Optical sensing is a powerful and non-invasive approach with wide-ranging applications in biomedical diagnostics, environmental monitoring, and food safety, among others. The sensing strategy aims to detect and quantify the interactions between a target analyte and the selective receptor immobilized on optical devices. However, when the targets are small and rare in quantities, achieving suitable specificity and sensitivity becomes increasingly challenging. Traditional biosensors rely on the use of fluorescence or secondary amplifying labels to measure biomolecular interactions, but the labeling process is time-consuming, labor-intensive, costly, and may result in false negative signals due to the blocking of reactive binding sites. An alternative label-free strategy based on plasmonic and photonic platforms has been extensively investigated to achieve ultra-sensitivity, high stability, and ultra-compact capabilities.

Surface plasmon resonance (SPR) biosensor is so far the most prevalent and well-established label-free detection technology, characterized by its real-time monitoring of biomolecular binding events and high-throughput imaging capabilities. SPR biosensor utilizes surface plasmon polaritons (SPPs), which are the collective oscillations of free electrons along a metal-dielectric interface excited by incident light with an appropriate wavevector satisfying the resonance condition. The excitation of SPPs generates strong electric field confinement that penetrates the sensing medium to depths of ~30-200 nm, enabling high sensitivity to variations in the surrounding refractive index (RI). Similar to any technology, the development of plasmonics faces challenges, which for SPP waves that are tightly confined at the metal interface are due to the strong non-radiative, Ohmic heating, losses in metals that limit propagation to only a few microns. This is known as the loss-confinement trade-off of SPP propagation and is one of the biggest research challenges in plasmonics research. Moreover, for SPR sensing in particular, these losses result in a broad linewidth which ultimately hinders the sensing capability, as any practical application demands substantial intensity variations or spectral shifts in the reflection or transmission spectrum. Since its first implementation in biosensing, various improved SPR-based sensing platforms have emerged to increase the RI resolution, allowing the detection of smaller bio-binding events. Progress has primarily focused on pursuing narrower resonance curves for higher precision discrimination and minimizing reflectivity to reduce the detection noise and achieve phase singularities. The best, cutting-edge plasmonic sensors to date, have achieved ultra-low limit of detection (LoD) in the range of 10⁻⁷ to 10⁻⁸ refractive index unit (RIU); they include plasmonic surface lattice resonance sensors and quantum-enhanced plasmonic sensors. However, obtaining such resolution necessitates picometers or millidegrees phase accuracy or a complex quantum-enhanced optical path to determine the sensing parameters.

This dissertation presents a comprehensive investigation of hybrid plasmonic-photonic multilayer systems and their applications in optical sensing. The thesis is systematically organized into seven chapters, each addressing specific aspects of the research. Chapter 1 provides the research background and the significance of plasmonic-based optical sensors. This chapter also clearly indicates the thesis objectives and provides a structural overview of the thesis. Chapter 2 delves into the theoretical framework and computational methodologies essential for understanding hybrid plasmonic-photonic multilayers. This theoretical foundation facilitates the analysis and prediction of the systems’ distinctive optical responses through rigorous simulation approaches. Chapter 3 offers a critical review of plasmonic-based label-free optical sensors. This chapter systematically addresses three key areas: (1) the fundamental principles and classification of biosensors and optical sensors, (2) critical performance parameters and various interrogation modalities, and (3) innovative strategies for enhancing sensing capabilities through advanced structural designs and novel optical phenomena. Chapter 4 focuses on the practical aspects, detailing the fabrication methodologies and characterization techniques employed in developing hybrid plasmonic-photonic multilayer structures. Chapters 5 and 6 present original research contributions through systematic investigations of two distinct hybrid plasmonic-photonic platforms. Chapter 5 examines a three-layer metal-dielectric-metal system, while Chapter 6 investigates a four-layer metal-dielectric-metal-dielectric lithography-free metasurface. Both designs demonstrate precise control over zero reflection points (ZRPs) for s- and p-polarizations. Through careful consideration of the constituent materials’ optical constants, these platforms enable strategic manipulation of ZRPs positions, facilitating the synergistic interaction between SPPs and photonic waveguide (PWG) modes. Chapter 7 provides a comprehensive conclusion of the research outcomes and forward-looking perspectives, describing promising avenues for future investigation.

