We have developed a time-correlated single photon counting system which has a modified photomultiplier tube as photon detector with properties of a high quantum efficiency, relatively large active area, and extremely small dark counts, for studying fluorescence characteristics of a single emitter. In this type of study, it is also important to collect emitted light from a single emitter. Therefore, we have developed an efficient light-collection system under high vacuum and various ambient gas conditions. This system has an immersion objective and an ionic liquid is used as a refractive index matching medium, and these are positioned in high vacuum. Using the measurement system, we have investigated fluorescence characteristics of single colloidal quantum dots and fluorescence dyes. Finally, we discuss the possibility for applications to various optical measurements using the measurement system.

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