Researchers have developed a 3D micro-printed microlaser sensor with directional light emission and ultrahigh sensitivity, offering a promising tool for early disease detection and lab-on-a-chip diagnostics.
Study: 3D micro-printed polymer limacon-shaped whispering-gallery-mode microlaser sensors for label-free biodetection. Image Credit: Max Acronym/Shutterstock.com
In a recent study published in Optics Letters, a team of scientists introduced a new method for creating whispering-gallery-mode (WGM) microcavities using 3D micro-printing. By shaping these tiny resonators into a limacon form, they achieved directional light emission while maintaining a high-Q factor—key for efficient light collection and integration into photonic systems.
Their approach enables the rapid, scalable production of complex optical structures from biocompatible polymers, and their goal is clear: to demonstrate that these limacon-shaped WGM microlasers can detect trace levels of biologically relevant molecules, pushing lab-on-a-chip technology toward real-world applications.
Background
WGM microcavities guide light along their edges using total internal reflection, which leads to intense confinement and enhanced optical interactions in extremely small spaces—ideal for sensing. However, traditional fabrication methods like femtosecond laser writing and electron-beam lithography offer precision but fall short on scalability and cost-efficiency.
To address this, the researchers employed a high-resolution 3D micro-printing technique that provides both design flexibility and compatibility with materials like SU-8, a transparent, mechanically stable polymer. This opens the door to rapid prototyping and scalable integration into photonic sensing platforms.
The Study
The team fabricated limacon-shaped microcavities from SU-8 using advanced 3D micro-printing, then doped them with Rhodamine 6G (Rh6G)—a dye known for its photostability and high quantum yield. Mixed at 1.5 wt% into the SU-8, the dye served as the gain medium, activated by optical pumping.
To test the microlasers, researchers illuminated them with a pulsed green laser at 532 nm. The focused beam triggered lasing in the WGM modes, and the resulting emission was collected via a multimode optical fiber and analyzed using a spectrometer with 25 pm resolution. They assessed performance metrics like Q factor, linewidth, and lasing threshold, while also studying the directional emission pattern in both air and water to simulate different sensing environments.
Numerical simulations complemented the experiments, modeling the electric field distribution and the effect of external refractive index changes on resonance behavior. These models helped fine-tune the geometry for improved sensitivity.
Results and Discussion
The results were impressive. The microlasers exhibited a low lasing threshold of 3.87 μJ/mm2 and a narrow emission linewidth of about 30 pm, translating to a Q factor near 2.0×104. Despite the asymmetric cavity shape, the lasing was stable and consistent across different pump energies.
A major highlight was the directional emission—a design feature that significantly boosts light collection efficiency. Far-field emission profiles showed strong, angle-specific peaks. Although the pattern shifted slightly when tested in water, the directional nature held steady, proving the system’s robustness across different environments.
The sensors also responded clearly to changes in the external refractive index. Both experiments and simulations showed a near-linear shift in resonance wavelength, with sensitivities around 20 nm/RIU for deformation coefficients between 0.4 and 0.5. This level of precision is critical for detecting subtle molecular interactions.
When applied to biological samples, specifically human immunoglobulin G (IgG), the sensors achieved an ultralow detection limit of ~70 attograms per milliliter (ag/mL). This sensitivity stems from the strong light confinement and high Q factor, allowing the device to detect minute resonance shifts as small as a few picometers. The results show real promise for applications in early disease detection, where biomarkers are often present at extremely low concentrations.
Conclusion
By combining the design flexibility of 3D micro-printing with the optical advantages of WGM microcavities, the researchers have developed a practical, scalable approach for building high-performance biosensors. These microlasers offer a powerful foundation for integrated, portable diagnostic tools that can rapidly and sensitively detect multiple biomarkers.
The study represents a significant step toward bringing optical biosensors into everyday healthcare, with meaningful implications for early diagnosis and personalized medicine.
Journal Reference
Wang Z., Raza M., et al. (2025). 3D micro-printed polymer limacon-shaped whispering-gallery-mode microlaser sensors for label-free biodetection. Optics Letters, 50(11), 3481-3484. DOI: 10.1364/OL.557384, https://opg.optica.org/ol/fulltext.cfm?uri=ol-50-11-3481&id=571264