high‑field region resides within the narrow holes, analyte penetration is limited, potentially reducing sensitivity. To address this, we propose an alternative configuration.

Case 2: Guided‑Grating‑Coupled BSW Configuration (GC‑BSW)

In the GC‑BSW design (Fig. 3), the sensing region is relocated to the bottom of the DBR. The TiO₂ layer adjacent to the solution is reduced to 30 nm, while all other layer thicknesses remain unchanged. A 2‑μm bio‑solution (n ≈ 1.333) is placed directly on the high‑index TiO₂ surface. BSW excitation occurs when s‑polarized light couples through the grating at θ ≈ 7° and λ ≈ 633 nm, forming a surface defect state that enhances the evanescent field at the solution interface.

Simulations show that the BSW field penetrates 2 μm into the solution—ten times deeper than in the DG‑BSW case—resulting in stronger interaction with analytes and higher angular sensitivity.

Results and Discussion

Reflectivity curves for both DG‑BSW and GC‑BSW configurations exhibit sharp, high‑Q resonances as a function of incident angle and wavelength (Fig. 4). DG‑BSW achieves a dip at θ = 4.3° (λ ≈ 660 nm), while GC‑BSW shows a resonance at θ = 7° (λ ≈ 633 nm). Angular sensitivity exceeds 1,190°/RIU (DG‑BSW) and 2,255°/RIU (GC‑BSW), surpassing conventional prism‑based sensors by an order of magnitude.

Azimuthal interrogation of GC‑BSW (Fig. 5) reveals a highly selective resonance that shifts significantly with minute refractive index changes. The field distribution shows strong overlap with the sensing layer, accounting for the superior sensitivity. The resonance width (FWHM) remains narrow (< 0.5°) even at elevated loss levels (γ_TiO₂ ≤ 10⁻³), confirming the robustness of the design.

Figure 6 demonstrates that increasing the polar angle narrows the FWHM while preserving resonance depth, enabling fine‑tuning of the sensor response. Loss analysis (Fig. 7) indicates that an extinction coefficient of κ = 10⁻⁴ yields the optimal balance between depth and width, while larger losses degrade the Q‑factor without shifting the resonance.

Azimuthal sensitivity calculations (Eq. 3) show that a Δn = 5 × 10⁻⁴ induces a 0.1° shift in DG‑BSW and a 0.2° shift in GC‑BSW, corresponding to the reported sensitivities. The GC‑BSW configuration consistently outperforms DG‑BSW in both sensitivity and FWHM, yielding a figure of merit (FOM) of 2,255°/RIU.

Overall, both grating‑coupled designs exhibit superior performance compared to traditional prism‑coupled BSW sensors, with no strict refractive index limits for the dielectric stack and scalable design parameters that allow operation across any desired spectral band.

Conclusions

We have demonstrated two compact, 2‑D hole‑array grating‑coupled BSW biosensors that achieve angular sensitivities of 1,190°/RIU (DG‑BSW) and 2,255°/RIU (GC‑BSW) at incident angles below 10°. The GC‑BSW architecture, in particular, offers a two‑fold sensitivity increase and a narrower resonance compared to DG‑BSW, while maintaining a low fabrication footprint. These designs pave the way for integrated, high‑performance “lab‑on‑chip” biosensing platforms that can be tailored to any wavelength by adjusting grating and DBR parameters.