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Enhancement and quenching of the fluorescence of single CdSe/ZnS quantum dots studied by confocal apertureless near-field scanning optical microscope

We have studied the influence of Si atomic force microscope (AFM) probes on fluorescence of ZnS overcoated CdSe quantum dots (QDs) in an apertureless near-field scanning optical microscope (ANSOM). In these ANSOM measurements, the excitation light polarization and probe preparation procedure strongly affect the QD fluorescence. When the excitation light polarization is orthogonal to the probe axis (and parallel to the substrate surface), we detect 50 to 80% fluorescence quenching, with a HF-etched Si probe scanning ~10 nm above the sample. With polarization of the excitation collinear with the probe axis, the optical field is amplified many times in the near-field zone, and the net result is a 2-4 times fluorescence enhancement. In this work we utilize a home-built, non-contact AFM and confocal ANSOM microscope under total internal reflection of Ar+ laser beam excitation.