Investigation on achieving super-resolution by solid immersion lens based STED microscopy

The feasibility of stimulated emission depletion (STED) microscopy using a solid immersion lens was investigated. First, the theoretical feasibility of the considered system is discussed based on a vectorial field algorithm that uses a stratified medium composed of a SIL air-gap and test sample. Using the simulation, we verified that evanescent waves with much higher spatial frequencies corresponding to the high numerical aperture in the air-gap can be utilized to achieve a higher resolution than a confocal fluorescent image without a depletion beam. The simulated expectation was supported by actual imaging on two types of samples: fluorescent beads with a 20 nm diameter and an actin sample with a filamentous structure. The lateral resolution of the system was determined to be 34 nm via the imaging results on the nano-beads. The system was quite promising for achieving nano-scale surface imaging of biological samples; an even higher resolution was achieved by adjusting the wavelength and the intensity of the depletion beam.