High-Efficiency Fluorescence Modulation

Optical interferometry, widely used in biosensing and medical imaging, has traditionally relied on coherent light sources to generate precise optical responses. This paper challenges that paradigm by demonstrating that embedding solid-state light emitters (Cr³⁺:MgO) within plasmonic interferometers can generate coherent surface plasmon polaritons (SPPs) even under incoherent illumination. The study showcases how fluorescence interference patterns can be exploited for biochemical sensing, eliminating the need for precisely aligned and calibrated external light sources. The ability to perform sensing without external coherence constraints enhances system robustness and reliability, making this approach ideal for applications in biosensing, drug discovery, and environmental monitoring. The work is particularly relevant for the development of compact, cost-effective, and high-throughput optical sensors that can operate under variable illumination conditions.

1. Elimination of Coherence Requirements – Unlike conventional plasmonic interferometers, this design enables interferometric sensing without coherent, broadband, or precisely aligned light sources.
2. Integrated Light Emitters – Embedding Cr³⁺:MgO within nanoapertures allows direct SPP excitation, enhancing device stability and simplifying fabrication.
3. Same-Side Excitation and Detection – Enables measurements in open environments without microfluidic channels, solving alignment and sample delivery issues.
4. Highly Sensitive Biochemical Sensing – Demonstrates detection of minute refractive index changes (sensitivity ~2600% RIU⁻¹, spectral FOM ~440 nm RIU⁻¹).
5. Scalability and Miniaturization – Allows integration into compact, low-cost, high-throughput sensor arrays for portable and point-of-care applications.

Plasmonic Interferometry without Coherent Light
A central finding of this study is that plasmonic interferometry can be achieved without the need for an external coherent light source. By embedding Cr³⁺:MgO emitters directly into the nanoholes of plasmonic interferometers, we demonstrated that surface plasmon polaritons (SPPs) can be excited with high coherence regardless of the coherence state of the external illumination. This significantly reduces the complexity and cost of interferometric biosensors, as it eliminates the need for carefully aligned lasers or broadband coherent sources.

Fluorescence-Based Interference for Sensing Applications
The study shows that fluorescence emission from embedded light sources undergoes interference due to SPP-mediated optical paths, creating measurable intensity modulations. This effect is harnessed for sensing applications, where variations in the refractive index of the surrounding medium modify the interference conditions, leading to detectable spectral shifts. The ability to measure biochemical analytes using fluorescence modulation opens new possibilities for high-sensitivity biosensors.

Independence from Illumination Conditions
A major advantage of this approach is its robustness against changes in illumination conditions. The experiments demonstrated that fluorescence modulation remains unchanged for various incident angles, spatial coherence levels, powers, and spectral bandwidths of the excitation source. This makes the proposed sensor architecture highly adaptable to different environments, including scenarios where precise illumination control is challenging.

High Sensitivity in Biochemical Detection
We performed biochemical sensing experiments where small refractive index changes can be detected using modulated fluorescence intensity. The system achieved a figure of merit (FOM) of 2600% RIU^-1 and a spectral shift sensitivity of 440 nm RIU^-1. These values are significantly higher than those of conventional plasmonic interferometers, demonstrating the practical viability of this approach in applications such as medical diagnostics and environmental monitoring.

Open-Surface Micro-Droplet Sensing for Point-of-Care Applications
Unlike traditional plasmonic interferometry, which requires structured microfluidic sample delivery, the proposed system enables open-surface sensing. The ability to detect biomolecular interactions in micro-droplets without a controlled flow system greatly simplifies sensor design. This feature is especially relevant for point-of-care diagnostics, where rapid, label-free biochemical detection in small sample volumes is crucial.

This study fundamentally challenges the conventional requirement of coherence in plasmonic interferometry, demonstrating that embedded solid-state emitters can enable highly sensitive biochemical sensing with incoherent light. The integration of Cr³⁺:MgO emitters into nanostructured plasmonic interferometers allows for a robust and highly sensitive detection platform, which is immune to variations in excitation conditions. The results highlight the potential of this technology for developing compact, low-cost, and high-throughput optical sensors, with applications ranging from clinical diagnostics to environmental sensing.

Future work may focus on extending this approach to different classes of light emitters, such as semiconductor quantum dots or electrically driven sources, to further enhance performance and integration possibilities. Additionally, expanding the spectral range and optimizing device architectures could improve detection limits and selectivity, enabling even broader applications in biomedical, environmental, and industrial sensing.