Ultra-Sensitive Whispering Gallery Mode Resonators for Photoacoustic Bio-Sensing and Imaging

Whispering gallery mode (WGM) micro resonators are robust optical devices that have high sensitivity, high-quality factors, small mode volume, and strong light-matter interac- tion. This report delves into the extensive applications of WGM microresonators, with a specific focus on their pivotal role in photoacoustic biosensing and imaging. Even though the microresonators themselves can detect nanometer-sized particles utilizing a shift in res- onance, they have to rely on evanescent field interactions with molecules on the surface, while molecules outside the range remain undetected. This report introduces a novel ap- proach, proposing the generation of photoacoustic signals with a pulsed laser and the detec- tion of photoacoustic signals through the microresonator, to enable subsequent imaging or biomolecule detection. Early detection of pathogens, particularly bacteria, is essential for accurately determining the stage of wound infection. Different types of bacterial colonies dominate at different stages of infection due to changes in the wound environment—such as pH, oxygen levels, and the immune response. Identifying the dominant bacteria at an early stage can guide appropriate treatment decisions and help avoid unnecessary use of broad-spectrum antibiotics, which contributes to the growing global issue of antibiotic resistance. This is especially critical in developing regions like Asia and Africa, where access to advanced medical diagnostics is often limited. While traditional methods like PCR or genome sequencing provide accurate bacte- rial identification, they are typically bulky, expensive, and time-consuming. Our approach offers a faster, more accessible alternative. In our current work, three kind of wound bacteria samples are introduced into the microbubble resonator sensor and selectively excited by a pulsed laser at a wavelength specific to the bacterias. Our technique enables selective detec- tion and classification of bacterias by exploiting their unique optical absorption properties. While our current work focuses on controlled flow conditions, it can be potentially miniatur- ized for smart wound dressings, wearable patches and AI assisted monitoring system. The high sensitivity of whispering-mode-resonator makes it an excellent candidate for the high throughput bacteria detection across an extended sensing volume laying the foundation for future non-invasive, contactless wound infection bacteria sensing. In the latter section of the report, we detail the utilization of a packaged microbubble res- onator for imaging, leveraging the acquisition of photoacoustic signals. Even though we have been using tapered optical fiber for coupling light into microresonators, its’ instability and fragility pose limitations when applied in practical applications. The packaging of WGM sensors for practical using MY-132-A, a material distinguished by its low refractive index compared to alternatives. This advancement enables the effective application of the pack- aged device in photoacoustic imaging techniques, offering a comprehensive understanding of the operational capabilities of WGM microresonators in real-world scenarios.