This research endeavors to explore and elucidate the complex interplay of hybrid SPP and PWG modes within multilayer structures in the context of spectroscopic ellipsometry (SE) as a sensing strategy. Previous works have shown the advantages of SE as arising from its phase sensitivity. Our approach goes a step further by harnessing the essence of ellipsometry as a measurement of the complex reflectance ratio of p- to s- polarizations. This innovative approach enables the synergistic combination of i) plasmonic modes; ii) photonic modes; and iii) interferometric-like, phase-sensitive relative polarization state sensing. This method therefore enables the rational, and well-coordinated control, of the tunable topological darkness positions of p- and s- polarizations and holds significant promise for revolutionizing biosensing technologies across diverse science, medicine, and industry fields. The study addresses the following key objectives:

1) Hybrid SPP–PWG Mode Establishment in P-Polarization
We investigate the hybridization of SPP and PWG resonances in p-polarization, aiming to achieve enhanced electric field intensity and penetration depth while reducing optical losses. This lithography-free multilayer platform offers a novel approach to balancing the inherent loss-confinement trade-off in plasmonic systems. The hybrid mode facilitates suppression of reflection and induces abrupt phase jumps, providing a versatile tool for precise tuning of p-polarized ZRPs.

2) Ellipsometry Sensing and Incorporation of S-Polarized PWG modes
We comprehensively analyze the multilayer structure using SE which is a technique ideally suited for quantitative study of the optical properties of thin films and multilayers. Importantly, SE eliminates experimental errors associated with intensity-based methods, such as transmission and reflection spectroscopy, because by using the ratio of p- to s- intensities it becomes self-referenced and insensitive to light source fluctuations. Critically, we then introduce s-polarized PWG modes into the sensing scheme in an engineered and optimized manner to the overlap of p- and s-polarized ZRPs. This approach enables previously unexploited capabilities inherent to SE resulting in a significant increase in sensitivity for the amplitude ratio (Ψ) and phase difference (Δ) spectra and enabling high-resolution sensing capabilities.

3) Numerical Validation of Experimental Results
To develop a robust understanding of the novel concept of orthogonally polarized ZRP overlap, we employ rigorous numerical simulations. Experimental findings are corroborated using Fresnel coefficient calculations and the fully-vectorial Finite-Difference Time-Domain (FDTD) methods. Given the complexity of the multilayer structure used, we conduct comprehensive parametric analyses to explore the system’s tolerance to variations in optical constants, layer thicknesses, material densities, and surface roughness. This investigation aims to establish the robustness and stability of the proposed sensing strategy.

4) Label-free Real-time Monitoring
Leveraging the unique optical responses of hybrid plasmonic–photonic systems, we seek to refine label-free detection methodologies. Our goal is to enable real-time monitoring of trace biomolecular interactions without the need for fluorescent or radioactive labels, thereby preserving the native state of the molecules under investigation and facilitating the observation of dynamic processes in biological systems.

5) Development of an Innovative “Tune and Reset” Strategy
We propose and validate a novel approach for recalibrating measurement scales to their optimal sensing operation points. This method involves precise modulation of the angle of incidence (AoI) while maintaining a record of the total accumulated RIU change. The strategy aims to combine an ultra-low LoD with an exceptionally broad sensing dynamic range, addressing a longstanding challenge in biosensing technologies.

6) Practical Biosensing Applications
To demonstrate the practical utility of our platform, we apply it to specific biosensing challenges. Initially, we focus on the specific recognition of SARS-CoV-2 spike (S2) protein through surface modification, tracking the complete biofunctionalization process, followed by resetting to optimal sensing conditions and performing dose-dependent assays. Furthermore, recognizing the importance of detecting small biomolecules for real-time clinical diagnostics, we investigate the affinity binding between ampicillin (a penicillin-based antibiotic with a molecular mass < 500 Da) and its specific aptamer.

In conclusion, by leveraging the superb LoD, stability, and large reaction area of the multilayers, the proposed hybrid plasmonic-photonic platforms based on topological darkness intersection open new avenues for the development of versatile platforms for ultrasensitive biosensors. Furthermore, this study not only advances our fundamental understanding of complex light-matter interactions in stratified media but also provides a solid foundation for the development of novel optical devices that harness the unique properties of topologically protected photonic modes.