New dGMR Sensor Breaks Records for Refractive Index Sensitivity in Compact Design

Breakthrough in Optical Sensing Technology

Scientists have developed a groundbreaking optical sensor that leverages differential guided-mode resonance (dGMR) to achieve unprecedented sensitivity in detecting changes in refractive index. This innovation opens the door to more advanced, real-time monitoring systems in both medical and environmental fields.

The Significance of Refractive Index Sensors

Refractive index sensors are essential tools for identifying minute shifts in optical properties. Their high sensitivity makes them crucial across various domains, including medical diagnostics and environmental monitoring. Traditional techniques such as immunofluorescence and mass spectrometry, while reliable, often require complex hardware and extensive sample preparation. Even optical refractive index sensors, despite their sensitivity, face challenges related to intricate setups and limited portability.

How the dGMR Sensor Functions

The dGMR sensor overcomes these limitations by engineering nanometre-scale thickness variations in a planar waveguide on a metal substrate using the Kretschmann configuration. Incident light excites surface plasmon polaritons at the metal surface, which then couple into guided-mode resonances within the waveguide. Introducing two slightly different thicknesses in the waveguide creates two resonant modes that shift relative to each other as the surrounding refractive index changes.

These shifts manifest as ring-shaped dark stripes in reflected light, easily captured by a standard CMOS camera. By tracking the position of these rings, the sensor achieves extremely precise measurements—up to one million pixels per refractive index unit (RIU), which is three orders of magnitude better than previous imaging-based approaches.

Unlike conventional resonance sensors, the dGMR’s sensitivity is primarily governed by the difference in thicknesses in the waveguide, allowing for high performance without overly complicated optical setups.

Fabrication and Chip Designs

To demonstrate the concept, researchers fabricated two types of sensor chips: one with pixelated arrays created through electron beam lithography, and another with continuous thickness gradients formed via plasma-enhanced chemical vapor deposition. Both methods provided nanoscale precision in controlling waveguide thickness.

Testing showed the sensor's versatility. On continuous gradient chips, even small refractive index changes caused spatial shifts of hundreds of pixels in the resonance rings. The response remained linear, enabling accurate, quantitative measurements.

Adjusting the incident illumination angle extended the dynamic range to cover wider refractive index intervals, useful for applications like tracking glucose concentrations. The figure of merit, defined as sensitivity divided by the full width at half maximum of the resonance peak, reached 104 RIU⁻¹, showcasing the sensor’s superior performance compared to conventional surface plasmon resonance sensors.

Practical Demonstrations and Applications

Practical demonstrations included measuring glucose solutions with concentration errors below 0.5%, monitoring polydopamine film deposition, and detecting nanomolar biotin via streptavidin-functionalised chips. Both pixelated and gradient designs produced clear, interpretable patterns linked to analyte concentration.

A compact prototype was built, measuring just 20 x 14 x 8 cm, integrating a dGMR chip with a smartphone camera. The device successfully mapped humidity patterns by detecting local refractive index changes, highlighting its potential for field-based environmental monitoring.

Microfluidic channels expanded the platform’s capabilities, enabling multi-channel sensing arrays for simultaneous biochemical assays. The sensor’s high sensitivity at low concentrations supports early disease detection. Additionally, the ability to encode refractive index data into QR code-like patterns shows potential for secure optical data storage and transmission.

Future Prospects

The research team is currently focused on refining fabrication for scalable production, further miniaturizing the system, and developing intuitive software for pattern analysis. Expanding the platform to support multi-parameter sensing could make it even more versatile for real-time monitoring in medical, environmental, and biochemical applications.

With its combination of ultra-high sensitivity, wide dynamic range, and compact design, the dGMR sensor sets a new benchmark in optical sensing technology.