Plasmonic microscopy has been widely used in the imaging and analysis of nanosized objects in various applications. For effective identification of nanomaterials, inherent information such as refractive index, sample sizes, and orientations are required, and obtaining such information directly from phase shift in surface plasmon resonance is believed to be superior in terms of sensitivity. Among the models used for phase quantification, surface plasmon resonance interferometry has enabled phase imaging but at a high cost of complex optics. Besides, it has a limited spatial resolution that affects its overall sensitivity.
Previously, treating surface plasmon resonance microscopy as scattering interferometry has been identified as a promising solution for obtaining phase information and improving the spatial resolution without the need for additional optics. An image processing algorithm has been effectively incorporated in interferometric plasmonic microscopy with a remarkable improvement in its sensitivity and resolution. Unfortunately, this approach has become less popular due to information loss during filtering steps in the image reconstruction algorithms.
To address these issues, researchers at Shanghai Jiao Tong University: Dr. Yuting Yang, Chunhui Zhai (Ph.D. student), Qiang Zeng (Ph.D. student), Dr. Ab Lateef Khan, and Dr. Hui Yu developed a new method for obtaining the quantitative amplitude and phase images in interferometric plasmonic microscopy. First, the principle of plasmonic imaging under different focal conditions was established. Next, a holographical reconstruction algorithm was used to represent the retrieving amplitude and phase images. Furthermore, the authors determined the correlations between the nanoplasmonic functions and structures as well as that between the phase images and refractive index of the nanoparticles. In what appeared as validating the proposed approach, its potential applications in probing plasmonic near field and super-resolution imaging was investigated. The work is currently published in the ACS Nano journal.
The authors observed that operating the plasmonic microscopy over the surface plasmon resonance angle separated the twin images. This further allowed for accurate mapping of the amplitude and phase distribution of the surface plasmons near the fields. Due to the enhanced imaging capabilities, it was possible to directly visualize the complex surface plasmon fields emanating from the interaction with nanowires and nanoparticles without the need for nanoscopic scanning probes. Unlike the conventional methods, the proposed approach has several advantages including a large field of view, good compatibility for biosensing, no need for nanoscopic probe, and a high imaging rate.
Based on the research findings, proof of the concept was demonstrated through theoretical and experimental analysis. The results suggest that the proposed method will enhance future applications of nanoparticles and super-resolution imaging. Thus, Dr. Hui Yu (ipmyu.sjtu.edu.cn), the lead author in a statement to Advances in Engineering expressed his confidence the importance of the new method quantitative amplitude and phase imaging capabilities in enhancing future nanoplasmonic studies and various sensing applications.
Yang, Y., Zhai, C., Zeng, Q., Khan, A., & Yu, H. (2019). Quantitative Amplitude and Phase Imaging with Interferometric Plasmonic Microscopy. ACS Nano, 13(11), 13595-13601.