Observation of DNA fixed on a sapphire substrate by a collection mode near-field optical microscope

We present the observation results of a single string DNA (deoxy ribo nucleic acid) obtained by collection mode near-field optical microscope (NOM) operated under constant distance mode with the optical near field intensity as the feedback signal. The observed width of the narrowest string is 4 nm. To our knowledge, this is not only the successful optical observation of a single string DNA by purely optical means but also under such high resolving capability. These are attributed to the special care taken during the sample preparation to avoid unwanted scattered light and to the probe which were specially designed to enhance the local near field interaction. All these factors make the probe picks up efficiently the high spatial frequency components of the scattered near field at the same time rejecting effectively the lower spatial frequency components.