Advanced Microscopy

We develop advanced optical techniques that detect linear scattered light from small particles for studying important physical and chemical processes in living systems. The high sensitivity and the fast acquisition rate of our optical systems enable direct observation of the rapid motions of individual small particles in three dimensions.
Linear scattering and absorption are the most fundamental light-matter interactions. They have been used as the contrast mechanism for optical microscopy because of their simplicity and reliability. The high spatial and temporal resolution of optical microscopy has made it an indispensable tool in the study of life science. Scientists have found that many disease-related phenomena originate from the interplays between individual molecules. However, conventional scattering-based or absorption-based optical microscopy does not have sufficient sensitivity to observe nanoscopic objects or single molecules. This is because, for a nano-sized object, the scattering and absorption signal is usually very weak compared to the background. Therefore, detection and imaging of small particles or single molecules have been heavily reliant on fluorescence labeling. Modern scientific light detectors and cameras have made single-molecule fluorescence measurements as routines in laboratories. Unfortunately, the precision, speed, and observation time of the fluorescence measurements are restricted by photobleaching and saturation of the signal as the result of the fundamental photo-chemistry and photo-physics of the fluorophores.

We believe the next-generation optical microscope technique should exploit the most basic light-matter interaction, that is, to take advantage of linear scattering and absorption (extinction) of light for image formation. In many circumstances, scattering-based approaches avoid the aforementioned limitations of the fluorescence method. A linear scattering signal is stable and indefinite, facilitating long-term observation. The strength of a linear scattering signal can be increased by raising illumination intensity. In addition, given a sufficient sensitivity, scattering-based imaging requires no label, allowing us to study living systems in their most native forms. To enhance the sensitivity of scattering-based imaging, we design the optical system and detect the signal by widefield interferometry (e.g., iSCAT microscopy and COBRI microscopy). Our current methods have the sensitivity to directly observe the dynamics of individual particles of 10 nm. In addition to the high sensitivity, the interferometric measurements also enable high precision at an ultrahigh image acquisition rate (100,000 frames per second). The ultrahigh speed and ultrahigh precision of scattering-based optical microscopy offers the opportunity to investigate dynamics in living systems with unprecedented clarity.
More information:
iSCAT microscopy at 500,000 Hz
COBRI microscopy for single virus tracking at 100,000 Hz
High-speed imaging and tracking of 10 nm gold nanoparticles