High-speed nano-polarimetry for real-time plasmonic bio-imaging

Plasmonic nanoparticles are excellent nonbleaching probes for bio-imaging. Due to their anisotropic properties, polarization analysis of individual nanoparticles allows for revealing orientational information, plasmon mode assignment, and the local microenvironment. Previous implementations utilize mechanical rotation of conventional polarizers to align the polarization angles with specific axes of nanoparticles. However, the manufacturing defects of the polarizer ( e.g. , non-parallelism) limit the measurement stability ( e.g. , beam wobbling) in polarimetric imaging, while the mechanical rotation limits the measurement speed, and thus hinders accurate, real-time acquisition of individual nanoparticles. Here, we demonstrate a high-speed nano-polarimetric system for stable plasmonic bio-imaging by integrating our voltage-tunable polarizer (VTP) into a microscope. The angular rotation of the polarization (0∼π) can be realized by applying voltage on the VTP. We show that our voltage-tunable system offers high extinction ratio (∼up to 250), and uniform transmission (∼55p) over a large input power range (<5% deviation for input power from 50 μW to ∼20 mW). Meanwhile, the transmission polarization can be rapidly tuned with a response time up to 50 ms. Compared to conventional polarizers, our system is able to provide reproducible and high-speed polarimetric images of individual nanoparticles with sub-pixel spatial precision. Such a polarimetric nanoimaging system could be a useful tool for real-time single nanoparticle bio-imaging with both high stability and time resolution.