A low-temperature scanning confocal and near-field optical microscope

Abstract

A scanning near-field optical microscope equipped with an Al-coated glass fiber tip for operation in a He cryostat is described. The instrument is designed for fluorescence detection of nanoscopic particles (single dye molecules and nanocrystallites), and allows optical imaging and spectroscopy of surfaces or thin objects with subwavelength spatial resolution at low temperatures. A shear-force control loop keeps the probing fiber tip aperture at a constant distance, i.e., a few nanometers away from the sample. The shear-force method results in a vertical stability of better than 2 nm. For light collection and confocal imaging, a transmission objective with a high numerical aperture is operated inside the cryostat. For rapid sample inspection, and in cases where no high lateral resolution or topographic imaging is necessary, the instrument can also be used without a tip as a low-temperature conventional or confocal microscope, due to the large scan range of an improved scanner. Details and characteristics of the experimental setup as well as first results are presented. First shear-force images of test patterns taken at low temperatures down to T=22 K, as well as near-field optical fluorescence images of low concentrated sulforhodamine molecules dispersed in a thin polymer film at T=75 K and room temperature, are shown. With this instrument, confocal images of CdS nanocrystallites were taken at 20 K.