In the fields of advanced manufacturing, biomedical research, and new material research and development, accurate measurement of the refractive index (RI) and thickness of materials is crucial for monitoring chip production yield and exploring novel material synthesis methods. However, existing measurement tools still suffer from inherent limitations. Traditional "nano scales" represented by atomic force microscope (AFM) and transmission electron microscope have extremely high accuracy, but their point-by-point scanning mode results in slow speed and potential risk of damage to the sample. Meanwhile, high-sensitivity RI measurement techniques based on surface plasmon resonance (SPR) often exhibit a measurement accuracy limited to approximately 10⁻⁴ RIU in practical applications, hindering the detection of smaller RI changes. For this purpose, researchers have designed more complex SPR excitation structures. Although these methods have enhanced sensitivity, speed, and miniaturization, they have led to an over-reliance on precision nano-manufacturing, where high costs have become a stumbling block to their widespread applications.
Surface plasmon resonance holographic microscopy (SPRHM) emerges as a novel optical characterization technique. By combining digital holographic microscopy with SPR microscopy, SPRHM leverages dedicated demodulation algorithms to extract diverse physical properties from measured signals. SPRHM offers distinct advantages, including label-free, non-invasive, full-field, real-time measurement with high sensitivity. However, current implementations often suffer from poor system simplicity and limited measurement sensitivity, which restricts their practical applications. Designing a SPR holographic microscope that is both user-friendly and capable of higher sensitivity has therefore become a critical challenge.
In a new paper published in Light: Advanced Manufacturing , a team of scientists, led by Professors Jiwei Zhang and Jianlin Zhao from Northwestern Polytechnical University, China, has successfully developed a versatile SPR holographic microscope based on objective-coupled SPRHM. The microscope features a modular design equipped with a motorized angle scanning module, including beam shaping and control, SPR excitation, imaging, and hologram recording modules. This not only achieves a high degree of integration of the opto-mechanical structure but also makes it easy for component replacement and system upgrades. To suppress environmental noise, the team rigidly coupled standardized cage components with vibration-isolation pillars and sealed system in a dust‑tight housing, ensuring signal stability for long-term measurements. This microscope enables practical deployment outside conventional laboratory settings, providing a robust tool for diverse application scenarios.
The team further introduced an optimized Ag-Au bilayer SPR excitation configuration. This design leverages the superior excitation efficiency of Ag as well as the chemical stability of gold, achieving a perfect balance between high sensitivity and long-term reliability. To maximize the potential of the configuration, the team developed an optimization algorithm that precisely calculates the ideal bilayer thickness for specific samples. Leveraging the custom-built microscope platform and the designed excitation configuration, the team monitored the evaporation dynamics of different ethanol–water mixtures, successfully tracking small RI changes with ultrahigh sensitivity. The results demonstrated a remarkable RI measurement resolution of 2.58 × 10⁻⁷ RIU.
The team also developed a thickness demodulation workflow for atomic layer materials. The method determines the resonance angle through incidence angle scanning, which is then combined with phase information retrieved via holographic reconstruction to achieve pixel-by-pixel thickness profiling across different regions of a graphene terrace. The method achieves a sub-nanometer thickness profiling resolution of 0.6 nm, establishing the microscope as a novel, precise, and reliable platform for characterizing the thickness of atomic layer materials.
“As the SPR holographic microscope prototype bridges laboratory verification and practical applications, this technique establishes a new platform for noninvasive, label-free, wide-field, and high-throughput detection with an ultrahigh sensitivity. We anticipate progressive impacts across biological research, in situ nanomaterial inspection, and electrochemical studies,” the scientists forecast.
Light: Advanced Manufacturing
Refractive Index and Thickness Measurements with Ultrahigh Sensitivity via Versatile Surface Plasmon Resonance Holographic Microscope