Plasmonic Interferometry for Optical Material Characterization

This paper presents a plasmonic refractometer based on plasmonic interferometry, which enables the precise measurement of dielectric functions over a broad spectral range. The device consists of an array of slit-groove (SG) plasmonic interferometers fabricated in thin metal films (silver or gold) and operates without the need for traditional coupling optics such as prisms or gratings. The interferometric principle exploits surface plasmon polaritons (SPPs) generated by nanogrooves, which interfere with direct optical transmission through the slits, producing measurable plasmonic interferograms. These interferograms encode information about the optical properties of the adjacent dielectric material, allowing for direct extraction of refractive index dispersion across visible wavelengths (400–800 nm). This method stands out by eliminating the need for a priori dispersion models, making it a powerful and flexible alternative to conventional spectroscopic ellipsometry and SPR-based refractometry.

One of the most significant findings of this work is the demonstration of high sensitivity and spectral resolution. By carefully analyzing plasmonic interferograms, the refractometer achieves a refractive index resolution better than Δn ≈ 10^-3, sufficient to differentiate materials with small optical contrasts, such as water, methanol, and ethanol. The extracted refractive indices and dielectric functions for these materials are in excellent agreement with reference values, validating the robustness of the method. Furthermore, the device allows for simultaneous measurement of both the real and imaginary parts of dielectric functions, enabling characterization of both transparent and absorbing materials. This level of precision and broadband capability is rarely achievable in conventional SPR biosensors, which typically rely on a single-wavelength resonance condition.

Beyond sensitivity, the study highlights the compactness, scalability, and high-throughput potential of plasmonic refractometry. The fabricated device integrates an 80 μm-deep microfluidic channel, ensuring precise sample delivery and uniform contact with the sensing region. Moreover, the use of high-density plasmonic interferometer arrays (>10⁴/cm²) enables rapid, parallel measurements, a major advantage over conventional prism-based refractometers and ellipsometers, which require large sample volumes and complex alignment procedures. This miniaturized, CMOS-compatible platform opens the door for on-chip, real-time refractometric sensing and integration into portable optical metrology tools.

The impact of this work extends across multiple disciplines, particularly in plasmonics, optoelectronics, and biochemical sensing. The ability to perform broadband, high-resolution refractometry on ultra-small sample volumes makes this technique attractive for lab-on-a-chip applications, material characterization, and chemical or biological diagnostics. In contrast to standard SPR techniques, which are mainly used for biomolecular detection, this plasmonic interferometry-based method provides quantitative dielectric function measurements, making it valuable for fundamental materials research, thin-film metrology, and integrated photonic circuits. Additionally, the design eliminates the reliance on external coupling optics, reducing the complexity and cost of implementing high-sensitivity plasmonic sensors.

In summary, this paper introduces a transformative approach to plasmonic refractometry, bridging the gap between high-sensitivity optical biosensing and broadband material characterization. By leveraging plasmonic interferometry, it achieves a combination of high spatial resolution, broad spectral range, and miniaturization, setting a new benchmark for integrated optical metrology. This research has the potential to influence the development of next-generation spectroscopic tools, with far-reaching applications in nanophotonics, optoelectronic device fabrication, and portable sensing technologies.

This paper introduces a plasmonic refractometer that leverages plasmonic interferometry to precisely determine the optical dielectric functions of various materials across a broad spectral range. By employing slit-groove plasmonic interferometers, the device achieves high-resolution refractometry without requiring traditional bulky optical elements like prisms or gratings. The study demonstrates excellent agreement between extracted refractive indices and reference values, validating the method’s accuracy. Additionally, the integration of a microfluidic channel ensures precise sample delivery, while the high-density interferometer array enables parallel, high-throughput sensing. Compared to traditional techniques such as SPR biosensing and spectroscopic ellipsometry, this approach stands out for its broadband capabilities, miniaturization potential, and independence from predefined dispersion models. These advancements make it an ideal candidate for next-generation optical metrology tools, spanning applications in material characterization, thin-film analysis, and lab-on-a-chip technologies.

Looking ahead, further refinements could enhance sensitivity and extend operational wavelength ranges. The use of alternative plasmonic materials—such as aluminum for deep-UV sensing or titanium nitride for near-infrared applications—could expand the refractometer’s utility. Additionally, integrating machine learning algorithms for real-time data analysis could improve the speed and accuracy of dielectric function extraction. The design is also well-suited for CMOS-compatible on-chip integration, paving the way for miniaturized, portable spectroscopic sensors for use in biochemical detection, pharmaceutical research, and environmental monitoring. With these developments, plasmonic interferometry-based refractometry has the potential to revolutionize optical sensing and metrology, offering a powerful, scalable, and cost-effective alternative to conventional refractometric techniques.