A substrate-free silicon nitride membrane converts its own photoluminescence into a sensing signal, sidestepping a major practical hurdle for ultranarrow quasi-BIC resonances.
Paper: Photoluminescence-Driven Quasi-BICs in Photonic Nanomembranes for Active Refractometric Sensing. AI-generated abstract conceptual image created using ChatGPT/OpenAI
A paper recently published in Nano Letters developed an emission-based active refractometric sensing platform that integrates a freestanding photonic nanomembrane supporting high-Q quasi-bound states in the continuum (quasi-BIC) resonances.
Importance of Freestanding Architectures
In refractometric sensing, the spectral resolution of a resonance and the confinement of optical-mode energy in the near field govern device performance. Conventional plasmonic sensors use confined electromagnetic fields at the dielectric-metal interface.
These strong near fields provide localized sensitivity to minor analyte binding events, but metals also absorb light because of inherent ohmic losses. In plasmonic platforms, this absorption broadens the resonance linewidth and limits the figure of merit (FOM).
All-dielectric metasurfaces and nanostructures use low-loss dielectrics to produce narrower line widths. Yet, when a substrate supports these structures, the resonant mode profile extends deeply into the inactive bottom layer.
This field penetration enlarges the overall mode volume, introduces radiation leakage, and dilutes surface sensitivity. Freestanding photonic nanomembrane architectures can address this limitation.
Removing the substrate creates a symmetric air/nanomembrane/air index profile when no analyte layer is present. For quasi-BICs, this out-of-plane symmetry reduces out-of-plane radiation leakage.
The freestanding structure maintains the high Q-factor associated with all-dielectric photonic structures while confining sensing to a short distance from the surface, similar to plasmonic platforms. Water on the top surface breaks this symmetry, but the lack of a solid substrate still limits downward leakage and preserves high-Q resonances.
These optical advantages do not remove the difficulty of measuring such narrow resonances.
Impact of Passive Optical Interrogation
Currently, most photonic sensing platforms depend on passive optical interrogation. High-Q quasi-BIC resonances can have subnanometer line widths, so resolving them requires complex instrumentation with highly stabilized external sources and high spectral resolution.
Passive optical configurations are also sensitive to the incident-beam angle. Minor beam divergence, angular deviations, or mechanical vibrations can smooth out or completely obscure the high-Q resonance in measured spectra.
Such strict requirements for precise angular calibration and complex optical setups hinder the practical use of quasi-BIC sensors.
The Proposed Refractometric Sensing Platform
The researchers demonstrated an active sensing platform that couples the defect-related photoluminescence (PL) of a freestanding silicon nitride (SiN) nanomembrane to quasi-BIC modes. The sensor uses emission-based active sensing rather than passive reflection or transmission.
The team used the defect-related PL of the SiN nanomembrane as the internal emission source, which was strongly modulated by and coupled to engineered quasi-BIC modes. The freestanding nanomembrane confined electromagnetic fields near and within the nanostructure, supporting high-Q resonances.
Compared with most passive optical sensing systems, the active approach uses a simpler measurement setup and allows direct detection of high-Q PL peaks. A 488 nm continuous-wave laser excited the PL, which was collected through the same 10× objective. The setup still requires optical excitation and spectroscopy, but the emission readout avoids angle-matching the interrogation beam to the quasi-BIC resonance.
The SiN nanomembranes in their prepatterned form were obtained from commercial sources, and low-damage reactive-ion etching was used to fine-tune the membrane thickness.
Every 500 × 500 μm² freestanding window contained several distinct 25 × 25 μm² patterned regions. The region studied here comprised a 50 × 50 hole array with a final membrane thickness (T), hole diameter (D), and lattice period (P) of 125, 250, and 500 nm, respectively.
To identify the resonances associated with the observed optical response, the authors used rigorous coupled-wave analysis to map the resonance characteristics of the photonic nanomembrane. The nanomembrane refractive index dispersion was taken from experimental measurements.
In the simulation, the incident angle was varied along the y-axis, with the electric field along the x-axis. The authors characterized the nanomembrane's photonic resonances using the simulated absorption spectrum.
Changes in absorption mainly tracked the resonant field strength of the supported optical modes. Finite-size effects also produced the multiple fine PL peaks observed experimentally.
The Platform Effectiveness
The researchers demonstrated active refractometric sensing with the freestanding photonic nanomembrane. The emission readout reduced the need for precise angular positioning and beam collimation used in passive schemes to resolve ultranarrow resonances.
The no-substrate architecture tightly confined near-field electromagnetic energy to the nanostructure surface and reduced substrate-induced out-of-plane radiation leakage.
The photonic nanomembrane yielded a 0.17 nm linewidth (Q ≈ 4000) in air and retained a narrow 0.26 nm linewidth in a water/nanomembrane/air configuration. In pure water, it resolved refractive-index changes as small as Δn = 1 × 10-³ and achieved a FOM of 250, substantially above representative values reported for passive quasi-BIC sensors in the visible regime.
Glucose solutions from n = 1.333 to 1.396 produced systematic redshifts, and returning to pure water restored the initial spectral position. Liquid filling inside the nanoholes was not directly verified. A leaky mode had greater bulk sensitivity, but the quasi-BIC's narrower line gave the higher FOM.
Numerical analyses beyond bulk index sensing showed a strongly localized surface sensitivity profile, with up to 39.2 times higher response at the nanomembrane interface than at a distance of 200 nm. These simulations suggest that the design could detect ultrathin surface-binding events and small analytes. The study experimentally tested bulk refractive-index sensing, not surface-binding detection.
Overall, the active quasi-BIC nanomembrane design combines emission-based readout with substrate-free high-Q sensing. The authors describe the method as suitable for larger-scale use, though they did not test manufacturing scale-up in this study.
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