Low-temperature near-field spectroscopy of CdTe quantum dots

A near-field optical microscope has been developed for operation at low tem perature. This microscope is used to study the photoluminescence of CdTe-ba sed quantum dots. Spectra collected upon approaching the optical tip into t he near-field region of the sample reveal the evolution from a broad far-fi eld luminescence band - that is typical for a large ensemble of dots - to a near-field structure made up of a few sharp peaks originating from individ ual dots. Experiments carried out in the excitation-collection mode through the optical tip allow study of the effect of an increase in excitation pow er on the near-field spectra. It is found that upon increasing the excitati on by two orders of magnitude, a spatially resolved spectrum progressively transforms back into a broad 'far-field-like' spectrum. Photoluminescence i mages taken by scanning the sample under the tip are used to discriminate v arious contributions coming from individual dots.