Scanning Resonator Microscopy

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Summary Photoactivated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM) have revolutionized the super-resolution field by introducing approaches that are reasonably mastered and implemented in the end-user laboratory. Their impact is now felt across the disciplines where their unique capabilities are being applied to an ever-expanding spectrum of important problems. Near-field scanning optical microscopy (NSOM) is another super-resolution technique that has attributes complementary to those of PALM and STORM. NSOM is a scanning probe technique that uses specially fabricated fiber optic probes to measure super-resolution fluorescence and sample topography, simultaneously. This is particularly useful in the biological sciences, where cell shape and morphological features can be compared directly with species specifically labeled in the fluorescence image. To date, however, the impact of NSOM in the biological sciences has been modest. This is mainly due to burdensome implementation requirements and poor performance of the fiber optic probes. To overcome these challenges, we propose a completely new approach for integrating optical contrast mechanisms with atomic force microscopy (AFM). Scanning resonator microscopy (SRM) uses a small dielectric microsphere attached at the end of conventional AFM probe for super-resolution imaging. The approach exploits whispering gallery mode (WGM) resonances excited in the attached resonator to sense or excite sample properties. Unlike conventional NSOM, the probes are easily assembled under a dissecting microscope and SRM requires only minimal modifications to commercial AFM platforms. This should enable widespread adoption in the end user lab. We have developed a prototype SRM and demonstrated the feasibility of this approach by simultaneously quantifying sample refractive index and topography of thin films with super-resolution. The overall goal here is to develop the next generation SRM capable of simultaneous fluorescence, refractive index, and topography measurements on complex biological samples. The research plan proposed here will: (Aim 1) develop a “ride along” fiber optic coupler for SRM tip excitation that enables super-resolution fluorescence imaging on thick biological samples and (Aim 2) test, validate, and benchmark SRM performance on a real biological system by studying annexin VI localization in fixed human arterial smooth muscle cells following calcium stimulation. The successful completion of this work will introduce a new super-resolution tool that can easily be adopted in the end user lab, with unique capabilities complementary to existing technologies. For the future, it is easy to envision additional sensing capabilities being integrated with SRM through specific coatings or modifications of the optical resonator at the tip end.