Non-Uniform Microlens Array Based on Photonic Nanojets for Remote Raman Sensing of Subsurface Analytes

Raman spectroscopy is a powerful technique for surface molecular analysis due to its ability to provide molecular fingerprint information. However, its application to subsurface analytes is limited by destructive or invasive methods that compromise the detection accuracy. To address this, we introduce a non-uniform microlens array based on the photonic nanojet (PNJ) principle to realize subsurface remote Raman sensing. Using finite element simulations, the microlens design was optimized with a central lens radius of 5 μm and side lenses of half this radius, achieving a 52% increase in the focal length and a subwavelength spatial resolution compared to a single microlens. The non-uniform design also enhanced the Raman intensity by 85%, enabling sensitive detection of the subsurface analytes. The design’s versatility was validated with a rectangular microlens array, which showed similar improvements. Fabrication using 3D printing produced experimental results closely aligned with those of simulations, with focal length deviations of less than 9% at 1550 nm. These findings demonstrate that non-uniform microlens arrays are scalable, non-invasive, and effective tools for Raman spectroscopy, offering potential applications in biomedicine, materials science, and environmental monitoring, advancing the capabilities of subsurface sensing technologies.